Hot dip galvanised steel is reliably painted every day using a number of different systems.  A major benefit of this “Duplex System” is that the whole considerable outperforms the sum of the parts and warranties of up to 50 years are being given on some projects. 

Surface preparation is often required prior to painting especially if a weak sodium di-chromate bath (or “chromate bath”) has been used to passivate the surface.  If the chromate bath is not used it is recommended that the steelwork be painted within 10 hours of galvanising and is always protected from the elements, a practice often used on large infrastructure projects such a light poles.

If the chromate bath has been used (normally the case), reliable surface preparations generally include etch primers or whip blasting but the preparation needs to be matched to the paint system. 

 For other paint systems we recommend contacting paint manufacturers.

Yes you can but you need to know whether it is protected from cleansing rain salt spray etc and if it is unprotected in the ground, you will also need soil parameters such as resistivity and moisture content.

For a precise answer from an experienced engineer in Canberra call Mal Wilson from Advanced Structural Designs on Ph 02 6161 2171.

You’re probably thinking we specialise in commercial buildings rather than agricultural structures and you are right, but Mal Wilson did carry out quite a deal of research in order to try to justify some very marginal steel silos, conveyors and elevator towers in a grain handling facility at Temora in 2001 and carried out a forensic analysis of some silos in central NSW that failed in a wind storm​ in 2012.

The three predom​inant design considerations for the flat-bottomed silos were:

1. Bursting under hoop tension during loading
2. Buckling of the wall plates during unloading
3. Wind loading when empty

Bursting under hoop tension is a relatively easy check but care needs to be taken to ensure that discontinuities such as doorways are adequately catered for.

Wind loading is also a relatively simple check provided you have access to a finite element program supporting curved shell elements.  Local wind funnelling between silos is a well-known effect as is the possibility of vortices shedding from the leeward face.  We normally use NSW Universities wind engineers when model testing or expert opinions are required.

Buckling of the wall plates during unloading is potentially simple if you stick to the design recommendations in the many excellent papers written on the subject.  Mal has 16 design papers and a couple of good reference books that cover all aspects of design. 

In a typical situation of a lap bolted steel silo with vertical stiffeners account needs to be taken of :

1. Moment at the horizontal lap.
2. Relative stiffness of the walls and stiffeners (remembering the Poisson’s ratio effect on the walls).
3. Effect of bolt slippage.
4. Restraint at the base (against horizontal movement).
5. Effect of discontinuities such as doors and aeration ducts.
6. Beneficial effect of the grain pressure.

Unfortunately when you do take account of all of these effects a disturbing proportion of so called standard designs brought in from overseas do not comply with the Australian Design Codes.

We spoke to a couple of Australian designers working in the area but sadly they informed us that they did not take any account of items 1 to 5 and were unaware of the many design papers on the subjects.  The codes unfortunately cover loads rather than design techniques and fail to point designers in the direction of any relevant research.

One designer Mal spoke to from Queensland said he had been designing grain silos on high platforms for years and never taken any account of the possibility of earthquake loading.  This was particularly surprising as 3774 is crystal clear when it comes to loading and devotes a whole section to Seismic loading. 

The Temora job required a great deal of strengthening work to make it code compliant and left Mal with the distinct impression that designers of agricultural structures have a long way to go before they deliver the same degree of reliability as building designers.  One reason for this may be that there are many excellent and comprehensive books around on designing various types of buildings there are many fewer on silos.  The best single book Mal can recommend on this subject is “Guide to the Economic Design of Circular Metal Silos” by J Michael Rotter 2001 Spon Press.  You will also need a copy of a few European Codes to use it.

Mal has developed a number of excellent design tools that take the tedium out of designing and checking circular silos, so if you require some structural engineering appraisals or designs of silo’s near Canberra ACT near we would be happy to take it on.

​ 

If you are contemplating a refurbishment or extension we recommend insisting that the shop detailing be carried out by a local detailer.  This way the detailer has no problem with visiting the site with a tape measure and getting it right first time.

We have worked with all of the above and found them all to be very accurate and reliable detailers.

Class A1 concrete finish

It is firstly imperative that you understand that this type of formwork is normally only specified for monumental surfaces of relatively small area. For just what to expect and when it should be specified we suggest reading AS3610 and commentary.  The Cement and Concrete Association also has some useful publications on the subject. 

Some information that you won’t find in the above literature but that you may find useful is given below.

Rebates

It is important to specify what materials shall be used to form rebates as standard rebates used to be ripped from maple but are now often ripped from Miranti or other cheaper timber.  The trouble with some of these is that the surface left after ripping is often not smooth and this has a deleterious effect on the final product. Consider specifying clear strips of radiata pine but also check and approve all rebate timber prior to being used in the forms.

Plastic rebates leave a beautifully smooth finish but can cause serious problems where you have high daily temperature variations. The thermal coefficient of plastic can be as much as 20 times that of timber. 

Nails are generally used to hold the rebates in place and the specification may call for these to be punched filled and sanded smooth.  Covering nail holes is also a little more difficult with plastic rebates as the depression from the nail is wider and when sanding a filler it is difficult to get a smooth transition with such a slippery substrate.

Curing 

If you are looking for colour control A, we do not recommend wetting intermittently or covering with plastic or leaving the formwork on too long as all of these will tend to lead to a colour variation.  Consider the use of a water based acrylic curing compound complying with AS3799 such as Masterkure 404 but remember that some of these do leave a milky film on the surface that can take a while to vanish especially if it is rolled on rather than sprayed.

If your concrete is coloured forget these water based acrylic and go for a solvent based alternative such as Masterkure 402 and make sure you spray it.  It does not quite meet the Australian Standard for water retention but it will give you a superior finish.

If you are doing coloured precast work we normally recommend using CCS same day sealer and curing out of the sun and wind.

Bar Chairs

Give serious thought as to how you can keep the reinforcement in position without the need for bar chairs and take extreme care during the placement and vibration operations.  Make the contactor aware that you have a cover meter or concrete scanner and will reject any elements found with reinforcement cover outside the specification tolerances.

Formwork

For this class of finish the formwork needs to be very stiff and often has a reduced allowance for form tie holes or a detailed set out of the requirements.  Leakage of any type from formwork of an element with class A finish is a disaster so special sealing washers are required at the z​-bolt holes.  If you need to have the formwork specifically designed or require an structural engineer’s certification in the Canberra ACT just give Advanced Structural Designs a call on (02) 61612171.

We can normally locate the reinforcement and/or stressing in a concrete slab wall or beam by using a rebar covermeter.  Our covermeter is a fourth generation micro covermeter which we believe to be the best on the market.  As the name suggests the instrument can also used to confirm the concrete cover to reinforcement bars.  The results are extremely reliable where the bars are widely spaced but once the bar spacing is less than say twice the depth you are reading to, the bar location is harder to ascertain and the cover readings are unreliable.  You can also use concrete scanners to locate reinforcement and these are more reliable where there is congestion of reinforcement.

Bottom reinforcement can be located from the top of the slab to a maximum of 360 cover, but only in areas where there is no top reinforcement.  If it is necessary to miss bottom by drilling from the top it is sometimes necessary to locate the reo on the bottom of the slab and transfer it to the top through a pilot hole.  Again scanners which are becoming more common place are very useful especially where access can only be gained from above.

When location of prestressing is absolutely critical there are some other points to consider.  The first is that the prestress shop drawings are dimensioned and will normally give accurate offsets of live ends from the gridlines.  The second is that the chairs holding up the conduits normally have their base stapled to the formwork.  It is quite common to be able to find the staple locations which can in turn tell you where the strands are which can be a great confirmation of your remote sensing.  On some projects we have worked on clients have paid the builder to paint the tendon locations on the underside of the slas as soon as they are stripped which we consider is an excellent idea.

If you do need reinforcement or prestress tendons located, or the cover to reinforcement bars checked and you are in the Canberra region call Advanced Structural Designs on 02 61612171 or Mal Wilson on 0412 968574.

If you do not know much about the product we suggest you visit their website to see their standard details but to cut a long storey short AFS is two sheets of 9mm FC glued to castellated steel studs that can be stood and braced to act as permanent concrete formwork.  Ritek sold their original patent to AFS but now have a similar product on the maket but this is about the AFS system with which we have had more experience.  The walls can be reinforced vertically and horizontally where necessary.  Standard panel widths are 1200mm and heights are 2700, 3000 or 3600mm.

A few of projects in Canberra using this product would be Ivan Bullum’s development at the old Starlight Drive-in in Watson and Barry Morris’s Forum in the city or the Canberra Uni’s laboratory Building in Bruce (2012).  They are generally used for load bearing party walls and are generally 150 thick.  Whether they are reinforced or not will often depend on whether they are transferring loads to columns or simply carry loads to a lower floor or footing.

It is useful to look at costs up front to keep alternatives in perspective (costs in 2001 $). 

1       These values are very conservative and more testing is required

2       These values are extremely variable and dependent on brick density and face porosity as well as workmanship.

 This is of course only a small part of the storey, as you may need to add on Gyprock if you are unhappy with the finish or need to have a large number of power points and switches in the wall.  The variability of the Rw’s obtained on site is also a major issue for the brickwork options but there should be far less variability with Ritek. 

Ritek has recessed edges and can be taped and filled like any other FC product so if you are happy with the finish it can be competitive against double brickwork depending on your choice of brick.  If you are using the wall as a beam to transfer loads to columns below it will outperform the blockwork solution and give a superior final finish and acoustic performance.  This is where Ritek is at its most competitive; reinforced to transfer loads and where the clean flat durable surface represents the final product.

On the other hand, if you are battening out for gyprock in any case you will find the reinforced blockwork option appreciable cheaper.  Always remember to factor in increases in wall thickness at around $30/m^2 for every extra 100mm.

Our experience has been that many builder start out with the intention of using the FC face as the finished product only to find that they simply cannot achieve a suitable finish and end up slapping gyprock over it.

Construction issues

Having the right pump on site can be make pouring the concrete filling a safer and more manageable operation.  If you can get a 50 mm hose with volume control this is generally recommended.  We know Belconnen Concrete has one but there are probably others in town.  The pour should be done in maximum lifts of 600 to 1000 mm according to their literature with high slump concrete.  It is important to carefully police the strength of the mix required as builders will tend to order blockwork mixes which are far too weak for many applications.  The reason these mixes are suitable for block walls is that the blocks draw a large amount of moisture from the mix and thereby strengthen the concrete.

Pioneer supplied the mix for the Phoenix Development on Northbourne Avenue where they had a large number of blowouts at the base caused by the large lateral pressures from high slump concrete.  Eventually they settled for a mix that arrived on site with an 80 mm slump and had a super-plastisizer added to bring it to 150 mm.

If you have over 30mm of rain between standing and pouring the walls we suggest drilling holes in the base to let the water out after the rain.  Even if we could live with the weak concrete in the base of the walls (which we often can’t), water standing in the walls tends to weaken the FC bond to the studs and lead to blowouts at the base. 

Voids in the concrete filling are rare but obvious as the FC darkens with the wet concrete against it and unless the weather is exceedingly wet you will see the problem the following day.  Where repair to this type of defect are necessary they are time consuming and tricky if you are using the FC as your finished product so it is worth taking care in the filling operation.

An issue worth considering in design is the fact that it is difficult and time consuming to have wall reinforcement continue around the corner of these walls.  Whilst it represents good engineering practice and can add considerable lateral strength to a building it also increases the cost/m^2 and should be avoided where possible.  Hairpin reinforcement at corners is a more realistic proposition.

Setting up and bracing the wall system is crucial to its success as is cleaning it after the pour before any mortar dries.  If you need a dark coloured or gloss finish the Level 4 finish that is normally attained may not be sufficient for your requirements. 

The greatest misuse we see of this product on site is that there is often no thought on the part of the designers and steel fixers as to how the reinforcement is to be secured into position.  On a multi-story residential project in Barton we arrived to see the vertical reo being placed after the walls were poured and the results were appalling as the reo was anything but vertical.  The placement methodology should be clear on the drawings and the bars should be in and secured prior to the wall being poured.

If you have any particular queries related to this or any other building product or need some preliminary framing advice from a structural engineer in Canberra ACT or the surrounding district call Mal Wilson on Ph (02) 6161 2171.

The first thing to note about this, is that pouring a slab when rain is forecast is a conscious decision to put the quality of final product at risk.  A moderate chance of prolonged showers may pose a higher risk to the final outcome than a high risk of intermittent light showers as the slab can be protected for short intervals providing adequate covering and manpower is at hand.  We suggest that concreters read their insurance policies carefully before making the final decision.

The risk with rain-affected slabs is normally that the surface of the slab will collect too much water during the finishing process and result in a relatively soft, weak surface that is prone to dusting. The depth of this weak surface is often only 2 to 5mm but can be as much as 10 mm in extreme cases.

This problem is extremely important in industrial applications where surface wear is an important serviceability consideration.  A weak surface can also cause problems in residential construction where direct stick parquetry or tiling can delaminate the top 1 to 3 mm of the slabs surface.  This problem is caused by differential thermal, shrinkage or expansion stresses and can sometimes occur in a matter of weeks rather than years. 

The extent to which the surface of the slab may be damaged during rain depends on 

• Intensity of the rain.
• Timing of the rain relative to the initial and final setting of the concrete.
• Efforts made to remove excess water
• Efforts taken to protect the work
• Whether vibration or power trowelling took place

The time between pouring and initial setting of the concrete is generally around 2 to 3 hours and is the most critical in terms of ensuring a hard durable surface.  Every effort should be made to remove any additional water from the surface before finishing of the slab by rolling or dragging a hose over it.  If showers are intermittent it is sometimes possible to protect the work for a period to enable finishing in dryer weather.  Under no circumstances should cement powder be used to soak up the additional water.

More superficial damage can be done in the following 4.5 to 5 hours between initial and final set so protecting the surface before leaving the site is often a good idea.

If a builder wants to argue that the surface of the concrete is fit for purpose we generally score it with a coal chisel and compare the mark with a hard concrete surface.  This might not sound very scientific but there is no standard test for surface hardness.  Old fashion Schmidt Hammers used to give some indication if the problem was severe but over the last 10 years or so these have more of an indication of the density of the top 100mm.

To achieve high surface densities it is necessary to vibrate the concrete during placing.  Whilst this practice is universally accepted throughout most sectors of the building industry, residential builders often ignore this requirement as it is not expressly required by AS2780 unless the site is class H or E or if the slab forms part of the termite protection system. Further hardening of the surface is achieved during the power trowelling operation and you can expect weaker surfaces where this operation is not carried out.

How you fix the problem depends on the severity of the damage and the likely traffic and or coverings.  For example with superficial damage to a floor to be tiled we recommend water blasting at 3500 psi to remove the weak laitance and treating with a concrete hardener . We cannot responsibly recommend any treatment without first looking at the problem so for a full structural engineering assessment in Canberra ACT or the surrounding area call Mal Wilson  from Advanced Structural Designs on Ph 6161 2171 if you need a definite answer and full specification. 

The main advantages of prestressing are:

• Less overall building weight resulting in reduced material cost and footing size.
• Shallower beam sizes can lead to reduced floor to floor heights and reduction in façade costs
• Greater spans are possible, allowing greater fit out flexibility.
• Tighter and more reliable deflection control is both possible and economical.
• The relatively waterproof nature of prestressed concrete provides a superior barrier against water penetration over basement car parks and beneath exposed slab edges.
• The relatively crack free nature of a prestressed structure provides increased protection to the reinforcement and superior durability performance.
• Stripping times are generally less, enabling economies in the use of formwork.
• Large unplanned penetrations can sometimes be accommodated in between the stressing tendons without the need for trimming beams.
• The strands are generally simple to locate prior to coring.
• The deck prior to concreting is relatively uncluttered and safe to work on.

The main disadvantages are:

• The recess for the stressing live end at the ends of slabs and beams needs to be carefully patched, generally in a two stage procedure if they are to be texture painted.
• When an off form edge finish is required or the structure edge is not accessible pans are placed in the slabs which also require patching.
• Unauthorised coring of the slab can seriously weaken the structure when tendons are hit.
• Overall building horizontal shortening is increased. If this shortening is not planned for, it can lead to increased bending in columns and stress on façade elements.
• For very small projects (say less than 350m^2) stressing may be more expensive.
• For very short runs of cable (say less than 7m) stressing becomes inefficient due to draw-in losses.

In the last 5 years in Canberra working on moderate to large projects I have recommended stressing in about 95% of cases where the floors were cast in situ.  

If you have a particular project in mind in Canberra ACT and need a structural engineer’s opinion on the most economical framing option, call Mal Wilson from Advanced Structural Designs (02) 6161 2171.

Blistering or Efflorescence on the Concrete Surface

During the Power Trowelling Operation

Background

Concreters generally screed and bull float the concrete and wait for the bleed water to dissipate before they attempt to power trowel the surface of the concrete.  In normal conditions this often results in the power trowelling commences as soon as the concrete is capable of supporting the machine.

In hot, dry or windy weather when evaporation rates are above 1.0 litre per m^2 per hour it is often difficult to see the bleed water as it evaporates as quickly as it arrives and it is easy to misread the correct time to finish the surface.   The situation can be deceptive, as the rapidly drying surface is normally hard enough to support the power trowel machine (i.e. helicopter) well before all of the bleed water has reached the surface.

When concreting in hot weather it is extremely important to commence finishing the concrete at the correct time.  This is because commencing the finishing too early seals the surface of the concrete before all of the bleed water has risen to the surface.  This situation often results in small blisters of concrete around 20 to 30 mm in size and 1 mm deep (sometimes referred to as laitance) peeling off during the power trowelling operation.  In some cases you will see larger delaminations but the depth is always around 1 to 3 mm.   Another problem can be the discolouration of the surface of coloured concrete as the bleed water brings deposits of calcium carbonate and other salts to the surface.

Using Aliphatic Alcohol

In circumstances where evaporation rates are high and pouring must proceed we recommend the use of aliphatic alcohol to control the evaporation of bleed water.  The alcohol is generally applied after the screeding and bull floating operations. When the alcohol is applied the bleed water can be seen to build up under the surface of the aliphatic alcohol. The primary reason for its use is to prevent plastic shrinkage from occurring but it can also help with the finishing of the concrete by preventing the surface becoming too dry.

After the bleed water appears to have dissipated, the surface of the concrete should be poked with a finger to see if there is more bleed water just below the surface.  This water may be as far as 5mm or more below the surface.   If more bleed water is present, this normally indicates that the aliphatic alcohol has evaporated or runoff a sloping pavement and more needs to be applied.  If there is no bleed water beneath the surface it is generally a sign that power trowelling may commence.

Other options for controlling evaporation such as fog sprays, wind breaks and shading should also be considered where they can be achieved cost effectively.

Early Start

If the concreter has started the power trowel operation too early it is sometimes possible to flatten the angle of the blades to help release the bleed water and to steepen the angle on subsequent passes where a burnished surface is required in the specification.  It is far preferable of course is not to start power trowelling to early.

Mix Design

High amount of cement paste, fly ash, oxide colouring, fine sand or even entrained air in the mix can contribute to the rapid sealing of the surface during the power trowelling operation and it may be possible to reduce one or more of these by adjusting the mix design.  The use of a dry shake to colour the concrete can be particularly troublesome, as it will tend to seal the surface and soak bleed water at the same time.

If the mix is using a super-plasticizer the problem can often be severe as the bleed water has a tendency to arrive late and fast.  Whenever a super-plasticizer is used the concreter needs to be involved in the decision and informed as to the likely behaviour of the mix as he may need to wait longer and have more finishers available.

The Weather

When forecasts indicate evaporation losses higher than about 1.2 l/m^/hr it is generally preferable to postpone the pour until the weather is more conducive to attaining a high quality finish.  Below is a chart (based on ACI 305) that you can use to estimate likely evaporation rates for any given relative humidity, air temperature concrete temperature and wind speed.

Seriously consider wetting down the surrounding area where dust is a problem as dust landing on the slab can rapidly soak up any bleed water and contribute to the problem.  Also ensure that the sub-grade beneath the slab (if pouring slab on ground) has been soaked but has no free water on the surface.  This will provide more bleed water to the surface and help with the finishing.

In a Canberra summer concrete temperatures at delivery rarely go above 30 degrees Celsius (typically only 3 to 5 days) but you could probably add 2 degrees for concretes carrying over 5% black oxide as the heat gain in the sun can be significant. We have a copy of CSR plot of air temperature and concrete temperature if you are interested, but your supplier should have batching temperatures of your mix.

With the high moisture retention rates being exhibited by some powered oxides (they are not all the same), there is even an argument for limiting pouring to days where evaporation rates are less than 1 l/m^2/hr when pouring coloured concrete.  Any such decision would of course need to fit within programming constraints.

All of the above information relates to blistering in hot weather which is when the problem is normally at its worst in Canberra.  If you are having these problems in cooler weather ignore the above advice and call to discuss.

The above is but a brief summary of what is a very complex topic, so if you have any further queries or need an onsite appraisal in Canberra ACT by an experienced structural engineer please do not hesitate to call Mal Wilson from Advanced Structural Designs Ph 02 6161 2171.

These can take a couple of different forms that are generally easy to tell apart but can, at times, act in unison.  These are either settlement cracks or plastic shrinkage cracks.

Settlement Cracks

These cracks are most commonly found on the sides of narrow columns at tie locations, or where sudden changes in the depth of slabs and bands occur.  They can also be seen to mirror the pattern of the top reinforcement in deep bands.    The normal cause of the problem is poor compaction but it is often wise to form the bottom half of very deep beams to slab soffit level as a separate pour.  This is also true of columns, which need to be poured separately to the slabs they support.

Plastic Shrinkage Cracking

Loss of water from fresh concrete where not prevented can cause cracking.  Most commonly the problem is surface cracking that results from the loss of water to the atmosphere.  It is worth remembering that water can also be lost to the sub-grade which can exacerbate the problems on the surface by preventing bleed water coming to the surface.  The sub-grade or sub-base should be soaked a day before the pour and again on the day if a membrane is not being used.  If you are using a plastic membrane about half of the literature suggests that this worsens the problem but I have yet to read a satisfactory reason as to why this may be the case.

Canberra tends to have a few more problems with this than other capital cities due to our relatively low humidity and high summer temperatures although wind speeds are also a major factor.  If you are operating on a dusty site controlling the dust with water tankers on the day you pour is also important as the dust also soaks up bleed water as it hits the slab and makes finishing difficult.

For an indication of exactly how much bleed water you are likely to loose on any particular day you can look up a chart where air temperature, wind speed, concrete temperature and relative humidity form the variables.  Once you have an evaporation of more than 0.6 litres/m^2/hr you have the possibility of plastic shrinkage cracking and need to use some aliphatic alcohol to control the situation.  Shading and wind breaks or fog sprays are also excellent solutions where practical.

The aliphatic alcohol needs to be applied both after bull floating and again as required during the finishing process.  If evaporation losses are predicted to be greater than 1.5 l/m^2/hr serious consideration needs to be given to putting off the pour until the weather improves.  All of this guidance is very general, as sub-grade conditions, membrane usage, oxide types, inclusion of super-plasticiser, aggregate grading etc all influence the amount and timing of bleed water to the surface.

It is sometimes possible for experienced concreters to revibrate the concrete after plastic shrinkage cracking has occurred.  This can be done provided the vibrator can still sink into the concrete under its own weight and must be carried out to the full depth of the crack. 

Mix Changes

If you are looking to change the concrete mix to help you out, there are a few things you can do vary the bleed rate but this represents a whole new topic.

Further Information

If you are in any doubt or require an onsite assessment by a structural engineer in Canberra ACT call Mal Wilson from Advanced Structural Designs (ph 02 6161 2171) about your particular problem, especially if you are pouring coloured concrete in summer as this summary doesn’t scrape the surface of what you need to know. 

Which is the best prestressed concrete design software?

I come from a background of designing prestressed concrete structures for the last 20 years starting with hand calculations and spreadsheets on anything from concrete slabs to incrementally launched curved prestressed bridges.  It is amazing how far some judicious load balancing can get you but eventually everyone has to deal with Secondary (Hyperstatic) effects and the number crunching starts to become both tedious and complex. 

After moving to TTW in around 1995 I started using their program for concrete floors developed in house called RAPID which was written by Graeme Deaker and sold commercially.  It had recently won an Institution of Engineers Australia design award and was being marketed by the Standards Association of Australia.  Frankly this was in my view the best program on the market and was a year or two ahead of the opposition. The disappointment I had with the program was that no significant development work was carried out in the 6 years I was there and the program was overtaken by other products some of which had been developed later on a windows platform (RAPID was developed in Fortran on a DOS platform).  I imagine that people who bought into this program commercially were feeling like they had been let down by the developers so a sense of commitment it is certainly something I try to gauge when I talk to program developers.  It is also useful to know whether these developers are in a position to make commercial decisions about the products future. Rapid has not been available commercially since the mid 1990’s and is now only an in house program used by TTW.

I took a week off work to review what prestressed post-tensioning concrete design software I should be using in 2005.  Well, actually it was 2 weeks but I stressed 3000m2 of complicated transfer floors twice with 2 lots of software so that’s a decent weeks work.  I started by asking anyone whose opinion I thought was worth something and came up with the following candidates.

PT3D – 3D program developed by Inducta who’s Slabs programs sets the industry standard for reinforced concrete.

ADAPT – This one wasn’t recommended but they advertise enough so I thought “What the hell!”

RAM Concept – 3D program with very strong support from some quarters.  Used to be called Floor and Floor2 most codes are covered.

RAPT – Industry standard for quite some time most codes covered but only 2D.

2D or 3D

The first question to ask yourself these days is “Is 3D worth the trouble?” Let’s face it we have survived without it for years but we have also been guessing  a fair amount of the time when it comes to things like which really is the stiffest load path and by how much and how crack control may be being compromised by stiffer support elements such as stair and lift shafts.  3D is certainly slower but it is also more rigorous and treats the building holistically rather than piecemeal.  It also keeps the calcs in one file and is less likely to contain geometric errors when it is based on the CAD drawing of the building outline.  

The other point worth considering is whether you can you make use of the info that you never had before.  For example these programs can generally give you;

• The total amount of concrete in the slab.
• The total length of strand used.
• A full schedule of duct supports. 
• Some indication of the restraint to average floor compression afforded by the support system.
• Some idea of moments induced in columns due to slab creep and shrinkage.

In addition to this you get an excellent 3D rotatable view of the slab which allows you to quickly weed out any geometrical inconsistencies.  The fact that you based the model on the actual drawing also assists in this regard.  Another point is that the whole slab is in one file so you are not messing around with curious file names for strips that are at some odd angle to a grid reference.

Of course the other intangible is the wow factor when your client wonders in and you have a 3D model of his structure on the screen showing all the tendons.  They leave with the impression that you are on the cutting edge when it comes to prestress. I firmly believe that in five years time all buildings with a substantial footprint will be designed this way.

I should also point out that you tend to get a different answer to what you may be used to and that reo layout may be somewhat more complicated in certain circumstances.  Take as an example compatibility torsion in an edge beam.  In a 2 D approach many designers ignore it and design their slabs with a simply supported end restraint.  This is often done to reduce congestion of shear/torsion ligatures in the edge beams especially where the support beams are narrow.   You can fiddle with end restraints in a 3D approach to achieve the same goal but you need to think about it more.

Thinking along the same line you can also have torsion issues in the design of internal beams (or bands) especially if you are trying to use some column stiffness to bring down slab deflections.  This has been especially true since 2001 when the authors of AS3600 (see 7.6.4) effectively insisted on pattern loading every prestressed concrete structure while including prestress as a load rather than an internal action.

That is not to say that 3D is producing the wrong result, indeed quite the contrary is true but it will wake a few people up to the shortcomings of their current design approach.

Many people come to these new programs with the expectation that the distribution of moments will be based on cracked section stiffnesses but the reality is that they are straight linear elastic FE programs using gross section properties for all but deflection calculations (much like most 2D programs).

PT3D was just starting sales and I felt like a guinea pig for the Beta version, after 2 crashes in two hours so it never really made the first hurdle.  The programme is not an experience PT designer which does not fill me with confidence but he wasn’t a reinforced concrete designer either and “Slabs” is apparently a real winner so who knows?  Bottom line – needs work. 

The developer (Emil) emailed me after reading this and said  there program has had the glitches removed and pointed out that they have an association with Jeff Lind who is quite an experienced PT designer and is quite well regarded.  Emil invited me to have another go but I will probably won’t have another spare week for a while.   www.inducta.com.au

ADAPT The rep that I contacted new nothing about the opposition and not a lot more (or so it seemed) about ADAPT when questioned on finer detail, but he was happy to send some glossy brochures which were basically a repeat of what was available on the net.  The brochures (and net) seem to indicate that the programs were piecemeal affair and that if you wanted something as necessary as a 2D program to put you in the ball park it would cost extra.   I decided to strike them off my list not so much because of the price difference but more because their listed contact appeared to know so little about the product.  When I want help I want to be talking to someone who lives and breathes it.   www.adaptsoft.com/software.shtml

RAM Concept‘s Jim Trenerry was incredibly helpful, knowledgeable and buoyed by a spate of recent sales.  I annoyed the hell out of him for a week solid and the guy was dead set cheery every time he answered the phone which never rang out once. The program’s graphics are amazingly slick and the platform is logical, intuitive and rock solid.  You don’t get anything for nothing though and getting the model into the program takes some effort.  My tip is that care and precision when entering the model pays off in spades.  It helps a lot if you have a few CAD skills but if not expect a learning curve.  You could probably leave a lot of the hack work to a draftsman if you are not interested in being all hands on yourself.

The shortcomings were, no punching shear checks (what gives), incorrect modelling of torsion in deep beams and a strip wizard that was a little too skinny on features for my liking.  They say the first two will be fixed by October.  This is normally programmer speak for “Yeah we dream about that sort of stuff too.” but I saw this program a year ago and it is moving ahead like a steam train so they may well be right.

The other problem I have with this type of program is that it requires you to define design strips and pretend basically that it is not really a 3D program after all.  This is because the moments need to be smeared in accordance with the code and the code assumes you are designing in strips – talk about the cart pushing the horse. In a more logical world moment smearing would be more a function of support stiffness and the program should be working it out for itself.  Of course the people on the Code committee are not about to change the rules to keep programmers happy and this is basically an American program and their code has always lagged about a decade behind the rest of the world (unlike their steel code which sets the standard globally).  The problem here seems to be that they consider no matter how good the research is if it didn’t happen in the good old US of A, it didn’t happen.

In a perfect world RAM Concept would be producing long term deflection contours using cracked section properties but apparently this is some time off.  Actually they do go close to this by plotting cracked deflections along design strips.    They could do a little more work on graphical presentation of load diagrams but their graphics are so good in other areas this is me wanting it all now.

I loved the program, it is the future for sure, but the question is “Is the future really here yet or am I just being sucked in by the swank video parlour graphics?”  If your problems are always complicated and not too fluid it is almost certainly the program for you but if you do mostly dead simple stuff maybe not.  www.concsoft.com/ 

RAPT

What can I say, rock solid, does everything you’d expect a 2D program to do, works like a dream but it’s only 2D.  The strip wizard that would have sold the above program a million times over (well maybe not a million). 

Gil Brock is knowledgeable, helpful and is an experienced PT designer.  www.raptsoftware.com 

Recommendations

For most people Rapt will still be first choice because on a simple job you get where you’re going faster but if you have the dollars or you just stress slabs for a living buy RAM Concept as well for the tough stuff, you’ll love it.

Feedback

I do receive a fair amount of feedback on this article and perhaps I should point out to any European readers that the Swiss software CEDRUS is apparently the one to watch in that part of the world.  Dam near produces the drawings for you (or so they say).

This practice largely fell out of favour after AS3600 2009 effectively penalised mesh in suspended slabs by 20% due to it’s low ductility when compared with normal grade reinforcement.  The mesh did not change just the rules but they changed for good reasons.  You also cannot redistributed bending moments which results in a further loss of efficiency in continuous slabs especially under pattern loading.   Designing slabs with mesh reinforcement may still be a viable proposition that could produce minor cost savings when consideration is given to the following points.  

a)    Mesh should preferably be sufficient without additional reinforcement.

b)    The structural system should be predominantly a “one way slab” system.

c)     Lapping of mesh should be minimised.

d)   Lapping locations should be clearly documented so as to eliminate any possibility of top and bottom laps being coincident and so as to maximise usage and minimise cutting.

e)    Lapping should be achieved using bar splices so that each mesh remains in the same plane.

f)      Mesh lengths should be factor of the sheet length preferably using full sheets to minimise wastage.

A number of recent designs we have looked at for builders in the ACT ignore many of the above points and as a result create a system that is slow to construct, wasteful and burdened with problems in attaining the correct cover as multiple mesh laps (up to 8) occur at the same point and different height chairs are required for lapped and un-lapped areas of the top layer.  We have also seen engineers getting themselves into trouble by using software that fails to account for the AS3600 capacity reduction factors that apply to mesh.  The message here is to know what your software is capable of and know its limitations.  

Attempts to correct the problems by cutting mesh at the corners of double laps and leaving out additional reinforcement that will not fit invariably lead to weakening in local areas, which may be critical to the design.

Like all good ideas this one has its limitations so use it wisely and you may save time and money.  For me the 2009 changes mean that I would not consider it as an option unless steel fixing prices were sky high or the design was more about shrinkage control than strength.​​​​​.

You might think that this is a relatively straightforward topic but the number of times it goes wrong is beyond belief.  Over the years we have seen a number of pavements jack hammered up and redone because the saw cuts were carried out too late and the client could not accept the random crack pattern that resulted.  The reason there is a problem is normally because specification requirements are too loose or poorly policed.  The client is often told that there was a problem with the mix design or that it was caused by high shrink concrete, which is almost never the case.

The contractor hired to do the saw cutting is normally paid by the lineal metre and would prefer using a normal diamond tipped blade in normal working hours, which is never what is required. 

The concrete may need to be cut as early as 4 hours after finishing to prevent shrinkage cracks occurring.  We have found historically that 5 to 8 hours is normal in a Canberra summer and 8 to 15 in winter.  We normally specify that a small test area be set aside for testing the saw before commencing the cutting so that no work is damaged testing the timing.   Cutting should always be specified to be carried out with a “Green Concrete” saw blade as this will eliminate fretting or tearing of the concrete surface and allow the cutting to be carried out far earlier than with a normal diamond blade.  Check this on site, as these blades cost twice as much to run as they wear more quickly than a normal blade.

It is worth noting that if you are saw cutting coloured paving you should specify a dry cut.  This again cost more money as it again doubles blade wear but wet cutting the concrete when it is green has a tendency to stain it.

You may hear talk in the business of a “soft-cut” or “soff cut” saw that allows even earlier cutting of the concrete.  Our experience with these saws is generally good provided they have the right type of soft cut machine.  Certainly I consider that the hand held soft cut saws are more trouble then they are worth.  The saw used on one project we were involved in was hand held and the importer of the “Soff cut” saws says that they no longer import these saws due to their poor performance.  The importer does however recommend the use of their larger saws which he suggests are being used to great effect on a number of significant projects.  These larger saws are naturally held steadier during the cut and have a guide that compresses the exposed edges during cutting to reduce the tendency of the edge to tear.  

When saw cuts meet each other at acute angles drop the blade down deep and do not finish short of the intersection or a small triangle will attach to the adjacent slabs.  Anything less than 70 degrees should never be considered.

There are a few tricks you can use to minimise the possibility and consequences of edge fretting and if you have your structural engineering in Canberra carried out by Advanced Structural designs we will make sure that the documents address any potential problems.

For further information on this problem in Canberra ACT and surrounding region call Mal Wilson Ph (02) 6161 2171 he will be happy to help you out.

The Structure

The structure’s clear span is about 82 metres and you can park a 22m high Boeing 717 in it.  It is predominantly constructed of square hollow sections and consists of trussed arches supported on trussed cantilevered support frames with a knee brace added to transfer some moments out of the arch.

The major innovation in the structure was the use of bundled prestressing wires in the bottom chord of the trussed arches and in the outer chords of the support trusses.  These prestressing wires were stressed to impart compression into the truss chords effectively cancelling out the dead load deflections.  While the columns where stressed before erection, the arch bottom chords where stressed in two stages.  Both columns and the bottom chords where grout filled (columns from the base up) which both adds to their compressive strength and increases their fire performance according to the designer although the fire performance increase I’d thought would be marginal.

The major benefit of this approach was to save weight in the structure since steel SHS’s used have an ultimate strength of 450MPa compared with 1280MPa for the stressing wires.  This benefit along with the ability to control the initial compressive stress in the truss top chord and the support outer chord led to a claimed overall weight saving on the project of around 40%.

The approach was not without certain penalties as the bottom chord was very slender and required a great deal of additional lateral bracing before pairs of trusses could be lifted into place.  A special lifting jig was also required as the large spans could only be lifted at the extremities and the tendency for the paired trusses to flip over was quite pronounced.

The final product looks slender and attractive but on the day of inspection the trusses could be seen to move up and down 20 mm under a moderate breeze.  In normal circumstances this would be expected but given the gantry cranes hanging overhead we could not help but wonder whether this might become a serviceability issue.

The Hardstand

The hardstand was a design and construct job by Austress Freyssinet.  Since the tensile capacity of the concrete is generally the limiting factor in the strength of the pavement it is not that surprising that stressing the slab was found to be a viable solution.  We were surprised to hear that a major saving was reduction in base coarse thickness (of 400mm?) as base coarse under a rigid pavement is normally only deep enough to control pumping etc (say 200).  In any case the potential creep and shrinkage of such a pavement is considerable so the structure was isolated from the pavement and all services penetrating the pavement were fitted with compressible collars.  The real benefit of prestressing slabs like this is the removal of control joints which makes the slab a pleasure to drive forklifts on and reduces maintenance costs.​​​​​​​​​​

The construction was overseen by Construction Controls Alan Carey and completed in around 8 months and cost around $12million.

As an interesting footnote the new larger hanger being constructed on the RAAF side of the Canberra International Airport is STRARCH design due for erection later this year.  Big space did bid on this project as well but were too expensive.  If you need an independent engineering assessment of the various options for aircraft hangers contact Mal Wilson from Advanced Structural Designs on 02 6161 2171.

The Structure

The structure’s clear span is about 82 metres and you can park a 22m high Boeing 717 in it.  It is predominantly constructed of square hollow sections and consists of trussed arches supported on trussed cantilevered support frames with a knee brace added to transfer some moments out of the arch.

The major innovation in the structure was the use of bundled prestressing wires in the bottom chord of the trussed arches and in the outer chords of the support trusses.  These prestressing wires were stressed to impart compression into the truss chords effectively cancelling out the dead load deflections.  While the columns where stressed before erection, the arch bottom chords where stressed in two stages.  Both columns and the bottom chords where grout filled (columns from the base up) which both adds to their compressive strength and increases their fire performance according to the designer although the fire performance increase I’d thought would be marginal.

The major benefit of this approach was to save weight in the structure since steel SHS’s used have an ultimate strength of 450MPa compared with 1280MPa for the stressing wires.  This benefit along with the ability to control the initial compressive stress in the truss top chord and the support outer chord led to a claimed overall weight saving on the project of around 40%.

The approach was not without certain penalties as the bottom chord was very slender and required a great deal of additional lateral bracing before pairs of trusses could be lifted into place.  A special lifting jig was also required as the large spans could only be lifted at the extremities and the tendency for the paired trusses to flip over was quite pronounced.

The final product looks slender and attractive but on the day of inspection the trusses could be seen to move up and down 20 mm under a moderate breeze.  In normal circumstances this would be expected but given the gantry cranes hanging overhead we could not help but wonder whether this might become a serviceability issue.

The Hardstand

The hardstand was a design and construct job by Austress Freyssinet.  Since the tensile capacity of the concrete is generally the limiting factor in the strength of the pavement it is not that surprising that stressing the slab was found to be a viable solution.  We were surprised to hear that a major saving was reduction in base coarse thickness (of 400mm?) as base coarse under a rigid pavement is normally only deep enough to control pumping etc (say 200).  In any case the potential creep and shrinkage of such a pavement is considerable so the structure was isolated from the pavement and all services penetrating the pavement were fitted with compressible collars.  The real benefit of prestressing slabs like this is the removal of control joints which makes the slab a pleasure to drive forklifts on and reduces maintenance costs.​​​​​​​​​​

The construction was overseen by Construction Controls Alan Carey and completed in around 8 months and cost around $12million.

As an interesting footnote the new larger hanger being constructed on the RAAF side of the Canberra International Airport is STRARCH design due for erection later this year.  Big space did bid on this project as well but were too expensive.  If you need an independent engineering assessment of the various options for aircraft hangers contact Mal Wilson from Advanced Structural Designs on 02 6161 2171.

The National Museum of Australia Loop Canopy

I was not involved in the structural design until after the preliminary design was completed.  Andrew Spinelli, who was working with the Acton Peninsula Alliance asked if I could look at the structure with a view to reducing construction costs and bringing it back within budget.

By simple importing the node coordinates off the CAD drawing it was a simple exercise to create a 3 dimensional model for analysis using a structural design package (Spacegass).

Once the model was in the computer, the wind loads were introduced from eight discrete directions.  A three dimensional stability analysis was then carried out on all wind combinations.  The wind drags used in the calculations were independently verified by wind tunnel tests at Melbourne University.  The analysis gave an envelope of maximum moments at each point in the structure and the redesign resulted in a saving of over 30 percent in weight and 50 percent in weld lengths required.

Discussions with the fabricator led to splice points being selected at locations where the bending moments were relatively low.  Once this had been achieved 8mm fillet welds were carried out using flux cored MIG from cherry pickers.  The tubes were held in place by a series of clamps and backing/seating plates specifically designed for the job.  The final weld was a compound fillet weld with the root runs and the final runs being dye and magnetic particle tested.

Because the root runs were all that was necessary before the panels went on the two cranes required to hold the pieces in place were able to leave the site in less than two weeks.

If you need an innovative engineering approach to engineering design in Canberra ACT or the surrounding district contact Mal Wilson from Advanced Structural Designs on 02 6161 2171

After the recent Canberra bushfires a number of people rang to ask what damage has been done to their driveway slabs or house slabs and could they be reused.

Generally speaking slabs on ground don’t fair all that badly due to the fact that the maximum temperatures are above rather than below the fuel source and if there is damage it is often confined to the top 3 or 4 mm of the slab.  If there is any damaged concrete around from edge spalling we have a look at the top few mm for the colour changes that often occur with some of local (iron rich) aggregates.

We suggest cleaning the slab with heated high pressure water jet (over 3500 PSI) and see how it scrubs up.  If it is almost perfect you can use some Floorclean by MBT which should lift the remainder with 1 or 2 applications.  Floorclean is $200 plus tax for a 20 litre drum. 

If you think you have lost some surface hardness we can confirm your suspicion with a Schmitt Hammer or you can go straight to the next step and treat with a surface hardener which is around $30/m^2.

If you want a Rolls Royce solution (in terms of abrasion resistance) you can get an application like Ad-tex for around $30/m^2 that was used on Magnet Mart in Gungahlin.  This may need a non slip additive though if you are doing it in an exposed driveway.  The loss of surface strength is not just an issue for exposed slabs you will also find tiles and parquetry lifting if you fix them to a weak substrate so if you have any doubts have a professional assessment carried out.​

How does fire affect Concrete Structures?

When you first walk into the blackened remains of a fire ravaged concrete framed building it can be difficult to see past the devastation and look at what might be the framework of an entirely new building.  The fact is that after a fire most concrete buildings can have their structure repaired which can appreciably reduce the costs and time lost in rebuilding.

We are experienced in assessing fires in the Canberra region and can often quickly tell by the discolouration in the local aggregates what temperatures the fires have reached at various depths in the concrete and what the ramifications are for the long-term viability of the structure.  The concrete is of course not the only material that may have undergone change and we also assess the reinforcement or prestressing in the concrete to establish any changes in yield stress or effective prestress.

These effects can be quite variable as some of the steel may have been rapidly cooled during the fire-fighting operation, which can result in loss of ductility in certain areas.

Many other indicators such as the degree and type of spalling and cracking need to be assessed in a systematic way before repair procedures are established.  A correlation between surface hardness and underlying concrete strength may also prove a useful in establishing the extent of the damage. 

An experienced observer can often establish using burnt remains, an indication of the maximum heat of the fire as well as the duration of the burn.

Obviously peak temperature is only a small part of a very complex equation but to give some feel for what damage is being done as the temperature at any point increases we have listed some effects below.  Remember however that even when a fire with a peak temperature 850 degrees on the face of the concrete it may do very little damage if the duration is short.

Below 300 C 

No appreciable damage done

300 to 500 C

Damage to concrete requires careful assessment (especially above 400C).  Concrete will be weakened and some loss of modulus will have occurred.  Prestressing will be down to 50% of its strength above 400C and cold worked steel will be affected above 450C.

Above 500 C

At these temperature significant loss of strength occurs in the concrete and the modulus of the concrete is significantly reduced. Hot rolled steel is likely to recover its full yield strength even when heated to 600C but beyond this some strength losses will occur.

The full storey is a long and involved one so if you do require advice on a fire damaged structure in Canberra ACT or the surrounding region call Mal Wilson from Advanced Structural Designs on 02 61612171.

Canberra Fires Jan 2003 (Update)

After the Canberra bushfires a number of people rang to ask what damage has been done to their driveway slabs or house slabs and could they be reused.

Generally speaking slabs on ground don’t fair all that badly due to the fact that the maximum temperatures are above rather than below the fuel source and if there is damage it is often confined to the top 3 or 4 mm of the slab.  If there is any damaged concrete around from edge spalling we have a look at the top few mm for the colour changes that often occur with some of local (iron rich) aggregates.

We suggest cleaning the slab with a heated high pressure water jet (over 3500 PSI) and see how it scrubs up.  If it is almost perfect you can use some Floorclean by MBT which should lift the remainder with 1 or 2 applications.  Floorclean is $132 plus tax for a 20 litre drum. 

If you think you have lost some surface hardness we can confirm your suspicion with a Schmitt Hammer or you can go straight to the next step and treat with a surface hardener which is around $20/m^2.

If you want a Rolls Royce solution (in terms of abrasion resistance) you can get a spray on application like Adtex for around $30/m^2 that they used on Magnet Mart in Gungahlin.  This may need a non slip additive though if you are doing it in an exposed driveway.

If you have existing shrinkage cracks they will reflect through whatever you do. 

MGP Grades Vs F Grades

Anyone who has worked with timber will tell you that different species behave differently under varying types of loads, fail in different ways and generally behave like different materials.  This is not surprising as species have vastly different cellular structures.  The F Grading system attempts to lump all species (yes hardwoods and softwoods) together on the basis of flexural strength, and takes the lowest of all the remaining properties for the other parameters.

For the most part this system is acceptable as flexural strength is more often than not the most critical design parameter.  On the other hand Radiata, Slash and Caribbean Pine are similar in nature and represent a large and growing sector of the market.  It makes good commercial sense to group these together to make the most of their properties so that other properties such as stiffness and compressive strength are not effectively ‘dragged down’ by other timber species. The MGP grading system is an attempt to do just this.

To give you an idea of the benefits of the system take a look at the table below and compare the properties of F5 softwood with MGP10 (its equivalent flexural grade).

What the comparison shows is that if you are designing say a timber stud for compression you have a strength gain of 96 percent by using MGP equivalent.  If deflection is the governing criteria for a lightly loaded beam you have a potential reduction in deflection of 44 percent.  Increases in shear strength are even more dramatic but the F Grade values are known to be conservative and you may note that AS1684 (Residential Framing Code) allows much higher values to be used and differentiates between hardwoods and softwoods (see appendix A).

We are not implying that MGP graded timber is that much stronger or stiffer than its F Grade equivalent.  The difference is that we are allowed to use the additional strength because certain manufacturers have invested money in both testing and quality assurance systems to justify the greater values.

You will see advertisements in the magazines claiming MGP pine to be more stable and have reduced shrinkage but we have seen no evidence to suggest that this is anything more than “marketing hype”.

What’s Happening Around Canberra

I rang around today (January 2002) to see what percentage of suppliers carried MGP grades and found about 80% were carrying them (most exclusively) although nobody seemed to understand that there was any difference between the two grading systems.  The price variations between the F5 and MGP10 were not consistent, with MGP10 even being cheaper in some instances.  Bunnings Warehouse only carried MGP10 and MGP12 which were often sold at the same price as they were bundled together on delivery.  AAJA carried mainly MGP10 and advised us that builders never wanted to pay the 10 to 12 percent premium on the MGP12 (note it is 72% stronger and 27% stiffer).  This is probably because builders are too far down the design process to take any advantage from the strength or stiffness gain.

FAW on the other hand had never heard of MGP and carried F5 90*45’s a little cheaper than most of the others.

What we do

Our current policy is to use the MGP grading system in design but to give an equivalent F grade where flexural strength proves to be the governing design factor.  This approach gives the builder the option to source cheaper timber where appropriate. 

Conclusions

For all builders there are opportunities to save money by having their designers use the MGP grading system, especially heavily loaded wall framing (such as in three storey walk-ups)where compressive capacity may govern or lightly loaded beams where stiffness governs.  Relative pricing changes daily so we give options wherever we can.

With other timber species and many innovative products on the market such as Hyspan, Hybeam, Posi-STRUT, Pryda Longreach, LVL, Easybeam and various cold formed steel solutions, the best answer is not always obvious and may depend on manpower and cranage available durability or bulk discounting by suppliers.

If you need some specific answers or just want a few options on floor, roof or wall framing  from a structural engineer in Canberra ACT call Mal Wilson from Advanced Structural Designs on 02 6161 2171.

This is a good question because the two seem to be used interchangeably on Projects but they are not the same.  Both of the above are timber I shaped beams commonly used for joists and rafters in Australia along with a few other lesser known.  Hybeam is most often specified probably because their design software is by far the best we have seen and they are very widely available.

The Hybeam has a ply web and an LVL top and bottom flange while the Hyne I Beam has a oriented strand board  (OSB) web and MGP15 finger jointed flanges.  The LVL flanges can carry 33MPa or more in tension but the M15 pine is only worth 22.7MPa which starts alarm bell ringing straight away.

Let’s look more closely at a Hybeam HJ300 Vs a Hyne I Beam HI30068.  The Hybeam flanges are 63 by 39 whereas the Hyne I Beam is 68 by 34 so the Hyne I Beam has 5% less timber in the flanges.

Perhaps the two most significant performance indicators are EIx and Mx which measure the stiffness and strength in bending respectively.

On comparison of published data the Hybeam is 70% stronger than the Hyne I Beam but falls 15% short on stiffness.  For lightly loaded joists and almost all rafters, stiffness rather than strength may be the governing criterion which means that for the many applications the Hyne-I-Beam will span further.

One of the features of Hyne I Beams is that they are also available in a wider 88 mm flange which although it still falls 25% short on strength (on the above comparison) is some 50% stiffer than the Hybeam.

The bottom line here is if you need to make a change from one to the other economic or supply reasons you do need to talk with your engineer to get the OK.

If you need a comparison done by a structural engineer in Canberra ACT or the surrounding region or just need advice on timber framing call Mal Wilson from Advanced Structural Designs on Ph 02 6161 2171.

Shade Cloth Structures

Shade cloth structures are becoming more commonplace every day as people become more conscious of exposure to the sun.  Until recently there had been little guidance on their design, but after a couple of spectacular failures in Brisbane the University of Queensland was given funds by the government to carry out some research. This research has resulted in a number of excellent technical papers that allowed designers to take a more reliable approach to design.

From a structural standpoint the shade/hail canopies come in three basic designs.

Tied at Discrete points

These are probably the most common type, where the shade cloth is tied down to a rigid frame.  I was an expert witness for the defence recently where a local consultant had condemned one of these structures on the grounds of insufficient strength.  We were able to demonstrate that the support frames could afford to be very light as wind pressures were unable to build under the canopy because the fabric on the leeward edge flutters and allowing air to escape.  This is particularly true when the shade cloth is relatively loosely draped. 

In the case of these structures, the normal design guidelines can be unnecessarily conservative and engineering judgment needs to be exercised. 

Flat Canopies Tied to Cantilevering Posts

We have had cause to look at these closely after a recent failure at a local swimming pool.  A cantilevering steel post failed in a brittle manner at a butt weld at the base and the post shot across the shallow end of a pool full of swimmers.  Amazingly no one was badly injured but it could easily have killed people.   We carried out a three-dimensional second order analysis of the structure using cable elements to model its exact behaviour during the wind storm.

The first point to note about this type of structure, is that for the fabric to stay taut it is necessary to design the pattern of the fabric such that the tension locked into the cables maintain tension across the fabric itself. Tensioning the structure is therefore an important part of the design and construction process, as it affects not only the serviceability performance (tautness) of the structure but also the ultimate strength of the support structure under external loading such as wind or hail.

To carry load, the canopy support cables need to deform into roughly a circular arc. As more deflection takes place the profile is able to carry greater loads perpendicular to the drape (for any given cable tension).  The large deflections result from support cables elongating under load and the support posts flexing inwards towards the canopy. For this reason having a relatively flexible structure can be quite desirable as it results in increased cable drapes and therefore less load on the supports and foundations under heavy live loads.  Like most things in life it is not all up side as the deflected cloth captures wind like a spinnaker increasing the net pressure coefficient. 

In the case of the swimming pool canopy we modelled, the pitch was relatively flat and the lift on the canopy was theoretically quite low which in turn meant loads were low.  However the flexibility of the windward half of the structure led to high deflections.  These deflections more than doubled the pitch of the canopy which in turn doubled the wind loads.  In this particular case high deflections were problematic rather than beneficial and the windward cables required stiffening along with stiffening of the windward posts.

These structures are relatively unregulated in Australia and we estimate that 95% are erected without any degree of certification by designers.  This is probably because authorities view them as a low risk of physical danger since the fabric is light.  Most installers have a ready reckoner for post sizes based on area of clothe that they read off a table.  I am not sure what design assumptions have been made in the tables as none are generally listed with the tables but the post sizes given are generally independent of many basic design parameters such as.

• Fabric cut (edge shape)
• Desired fabric pretension
• Slope of fabric
• Porosity of fabric

The designs in some cases we have checked can be dangerously unconservative and this is sometimes due to the charts themselves or because installers have no idea of the assumptions in the charts or even the fundamental design principles on which they are based.  A good example of this is where the charts for a 4 post structure is used to construct a number of connected shade structures sharing central posts.  In such cases the central posts are often over designed but the end posts under designed as post draw in is no longer from each end which results in higher moments on the posts.  At other times no account is taken of escapement factors which can drastically increase wind pressures.

To give an example of just how poorly understood the design of shade structures can be you need go no further than to ask a number of experienced engineers what pressure factor to use to account for the porosity of say a Coolaroo 95 fabric at a 7 degree pitch and the answers will vary from 0.14 to SAY 0.6 (the smart ones won’t know).  This is because

• The definition of porosity is not universal
• Porosity varies considerably with tension (both pretension and tension from load)
• Pressure coefficients vary markedly with wind speed (due to turbulence).
• Consideration may be given to the cloth being wet under wind load.  We have all heard of a boat coming home with a wet sail.

Much of the data gathered on this topic has been at low wind speeds and is thought to be wildly un-conservative by researchers who have carried out wind tunnel tests on the cloths at design wind speeds so designers do need to be careful when picking up older technical papers.  This having been said if all designers did opt for a pressure coefficient of 0.6 much of the economy and elegance of the modern shade clothe structure would be lost.

Standard design charts we have seen can be even more bizarre when it comes to designing footing systems.  One set from Queensland recommends bored pier footing sizes independent of any soil parameters and one from New South Wales has footing sizes varying with only two soil types (cohesive and non cohesive) and completely independent of post height.  How the post hole footing size can be unrelated to the moment being delivered is a complete mystery to us but a new one goes up every day using these charts so they must be right, right?

For many fabricators and installers this technical talk is all irrelevant because they are smart enough to have the client sign a waiver stating that the structure is not warranted against damage in high winds.  The definition of high wind is not explicit but it should be taken as read that if the structure was damaged ipso facto the wind was indeed high.

When design engineers are asked to certify these shade cloth structures what the client is often saying (without actually saying it) is that he’d very much like our $10million PI insurance policy underpinning this poorly thought out under designed debacle that he bought for a song so it is not something we do lightly.  

Shaped canopies with cables forming opposing curves

These structures are designed to have curves built into their geometry by designing in a series of opposing curves providing stability for each other.  They can be fashioned into very interesting shapes which they tend to hold well under wind loads.  The design process requires a close interaction between the architects and the structural engineer and results are generally structurally efficient and spectacular.  

The only downside is that they are generally more expensive to design and construct than the others.

If you require advice on shade canopies from an experienced structural engineer in Canberra ACT call Mal Wilson from Advanced Structural Designs on (02) 6161 2171.

The approval authority in the ACT is Building Electrical and Plumbing Control (BEPCON) Ph 62071923 and they will do what they call a conveyancing package on a commercial or residential block to let you know what is approved.  For this you will need to fill in a form that they will fax to you and you will also need a letter of authority from the building owner.

For residential blocks they promise all drawings and Certificate of Occupancies (C of O’s) relevant to the block and a short report including other items they believe are of interest for a reasonable fee.  If the C of O has been issued for the aspect of the building you are interested in then from BEPCON’s perspective it has been approved. The process takes 3 to 4 days normally but can be speeded up if you pay higher fees (within 24 hrs for triple the fee).

For commercial blocks you receive the same as above for more money but they only include the site survey drawing.  If you need to review or copy the drawings for a commercial building you will need to go out there (Cnr Lysaght and Hoskin St Mitchell) and search yourself.  If you want to save yourself some time and money get the drawings cut to CD and pick them up the next day.

If the commercial building was originally a government owned building it may not have required BEPCON approval and the drawing whereabouts are a little more complicated but they are available.  We have never been unable to locate drawings for government buildings but occasionally security clearances are also required to access the drawings.

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Remember if you need a structural engineer in Canberra (ACT) call Advanced Structural Designs on 02 6161 2171.