Advanced Structural Designs

ACN 097 789 87 92 Vasey Cres CAMPBELL ACT 2612

Telephone 61612171 Facsimile 61612170

Email mal@structuraldesigns.com.au

 

 

 

Shade Cloth Structures

Shade cloth structures are becoming more commonplace everyday 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.

v    Fabric cut (edge shape)

v    Desired fabric pretension

v    Slope of fabric

v    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 wont know). This is because

v    The definition of porosity is not universal

v    Porosity varies considerably with tension (both pretension and tension from load)

v    Pressure coefficients vary markedly with wind speed (due to turbulence).

v    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 hed 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 architect 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) 61612171.

 

Home FAQs