 |
Continual Professional Development (CPD).
This section is continued over 3 pages... 1 | 2 | 3
Basic Support Systems Ideas.
Conventional structures have an internal rigidity to be stable. Fabric structures being mainly fabric and cables have little or no rigidity and therefore must rely on their form and internal pre-stress to perform the same function.
To resist these loads we have to put equal loads into the fabric for it to remain stable, the pre-stress.
Because fabric structures rely on internal tensile forces to remain stable their behaviour is more complicated than their conventional counterparts and therefore they are more difficult to design. The significant changes in their geometry means they are non-linear even though the fabric remains more or less stable. If properly designed this is a desirable quality that increases their ability to carry load as they deform from the effect of live loads. Fabric Structures are more capable in this respect due to their very high strength weight ratio than equal spans of concrete or steel.
As a rule of thumb spans greater than 15 metres should be avoided however, much greater spans can be achieved by reinforcing the fabric with webbing or cables.
These loads have to be transmitted into our support structure. A support structure might be edge tripods, central masts or push-ups, we might decide to suspend the fabric in some form to give a very large mast free area. In a closed system the edge will be supported continuously by a "Kader" in an extruded section.
Fabric to Support System Connections.
The final part of this design process is how to attach the fabric to the support system. Care must be taken not only to provide a path for the load to easily flow into the support structure but to allow flexibility in the connection for displacement and rotation. During the erection process the whole structure will probably experience loads greater than the snow and wind effects during its working life span. This is because of the uneven loads imposed as the structure is assembled and tensioned.
Some fabrics can develop creep or elongation due to the type of weave or coating on the weave, heat and moisture. This should be considered during analysis (which we are not covering today) and has a direct effect on the connection system. Creep will induce a loss of pre-stress tension in the fabric which will in turn mean that it can develop ponds of water on its surface and will flap in the wind. This loss of tension is dangerous for the stability of the structure and if not regularly maintained will lead to a failure of the structure. Connections from the fabric to the support system should always be adjustable. Teflon coated fabrics require re-tensioning once the fabric has settled over a period of a few weeks.
Understanding Fabrics.
Fabric, being flexible and normally woven, is a ‘live’ medium that stretches across the diagonal to a greater extent than in the direction of the weave, one can most easily see this with a piece of netting.
Certain fabrics can have more stretch in one direction than in the other. This is due to the threads of the weft being woven in and out of the warp threads which are already tightened. If an uneven load is applied only on the weft threads, they will straighten and crimp the warp threads, subtly changing the shape of a canopy or part of a canopy. This must be allowed for in the patterning of the fabric and the erection of the canopy. Distortion is more evident in material where the threads are first coated and then woven. Where the base fabric is woven and then coated, the coating applied to both sides of the base fabric helps to keep the threads at their original spacing.
There are many advantages and disadvantages with different fabrics, which is a seminar in its own right, however, there are three basic external fabric types most commonly used in fabric structures.
External fabrics.
These are:
 PVC coated polyester (polyvinyl chloride),
Silicon coated glass and
Teflon coated glass P.T.F.E. (polytetrafluroethylene).
The polyester and glass are the woven element which we call sub-strait, the PVC, Silicon and Teflon are coatings applied to this.
PVC coated polyester is the least expensive and has a design life of 15 to 20 years due mainly to ultra violet attack. Besides cost it has the advantage of being very robust, easier to ship and erection can be de mountable. It also comes in many colours.
Silicon glass has a much higher tensile strength than polyester; however, being glass is brittle. To over come this, the fibres are made to a very small diameter but are still subject to damage from repeated flexing. The advantage of glass is that it is not vulnerable to ultra violet attack which gives it a 30+ year design life. Both silicon and Teflon are almost completely chemically inert, resistant to moisture and micro-organisms and have good self cleaning properties. Silicon Glass is less expensive than the Teflon glass, however the Teflon has a better "self cleaning" properties. One other point about Teflon coated glass fibre is that it is difficult to handle and transport in large pieces because when sharply folded the Teflon coating visibly "bruises".
Internal fabrics.
All types of fabric can be used if suitably fire retarded. The most commonly used is PVC coated glass cloth due to its easy maintenance and very good fire resistance. Untreated cotton can be used which will burn out in a flash and will not drop hot plastics on anyone below. Lycra materials can be used for awkward or ad-hoc shapes.
Appropriate Applications.
I will start this from a negative standpoint of what does not work particularly well.
Open system structures by their nature move in heavy weather conditions. Trying to join rigid walls to a free standing open system structure requires a flexible jointing system between roof and walls which usually looks untidy.
Trying to successfully control water from an open system structure is difficult and requires guttering as a conventional building which again looks unsightly and loses the advantage of the beautiful free form. One can use a foam filled cylindrical thickening attached to the edge of the fabric to direct water towards the membrane plate, which incorporates a drainage point leading back to the mast along the boom. Most of the time one simply orientates the canopy form so that the main routes under it occur beneath high points or edges that will not shed water. This does not effect a closed system structure which is relatively easy to drain.
Water drainage via membrane plates, booms and down through mast.
The masts, tripods and booms of the supporting structure can be used as down-pipes in order to allow water to be led away without other visible appendages.
An open system structure looks best when used as an independent stand-alone statement. If walls are required these need to be designed separately using the tensile structure as a primary roof.
If an open system structure is to abut a building then care needs to be taken with loadings. Often the building will need to be reinforced to take the pre-stress. It is important to take this into early consideration and undertake a load analysis which will cost money at pre-tender stage. The only other disadvantage is thermal values which limit use. This can be overcome with thermal lining and double skins at the cost of translucency.
The disadvantages are far outweighed by the advantages, their sculptural forms, environmental sensitivity and magical luminosity. Tensile fabric structures engender and incorporate all the balance of nature, covering areas using the minimal of materials in a most cost-effective way.
Cost implications.
Early involvement of a Contractor will help greatly in maintaining cost, however, designing within a budget is essential for Architect and Contractor alike and nearly all projects are put out to closed tender so the advantage a Contractor has in knowing the budget does not help in this competitive industry. It is worth noting that main contractors largely regard tensile structures as complicated tents and having little knowledge of them, will try and avoid them.
Life cycle costs:
The most economic material to use is nearly always going to be PVC coated polyester if maintenance and renewal schedules are in place. The additional initial cost of the glass fabric approx. 50% more expensive for their extended life of approximately 10 years is difficult to justify unless a decision is taken not to maintain a structure.
Current Case Study.
Car show room and surrounding area.
Objective was to create a cost effective highly visual sales showroom and surrounding point of sale areas.
Their initial budget for the tensile structure over the sales showroom was £80,000.
The ground area to be covered was 660m². This gave a budget cost per square metre £121.
The actual cost excluding foundations was calculated as £145m².
The sequence of events on this project was fairly typical:
Architect rings to discuss.
Sends whatever he has; normally a plan with an area shaded in and an idea of the
form he wants.
We advise on ideas against costs over the phone to evaluate budget restrictions.
He accesses web page to review existing structures for ideas.
Together we firm up some basics and then we normally visit the site, 1½ hour.
We pull together some quick visualisations for them to present to the client and
planning authority.
If the price and scheme are feasible a decision will be taken on whether to award us the
contract or to put it out to tender. Up to this stage no fee is chargeable.
If the decision is to go to tender we will help with specifications or for a fee write the tender
documents. If we perform this function we cannot tender but will help co-ordinate the
tenders and sit on the tender committee.
This particular project is presently in the planning stage and will probably not be tendered due to the competitiveness of our pricing.
The tensile structure is being used as a primary roof over a secondary flat roof.
The structure will be erected in 8 days and will allow the secondary structure to be built under it saving down time due to the weather.
The fabric will be lit from the underside making a highly visual statement.
If successful after a trial, several such sale rooms could be commissioned, with savings on the design and engineering costs.
It was decided to use PVC material due to last 15 years, as their corporate image will probably change during that time.
It is worth noting that life cycle expectancy is very important in choosing the correct materials. Often it is more expedient to use a PVC rather than a teflon and replace every 15 years.
PVC Coated Polyester.
Sound Transparent.
Reflectivity 70%.
U Value 4.5w single layer of fabric.
Translucency 12%.
Fire rating PVC will self extinguish and will not produce flaring droplets (BS5867).
U.V. PVC will deteriorate between 15-20 years.
Silicon & Teflon glass are not affected; assume 25 + years.
Costs PVC structure 50% less than Teflon structures.
Strength Structural fabric will have tensile strength of approximately 10 tonnes/linear metre.
Safety Factor on loads of 4 to 6 times.
Vandalism Design around the problem, connections above arm-reach etc.
Cleaning Once a year.
Condensation Control of ventilation and or flow if second skin is used.
Colour PVC & Silicon glass can be made to any colour. Teflon glass white only.
Repair All fabrics can be patched.
Lighting Up-lighting recommended.
Considerations in designing initial concept.
Site:
Location (Wind and snow loads)
Foundations
Drainage
Canopy:
Acoustic Performance
Fire resistance
Thermal values
UV attack
Condensation
Translucency
Reflectivity
Lighting
Life span Costs:
Maintenance
Access
Vandalism
< previous page | next page >
This section is continued over 3 pages... 1 | 2 | 3
|
|
|
|
|
|
