© Augusto Da Silva/Graphix-images – Architect: Wilmotte & associés
Structures made of stretched fabric are few and far between. They require very specific skills. Several CSTB teams have been asked to help evaluate the glass fabric structure used in the Montrouge “Evergreen” operation, including restructuring and construction of 78,000 m² of offices.
The white fabric roof in a 2-hectare garden right in the middle of the tertiary campus forms an outer roof protecting the glass roof of a company restaurant. CSTB “Structure and Fire” experts examined the request for an ATEx made by the Pfeiffer Company responsible for construction of the structure. They were supported by CSTB specialists in Aerodynamics and Climate, and Skin and Coatings.
The first step before making an expertise of the mechanical strength of the 1,000 m² stretched fabric conceived by the Wilmotte & partners architecture office, was to identify all existing forces and stresses. The contractor commissioned CSTB’s Climatology and Aerodynamics Department that carried out its consulting mission in Nantes, where tests characterised and quantified pressure fields in the structure. An essential preliminary study since fabric deforms under the action of wind and there is a risk of local deformations due to turbulence or severe weather loads with the formation of snow pockets. All resultant stresses in the membranes were determined using special purpose software, using the results from the wind tunnel study.
The Structures Fire Safety Department then assessed the safety, feasibility and risks of tearing. It worked in cooperation with the SOCOTEC inspection office and CSTB roof specialists, with the objective of validating a non-traditional technique based on experiments and expertise. What is the real mechanical strength of the fabric after allowing for aging and installation conditions? What is the creep? Do tests carried out on models confirm values of the fabric strength calculated by the design office?
Evaluation by expertise and experiments
The first major query concerns creep of the fabric, in other words its extension under the constant load of prestressed steel tie rods. Inspections and regular adjustments of the structure should help to answer this question. Another major query is how does the glass fabric change in time? Samples were analysed to determine the durability of its mechanical strength through biaxial tests performed in Germany. Samples of the former fabric were also analysed to get a better understanding of the durability of the former material, which has a lower performance than the new material.
The favourable ATEx issued by CSTB then allowed the contractor to manufacture and install the Montrouge outer roof in accordance with a quality plan that it wrote internally. A particular comment was made for the transport, handling and placement of the fabric: “The shape of the fabric is determined using patterns, and then by cutting and welding strips, just like a boat sail, says Patrick Delmotte, CSTB engineer. This large and fairly heavy fabric is entirely manufactured in the workshop, and then delivered to site in a single piece. It is also coated with PTFE, in other words polytetrafluoroethylene. All the transport and handling problems that might be expected then follow…” PTFE is better known under some of its trademarks like Teflon, and has a very low coefficient of friction and excellent anti-adhesive properties. In everyday life, it is applied as a coating on some cooking utensils and in the form of plumbing tape to make threaded assemblies leaktight. In Montrouge, it will prevent rain, dust or other residue from sticking to the fabric. But this coating is fragile. Therefore, it was essential to take great care in folding, storing and placing the fabric, obliging the contractor to scrupulously respect the phasing of installation operations.
|Location:||50 avenue Jean Jaurès, Montrouge (92)|
|Client:||CEREP Montrouge SARL|
|Project Manager:||Wilmotte & Associés|
|Engineering design office:||RFR|
|Construction design office:||IF Ingenieurmeinschaft|