Tensile Structure Design, Engineering & Fabrication

A tensile roof has no stiffness of its own. It gets its strength from double curvature and pre-stress, so the geometry is not styling — it is the structure. That has a practical consequence: the drawing office, the patterning table, the welding bay and the tensioning crew are all working on the same structural decision at different stages, and an error in any one of them only becomes visible at the next.

Patterning that is compensated wrongly produces panels that will not come up to tension. A seam welded wrongly holds for a season and then opens. A frame drilled on site instead of in the shop leaves bare steel at the connection. None of these are visible at the stage where they are made, which is the argument for keeping all of them under one roof. Ekra Decor is both the designer and the tensile structure fabricator on its own projects, and subcontracts none of it. What we take on commercially, and where the line between our scope and yours is drawn, is set out on the tensile structure contractor page; this page is the method behind it.

Form finding generates the equilibrium surface for a given set of supports, edges and pre-stress. The result is an anticlastic surface that carries both downward load and uplift and drains to its low points. Only once that surface exists is there anything meaningful to analyse, which is why a catalogue shape scaled to fit a different span is not the same product.

Form finding and analysis are run in ixCube and ANSYS. The analysis checks the found shape against wind, and against rain or snow where the location calls for it. Membrane stresses, cable forces and steel sections are results of that run.

Design pressures, membrane stresses, cable forces and deflections are project-specific and come out of the analysis for your site. They are issued as drawings and figures before anything is cut, and we do not publish representative values here, because a generic number on a web page is not a design.

Before anything is detailed there is a 3D render and a walk-through of the geometry for sign-off, so the form is agreed while changing it is still cheap. After that the model becomes two sets of drawings: general-arrangement and workshop drawings for the steel, and the developed membrane surface for the cutting floor.

Which drawings you get, and when, is worth settling in the quotation rather than afterwards — it is one of the questions to ask before signing. Where a municipal or authority submission is needed, drawings are prepared in the form the approving authority expects.

A doubly-curved surface cannot be flattened without distortion. So the surface is developed into narrow strips that each flatten with an acceptable error, and the strip width is limited by the width of the fabric roll — the roll dictates the panel layout, not the other way round.

Each flat panel is then cut slightly smaller than its geometric size, because the fabric stretches to shape when it is tensioned. That allowance is the compensation, and the values come from biaxial tests on the specific fabric being used, because warp and weft stretch by different amounts. Compensation is material-specific: it cannot be carried over from one fabric to another, which is why the membrane grade has to be fixed before patterning starts, not after.

Membrane panels are plotted and cut on a CNC table, each marked with its panel number, orientation and seam allowances. The warp direction is aligned consistently across the roof, because the two fabric directions do not behave alike under load.

Seaming follows the material rather than a house preference:

A correct weld is as strong as the parent material, which is why the seam is welded rather than stitched or bonded — the longer argument is in how a tensile structure actually gets built.

Three checks, all of them before the panel is packed, because a fabrication error found on site is a crane, a crew and a programme standing still: weld strength on samples, panel dimensions against the patterning, and hardware and hems against the drawings. Defects are corrected in the workshop rather than adjusted for during installation.

This is also where material substitution would show up. Membrane is specified by grade and finish, and the certifications and registrations behind the company are listed on the certifications page.

Foundations come first: excavation, reinforcement, casting, and then curing — the one part of the programme nobody can compress, whatever the pressure on the date. Because the steel arrives already fabricated and galvanised, erection itself is quick: stand the frame, align it, check the levels.

The membrane is then lifted, drawn out, connected to the edge cables and fittings, and brought up to its engineered pre-stress in a controlled sequence. That last step is what turns a slack sheet into a stiff structural surface, and it is done by the crew that fabricated it, to a method statement rather than by feel. Realistic durations are set out in how long a tensile structure takes.

Survey, engineering, drawings, fabrication, installation and tensioning are run by one in-house team, so there is no hand-off point at which responsibility becomes arguable. Site survey and wind-load analysis, 3D CAD membrane form finding and patterning in ixCube and ANSYS, high-frequency fabric welding, structural steel fabrication and on-site tensioning are all ours.

On the client side, the single most useful thing for the programme is a nominated person who can approve drawings. Fabrication cannot start until drawings are approved, so an approval sitting with an unnamed committee is the most common cause of a date slipping — more common than anything that happens in the workshop.