Tension and cable structures
Tension and cable structures carry load almost entirely in tension, using cables, membranes and masts to span long distances with very little material. We design these structures for clients across the UK.
Why is a cable so efficient?
Because it only carries tension, using its whole cross section fully, with no bending. That lets a slender cable span distances that would need a very large beam, which is why cable and tension structures cover long spans with very little material.
How a tension structure carries load
A cable is the most efficient structural element there is, because it can only pull. It has no bending stiffness and cannot push, so it carries load purely in tension along its length, and a material in pure tension uses every fibre of its cross section fully. This is why a slender cable can span distances that would need a huge beam. In a cable roof or a tensile membrane the load is carried by cables and fabric working in tension, gathered to masts and then down to anchorages in the ground; the masts work in compression and the ground anchors resist the pull, so the whole system is a balance of tension and compression held in equilibrium.
Because a cable takes the shape its loads give it, the geometry is not chosen freely; it is found. A single cable under gravity hangs as a catenary, and a fabric roof stretched between high and low points takes a saddle-shaped, doubly curved surface. That double curvature, curving up one way and down the other, is what gives a membrane its stiffness and stops it flapping, because one set of curved cables is pretensioned against the other. We carry out form finding to establish the equilibrium shape, then check that shape under wind, snow and asymmetric loads, all to the Eurocodes and the relevant tensile structure guidance.
Pretension, stiffness and stability
A tension structure is only stable if it is prestressed. Without pretension a cable or membrane would go slack the moment the load reversed, and a slack cable is a mechanism, not a structure. So the cables and fabric are tensioned against each other and against the masts and anchors, and this pretension is what gives the structure its stiffness and stops it fluttering or inverting under wind uplift. The level of pretension is a design decision: too little and the structure moves and flaps, too much and the cables, fabric and anchors are overstressed for their whole life. We set the pretension to keep the surface taut under every load case without overloading any element.
Stability under reversing loads is the heart of the analysis. Wind can lift a light membrane roof far harder than gravity pushes it down, so every cable must stay in tension as the load swings from downward snow to upward suction, and the analysis is non-linear because the shape itself changes as the load changes. We model the structure in its found shape, apply the load cases, and confirm that no cable goes slack and no member is overstressed, while keeping deflections within limits the fabric and its supports can accept. The masts and their guys or props are designed for the compression and the out-of-balance pull they carry.
Anchorage, connections and where they suit
Everything a cable structure pulls on has to be held down, so the anchorages are as important as the cables. The tension collected at each mast and cable end is delivered to foundations that resist it, often large concrete blocks or tension piles working against uplift and horizontal pull, and getting these anchor foundations right is usually the largest part of the substructure. Cable end fittings, sockets, clamps and the connections to masts and edge cables are designed for the full cable force with a suitable margin, because a single failed termination can release the whole prestress.
Tension and cable structures suit stadium roofs, canopies, walkway covers, exhibition and event structures and long-span lightweight roofs where minimum material and a striking form are wanted. They are exceptionally light for the area they cover and open up spans that no beam could reach economically. The limits are movement, durability and detailing: these structures flex and move under load far more than a stiff frame, the fabric has a finite life and must be specified for the exposure, and the anchorages, terminations and pretension demand precise design and installation. We handle the form finding, the non-linear analysis and the anchorage design so the built structure holds its shape.
What we check.
The points our calculations resolve for a project like this.
- Form finding to establish the equilibrium shape of cables and membrane
- Pretension level that keeps the surface taut without overstressing elements
- Non-linear analysis under reversing wind, snow and asymmetric loads
- Ensuring no cable goes slack as load reverses, and controlling deflection
- Mast, guy and prop design for compression and out-of-balance pull
- Anchorage foundations, cable terminations and end fittings for full load
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
Why is a cable so efficient?
Because it only carries tension, using its whole cross section fully, with no bending. That lets a slender cable span distances that would need a very large beam, which is why cable and tension structures cover long spans with very little material.
What stops a fabric roof from flapping?
Pretension and double curvature. The fabric is stretched into a doubly curved, saddle shape and prestressed so one set of curved cables tensions against the other. That prestress gives it stiffness and stops it fluttering or inverting under wind.
Is wind or snow the bigger problem?
Often wind, because it can lift a light membrane far harder than gravity presses it down. Every cable must stay in tension as the load swings from downward snow to upward suction, which we confirm through a non-linear analysis of the found shape.
What holds the cables down?
The anchorages, usually large concrete blocks or tension piles that resist the pull and uplift, plus masts in compression. The anchor foundations are frequently the largest part of the substructure, and we design them and the cable terminations for the full force.
Related in structural frames & systems.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project