Pleated    Structures

Blunt pleats

Blunt pleats

The problem

This page deals with the issue of how to eliminate areas of high tension in pleated structures made using flat sheets of material.

Use of flat, unpatterned material usually requires that the ridge and valley cables be tensioned so they are completely straight - in order to avoid the membrane forming doubly-curved surfaces.

In structures where there is a membrane prestress, this means that the tension in the cables needs to be very large by comparison.

While pleated structures require minimal membrane prestress, some prestress is still needed - just to take out wrinkles in the fabric.

Consequently, in order to tension the cables with enough force to make them straight considerable force needs to be applied.

The side-effects of this include the need for thicker cables, thicker struts and larger ground anchors - all of which contribute to greater cost.

If insufficient forces is applied to the cables then they will form catenary curves - under the prestress force applied by the membrane.

The formation of catenary curves means that a flat membrane will not fit smoothly in over the resulting structure without stretching.

The fabric is likely to be stretched at either end of each pleat.

This will create stress in the fabric where it contacts the ridge and valley cables - resulting in an increased probability of it tearing or failing at those points.

High membrane tension at each end of the peats is likely to be accompanied by low membrane tension in the middle of them. Such areas of low membrane tension may result in the membrane flapping against the cables. This is likely to results in cumulative membrane damage.


Of course, the conventional solution to this problem involves patterning the fabric.

Patterning results in a much more even distribution of tension along the pleat.

However there are some disadvantages to patterning:

  • It is time-consuming and expensive to do;
  • It often requires specialised equipment;
  • It creates weaknesses in the fabric;
  • It creates potential leaks;
  • It wastes fabric;
  • It makes the fabric harder to reuse.

Between them, these disadvantages are enough to motivate a search for methods of construction which do not require patterning.

Blunt pleats

In a case of pleated structures, doubling up the ridge and valley cables and inserting spacers can result in pleats with what appear to be blunt edges.

Taking the 'edges' off the middle of the pleats results in ridge cables that form natural catenary curves - while still retaining the ability to use a flat membrane to cover them.

The digrams here illustrate the pattern:

Blunt pleats - tensile structure model

Here a tensile structure employs twin ridge and valley cables with single strut spacers.

There approximate the desirable curved oval shape of blunt pleats.

More information about this model is available here.

Blunt pleats

Blunt pleats - cables separated by strut spacer


Curved pattern
Linear approximation

A curved pattern would distribute forces best along the length of the cable.

In practice, this is approximated - using a series of straight line segments.


Use of this pattern allows membrane patterning to be dispensed with - but there are a number of disadvantages to using it:

  • the number of ridge and valley cables is doubled;
  • more struts are used - increasing the weight;
  • more strut material is placed in contact with the membrane;
  • more strut-to-cable joins are produced, increasing complexity;
  • adjusting cable tension involves more maintenance problems;
  • cables can no longer be tightened as far as they will go;
  • external strut material interferes with drainage.
The problems are a bit different for ridge and valley cables.

The idea makes more sense for ridge cables - because fewer disadvantages apply in that case.

In particular:

  • adjusting the struts is less likely to involve climbing on the structure;
  • the struts are less exposed to the elements;
  • the struts are less likely to interfere with drainage or create puddles.


The essay on softer boundaries is related to this one.

Tim Tyler | Contact |