There are two major factors that determine the performance of the sheet, stiffness and maximum stress.

Sheet stiffness will determine the deflection of the sheet under load conditions. It will also determine the load at which the sheet will pop out of the fixing rebate.

There is a limit to the maximum stress that the material can sustain. Failure of the sheet will be due to cracking. The point at which this occurs can be predicted by finite element analysis and conformed by test.

The deflection of the sheet can be calculated by formula and so the failure of a sheet due to pop out can be predicted. The formula however will not indicate when the stress limit has been reached and is therefore limited without additional information. This comes from both finite analysis and practical test.

Deflection = 5 x q x s4 q a 3

384 x E x I x 8 G x d

Where; q is the uniformly distributed load, s is the span of the sheet, E is the stiffness modulus of the material, I is the second moment for the sheet a is the length of the sheet, d is the thickness and G is a stiffness modulus.

Limiting parameters

For practical purposes limits are normally set for the glazing depending on the application.

A deflection limits is normally required. If the sheet deflects too much it looks unsightly. There is an increased risk of the ingress of water due to the deformation of seals and capping.

A safety factor against pop out and buckling is normally applied. This safety factor covers imperfections in the sheet should they occur, variance form the theoretical model and most importantly quality of installation. If a sheet is badly cut, or the edge engagement of a sheet is not as designed, the safety factor will go some way to offset these problems.

We would normally recommend a deflection limit of 1/25 span and a safety factor of 1.5 with a minimum effective edge engagement of 15mm as good design practice for weather tight building applications.