Colorcoat-Online - Functional Design

Functional Design

A key purpose of the building envelope is to enclose the interior of the building and protect it from the elements. The envelope does not, in most cases, provide primary support to the building, however the need for it to withstand the worst of the weather has structural implications for the performance of the cladding. Here we explain how the envelope interacts with other structural aspects of the building, basic loading considerations and design considerations for low pitched roofs as well as protecting against the effects of lightning. Under current regulations, safety in all its guises is vital to the functional design of a building, so this is also addressed. The structural integrity of the envelope is very important and whilst the advice given is intended to provide a first point of reference, it does not replace the need for a qualified structural engineer.

Building structure

Throughout history, man has built to keep out the elements. Today, the prime functional purpose of the building envelope is still to enclose the interior of the building and protect it from the elements. The envelope does not, in most cases, provide primary support to the building, but the ability to withstand the worst of the weather can have significant implications for the structural performance of the cladding.

Structural implications of cladding systems

Here the stuctural implictions of each cladding system is explored.

Calculating loadings on cladding

In designing a metal-clad building, the following loads should be considered; dead load, uniform imposed load, concentrated imposed load and wind load.

Design loads

To determine the design load for a particular building, first establish the relevant case i.e. roof with access, roof without access or walls. Then for the relevant case, the design load to be used is the largest of the loading situations (windload, snowload, distributed or concentrated load) given in the table:

Load/span tables

In practice, pre-finished steel cladding and secondary steelwork must be designed to withstand both positive and negative (wind-suction) loads. To help the building designer, roof and wall cladding system manufacturers publish load/span tables for both positive and negative loads, giving the maximum allowable spans for specific profiles under given design loads.

Structural design rules

Practically, the design rules are considered by the individual roof and wall cladding system manufacturers when designing profiles, and form the basis of published load-span tables.

Thermal expansion of steel cladding

The thermal expansion coefficient of steel used for roof and wall cladding is 11.3 x 10-6 per °C. In practical terms, this means that, over a 50°C temperature range, an 8 m long sheet will expand by 4.5 mm. Since the whole of the building structure flexes to accommodate this very small movement, thermal expansion of prefinished steel sheets, unlike some alternatives, rarely presents any problems.

Lightning protection

Lightning is a natural phenomenon which is formed as a result of a natural build-up of electrical charges within storm clouds which discharge to earth. The magnitude of the electric current created can range from around 3000 and 200,000 amps with potentials of 100 million volts. The calculated probability of structurally damaging lightning strikes is once in every 500 years.

Design guidance for roofs

The primary purpose of roof cladding is to maintain a weather-tight envelope for the effective life of the roof. This means both maintaining water-tightness and protecting the roof cladding material from undue and premature degradation. For these reasons, all roofs should be designed to avoid retention or ponding of water.

Safety in design

To make the management of hazards and minimisation of risk during construction and life of a building easier are increasingly important considerations for the building designer. The introduction of the Construction (Design & Management) Regulations 1994 (CDM) has placed a duty on the building designer to consider the safety implications of the design at all stages.