Limit state design philosophy

M R Scaffolding wins Gold Award for considerate working practices

The ‘basis of design’ employs calculations to check that the stresses due to loading at critical points have a very low chance of exceeding the material strength. This ensures that the safety of a structure is never compromised and that inherent redundancy is present in the design and in the final structure.

The scaffold engineering sector generally uses Permissible Stress Design for structures, whereas all other structural engineering branches have now switched to the Limit State Design method.

The two different design philosophies have their own advantages and disadvantages when considered with respect to the design of scaffolding. However, it is evident in comparison that the advantages of limit state design far outweigh the benefits of permissible stress design.

The scaffold engineering sector traditionally always adopts the permissible stress method for the added reason that site personnel generally do not understand the concepts of the limit state approach, while they do understand the terms ‘allowable’ and ‘safe working load’.

It is of my opinion that site personnel do not need to understand the concept of limit state design; they just need to know what load can be applied to the structure before the factors are applied – the serviceable load.

The terminology is just slightly different but can be portrayed in the same manner. The serviceable load

(design working load) is something which is indicated on the design and is managed on-site by the principal contractor. Therefore, site personnel and scaffolders do not have to consider the inner workings of the basis of design; they just need to know the load permitted.

I don’t believe that understanding of ‘allowable’ or ‘safe working load’ is a founded reason for not adopting limit state design as site personnel do not often understand the concept of either kilo-Newton per square metre or kilo-Newtons and do not need to if a design is produced.

A good way of viewing this argument in a different light is considering permanent works construction. If permanent works operatives do not understand the concept of limit state design, then why should a scaffolder? Would a steel fixer understand how the reinforcement bar had been calculated and derived? It is irrelevant as to whether or not the site personnel understand the basis of design because they just need to know the specification of the materials. Therefore, I see no reason why it cannot be adopted in temporary works design.

The other reason why scaffold engineers believe that limit state cannot be adopted is that all materials and components are stated in terms of ‘permissible’. However, in order to obtain the permissible, an ultimate value has obviously been obtained through testing.

All scaffolding products will be tested to failure with 95 per cent confidence limits. If the factor of safety is known, then the characteristic / ultimate value of resistance can be obtained by multiplying the permissible by the factor of safety. In addition, some manufacturers state both ultimate and permissible values. The ultimate value can be obtained directly from the manufacturer or the relevant British Standard.

The limit state approach is much more comprehensive and provides a much more versatile basis of design, where design can lead to more efficient structures, resulting in reduced construction costs. This, in addition to all other engineering sectors adopting limit state design, all codes requiring limit state design and the possible downsides of not adopting limit state design, means that the permissible stress method should remain a thing of the past. As scaffold engineers, we should move into a new era and embrace the change with open arms. I am sure that the change would cause some trepidation by the sector but this would soon be overcome when the benefits to be had are seen.