Limit State Design shares the same acronym – LSD with the psychedelic drug Lysergic (Acid) Diathylamide, the “Lucy in the Sky with Diamonds” of the Beatles, even if John and Paul denied the association. Likewise more than a few Structural Engineers that studied in the “Swinging Sixties” are in denial over the merits of LSD, a design philosophy at variance with the time honoured pre-computer age method of ASD – Allowable Stress Design.
The term Limit State merely indicates a boundary condition beyond which the whole structure or an element thereof is considered to be no longer fit for purpose. This may arise in many ways but the principal Limit States to be considered are firstly one of Ultimate Strength and secondly of Serviceability. In the former we are concerned with the maximum loading capacity of the structure (at imminent collapse) – the Ultimate Limit State. In the case of the latter we are concerned more about the appearance of the structure; not only damage to finishes and the durability of the shell but also the detrimental effects on occupants should large deflections or deformations take place. These definitions are those of the structural engineer, whereas an owner or a developer may have his own views and it may well be acceptable for a building to have reached its “Limit” and be ready for redevelopment long before its structural fabric has deteriorated; as was often the case in the Johannesburg CBD when buildings were torn down after only a few...
