All structures are subjected to loading from various sources. The main categories of loading are: dead, imposed and wind loads. In some circumstances there may be other loading types which should be considered, such as settlement, fatigue, temperature effects, dynamic loading, or impact effects (e.g. when designing bridge decks, crane-gantry girders or maritime structures). In the majority of cases design considering combinations of dead, imposed and wind loads is the most appropriate.

Most floor systems are capable of lateral distribution of loading. In situations where lateral distribution is not possible, the effects of the concentrated loads should be considered with the load applied at locations which will induce the most adverse effect, e.g. maximum bending moment, shear and deflection. In addition, local effects such as crushing and punching should be considered where appropriate.

In multi-storey structures it is very unlikely that all floors will be required to carry the full imposed load at the same time. Statistically it is acceptable to reduce the total floor loads carried by a supporting member by varying amounts depending on the number of floors or floor area carried. Dynamic loading is often represented by a system of equivalent static forces which can be used in the analysis and design of a structure.

The primary objective of structural analysis is to determine the distribution of internal moments and forces throughout a structure such that they are in equilibrium with the applied design loads.

Mathematical models which can be used to idealise structural behaviour include: two-and three-dimensional elastic behaviour, elastic behaviour considering a redistribution of moments, plastic behaviour and non-linear behaviour. The following chapters illustrate most of the hand-based techniques commonly used to predict structural member forces and behaviour.

In braced structures (i.e. those in which structural elements have been provided specifically to transfer lateral loading) where floor slabs and beams are considered to be simply supported, vertical loads give rise to different types of beam loading. Floor slabs can be designed as either one-way spanning or two-way spanning as shown in Figures 1.12(a) and (b).

Figure 1.12

In the case of one-way spanning slabs the entire load is distributed to the two main beams. Two-way spanning slabs distribute load to main beams along all edges. These differences give rise to a number of typical beam loadings in floor slabs as shown in Figures 1.13.

Examples in structural analysis 12