Structural Analysis | Load Consideration in Structure Design

The structural design of buildings, whether of structural steel or reinforced concrete, requires the determination of loads acting, the overall proportions and dimensions of the supporting framework and the selection of the cross sections of individual members. In most cases the functional design, including the establishment of the number of stories and the floor plan, will have been done by an architect, and the structural engineer must work within the constraints imposed by this design and different load consideration on building.

In previous article we have already discussed about fire proof building design and cyclone resistant building design. In this article we are going to discuss load consideration in a typical building design.

Weather you are designing a building or analysing a building, different type of loads are to be considered. The forces that act on a structure are called loads. They belong to one of two broad categories: 

Dead load 
Live load
( theconstructor.org explains types of loads on structure in this article and civileblog.org also discuss about 12 different types of loads in this article)
Dead loads are those that are permanent, including the weight of the structure itself, which is sometimes called the self-weight. In addition to the weight of the structure, dead loads in a building include the weight of nonstructural components such as floor coverings, partitions, and suspended ceilings (with light fixtures, mechanical equipment, and plumbing). All of the loads mentioned thus far are forces resulting from gravity and are referred to as gravity loads.

Live loads, which can also be gravity loads, are those that are not as permanent as dead loads. They may or may not be acting on the structure at any given time, and the location may not be fixed. 

Examples of live loads include furniture, equipment, and occupants of buildings. In general, the magnitude of a live load is not as well defined as that of a dead load, and it usually must be estimated. In many cases, a structural member must be investigated for various positions of a live load so that a potential failure condition is not overlooked. 

If a live load is applied slowly and is not removed and reapplied an excessive number of times, the structure can be analyzed as if the load were static. 

If the load is applied suddenly, as would be the case when the structure supports a moving crane, the effects of impact must be accounted for. 

If the load is applied and removed many times over the life of the structure, fatigue stress becomes a problem, and its effects must be accounted for. 

Impact loading occurs in relatively few buildings, notably industrial buildings, and fatigue loading is rare, with thousands of load cycles over the life of the structure required before fatigue becomes a problem. 

Wind exerts a pressure or suction on the exterior surfaces of a building, and because of its transient nature, it properly belongs in the category of live loads.  

Because of the relative complexity of determining wind loads, however, wind is usually considered a separate category of loading. Because lateral loads are most detrimental to tall structures, wind loads are usually not as important for low buildings, but uplift on light roof systems can be critical. 

Although wind is present most of the time, wind loads of the magnitude considered in design are infrequent and are not considered to be fatigue loads.

Earthquake loads are another special category and need to be considered only in those geographic locations where there is a reasonable probability of occurrence. 

A structural analysis of the effects of an earthquake requires an analysis of the structure’s response to the ground motion produced by the earthquake. Simpler methods are sometimes used in which the effects of the earthquake are simulated by a system of horizontal loads, similar to those resulting from wind pressure, acting at each floor level of the building.

Snow is another live load that is treated as a separate category. Adding to the uncertainty of this load is the complication of drift, which can cause much of the load to accumulate over a relatively small area.

Other types of live load are often treated as separate categories, such as hydro-static pressure and soil pressure, but the cases we have enumerated are the ones ordinarily encountered in the design of structural steel building frames and their members.