Structural analysis is the process of determining the forces and deformations in structures due to specified loads so that the structure can be designed rationally, and so that the state of safety of existing structures can be checked.

In the design of structures, it is necessary to start with a concept leading to a configuration which can then be analyzed. This is done so members can be sized and the needed reinforcing determined, in order to: (a) carry the design loads without excessive deformations (serviceability or working condition) ; and (b) prevent collapse before a specified overload has been placed on the structure ( safety or ultimate condition).

Since normally elastic conditions will prevail under working loads, a structural theory based on the assumption of elastic behavior is appropriate for determining serviceability conditions. Collapse of a structure will usually occur only long after the elastic range of the materials has been exceeded at critical points, so that an ultimate strength theory based on the inelastic behavior of the materials is necessary for a rational determination of the safety of a structure against collapse.?Nevertheless, an elastic theory can be used to?determine a safe approximation to the strength of ductile structures, and this approach is customarily followed in reinforced concrete practice.

Looked at critically, all structures are assemblies of three dimensional elements, the exact analysis of which is a forbidding task?even under ideal conditions and impossible to contemplate under conditions of professional practice. For this reason, an important part of the analyst's work is the simplification of the actual structure and loading conditions to a model that is susceptible to rational analysis.

Thus, a structural framing system is decomposed into a slab and floor beams that in turn frame into girders carried by columns that transmit the loads to the foundations. Since traditional structural analysis has been unable to cope with the action of the slab, this has often been idealized into a system of strips acting as beams. Also, longhand methods have been unable to cope with three-dimensional framing systems, so that the entire structure has been modeled by a system of planar subassemblies, to be analyzed one at a time. The modern computer methods have revolutionized structural analysis by making it possible to analyze entire systems, thus leading to more reliable predictions about the behavior of structures under loads.

Actual loading conditions are also difficult both to determine and to express realistically, and must be simplified for purposes of analysis. Thus, traffic loads on a bridge structure, which are essentially both of dynamic and random nature, are usually idealized into statically moving standard trucks, or distributed loads, intended to simulate the most severe loading conditions occurring in practice.

Similarly, continuous beams are sometimes reduced to simple beams, and rigid joints to pin-joints; filler-walls are neglected; shear walls are considered as beams. In deciding how to model a structure so as to make it reasonably realistic but at the same time reasonably simple, the analyst must remember that each such idealization will make the solution more suspect. The more realistic the analysis, the greater will be the confidence it inspires, and the smaller may be the safety factor ( or factor of ignorance).

The most important use of structural analysis is as a tool in structural design. As such, it will usually be a part of a trial-and-error procedure, in which an assumed configuration with assumed dead loads is analyzed, and the members are designed in accordance with the results of the analysis. This phase is called the preliminary design; since this design is still subject to change, usually a crude, fast analysis method is adequate. At this stage, the cost of the structure is estimated, loads and member properties are revised, and the design is checked for possible improvements. The changes are now incorporated in the structure, a more refined analysis is performed, and the member design is revised. This process is carried to convergence, the rapidity of which will depend on the capability of the designer.

An efficient analyst must thus be in command of the rigorous methods of analysis, must be able to reduce these to shorteut methods by appropriate assumptions, and must be aware of available design and analysis aids, as well as simplifications permitted by applicable building codes. An up-to-date analyst must likewise be versed in the bases of matrix structural analysis and its use in digital computers as well as in the use of available analysis programs or software.