If computation shows that the reactions balance the loads (weight of structure, occupants, stored materials, vehicles, wind, and earthquake forces), the structure is in static equilibrium.

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The next step is determination of internal forces and unit stresses in the components of the structure. Finally, if necessary, the deformation of the structure as a whole and of its components may be calculated.

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These steps are facilitated by use of principles and concepts such as the law of equilibrium. Many of these tools are based on the assumption that the structure is elastic under loads; that is, stress is proportional to strain.

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The structural analysis for stresses and strains in frame members includes consideration of static loads, such as weight of the members and floors, content of the building, and live loads (considered to be static), and dynamic loads such as wind and earthquakes.

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Because of the nature of wind and the lack of better information, wind forces are considered to be statically applied to the structure. The manner of calculating the loads and location of lines of action on the building face are prescribed by local building codes.

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For other types of irregular buildings having abrupt changes in plan and elevation, dynamic methods of analysis must be used in order to determine the internal stresses on all the components of the structure.

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These dynamic methods of analysis require an understanding of the principles of structural dynamics and vibrations of buildings. As such, because of the complexity of the methods, highly sophisticated computers are required to determine the solution of the many equations of motions of the structure.

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Structural design is the selection of materials and member type, size, and configuration to carry loads in a safe and serviceable fashion.

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Structural design involved at least five distinct phases of work; project requirements, materials, structural scheme, analysis, and design. For unusual structures or materials as sixth phase, testing, should be included.

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Before starting design, the structural engineer must determine the criteria for acceptable performance. The loads or forces to be resisted must be provided. For specialized structures, this may be given directly, as when supporting a known piece of machinery, or a crane of known capacity.

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For the structural to be serviceable or useful, deflections must also be kept within limits, since it is possible for safe structures to be uncomfortably “bounce.”

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Very tight deflection limits are set on supports for machinery, since beam sag can cause driveshafts to bend, bearings to burn out, parts of misalign, and overhead cranes to stall.

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Limitations of sag less than span/1000 (1/1000 of the beam length) are not uncommon. In conventional buildings, beams supporting ceilings often have sag limits of span/360 to avoid plaster cracking, or span/240 to avoid occupant concern (keep visual perception limited).

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Beam stiffness also effects floor “bounciness,” which can be annoying if not controlled. In addition, lateral deflection, sway, or drift of tall buildings is often held within approximately height/500 (1/500 of the building height) to minimize the likelihood of motion discomfort in occupants of upper floors on windy sway.

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