The single most important seeming principle in seismic design is to ensure that the structural components and systems are adequately tied together to perform as a structural unit. However, due to uncertainties surrounding how sturdy structural system can be achieved, other special considerations are applied in the seismic design to guard against failure. These considerations include:
The concept of over strength is a realization that a shear resisting system’s ultimate capacity is usually significantly higher than required by a design load as a result of intended safety margins. At the same time, the seismic ground motion (load) is reduced by the seismic response modifier factor (R-factor) to account for ductile response of the building system, among other things. Thus, the actual forces experienced on various components (i.e. connections) during a design level event can be substantially higher, even though the resisting system may be able to effectively dissipate that force. Therefore, over strength factors have been included in newer seismic codes with recommendations to assist in designing components that may experience higher forces than determined otherwise for the building lateral force resisting system. In essence, the over strength concept is an attempt to address the principle of balanced design. It strives to ensure that critical components, such as connections, have sufficient capacity so that the overall lateral force-resisting system is able to act in its intended ductile manner (i.e., absorbing higher-than design forces). Thus, a premature failure of a critical component (i.e., a restraining connection failure) is avoided. This requires adequate designer judgment in addition to code requirements.
This was postulated to address the reliability of lateral force-resisting systems by encouraging multiple lines of shear resistance in a building. Redundancy is an area where exact guidance does not exist and the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
Deflection amplification has been applied in past and current seismic design codes to adjust the deflection or story drift determined by use of the design seismic shear load (as adjusted downward by the R-factor) relative to that actually experienced without allowance for modified response (i.e., load not adjusted down by the R-factor). This often requires multiplying the value of seismic shear load, V by a factor and the situation is onerous based on the susceptibility of the building to collapse. This application also requires designer judgment.
Irregularities are related to special geometric or structural conditions that affect the seismic performance of a building and either requires special design attention or should be altogether avoided. In essence, the presence of limits on structural irregularity speaks indirectly of the inability to predict the performance of a structure in a reliable, self-limiting fashion on the basis of analysis alone. Therefore, many of the irregularity limitations are based on judgment from problems experienced in past seismic events.
Irregularities are generally separated into plan and vertical structural irregularities. Plan structural irregularities include torsional imbalances that result in excessive rotation of the building, re-entrant corners creating “wings” of a building, floor or roof diaphragms with large openings or non-uniform stiffness, out-of-plane offsets in the lateral force resistance path, and non-parallel resisting systems. Vertical structural irregularities include stiffness irregularities (i.e., a “soft” storey), capacity irregularities (i.e., a “weak” storey), weight (mass) irregularity (i.e., a “heavy” storey), and geometric discontinuities affecting the interaction of lateral resisting systems on adjacent storeys.
The concept of irregularities is associated with ensuring an adequate load path and limiting undesirable (i.e., hard to control or predict) building responses in a seismic event. Again, experienced designers generally understand the effect of irregularities and effectively address or avoid them on a case-by-case basis. For typical single-family housing, all but the most serious irregularities (i.e., “soft storey”) are generally of limited consequence, particularly given the apparently significant system behavior of light-frame homes (provided the structure is reasonably “tied together as a structural unit”). For larger structures, such as low and high-rise commercial and residential construction, the issue of irregularity and loads becomes more significant. Because structural irregularities raise serious concerns and have been associated with building failures or performance problems in past seismic events, the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
This issue may be handled through specification of materials that have similar deformation capabilities or by system detailing that improves compatibility. For example, a relatively flexible hold-down device installed near a rigid sill anchor causes greater stress concentration on the more rigid element. The solution can involve increasing the rigidity of the hold-down device (which can lessen the ductility of the system, increase stiffness, and effectively increase seismic load) or redesigning the sill plate connection to accommodate the hold-down deformation and improve load distribution. There is little definitive design guidance on deformation compatibility considerations in seismic design of wood-framed buildings and other structures. The designer judgment may also be applicable here.
ACI-318 (Residential Structural Design Guide), Chapter 3: Design Loads for Residential Buildings