NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures - Part 2: Commentary (FEMA 450-2 / 2003 Edition)

The goal of the “Provisions” is to present criteria for the design and construction of new structures subject to earthquake ground motions in order to minimize the hazard to life for all structures, to increase the expected performance of structures having a substantial public hazard due to occupancy or use as compared to ordinary structures, and to improve the capability of essential facilities to function after an earthquake. The “Provisions” provides the minimum criteria considered prudent for the protection of life safety in structures subject to earthquakes. The “Provisions” document has been reviewed extensively and balloted by the architectural, engineering, and construction communities and, therefore, it is a proper source for the development of building codes in areas of seismic exposure. Some design standards go further than the “Provisions” and attempt to minimize damage as well as protect building occupants. The “Provisions” document generally considers property damage as it relates to occupant safety for ordinary structures. For high occupancy and essential facilities, damage limitation criteria are more strict in order to better provide for the safety of occupants and the continued functioning of the facility. Some structural and nonstructural damage can be expected as a result of the “design ground motions” because the “Provisions” allow inelastic energy dissipation in the structural system. For ground motions in excess of the design levels, the intent of the Provisions is for the structure to have a low likelihood of collapse. It must be emphasized that absolute safety and no damage even in an earthquake event with a reasonable probability of occurrence cannot be achieved for most structures. However, a high degree of life safety, albeit with some structural and nonstructural damage, can be achieved economically in structures by allowing inelastic energy dissipation in the structure. The objective of the “Provisions” therefore is to set forth the minimum requirements to provide reasonable and prudent life safety. For most structures designed and constructed according to the “Provisions,” it is expected that structural damage from even a major earthquake would likely be repairable, but the damage may not be economically repairable. Where damage control is desired, the design must provide not only sufficient strength to resist the specified seismic loads but also the proper stiffness to limit the lateral deflection. Damage to nonstructural elements may be minimized by proper limitation of deformations; by careful attention to detail; and by providing proper clearances for exterior cladding, glazing, partitions, and wall panels. The nonstructural elements can be separated or floated free and allowed to move independently of the structure. If these elements are tied rigidly to the structure, they should be protected from deformations that can cause cracking; otherwise, one must expect such damage. It should be recognized, however, that major earthquake ground motions can cause deformations much larger than the specified drift limits in the “Provisions.” Where prescribed wind loading governs the stress or drift design, the resisting system still must conform to the special requirements for seismic-force-resisting systems. This is required in order to resist, in a ductile manner, potential seismic loadings in excess of the prescribed loads. A proper, continuous load path is an obvious design requirement for equilibrium, but experience has shown that it often is overlooked and that significant damage and collapse can result. The basis for this design requirement is twofold: 1. To ensure that the design has fully identified the seismic-force-resisting system and its appropriate design level and 2. To ensure that the design basis is fully identified for the purpose of future modifications or changes in the structure. Read more