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Figure 2.43. Design basis earthquake ground motions; 10% probability of exceedence in 50 years, corresponding to a return period of 474 years, typically rounded to 500 years.

determining design forces, considerable energy dissipation through repeated cycles of inelastic straining is assumed. The reason is the large demand exerted by the earthquake and the associated high cost of providing enough strength to maintain linear elastic response in ordinary buildings. This unusual limit state means that several conveniences of elastic behavior, such as the principle of superposition, are not applicable. This is the reason why seismic provisions contain so may provisions that modify customary requirements for proportioning and detailing structural members and systems. It is also the reason for more stringent construction quality assurance requirements.

Figure 2.44. Maximum considered earthquake ground motions; 2% probability of exceedence in 50 years, corresponding to a return period of 2475 years, typically rounded to 2500 years.

2.4.1.1. Importance Factor I

The purpose of this factor, I, is to specifically improve the capability of essential facilities and structures containing substantial quantities of hazardous materials to function during and after design earthquakes. This is achieved by introducing the occupancy importance factor of 1.25 for seismic use group (SUG) II structures and 1.5 for SUG III structures. This factor is intended to reduce the ductility demands and result in less damage. When combined with the more stringent drift limits for such hazardous facilities, the result is improved performance of such facilities.

2.4.1.2. Redundancy

This factor applies for structures in seismic design categories (SDC) D, E, and F. The value of this factor varies from 1.0 to 1.5. It has the effect of reducing the R factor for less redundant structures, thereby increasing the seismic demand. The factor recognizes the need to quantify the issue of redundancy in the design. It should be noted that many nonredundant structures have been designed in the past using values of R that were intended for use in designing structures with higher levels of redundancy. The intent of redundancy factor is to prevent such misuse.

2.4.1.3. Elements Supporting Discontinuous Walls or Frames

The purpose of the special load combinations is to protect the gravity load-carrying system against possible overloads caused by overstrength of the lateral force-resisting system. Either columns or beams may be subject to such failure; therefore, both should include this design requirement. Beams may be subject to failure due to overloads in either the downward or upward directions of force. Examples include reinforced concrete beams or unbraced flanges of steel beams or trusses. Hence, the provision has not been limited simply to downward force, but instead to the larger context of vertical load. An issue that has not been fully addressed in the ASCE-7 is clarification of the appropriate load case for the design of the connection between the discontinuous walls or frames and the supporting elements.

2.4.1.4. Special Seismic Load Combinations

Some elements of properly detailed structures are not capable of safely resisting ground shaking demands through inelastic behavior. To ensure safety, these elements must be designed with sufficient strength to remain elastic. The Qo coefficient approximates the inherent overstrength in typical structures having different seismic force-resisting systems. The special seismic loads, factored by the Qo coefficient, are an approximation of the maximum force these elements are ever likely to experience. ASCE-7 permits the special seismic loads to be taken as less than the amount computed by applying the Qo coefficient to the design seismic forces when it can be shown that yielding of other elements in the structure will limit the amount of load that can be delivered to the element. A case in point is the axial load induced in a column of a moment-resisting frame from the shear forces in the beams that connect to this column. The axial loads due to lateral seismic action need never be taken greater than the sum of the shears in these beams at the development of a full structural mechanism, considering the probable strength of the materials and strain-hardening effect. For frames controlled by beam hinge-type mechanisms, this would typically be 2Mp/L, where for steel frames, Mp is the expected plastic moment capacity of the beam as defined in the AISC Seismic specifications. For concrete frames, Mp is the probable flexural strength of the beams. L is the clear span length for both steel and concrete beams. In the context of seismic design, the term capacity means the expected or median anticipated strength of the element, considering potential variation in material yield strength- and strain-hardening effects. When calculating the capacity of elements for this purpose, material strengths should not be reduced by capacity or resistance factors.

Where earthquake forces are applied concurrently in two orthogonal directions, the 5% displacement of the center of mass should be applied along a single orthogonal axis chosen to produce the greatest effect, but need not be applied simultaneously along two axes (i.e., in a diagonal direction).

Most diaphragms of light-framed construction are somewhere between rigid and flexible for analysis purpose, i.e., they are semirigid. Such diaphragm behavior is difficult to analyze when considering torsion of the structure. As a result, it is believed that consideration of the amplification of the torsional moment is a refinement that is not warranted for light-framed construction.

The intent is not to amplify the actual, i.e., the calculated torsion component, but only the component due to accidental torsion. There is no theoretical justification to further increase design forces by amplifying both components together.

2.4.1.6. Relative Displacements

The design of some nonstructural components that span vertically in the structure can be complicated when supports for the element do not occur at horizontal diaphragms. Therefore, story drift must be accommodated in the elements that will actually distort. For example, a glazing system supported by precast concrete spandrels must be designed to accommodate the full story drift, even though the height of the glazing system is only a fraction of the floor-to-floor height. The condition arises because the precast spandrels will behave as rigid bodies relative to the glazing system and therefore, all the drift must be accommodated by the joint between the precast spandrel and the glazing unit.

2.4.1.7. Special Requirements for Piles and Grade Beam

Anchorage of the pile into the pile cap should be conservatively designed to allow energy-dissipating mechanisms, such as rocking, to occur in the soil without structural failure of the pile. Precast prestressed concrete piles are exempt from the concrete special moment frame column confinement requirements since these requirements were never intended for slender, precast prestressed concrete elements and will result in unbuildable piles. These piles have been proven through cyclic testing to have adequate performance with substantially less confinement reinforcing than required by ACI 318. Therefore, a transverse steel ratio reduced from that required in frame columns is permitted in concrete piles. It should be noted that confinement provided by the soil improves the behavior of concrete piles.

Batter pile systems that are partially embedded have historically performed poorly under strong ground motions. Difficulties in examining fully embedded batter piles have led to uncertainties as to the extent of damage for this type of foundation. Batter piles are considered as limited ductile systems and should be designed using the special seismic load combinations.

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