Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 414

414 Earthquake Engineering for Structural Design

Figure 9.19 Inelastic force-displacement curve (NEHRP, 2003)

the design seismic lateral forces correspond to the level causing the formation of a first
plastic hinge in the most heavily loaded element in the structure. The loading
increasing causes the formation of additional plastic hinges and the structure capacity
increases until the maximum is reached. The overstrength capacity obtained by this
inelastic action provides the reserve strength necessary for the structure to resist the
extreme motions of the seismic forces which may be generated by the actual ground
motions. It may be noted that the structural overstrength described above results from
the development of a sequential plastic hinging in a redundant structure. Figure 9.19
explores some of the factors which contribute to produce the structural overstrength:
material overstrength (actual material strength being higher than the reduced nominal
strength) and additional overstrength (by selecting sections exceeding those strictly
required by the computations). The result is that structures typically have a much
higher lateral resistance than the one specified by the codes (NEHRP, 2003).
The main problem is how to optimize these aspects in structural design. The
transformation of the elastic response spectrum into an inelastic response one is the
best way to follow in a simplified methodology, which uses the response spectra. The
inelastic response spectra can be obtained in seismic codes by modifying the elastic
response spectra by means of a factor, namely the q- factor , which takes into account