Report - PEER - University of California, Berkeley

approximation to the uniform hazard spectrum (UHS) is improved by using twoindependent parameters to build the spectrum: examples includes the use of twohazard maps, one for an acceleration-related parameter and the other a velocityrelatedparameter, as in the 1984 Colombian code and the 1990 Canadian code. InIBC 2000, the UHS is constructed using the NEHRP approach of maps of spectralacceleration at 0.2 and 1.0 second. A less elegant — and indeed less effective —approach has been adopted in EC8 (CEN, 2003), prompted by resistance to facingengineers with a second ground-motion parameter: two spectral shapes are presented,one for regions only affected by earthquakes of magnitude M s < 5.5 and the other forregions affected by larger events, and each country will adopt the more appropriateshape in its National Application Document.For regions affected by significantly different sources of seismicity, such asmajor subduction zones and moderate magnitude crustal earthquakes, the variation ofthe bedrock spectral shape across a country may be pronounced. In the 1994 Spanishcode, the variation is included via a coefficient K — mapped as contours on top of thebasic zonation map — that represents the degree of influence of large offshoreearthquakes and raises the long-period spectral ordinates accordingly. NeighbouringPortugal adopts a more radical approach, defining two separate spectra — one forlocal events and one for large, distant earthquakes — each of which must beconsidered separately in design. The 1989 Chinese code also defines separate spectralshapes for near-field and far-field earthquakes. EC8 also envisages such a possibility:“When the earthquakes affecting a site are generated by widely differing sources, thepossibility of using more than one shape of spectra should be contemplated toadequately represent the design seismic action. In such circumstances, differentvalues of PGA will normally be required for each type of spectrum and earthquake”(CEN, 2003).Another shortcoming with the acceleration spectra specified in most currentseismic codes is that they cannot be converted to displacement spectra because thisresults in excessively large long-period ordinates due to the absence of a branch inwhich the ordinates decay as 1/T 2 . A notable exception to this is EC8 in which theacceleration spectrum is specified to be compatible with the displacement spectrum,following the proposal of Bommer et al. (2000). The 2003 NEHRP guidelines alsoinclude a transition to constant displacement ordinates, although at much longerperiods (ranging from 4 to 16 seconds, depending on the modal earthquake magnitudeobtained from disaggregation) than the 2.0 s adopted in EC8.2.2.2 Vertical Spectral ShapesThe importance of the vertical component of the ground motion in terms of structuraldamage is a subject of ongoing debate; many current codes do not consider verticalground acceleration at all and those that do simply specify the vertical accelerationspectrum to be equal to ½ or ⅔ of the horizontal spectrum. An exception to this is thecurrent version of EC8 (CEN, 2003), in which the vertical spectrum, based on thework of Elnashai (1997), is specified independently from the horizontal spectrum.472