On 9 January 1857, a large earthquake of magnitude 7.9 occurred on the San Andreas fault, with rupture initiating at Parkfield in central California and propagating in a southeasterly direction over a distance of more than 360 km. Such a unilateral rupture produces significant directivity toward the San Fernando and Los Angeles basins. Indeed, newspaper reports of sloshing observed in the Los Angeles river point to long-duration (1-2 min) and long-period (2-8 sec) shaking. If such an earthquake were to happen today, it could impose significant seismic demand on present-day tall buildings. Using state-of-the-art computational tools in seismology and structural engineering, validated using data from the 17 January 1994, magnitude 6.7 Northridge earthquake, we determine the damage to an existing and a new 18-story steel moment-frame building in southern California due to ground motion from two hypothetical magnitude 7.9 earthquakes on the San Andreas fault. Our study indicates that serious damage occurs in these buildings at many locations in the region in one of the two scenarios. For a north-to-south rupture scenario, the peak velocity is of the order of 1 m • sec ‫1מ‬ in the Los Angeles basin, including downtown Los Angeles, and 2 m • sec ‫1מ‬ in the San Fernando valley, while the peak displacements are of the order of 1 m and 2 m in the Los Angeles basin and San Fernando valley, respectively. For a south-to-north rupture scenario the peak velocities and displacements are reduced by a factor of roughly 2.

Using state-of-the-art computational tools in seismology and structural engineering, validated using data from the Mw=6.7 January 1994 Northridge earthquake, we determine the damage to two 18-story steel moment-frame buildings, one existing and one new, located in southern California due to ground motions from two hypothetical magnitude 7.9 earthquakes on the San Andreas Fault. The new building has the same configuration as the existing building but has been redesigned to current building code standards. Two cases are considered: rupture initiating at Parkfield and propagating from north to south, and rupture propagating from south to north and terminating at Parkfield. Severe damage occurs in these buildings at many locations in the region in the north-to-south rupture scenario. Peak velocities of 1 m.s−1 and 2 m.s−1 occur in the Los Angeles Basin and San Fernando Valley, respectively, while the corresponding peak displacements are about 1 m and 2 m, respectively. Peak interstory drifts in the two buildings exceed 0.10 and 0.06 in many areas of the San Fernando Valley and the Los Angeles Basin, respectively. The redesigned building performs significantly better than the existing building; however, its improved design based on the 1997 Uniform Building Code is still not adequate to prevent serious damage. The results from the south-to-north scenario are not as alarming, although damage is serious enough to cause significant business interruption and compromise life safety.

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We expound on the nature of collapse of one class of tall buildings (steel moment frame buildings) under earthquake excitation. Using a parametric analysis of a couple of index buildings subjected to idealized ground motion histories, we establish the ground motion features that cause collapse in these structures. Systematically mapping damage localization patterns, we track the evolution of the collapse mechanism. We demonstrate the existence of a select few preferred mechanisms of collapse in these buildings and describe the associated physics using wave propagation through a shear beam. A simple theory based on work-energy principles can identify these mechanisms. Figure 1. (a) Existing building (1982 UBC design) FRAME3D model; (b) Existing building typical floor plan; (c) Redesigned building (1997 UBC design) typical floor plan.

This paper describes the three-dimensional nonlinear analysis of six 19-storey steel moment-frame buildings, designed per the 1997 Uniform Building Code, under strong ground motion records from near-source earthquakes with magnitudes in the range of 6.7-7.3. Three of these buildings possess a reentrant corner irregularity, while the remaining three possess a torsional plan irregularity. The records create drift demands of the order of 0.05 and plastic rotation demands of the order of 4-5% of a radian in the buildings with reentrant corners. These values point to performance at or near 'Collapse Prevention'. Twisting in the torsionally sensitive buildings causes the plastic rotations on the moment frame on one face of the building (4-5% of a radian) to be as high as twice of that on the opposite face (2-3% of a radian). The asymmetric yield pattern implies a lower redundancy in the lateral force-resisting system as the failure of the heavily loaded frame could result in a total loss of resistance to torsion. the structure. The wave has a forward pulse and a reverse pulse. The constructive interference of these pulses travelling up and down the building results in yield localization and the formation of a kink in the building. Such a kink could cause large P-effects compromising the stability of the structure. Using simplified assumptions, such as a uniform shear building and an idealized ground motion pulse, it is possible to predict when and where along the height of the structure this kink is likely to occur. However, for real buildings, especially of the irregular kind, subjected to threecomponent ground motion, it would be practically impossible to use wave theory to determine the extent and distribution of yielding. The only alternative is to build a detailed three-dimensional finite-element model of the building.Near-source ground motion from large earthquakes has been recorded only in recent times. Their effects on both regular and irregular tall buildings are virtually unknown. Near-source factors have been introduced in the 1997 Uniform Building Code, UBC97 [3] to account for some of the special features of near-source ground motion (it should be pointed out that the more recent International Building Code, IBC2006 [4], does not explicitly specify near-source factors for near-source conditions; higher ground motions at sites close to active faults are specified in the maximum considered earthquake ground motion maps instead). Due to lack of data at this stage, it is not clear how buildings designed according to these guidelines will fare in a near-source event. This raises many fundamental questions that need to be carefully addressed: what is the ductility demand on code-designed tall buildings under near-source ground motion similar to what has been recorded in recent times during the 1994 Northridge earthquake, the 1995 Kobe earthquake, and the 1999 Turkey and Taiwan earthquakes? What kind of interstorey drifts and permanent roof offsets can be expected during such events? How do irregular features in ...