Analysis of a Six Story Steel Moment Frame Building in Santa Monica (SAC Building Site 7)

M. D. Engelhardt and Keedong Kim


This report summarizes the results of an analytical study of a six story steel moment frame building located in Santa Monica, California. This study was conducted as part of the SAC Task 3.1 as Building Site 7.

This building sustained significant damage to its steel moment frame joints in the 1994 Northridge Earthquake. The primary form of damage observed in this building was the fracture of beam flange welds, typically at the beam bottom flange. A large number of the building's 120 moment frame joints experienced some damage.

Analytical studies were conducted to investigate the predicted response of the building in the Northridge Earthquake, and its predicted resonse under other strong ground motion records. Three types of analyses were conducted in this study: two-dimensional elastic analysis, three-dimensional elastic analysis, and two-dimensional inelastic analysis. A principal objective of the study was to determine if the damage observed in this building was in any way predictable by structural analysis, and if structural analysis could be used as a tool to guide building inspections.

The primary ground motion record used in the analysis of this building was the Santa Monica City Hall record from the 1994 Northridge Earthquake. The record site is very close to the building site, and this record was considered the best estimate available of the actual ground motion experienced by the building during the Northridge Earthquake. In addition to the Santa Monica City Hall record, a variety of other strong motion records were used in the analysis. This included other records from the Northridge Earthquake, as well as records from other strong earthquakes.

The results of both the elastic and inelastic analyses using the Santa Monica City Hall record showed some degree of correlation between the predicted structural response and the damage observed at the joints. There were also numerous exceptions to this trend. Nonetheless, the results of this study suggest that an elastic structural analysis could have been used as a useful guide for inspecting this building. It appears that the chances of locating a damaged joint would be increased by first inspecting joints with the highest predicted beam moment demand capacity ratios (DCRs). For joints with similar levels of beam moment DCR, the chances of locating a damaged joint would be increased by first inspecting the joints for the heaviest beams.

The inelastic analysis indicates that the maximum beam plastic rotations experienced in this building during the Northridge Earthquake were on the order of 0.010 rad. These rather modest inelastic deformation demands suggest that the moment frame joints performed quite poorly.

For the other ground motions considered in this study, the analyses predict significantly greater structural demands on this building (beam moment DCRs, beam plastic rotations, interstory drift ratios, etc.) than were predicted using the Santa Monica City Hall Record. Much higher levels of damage might be expected in this building under these other strong ground motions, than was experienced in the Northridge Earthquake. The simulated Elysian Park record appeared to be particularly damaging to this building.

Excluding the Elysian Park record, the maximum beam plastic rotations developed in this structre under a variety of very strong ground motions were on the order of 0.02 to 0.03 rad. The Elysian Park record developed a maximum beam plastic rotation of about 0.04 rad.

No attempt was made in this study to model the hysteretic response of a damaged joint. Therefore, no conclusions can be drawn on the consequences of connection damage on the response of this structure in future strong earthquakes.

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