Excavations in dense urban areas that are seismically active are poorly covered by existing building codes, said Arup principal Nick O’Riordan at a joint British Geotechnical Association and Society for Earthquake and Civil Engineering Dynamics meeting last night.
“Surrounding buildings have an effect on the excavation and the excavation will also have an effect on the existing buildings but analysing the relationship and potential impact of a seismic event during the excavation is a challenge,” he said.
O’Riordan is currently working as part of a team constructing the Transbay Transit Center in downtown San Francisco. The public transport interchange links underground rail services with an elevated bus station and park and is surrounded on all sides by a number of different age and height buildings.
“The center involves an excavation 20m deep and 60m wide,” explained O’Riordan. “Analysis of the first mode period [seismic event response time] of the buildings and utilities surrounding the site showed they would vary from 1.2 up to 4.77 seconds so would all react differently to a seismic event. With a three year construction period we had to consider the potential impact of an earthquake while the excavation was open.”
As a result of the limited codes, the team needed to find alternative ways of finding out more. It considered static analysis but found that 2D analysis was unrealistic and 3D finite element analysis was the best option, although assumptions had to be made and complex modelling was needed to give results that could be used for design.
Analysis of the impact of an earthquake during construction on the tallest building at the site showed that through use of a secant piled buttress to prevent impact on the building, the response to the earthquake of the structure was improved. However, the shove effect of the building had a significant and permanent effect on struts placed lower in the excavation.
O’Riordan said that existing design codes do not offer enough guidance for construction under these conditions. Considerable money had been invested in the design for the Transbay Transit Center to reassure the development’s neighbours that the construction could be carried out safely but not all projects can afford this and it may limit future development without development of design codes, he said.