Implications of changes of building use on seismic performance of adjacent structures

Widespread damage within urban centres (e.g. in the 2011 Christchurch Earthquake) and increasing urbanisation are key motivating factors in better understanding the seismic structure-soil-structure interaction (SSSI) between closely-spaced adjacent building structures. It has been known for some time that SSSI can affect the dynamic response of adjacent structures either beneficially or detrimentally, on ground which is responding elastically. More recent work based on geotechnical centrifuge testing has further demonstrated that SSSI can also strongly influence the post-earthquake settlement and rotation of structures, associated with the foundation behaviour. Although these foundation responses do not necessarily affect the collapse damage condition (life safety), they can be very damaging in terms of preventing use of the building and in extreme cases, necessitating demolition and rebuilding. This paper will consider the effects of a change of use being made by the owners of one building within a small group of initially identical low-rise structures, with the change resulting in an increase in mass (and hence increase in natural period and reduction in footing factor of safety). Potential mitigation of any negative effects induced on any of the structures due to the change will also be considered using ground modification (densification) and foundation strengthening (extending foundations from strips to a raft). In urban areas mitigation is complicated by the fact that ownership of the adjacent structures will most likely be different, and so mitigation will be limited to the structure with the change of use and the ground beneath it. Numerical simulations using the Finite Element method will be conducted for the case when the subsoil is non-liquefiable, utilising a soil model which has previously been validated for SSSI problems against dynamic centrifuge testing data. It will be demonstrated that the decision to alter just one structure in a group may have detrimental effects on its own performance, but will also affect the response of the adjacent structure, either detrimentally or beneficially, depending on the type of foundation (strips or raft). Densification beneath the modified structure can neutralise at least some of these effects to the extent of also protecting the neighbouring structure. This is in contrast to improving the foundation type by extending the strips to a raft, where the impact on both structures is always detrimental. It is clear that the effects of SSSI mean that the performance of neighboring buildings in dense urban areas should be considered when making decisions about change of use or otherwise modifying the seismic performance of individual buildings within the group.