Summary
The subject office building was reassessed using a different approach. The seismic rating was revised accordingly to 67%NBS (previously less than 34%NBS). The earthquake-prone label was removed from the building. The outcome was also verified against a real earthquake that shook the building recently.
Background
The client is a leading Property Group with a portfolio of high-profile commercial buildings across New Zealand. The client’s office was situated in a 10-storey office building. Engineer A was first engaged by the client to provide a Detailed Seismic Assessment (DSA) of the subject building. Engineer A reported a seismic rating of less than 34%NBS, which meant the building was earthquake prone.
Then the client approached Engineer B for an independent review of the initial assessment and subsequent reassessment using a different approach.
Building Description
The subject building is a 10-storey reinforced concrete structure built in the 1960s, with a basement and shallow foundations. Plant and tank rooms are located on the roof level of the existing building. The basement, ground and first floor levels consisted of in-situ concrete slabs reinforced with hot-rolled steel reinforcing bars. The levels above the first floor were constructed with precast floor elements that span across the building in the transverse direction.
Most of floor beams have a corbel that provides seating support to the precast concrete floor elements. The structure consisted of a perimeter frame above the first floor, with closely spaced columns and deep and short beams. There are three main exposed building facades with windows between the perimeter columns and beams. There are also internal frames that run in the longitudinal direction. The lift shaft, stair well and toilet areas are separated by in-situ concrete walls. Figure 1 shows the typical structural layout of the building.
What did Engineer A do?
Engineer A assessed the building by performing a pushover analysis without the shear walls at the stairs, lift and toilet areas at one side of the building as shown in Figure 1. As the building was tall and irregular in plan, due to the shear walls to one side only, the higher-mode effect was significant. A pushover could not be used as it would give inaccurate results.
Without the shear walls, the analysis results showed that the perimeter and internal frames were over loaded. However, in reality, most of the loads would go into the shear walls. Engineer A concluded that the building was earthquake prone as the seismic rating was less than 34%NBS, limited by the critical structural weakness – the shear failure of the perimeter beams of the building.
How did Engineer B improve the work?
Engineer B then reassessed the building using a different approach. This time, he decided to model all the shear walls to understand the true behaviour of the building. From the initial assessment by Engineer A, Engineer B already knew that the higher-mode effect was dominant, so a pushover would not be suitable. He conducted nonlinear direct-integration time-history analysis to capture the higher-mode effect, as well as the post-yield ductility and energy dissipation characteristics of both the shear walls and moment-resisting frames. To perform the analysis, he used a range of real earthquake ground motion records selected by the project geotechnical engineer in accordance with the relevant loading standard requirements. To achieve realistic results, all the earthquake records were scaled based on the target spectrum being the site-specific 500-year spectrum (instead of code-prescribed spectrum) developed by the geotechnical engineer.
Figure 2 shows a selected earthquake ground motion accelerogram for the time-history analysis. Figure 3 shows components of a selected scaled accelerogram matched to the 500-year target spectrum specific to the subject site. Figure 4 and Figure 5 show the variation of the building period and a selected plastic hinge rotation respectively with time during a particular earthquake.
The updated results from Engineer B indicated that the critical structural weakness was, in fact, the shear walls instead of the perimeter beams as reported by Engineer A.
Conclusion
The seismic rating was revised accordingly to 67%NBS (previously less than 34%NBS). The earthquake-prone label was removed from the building. The outcome was also verified against a real earthquake that shook the subject building recently.