Summary
A DSA (Detailed Seismic Assessment) was carried out for the subject office building. The seismic rating of the existing building in its original condition is 100%NBS. This was a positive outcome giving both the client and the tenants peace-of-mind. It also provided useful and robust information on the structural capacity and performance of the building, which allowed the client to plan for further works within the building.
Background
The client is a leading Property Group with a portfolio of high-profile commercial properties across New Zealand. They owned and managed a 16-storey office tower.
Engineer A was engaged to complete an Initial Seismic Assessment (ISA) of the building. An ISA provides a broad indication of the likely level of seismic performance of a building based on visual inspections and qualitative assessment, and thus there are limitations to the ISA. For example, due to the scale and complexity of the subject building, an ISA was not suitable.
A Detail Seismic Assessment (DSA) would be required despite the ISA rating being greater than 67%NBS. A DSA aims to establish the probable behaviour of a structure subjected to earthquake loading based on detailed quantitative analysis. Engineer B was engaged by the client to provide a DSA and retrofit design, as well as the structural design for the upgrade of building services in the building.
Building Description
The subject building is an existing 16-storey office building including a basement carpark level. The building was constructed in the late 1980s according to the property file held by the local council. The gravity system consists of precast concrete beams and columns that form moment-resisting and gravity-type frames at regular spacing in both the longitudinal and transverse directions. There are also 200mm-thick precast concrete walls around the building perimeter at the basement level.
There are both precast and in-situ concrete flooring systems. The regularly spaced concrete moment-resisting frames (MRFs) form the main part of the lateral system of the building and were originally designed as a fully ductile structure using the capacity design procedure to the relevant NZ design standards. At the basement level, the perimeter walls include two portions.
The first portion includes the pre-existing walls around the perimeter. The second portion was constructed in the late 1980s using precast concrete panels joined by in-situ concrete columns against the pre-existing walls. Any voids between the two portions were grouted.
As part of the foundation system, there are circular concrete piles, internal concrete ground beams, a 200mm-thick concrete slab on grade and perimeter strip footings of the walls. The piles were embedded 3 to 11.5m into the underlying bedrock.
How did Engineer B improve the work?
The Detailed Seismic Assessment (DSA) of this structure was conducted by Engineer B using a non-linear pushover analysis in accordance with the relevant guidelines and standards. This method allowed for the inelastic behaviour of the structure to be considered, and to assess the post-yield behaviour of individual structural elements within the building.
To verify the suitability of the pushover analysis, a modal analysis was carried out to understand the proportions of seismic mass involved in each principal mode. It was found that the structure is not sensitive to higher-mode effect, thus allowing the use of a pushover analysis in terms of the relevant guideline requirements. This was because the building was originally designed in the late 1980s with the intent to achieve regularity both in plans and elevations and avoid higher mode dominance.
The pushover is a displacement-based method of analysis, in which a target displacement was calculated in each principal direction. The structure was then ‘pushed’ in multiple steps to reach the calculated target displacement in each direction. The soil-foundation-structure-interactions were modelled using non-linear soil springs based on the p-y curves provided by the project geotechnical engineer. A seismic rating was generated for the building based on several criteria including the Ultimate Limit State (ULS) earthquake loading and performance-based Life-Safety (LS) acceptance criteria.
Figure 2 shows the idealized force-displacement backbone curve with the performance-based acceptance criteria, which may be defined at three different levels: Immediate Occupancy (IO), LS and Collapse Prevention (CP). The same type of curve shown in Figure 2 was used to define the relationship of moment (vertical axis) versus curvature (horizontal axis) for beams and columns.
Figure 2 – Force-displacement curve with acceptance criteria (same type of curve used to define moment-curvature relationship)
Results
Watch the animation below to see the formation and sequence of plastic hinges under the earthquake loading at 100%NBS. It should be noted that the green dots represent the development stage of plastic hinges between Points B and C shown in Figure 2.
What would happen if the structure was pushed to a target displacement greater than 100%NBS? Watch the animation below to find out. In this animation, the green dots represent plastic hinges with rotations between the IO and LS limits, while the blue dots show hinges of rotations between the LS and CP limits. The red dots indicate hinges of rotations exceeding the CP limit.
Figure 3 shows the actual pushover curve (base shear vs. roof displacement) of the building subjected to the earthquake loading greater than 100%NBS. It can be seen that the building possesses a high level of available ductility, and the lateral strength only started to degrade at a displacement~1m.
Figure 3 – Actual Pushover Curve (>100%NBS)
Conclusion
In conclusion, the seismic rating of the existing building in its original condition is 100%NBS. This was a positive outcome giving both the client and the tenants peace-of-mind. It also provided useful and robust information on the structural capacity and performance of the building, which allowed the client to plan for further works within the building.