Summary
The DSA (Detailed Seismic Assessment) was reworked significantly to address the issues identified through the peer review process. As a result, the final seismic rating was revised to 40%NBS (previously 25%NBS). This means that the subject apartment building is no longer earthquake-prone. Therefore, the earthquake-prone label was removed from the building, and there is no improvement work required by the council on the building under the Building Act 2004.
Introduction
Did you know most of seismic assessments in New Zealand are not peer reviewed or independently checked, and many %NBS ratings in New Zealand are either under- or overestimated? In many cases, an independent review is only completed when there is improvement work proposed on a building as it is a requirement of the council for a Building Consent application. We have prepared this case study to demonstrate the importance of a peer review by an independent engineer, and how it can make a difference to the final seismic assessment outcome.
The Client and Engineers
The client is a body corporate. It represents the owners of apartments in a six-storey residential building in a medium seismic risk area in New Zealand (see the seismic risk map here).
Engineer A first completed a Detailed Seismic Assessment (DSA) and concluded that the building was earthquake prone. Engineer A is a reputable multi-disciplinary engineering consulting firm.
Engineer B was then engaged by the client to carry out a peer review of Engineer A’s DSA. Engineer B is an independent seismic engineering specialist.
Building Description
The subject building is a six-storey RC frame and wall structure with a basement carpark and two small floors on top of the roof, used as plant rooms. The building was constructed in the 1980s. The lateral force resisting system consists of central lift and stair core shear walls and a perforated shear wall at each end of the building. The gravity system includes the walls, as well as gravity columns. Within the apartments, the floor system is of a composite type with steel decking filled with cast-in-situ concrete. In other areas, there is a cast-in-situ concrete slab and precast Unispan flooring. The building is mostly supported on a raft foundation including deep ground beams and a doubly reinforced slab. There are also strip and pad footings for concrete masonry walls and columns below the podium level. Figure 1 shows the typical structural layout of the building.
Figure 1 – Typical structural layout of subject building
Key Findings and Improvements to The Work
Engineer A completed the DSA, which was then peer reviewed by Engineer B. In Engineer B’s opinion, the significant issues in the DSA included the following:
Engineer A carried out the seismic analysis based on an assumed site subsoil class of ‘C.’ However, according to the Geotech report, the site subsoil class is Class D. Class C means a shallow soil site, and whereas Class D is a deep or soft soil site. The response of a building to ground shaking due to an earthquake is affected by the site subsoil conditions and building period. The building period is the time (in seconds) taken for a building to complete one cycle of oscillation (back-and-forth motion). Due to the correction of this subsoil class, the earthquake load demand increased.
However, the seismic weight from Engineer A’s analysis was also wrong, and as a result, the earthquake load demand was substantially overestimated. This was due to the floor slab modelled by Engineer A with a thickness 10 times of the actual thickness shown on the original structural drawings.
The material properties used by Engineer A were nominal values, which represented the fifth percentile values or lower bound of the strength test data. However, the actual strengths are likely to exceed the nominal values due to a number of factors such as conservative mix design, aging effect, and coarser cement particles that were used. Therefore, probable strengths should be used instead when assessing existing buildings in accordance with the MBIE seismic assessment guidelines and are higher than the nominal values. This means strength increases by around 50% and 8% for concrete and reinforcing steel respectively.
Engineer A assessed the building in accordance with NZS 3101: 2006 Concrete structures standard. However, NZS 3101 is for the design of new-build concrete structures, and Sections C2 and C5 of the MBIE seismic assessment guidelines should be used instead when assessing existing concrete buildings. There are significant differences between NZS 3101 and the MBIE guidelines. Also, the strength hierarchy at pier/spandrel joints and post-yield behaviour of the end perforated shear walls (critical) should have been assessed but were neglected in Engineer A’s assessment.
The floor diaphragms were not quantitatively assessed by Engineer A. The strut-and-tie method and the pseudo-Equivalent Static Analysis (pESA) should have been used in accordance with the MBIE seismic assessment guidelines (Section C5) and NZS 1170.5: 2004 Part 5: Earthquake actions - New Zealand to investigate the load path within the floor slab at each level to safely transfer the earthquake loads into shear walls.
Due to the building being supported on a raft foundation and soft subsoils, the effect of ground and foundation stiffness on the overall seismic performance of the building can be significant. Therefore, it was necessary to account for soil-foundation-structure interactions using appropriate non-linear soil springs as part of the seismic analysis of primary and secondary structural systems. However, Engineer A assumed rigid ground support and neglected the effect of soil-foundation-structure interactions on the assessment outcome.
Conclusion
The DSA was reworked significantly to address the issues found by Engineer B through the peer review process. As a result, the final seismic rating was revised to 40%NBS (previously 25%NBS). This means that the building is no longer earthquake-prone. Therefore, the earthquake-prone label was removed from the building, and there is no improvement work required by the council on the building under the Building Act 2004.