Summary
The subject office/warehouse building was assessed and rated at less than 34%NBS in its original condition – therefore, it was earthquake-prone. Subsequently, it was retrofitted to 75%NBS. The retrofit solution was very efficient and allowed the building to be fully occupied with minimal disruption during construction.
Background
The client is a Property Trust that owned and managed a four-storey office / warehouse building. They also occupied one floor within the building. Engineer A was engaged by the client to conduct a Detailed Seismic Assessment (DSA) and subsequent retrofit design of the building. The DSA would aim to establish the probable seismic capacity of the building in terms of %NBS (New Building Standard).
The %NBS rating indicates the life-safety risk compared to that of a new building built at the same site. The retrofit design would upgrade the DSA rating to a target rating of 75%NBS at minimum. Engineer B was then engaged by the client to carry out an external peer review of Engineer A’s work for a Building Consent application with the local council.
Building Description
The subject building is a four-storey RC (Reinforced Concrete) structure built in the late 1900s. The flooring system consists of suspended ribs and timber infills supported by prestressed precast shell beams. The shell beams and circular columns form the primary gravity structure of the building. The main lateral system includes the moment-resisting frames (shell beams and circular columns) and a precast shear wall at the rear of the building. Figure 1 shows the typical structural layout.
What did Engineer A do?
The Detailed Seismic Assessment (DSA) was conducted by Engineer A using computer-aided linear analysis based on the Modal Response Spectrum Method (MRSM). The earthquake loading was derived at the Ultimate Limit State (ULS). In terms of the ULS criteria, the structure may exhibit damages, but the collapse must be prevented to ensure life safety.
The probable capacities of structural members such as beams, columns and walls were calculated to the requirements of the relevant guidelines and standards. Then, a %NBS rating was generated for each member based on the ULS demands and probable capacities. The minimum rating of the structural members was the overall rating of the building – it was less than 34%NBS, meaning the building was earthquake-prone. The critical structural weakness was the tensile capacity of a floor diaphragm. To bring up the seismic rating to at least 75%NBS, Engineer A designed a seismic strengthening solution for the client. The solution involved a new steel Concentric-Braced Frame (CBF) at the front, beam-column and diaphragm-wall connections and a diaphragm strengthening system including externally bonded Carbon Fibre-Reinforced Polymer (CFRP) strips used in conjunction with a Post-Tensioning (PT) system with PT cables and cast-in-situ concrete anchorage blocks. The purposes of the proposed retrofits are summarized as follows:
The steel CBF was added to reduce the torsional response of the building and share the seismic loads from the precast shear wall at the rear.
The beam-column connections with steel plates, angles and epoxy anchors improved the positive bending capacity of the beams, thus allowing the shell beams and circular columns to function as a full Moment-Resisting Frame (MRF).
The diaphragm-wall connections consisted of steel equal angles and epoxy anchors and would aim to improve the shear transfer from the diaphragm to the wall.
The CFRP / PT system was used to improve the axial strength of the ‘ties’ in the strut-and-tie model of the floor slab subjected to in-plane earthquake loading.
At one particular floor level, it was difficult and disruptive to install CFRP strips directly on top of the slab due to the occupied floor and existing fit-outs. Therefore, the solution was CFRP strips installed to the underside of the slab in the longitudinal direction. However, they could not be practically installed in the transverse direction due to the concrete ribs, so PT cables were used and passed through the concrete ribs by drilling; they were anchored by cast-in-situ concrete blocks at the ends. The solution allowed the floor to be fully occupied with minimal disruption during construction. Figure 2 shows the proposed retrofit solution at the particular floor level.
Figure 2 – Proposed retrofit solution at one particular floor
What did Engineer B think?
Following the external peer review, Engineer B concluded that Engineer A’s solution was very efficient and would upgrade the seismic rating of the building to at least 75%NBS. In his review, he did not find any issues that would need to be addressed – this meant a high level of quality in Engineer A’s work deliverables. This was primarily due to a rigorous internal QA process conducted by Engineer A prior to the external peer review by Engineer B.
Conclusion
In conclusion, the building was rated at less than 34%NBS in its original condition – therefore, it was earthquake-prone. Subsequently, it was retrofitted to 75%NBS. The retrofit solution was very efficient and allowed the building to be fully occupied with minimal disruption during construction.