Summary
The seismic rating of the subject supermarket building in its original condition was revised to 6%NBS only (previously 67%NBS) due to a significant error. Subsequently, the carpark floor slab was strengthened using CFRP strips and steel plates around the slab edges to make the building safer and ultimately save human lives during a major earthquake.
Background
The client is a National Grocery and Liquor Retailer that owns and operates many large supermarkets across New Zealand. One of their largest stores was located at a two-storey RC (Reinforced Concrete) building that was built in the 1980s. The client engaged a reputable engineering firm to carry out a Detailed Seismic Assessment (DSA) of the building and design a strengthening solution to achieve 67%NBS.
Engineer A completed the assessment and design work. Then Engineer B conducted a detailed review of the work as part of the QA process.
Building Description
The building is rectangular in plan and has two floor levels with ~3000sqm at each level. The ground floor of the building is the supermarket. The first floor is a carpark for supermarket customers. The supermarket comprises a concrete slab on-grade at the ground floor level, perimeter precast RC walls, precast RC columns, U-shell beams, RC masonry walls and a hollow core concrete slab at the first-floor level (carpark). The carpark consists of steel portal frames supporting a lightweight roof.
Figure 1 – Carpark floor layout of subject building
What did Engineer A do?
Engineer A assessed the diaphragm using the strut-and-tie method in accordance with the relevant seismic assessment guidelines. A strut-and-tie model is like a truss structure used to represent a solid floor diaphragm. A strut is the part of the diaphragm loaded in compression, while a tie is the part of the slab acting in tension. From the review, Engineer B raised some queries. The tie (tension) capacity of the carpark floor diaphragm should be ~70kN/m. But Engineer A’s calculations showed a capacity of ~500kN/m.
Engineer A included the contribution of the prestressed strands in his calculations. These strands embedded in the precast hollow core flooring units ran only one way discontinuously and were already prestressed to almost the yield strength. Therefore, as a safe practice, they should not be considered as a contribution to the tie (tension) capacity of the diaphragm to resist earthquake loading.
How did Engineer B improve the work?
The tie (tension) capacity should be calculated from only the reinforcing steel in the slab. The prestressing strands should not be included. Non-ductile mesh in the slab has limited tension capacity as the steel mesh wires are already stressed to prevent concrete cracking due to temperature and shrinkage effect over time. The correction of the tie (tension) capacity led to a global tension failure of the carpark diaphragm. As a result, the diaphragm %NSB rating was revised to 6%NBS only (previously 67%NBS), and the floor slab was subsequently strengthened using carbon fibre-reinforced polymer (CFRP) strips and steel plates around the slab edges as shown in Figure 4.
Extensive in-situ CFRP pull-off testing was carried out to accurately determine the CFRP pull-off strength and establish the basis of compliance with the New Zealand Building Code (NZBC). The test results show that other design methods underestimate the effective axial stress of the CFRP by up to 50%. As a result, this resulted in a significant saving in the CFRP material required to meet 67%NBS.
Figure 4 – Slab retrofitted with CFRP
video of the in-situ CFRP pull-off test
The video below shows one of the CFRP pull-off tests performed on the actual concrete slab at the subject building. Watch the video and see how the externally bonded CFRP strip failed at the ultimate pull-off load.
Conclusion
In conclusion, the seismic rating of the existing supermarket building in its original condition was revised to 6%NBS only (previously 67%NBS). Subsequently, the carpark floor slab was strengthened using CFRP strips and steel plates around the slab edges to make the building safer and ultimately save human lives during a major earthquake.