Summary
An alternative retrofit solution was finally adopted by the client and would upgrade the seismic rating of the subject heritage hotel building to 100%NBS without extensive new construction required, such as concrete shear walls and foundations. Compared to the original retrofit solution by another engineer, the alternative solution is more cost-effective, constructible, environmentally friendly, less disruptive during construction and would preserve all of the Category 2 heritage features (i.e. the exterior brick facades).
Background
The client is a Property Group with a portfolio of commercial properties across New Zealand and overseas. They owned and operated a heritage hotel building, which is a nine-storey Reinforced Concrete (RC) structure with Unreinforced Masonry (URM) facades. The building has a Category 2 status according to Heritage New Zealand.
Engineer A completed a Detailed Seismic Assessment (DSA) and a seismic retrofit solution to upgrade the seismic rating to at least 67%NBS. Engineer B revisited the DSA and developed an alternative retrofit solution that was finally adopted by the client.
Building Description
The subject building is a nine-storey RC-framed structure with a basement. It was constructed in the 1930s. The perimeter of the building features a two-leaf URM cladding with a hollow internal leaf. At all the levels, the floors are one-way rib slabs. The building was founded on a slope in the longitudinal direction, featuring retaining walls at different levels. In both the longitudinal and transverse directions of the building, the primary lateral load resisting system consists of RC frames. Figure 1 shows the longitudinal building elevation.
Figure 1 – Building elevation
What did Engineer A do?
Engineer A completed a DSA and proposed a seismic retrofit solution accordingly. He concluded from the DSA that the seismic rating of the building was less than 34%NBS, which meant the building was earthquake prone.
For the DSA and subsequent retrofit solution, a linear dynamic analysis was conducted based on the Modal Response Spectrum Method (MRSM) to the relevant standard and guideline requirements. A ductility factor of unity was assumed – this meant a linear elastic behaviour in which the earthquake forces increase linearly, with the deformation based on the initial stiffness, with no step change considered for inelastic behaviour. As a result, the earthquake demands from Engineer A’s analysis were unrealistically high.
To upgrade the seismic rating to at least 67%NBS, extensive concrete shear walls and foundations were proposed by Engineer A in both the longitudinal and transverse directions in the front area of the building. Figure 2 shows the original retrofit solution proposed by Engineer A.
This retrofit solution was not desirable for the following reasons: Firstly, it would destroy the Category 2 heritage features, which are the exterior brick facades. These features are considered valuable cultural assets to the local community. Secondly, it would be more costly, difficult and disruptive to construct. Thirdly, it would require a great amount of energy and new materials, and cause high emissions into the atmosphere during both the production and construction phases.
Lastly, but not least, it would increase the overall building weight significantly and need additional foundations (shown in Figure 2 below), with excavations impacting on the surrounding environment.
How did Engineer B improve the work?
Engineer B revisited the DSA and found that in the transverse direction, a column sway or soft-storey mechanism would likely form at a particular floor level where the axial loads in the columns were at relatively low levels. Accordingly, Engineer B developed an alternative retrofit solution for 100%NBS, including the following significant features:
The potential plastic hinge regions of the columns were clamped using Carbon Fibre-Reinforced Polymer (CFRP). The CFRP clamping system would improve both the plastic hinge rotation capacity and shear strength of the columns, and hence the overall structural ductility and energy dissipation characteristics, leading to lowered earthquake demands in the building. Capacity design was carried out to suppress any undesired or brittle modes of failure, such as a frame shear failure and diaphragm collector failure.
Exterior brick facades were secured at all levels and isolated to accommodate 1.5 times of the expected peak seismic drift from the lateral column-sway mechanism.
The concrete beams were retrofitted with a steel strip plate, equal angles and epoxy anchors to the underside to improve positive bending capacities at both ends.
The rear retaining walls were thickened with a layer of reinforced shotcrete and anchored to the ground slope using soil nails embedded in the new shotcrete portion. As the building was founded on the slope, the soil nailing would improve the slope stability as well as reduce earthquake demands on the building in the longitudinal direction.
To establish the ‘tie’ actions in a strut-and-tie model of the diaphragm, the floor slabs were retrofitted with CFRP strips in both directions, anchored by steel plates around the slab edges. In some areas, slabs were partially demolished to allow a new reinforced concrete topping layer to be constructed.
The CFRP clamping system for the columns was new and outside the scope of any standards and guidelines in New Zealand. As a result, Engineer B carried out extensive international research and designed it accordingly. The problem was that the existing columns contain rebar lap splices immediately above each floor where a plastic hinge may form during a major earthquake. Due to the lap splices, strength degradation would occur after few cycles of loading based on experimental evidence.
To improve the deformation capacity for 100%NBS, a multi-layer CFRP wrap was designed to apply confinement pressure to the longitudinal rebar lap splices; it is a passive system that applies confinement pressure (in addition to that due to stirrups) only when the column starts to deform by rotation. Figure 3 shows the multi-layer CFRP clamping system.
Figure 3 – Multi-layer CFRP clamping system
Figure 4 shows the hysteresis loops of un-retrofitted and CFRP-retrofitted concrete columns from experimental tests. Charts a, d, g and j are from tests of un-retrofitted columns without and with lap splices of various lengths. Charts b, e, h and k are from tests of retrofitted columns (2 layers of CFRP) without and with lap splices of various lengths. Charts c, f, i and l are from tests of retrofitted columns (5 layers of CFRP) without and with lap splices of various lengths. The tested lengths of lap splices were 15, 30 and 45 times of the rebar diameter. Also, it should be noted that Charts a, b and c are for columns without lap splices. It can be seen that the post-yield ductility, strength and energy dissipation characteristics were significantly improved by way of CFRP clamping in all cases.
Figure 4 – Experimental test results – un-retrofitted and CFRP-retrofitted columns without and with lap splices
For the longitudinal direction, the soil nailing and reinforced shotcrete thickening of the existing retaining walls are shown in Figure 5 below. As the building was fixed to the ground slope in the longitudinal direction, most of the building would move with the ground in the event of a major earthquake. This would significantly reduce the seismic inertia forces and hence improve the seismic performance and overall rating of the building in the longitudinal direction.
Figure 5 – Soil nailing and reinforced shotcrete thickening of retaining walls at rear
Conclusion
In conclusion, Engineer B developed an alternative retrofit solution that was finally adopted by the client and would upgrade the seismic rating of the subject building to 100%NBS without extensive new construction required, such as concrete shear walls and foundations. Compared to the more conventional retrofit solution by Engineer A, the finally adopted solution by Engineer B is more cost-effective, constructible, environmentally friendly, less disruptive during construction and would preserve all of the Category 2 heritage features (i.e. the exterior brick facades).