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The Textile Centre Seismic Strengthening – Innovative Techniques to Minimise the Impacts on an Historic Structure

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The Textile Centre Seismic Strengthening – Innovative Techniques to Minimise the Impacts on an Historic Structure THE TEXTILE CENTRE SEISMIC STRENGTHENING – INNOVATIVE TECHNIQUES TO MINIMISE THE IMPACTS ON AN HISTORIC STRUCTURE N. RANJIT1, R. ROGERS1, O. SMITH1, S. KHATIWADA2 1 BBR Contech 2 PRENDOS SUMMARY An innovative seismic strengthening system was developed for the century-old Textile Centre building in Auckland’s suburb of Parnell, a former wool store constructed of brick masonry and lightly reinforced concrete with a 13,850m2 footprint. The building remains a historic city landmark and is home to high-flying technology, marketing and design companies. The design of the seismic strengthening system developed was optimised through close collaboration between the Architect, Structural Engineer and Contractors. The strengthening design for the perimeter walls consisted of a combination of vertical Post-Tensioning and Fibre Reinforced Polymer enabling the strengthening system to be installed largely from outside of the building. A key project objective was for the seismic strengthening to have minimal - preferably zero - impact on the building’s external physical appearance. PROJECT DESCRIPTION The Textile Centre consists of three distinct structures that were constructed independently between 1912 and 1928. The structures utilised different construction methods and materials but shared partition walls. In addition, the building was modified and strengthened several times over its service life. Large portions of the western and southern façade exhibit reinforced concrete frames with the balance of the external structure and party walls being UnReinforced Masonry (URM). Internal columns, beams and trusses are constructed from either steel or timber. The strengthening works were intended to improve the seismic performance of the structure to at least 67% NBS. To achieve this level of seismic performance, the perimeter walls and frames were upgraded to preclude brittle failure under lateral loads. The URM piers and spandrels required significant enhancement to achieve the required seismic performance. In addition, the connections between internal timber floors and the external and party walls required improvement so that they would act as diaphragms supporting the perimeter frames. Finally, collapse of the unrestrained parapets at the roof level was identified as a life safety risk and required strengthening. During the early stages of the project, the contractors, engineers and architects considered the preliminary design and collaborated to develop an optimized strengthening solution that eliminated externally bonded steelwork and site welding, thus simplifying the construction and resulting in significant cost reductions and a strengthening system that did not impact the external appearance of the building (Figure 1). Figure 1: The Textile Centre Building after strengthening works were completed. PRELIMINARY DESIGN SCHEME The preliminary tender design involved the addition of extensive steelwork to the exterior of the building. This required heavy steel elements to be bonded and bolted with site welded connections between the pier and spandrel steelwork. Preliminary Bonded Steel Tee-Sections Design During the Detailed Seismic Assessment of the building, the in-plane strength of a number of masonry piers was found to be particularly low and led to the seismic rating of the structure to be <30%NBS. All 13 piers on the western elevation required strengthening over full height of the building. Addition of tensile reinforcement was also required on all of the spandrel beams running between the piers to provide a desirable strong-spandrel-weak-pier mechanism (NZSEE, 2014). Approximately half of the spandrels in the southern façade were constructed from reinforced concrete and were deemed to have sufficient strength. All other spandrels on the western façade including those constructed of reinforced concrete were deemed to require additional reinforcing. The original tender design called for 180mm deep Tee-sections to be retrofitted to the piers and spandrel beams that were identified to require additional reinforcing (Figures 2 and 3). The Tee-sections were to be installed by saw cutting slots into the outside of the piers and spandrels with the Tee-section webs being inserted into the slots. Both webs and flanges were to be epoxy bonded and bolted to the existing masonry. The steel Tee-section was intended to act as a tension element and enhance the flexural capacity on the piers and spandrels. Figure 2: Preliminary Pier Strengthening Design Detail Figure 3: Preliminary Spandrel Strengthening Design Detail The original bonded steel strengthening scheme would have required extensive craneage to lift the heavy steel sections into place and mobile elevated working platforms to carry out all the saw cutting and epoxy application because fixed scaffold would have obstructed access to the building. This would have been a high-risk activity with significant impacts to tenants and local traffic. Furthermore, this system would not have worked on all the piers because, as was later discovered, some had reinforced concrete on the bottom two levels of the building and the existing reinforcement would be damaged by the saw cutting, potentially worsening the existing structure. Preliminary Parapet Strengthening Design URM parapets have been identified as particularly vulnerable to earthquake damage because seismic loads are greatest at the top of a building and parapets are often poorly connected to supporting elements (NZSEE, 2014). Consequently, parapets are often weak to out of plane seismic loading or displacement demands and can pose a significant life safety risk because they can collapse onto footpaths (Ingham, The Performance of Unreinforced Masonry Buildings in the 2010/2011 Canterbury Earthquake Swarm, 2011 b). The seismic assessment identified that the out of plane capacity of the parapet was insufficient and the collapse of the parapets posed a life safety risk. The original strengthening solution for the parapets consisted of a steel PFC section bolted to the top of the parapets spanning between the piers. The piers were to be strengthened with bonded steel Tee-sections similar to the original strengthening option described previously. These details can be seen in Figure 4 below. Piers that did not require strengthening over the full height had shorter Tee-beams in the top part only. The Tee-sections were intended to increase the out of plane flexural capacity of the piers to support the seismic loads from the parapet & PFC sections spanning between. Figure 4 Original Parapet strengthening detail and isometric of parapet and pier with embedded steel Tee section FINAL DESIGN SCHEME DEVELOPED BY THE PROJECT TEAM Pier Strengthening The strengthening design for the under strengthened piers which was adopted for construction involved adding axial load on these piers using Macalloy Post-Tensioning bars (PT bars) through the centre of the piers to induce compressive force on them. The Post-Tensioning strengthening method proved to be lower impact, simpler, safer and more economical than the preliminary bonded steel Tee-Section option. The installation method involved drilling 66mm diameter vertical holes through the centre of the piers from top of the parapet to the building foundation, circa 20m below. The Macalloy PT bars were then installed in the piers. Careful site set-out and monitoring during the coring operation was required to ensure that the holes remained centred in each pier. The URM piers were dry cored to avoid the risk of slurry escaping into tenant’s spaces through existing voids and cracks. Instead, compressed air was used to keep the drilling barrels cool. Industrial vacuums were used to capture all the dust and debris. Where drilling was required through concrete, water was introduced as cracks were less likely and the slurry could be controlled. The drilling activity involved noisy works which included some 250m of vertical drilling for the PT bars. The works were conducted with the tenants still occupying the building, and the proximity to nearby residential areas eliminated the possibility of conducting these works at night. Consequently, noisy works were restricted to Saturdays and short working windows between office hours and evenings to comply with the noise restrictions. After the drilling activity was complete, each hole was inspected with CCTV before the PT bars were installed from the parapet level as seen in Figure 5. The base of the PT bars was grouted over a bond length within the foundation to provide a dead-end anchorage. The bond length was designed using a similar design philosophy to a ground Figure 5: Installation of PT bars anchor system. At parapet level, concrete anchor blocks were cast in-situ to distribute the PT loads. The bars were then stressed and grouted over the full height. Spandrel Strengthening Having eliminated the externally bonded steel work for the piers, the project team worked to develop an alternative scheme to eliminate the externally bonded steel work to the spandrels, which presented similar construction and aesthetic concerns. The spandrel strengthening was intended to provide tensile elements to enhance the flexural capacity of the spandrels. The proposed alternative strengthening method involved the use of Fibre Reinforced Polymer (FRP) to replace the steel and provide the required tensile capacity. State of the art design procedures developed at the University of Auckland for the improvement of URM were used to design and size the FRP strips for each spandrel (Ingham, 2011
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