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 a). The FRP strips were completely hidden from the external façade after the reinstatement of plaster and paint. The quantity and type of FRP applied to each spandrel was varied depending on the specific demand determined by the analysis of the structure. The spandrel FRP was also tailored to accommodate various architectural building features that were to be retained.

Two different types of FRP system were employed to strengthen the spandrels at the Textile Centre. Reinforced concrete spandrels were strengthened using a surface bonded ‘wet-lay’ FRP system consisting of unidirectional, woven carbon fibre fabric from Sika’s SikaWrap Hex range. Masonry spandrels were strengthened using Near Surface Mounted (NSM) precured carbon fibre laminates from Sika’s Carbodur range.

Spandrel Strengthening with NSM FRP

The majority of the spandrel strengthening was achieved using precured carbon FRP laminates installed using the NSM method. which involves epoxy bonding thin FRP strips into slots cut into the spandrels. Figure 6 shows a strip being installed into a spandrel. The FRP strips were typically 50- 80mm wide and 1.2-1.4mm thick and installed in 120mm deep slots. To avoid damage to any existing steel reinforcing, this system was not employed on concrete spandrels. Some 950 metres of slots were cut through the spandrels, which were also completed in the short working windows available for noisy works.

One of the advantages of NSM FRP is its low weight, making it easy to transport and handle on site. The strips come in 100 metre rolls, which can be carried by hand. Once the slots were cut and prepared, the epoxy was injected to the back of the slots and then the FRP laminates were inserted by hand into Figure 6: Installation of NSM FRP the slots and ensuring a good bond was achieved. The epoxy adhesive was trowelled flush with the surface. Once the epoxy had cured, the surface was sanded and coated with a thin plaster layer, before being painted. Once completed, the spandrels displayed no visible sign of the strengthening works. All work on the spandrels was conducted from the fixed scaffold without the need for any craneage.

Spandrel Strengthening with Wet-lay Carbon FRP

A large proportion of spandrels on the western elevation of the building were constructed of reinforced concrete. Cutting slots for the NSM was not possible in these spandrels due to the presence of reinforcement. Thus, a surface bonded wet-lay FRP system was used instead of the NSM at these locations. The FRP strips were typically continuous along the spandrels on the face of the building. A unidirectional, woven carbon fibre fabric system (SikaWrap Hex 103C) was used as the wet-lay FRP system in these locations. This system could accommodate existing architectural features on the roof spandrels. The width of FRP in some locations was reduced and compensated by the addition of more layers of FRP as shown in Figure 7. Areas with surface bonded FRP were recessed before the FRP was applied, and then plastered and painted. Some 135 square metres of wet- lay FRP was applied to these spandrels.

The bond strength of the epoxy is significantly greater than the tensile and shear strength of the masonry or concrete substrate. A key design consideration was debonding of the FRP due to local failure within the substrate. Figure 7: Wet-lay FRP on a concrete roof Consequently, an anchorage system was spandrel designed to ensure the effective development length of the FRP in the spandrel spans near the termination points of each FRP strip. The strengthening of the FRP anchorage system involved applying surface bonded wet-lay carbon FRP on a large block of masonry within the bond length at the ends of the NSM or wet-lay FRP tensile strips. The wet- lay strengthened anchorage blocks developed the tensile forces by wrapping around the corner of the building and lapping with the NSM or wet-lay FRP over a sufficiently long bond length as shown in Figure 8.

Figure 8: Wet-lay FRP anchor blocks

Parapet Strengthening

The strengthening solution developed by the project team replaced the steel PFC sections spanning between piers with FRP strengthening of the capping beam. Piers that were strengthened over their full height with stressed PT bars did not require additional strengthening for the parapet loads. As seen in Figure 9, the strengthening system had minimal visual impact.

Forty-four piers required strengthening at the parapet level. Instead of the bonded steel Tee-sections, the out of plane strength of these piers was enhanced by installing 3m Figure 9: Parapet with strengthening works long PT bars from the top using a similar completed. method to that employed for the full height pier strengthening. The PT bars were grouted before FRP consisting of both NSM and ‘wet- lay’ Carbon and Glass FRP systems was applied along the top of the parapet. The FRP provided both flexural and shear reinforcement to the parapet cap and created a reinforced capping beam spanning between the piers.

Flexural strength was provided in most locations by NSM precured FRP plates similar to those used in spandrel strengthening. In the remaining locations where the parapet cap contained reinforcement, wet-lay technique was implemented with FRP fibres running parallel to the parapet as shown in Figure 10. Wet-lay glass fibre FRP from the Sikawrap range was applied with fibres running perpendicular to the parapet to provide shear reinforcement. The FRP system developed at the parapet cap varied significantly around the building with a variety of different dimensions and shapes. Some 500 metres of NSM and 750 square metres of wet-lay FRP was applied on the parapets along with 132 metres of PT bars.

Figure 10: A typical reinforced parapet strengthening detail

CONCLUSION

Innovative Seismic strengthening techniques were developed and implemented to enhance the Textile Centre Building, eliminating the need for obtrusive steelwork. Close collaboration between architects, engineers, contractors and the stakeholders proved vital for the successful delivery of the design and construction elements of the project. Significant design development and revisions were required during construction due to unexpected existing features were uncovered during construction. The innovative approaches adopted in design and construction generated a solution which resulted in improvements to the quality, efficiency, safety and economy of the project. The Seismic Strengthening project posed minimal disruption to stakeholders during the works and did not alter the exterior appearance of the building, which was a key architectural objective.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the contributions of the Building owners, Rockport Holdings LP; Project Architects, T Plus Architects; Project Main-Contractor, NZ Strong; and Drilling Sub-contractors, Lowery Supacutters.

REFERENCES

Ingham, J. (2011 a). Assessment and Improvement of Unreinforced Masonry Buildings for Earthquake Resistance. Auckland: University of Auckland. Ingham, J. (2011 b). The Performance of Unreinforced Masonry Buildings in the 2010/2011 Canterbury Earthquake Swarm. Auckland: University of Auckland. NZSEE. (2014). Section 10 - Detailed Assessment of Unreinforced Masonry Buildings. In Assessment and Improvement of the Structural Performance of Buildings in Earthquakes. New Zealand Society for Earthquake Engineering.