Seismic assessment and retrofit of heritage steel buildings in New Zealand

In the early 20th century, steel frame buildings were built to different standards from those used in modem construction. Riveted, built-up members and connections were often used, with joints and members encased in concrete for fire protection. These early steel buildings were designed based upon observations of past building performance rather than through detailed calculations and predictions of structural behaviour. The walls were infill masonry or concrete, either unreinforced or very lightly reinforced and floors were typically cast in place reinforced concrete. The strength and stiffness of the semi-rigid connections and masonry infill as well as the effect of floor slabs integral with their supporting beams were not well documented and probably not well understood. Examples of these structures can be found throughout the cities and towns of New Zealand and many are quite prestigious, in full service and often enjoying heritage status. Developing methods of assessing their seismic strength and serviceability is a major objective of the paper described herein. Assuming that their seismic strength is inadequate leads to the further objective of devising a retrofitting technique that will raise their strength to acceptable levels (without violating the heritage constraints). Seismic strength results from the resistance of the infill walls and steel frames, coupled together by the floor diaphragms. A search for candidate buildings due for demolition was undertaken in the hope of obtaining full scale, as-built beam, column and joint assemblies that could be removed and tested. When this was unsuccessful it was decided to adopt the archetypal, 1928, 9 storey Hope Gibbons building in downtown Wellington as a case study. It offered typical construction details, a near complete set of drawings and limited access to sample material properties. The experimental tests were conducted on half scale replicas of typical internal and external beam-column joints (including secondary beam stubs, integral composite floor slab and concrete encasing) to determine moment-rotation characteristics, failure mode, and effect of concrete slab, crack development and opportunities for retrofit methods to enhance seismic performance. This will be explained in the second part of the paper. The accuracy and validity of current NZSEE recommendations, June 2006, for seismic assessment of steel riveted joints was discussed and a new function is suggested to replace the current recommendations (Third part of the paper). The retrofit technique to enhance connection strength, using CFRP will be explained in the fourth part of the paper. As part of the strengthening plan, time-history and push-over analyses were completed to assess the as-built and retrofitted strength and stiffness of a typical building.