The HB Williams Library extension is on the northwest side of the existing library. The Northern part of the extension building is two storey and connects with a single storey southern part by a double height entrance atrium. Due to high seismic loading in Gisborne, the building was considered at Building Importance Level 3, and included a seismic resistant steel frame, portal frame and bracing across the building and on the roof.
CLIENT BRIEF/PROJECT CHALLENGES
Several investigations were conducted by WSP to analyse the risk of liquefaction and choose a suitable foundation type for the building. A shallow foundation was chosen in lieu of pile foundation to prevent uneven movement between the existing library and the additional building during a strong earthquake. A shallow foundation was also considered the most economical and effective option.
The HB Williams Library building is an example of WSP’s working experience designing Building Importance level 3 public structures. WSP was committed to delivering a resilient and sustainable structure to serve both library staff and Gisborne’s community. Providing durable seismic resistance system without interrupting overall aesthetic view of the building was one of the key factors. The challenge was to ensure a simultaneous response between the existing building and the proposed building under seismic load. WSP’s experienced geotechnical and structural engineers were passionate about working collaboratively to present the most economic solution for liquefaction risk at the site within a limited time frame.
The threat of liquefaction and lateral spreading due to the location and weak ground condition of the site were investigated by WSP’s geotechnical engineers. Based on geotechnical report, soil beneath the sand and gravel layer was cohesive, which significantly reduced the risk of liquefaction.
Lateral movement was also low since the site is 130 meters away from the Turanganui River. WSP recommended a shallow foundation bearing on the stronger soil layer which ensured even movement of new and existing library comprehensively.
An innovative shallow foundation bearing on four meters layer of sand and gravel was proposed as a cost-effective and robust seismic resisting foundation for the site. The proposed foundation reduced the cost of having more than a 27 metre pile. It also prevented the risk of differential movement between the existing and proposed library under Building Importance Level 3 lateral loading.