Detailed Seismic Assessment – Whanganui

LDE conducted a Detailed Seismic Assessment (DSA) and proposed preliminary strengthening measures for the heritage Power Board Offices building on Hill St, Whanganui. Originally built in 1926 with modifications in 1957, this three-storey reinforced concrete structure exhibits common design limitations of its era, relying on reinforced concrete shear walls and/or reinforced concrete columns for gravity loads.

The lateral system consists of concrete moment frames, concrete shear walls and concrete moment frames with in-fill unreinforced masonry brick.  Notably, the building displayed a significant torsional response attributed to the concrete stairwell, lift shaft, and other irregular reinforced concrete shear walls.

To assess the building’s seismic strength, we used both force and displacement-based methods, presenting a preliminary strengthening scheme.  Using SpaceGass software, we modelled the building with equivalent static and modal response techniques.  For assessing the concrete floor diaphragms, we followed the modified methods outlined in the Detailed Seismic Assessment Guidelines Part C5 Appendix.

Our recommended strengthening solution includes adding diagonal steel braces and ground beams to limit the building’s seismic drift. This approach was chosen due to the low displacement capacities of the façade, connections, and gravity columns.