Mid City Plaza, Napier
Engineering assessment finds unexpected construction materials
Mid City Plaza is a collection of interconnected buildings located between Emerson Street and Dickens Street in Napier. The buildings were constructed between 1920 and 1933 and have undergone various alterations.
The two story building on Dickens Street was designed by local architect EA Williams for the Hawke’s Bay Farmers Co-operative Association. It was built in the early 1920’s and survived the 1931 Napier earthquake.
An initial seismic evaluation was conducted in 2013 and a detailed structural analysis conducted in 2014. These assessments determined the Dickens Street building to be below 34% of the new building standard (NBS) and therefore earthquake-prone.
In 2017, engineering company ISPS Consulting Engineers were commissioned to design potential seismic strengthening solutions. Investigation of the building turned up some unexpected building materials. The first floor concrete slab was found to have been constructed without reinforcing: the ‘waffle’ slab floor (with voids formed by empty tin cans to reduce weight) was not uncommon in buildings of this age.
Strengthening solution proposed
The Dicken’s street building consists of reinforced concrete columns and beams. The side and rear elevations are a plain plaster finish over concrete. The original façade has been altered since the 1920’s with the addition of a veranda and alterations to the original steel framed windows.
ISPS Consulting Engineers provided designs for seismic strengthening to 70%NBS. These recommendations included:
- removing the unreinforced concrete first floor structure and replacing it with a new timber framed floor with a plywood diaphragm
- adding bracing at the first floor level
- building shear walls on the ground floor with new foundations tying in the existing walls and columns
Investigating the building’s heritage fabric
The buildings historical value is recognised through its listing as a Group 1 heritage item on the Napier District Plan and its inclusion on the New Zealand Heritage List as part of the Napier City Centre Historic Area.
As a heritage listed building, the work needed to take into account the building’s heritage fabric. A heritage assessment had been undertaken in 2013, and the interior of the building was reviewed with the Napier City Council when applying for project resource consent in 2016.
The most significant heritage fabric was found to be the exterior street facing façade. Fortunately, the proposed seismic strengthening works were all internal and didn’t impact the façade.
Funding the project
The upgrade project was awarded a $150,000 grant from Heritage EQUIP. This contributed towards the costs of removing the unreinforced first floor and replacing with a timber diaphragm structure, steel bracing on the first floor and constructing ground floor foundations and shear walls.
A positive future
The strengthening work ensures the Dickens Street building will be retained as an important contributor to Napier’s central city historic area. The building upgrade also helps ensure ongoing commercial viability of the building.