We're for ply

Published on 14 December 2021


Did you know that using ply in the renovation of heritage buildings can increase resilience to earthquakes?

When it comes to caring for Oamaru stone buildings, Waitaki leads the way. Members of our Property team are currently working on the stage 2 of the museum renovations where the addition of the ply diaphragm ceiling, which ties the external walls to the ceiling, has resulted in a major safety enhancement. Using ply has taken the seismic rating for the museum from 66% NBS (New Building Standard) which is based on the lowest performing area of the building, to 100% NBS, this is the same seismic resultants as a building constructed to today’s building standard.

We ‘plyed’ into the details with Toby from the Property Department:

1. was it a requirement to earthquake strengthen the museum?  No, the building already had a rating of 66% NBS (New Building Standard) and not classed as earthquake prone, the decision was made by council to go ahead as this building houses some important items relating to the history of the Waitaki district and to create a safer environment for public and staff and ensure the building survives for generations to come.

2. How did you know what work needed to be done to increase the seismic rating?  We had a DSA ( Detailed Seismic Assessment) done some time ago and this highlighted the critical areas of the building, from this the engineer produced the design to strengthen these areas to bring the building up to 100% NBS

3. So what had to be done to bring the building up to 100%?  The only area of the building that required strengthening work to bring the building up to 100% was tying the upper walls into the ceiling structure, This was done by bolting the perimeter ceiling joists and blocking to the external stone walls, the entire ceiling was the lined with structural plywood to create a ridged diaphragm that holds everything together.

4. Is this how it is normally done?  Yes, this is typical for an older Oamaru stone building, some also require structural steel frames but it all depends on the building. The main thing is tying the floors and ceiling/roof structures to the external walls to stop the collapsing in or out.

5. Is Oamaru stone a good building material to have been used for our buildings?  Yes, we are lucky that we had a good supply of this material, due to the size and weight of the blocks used and the techniques applied for construction most of our Oamaru stone building have great seismic resultants compared to other materials used in that time period.

The image shows the timber blocking being installed to take the edges of the ply sheets and on the far wall you can see where blocking has been installed between the ceiling joists and fixed to the exterior walls with threaded rods glued into the stone and then tied back into the ceiling with the ply sheeting, this will help prevent the external walls collapsing in or out during an earthquake. 

Also included are images from the DSA report, Using 3d modelling, the green and red areas show the critical areas that would require additional

strengthening to achieve 100% NBS


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