Seismic strengthening

Seismic strength: On completion of the project, the building will be strengthened to above 67% NBS.

The structural strengthening programme involves:

Foundations: The existing building is shallowly founded, with beam and pad foundations. These will be augmented with additional beams and bearing pads where new concrete shear walls have been added above.

Pumice block infill walls: There has been some strengthening in the early 1990s, with sprayed concrete overlays to the western wing walls. Polyplast overlays were applied over the remaining administration cavity block walls, along with some grouted bars, tying the cavity walls together. Additional ties, and steel angle mullions, will be inserted to further strengthen these walls.

Superstructure: Concrete walls will be added at key locations and existing diaphragms will be strengthened to take building loads to these locations.

A ductile strengthening system will be used, with U-shaped Flexural Plate (UFP) coupled shear walls. The UFPs consist of folded steel plates which roll when deformed and use up large amounts of seismic energy without losing strength. This reduces the forces on the surrounding structure, and on the foundation pads below.

Proposed structural strengthening for the Sir Howard Morrison Centre

Lateral bracing of roof: The existing steel trusses over the auditorium are prone to buckling. To mitigate this, the bottom chord of the truss will be removed where it meets the wall. To brace the roof, new steel L frames will be introduced. These are designed to yield before the truss chords are overloaded, keeping the truss stable and dissipating seismic energy.

Roof diaphragms: New steel trusses and a cross laminated timber (CLT) diaphragm will be introduced into the Concert Chamber roof, enabling it to carry the required theatre services loads. Connections between the auditorium sarking and the walls will be strengthened and sarking, absent from the centre of the auditorium, will be reinstated with thick ply.