Seismic upgrades to two significant buildings will be on show during Whanganui Heritage Month.

The Royal Wanganui Opera House and Whanganui War Memorial Centre have both had earthquake strengthening work in recent years and people are invited to look behind the scenes during tours of the buildings.

Whanganui District Council general manager property Leighton Toy said the architecturally significant War Memorial Centre was reopened in February after being closed for six months for a seismic upgrade to bring it up to 67 per cent of the New Building Standard (NBS). Earthquake strengthening must meet a minimum level of 34 per cent of the NBS.

"The work has involved securing non-load bearing masonry walls with additional steel poles and adding a reinforced shear wall structure on the forecourt near the main entrance," Toy said.

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"Structural steel frames have been inserted at the rear of the auditorium and in the Concert Chamber."

Built in 1960, the centre is a permanent memorial to the servicemen and women who lost their lives during World War II. In 1961 the New Zealand Institute of Architects awarded the building a gold medal. It is listed as one of the top 50 modernist buildings in the southern hemisphere.

Architect Gerald Cogan said visitors on the tour would be able to see "the interventions and the way in which these have been handled architecturally to retain the character and aesthetic of the building and its spaces".

The Royal Wanganui Opera House was seismically upgraded in 2015. The 119-year-old building is the last remaining working Victorian theatre in the southern hemisphere. The seismic upgrade brought the building up from 10 per cent of the NBS to +34 per cent.

"During work to upgrade the building's foundations, 120 tonnes of material was removed," Toy said.

"As work was within the restricted confines of the existing building, it was undertaken by hand and wheelbarrows.

"The installation of the new portal frame, within the proscenium arch, involved cutting two holes in the Opera House roof, through which the two 14-metre 'legs' of the frame were lowered and bolted between new steel column uprights.

"A new foundation beam beneath the edge of the stage was constructed to form the base of the new structural frame. In addition, the ceiling was covered in a layer of structural plaster which was tied back to the ceiling supports by 3870 stainless steel ties."

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Architect Bruce Dickson said a clever strengthening design meant there was very little visual change to the architecture of the building.

Contractors cut holes in the roof of the Royal Wanganui Opera House during earthquake strengthening work in 2015. Photo / File
Contractors cut holes in the roof of the Royal Wanganui Opera House during earthquake strengthening work in 2015. Photo / File

Free tours of both buildings will take place on Wednesdays, August 14, 21 and 28 and September 4. The Opera House tours will be at 9.30am-10.30am and the War Memorial Centre tours at 10.45am-11.45am. The architects and structural engineers who carried out the seismic upgrade work will lead the tours.