Summary
The subject heritage church building was assessed and rated at less than 34%NBS in its original condition. This meant an earthquake prone building. An innovative retrofit solution was subsequently developed and would upgrade the seismic rating to 67%NBS at minimum with minimal alteration to both the exterior and interior heritage features of the building – a positive outcome for both the client and local community who greatly value this Category 1 Heritage Church.
Background
The client is the owner of a landmark heritage church in New Zealand. The church is one of the oldest and most significant churches in New Zealand and has the Category 1 heritage status according to Heritage New Zealand.
As part of the previous work, Engineer A developed and proposed a more traditional retrofit solution to the client for the seismic upgrade of the building. The solution involved new steel frames in the nave area and would lead to alteration of important heritage features of the church. Therefore, Engineer A’s solution was rejected by the client. Due to the significance of this heritage building, both the exterior and interior heritage features could not be altered in any proposed building works – a huge challenge for the structural engineers.
Engineer B was then engaged by the client to undertake a Detailed Seismic Assessment (DSA) and then develop a different retrofit solution to upgrade the seismic rating to 67%NBS with minimal alteration to both the exterior and interior heritage features of the church. This would require minimising new construction work by utilising existing materials to their own advantage.
Building Description
The subject building has a large footprint and consists of two portions (original church and rear extension). The original church was constructed of unreinforced stone and brick masonry more than 100 years ago. It is a two-storey structure. The exterior walls feature stepping basalt stone buttresses with decorative Oamaru limestone. The interior features deep limestone and brick arches.
The rear extension was built of Reinforced Concrete (RC) frames with clay brick infill panels in the 1930s. The rear portion is a ~30m high multi-storey RC frame structure. The front and rear portions are separate from a seismic perspective due to an intermediate timber floor structure linking the two portions; the timber structure is considered to be ‘flexible’ compared to masonry and concrete.
The roof structure for both portions includes timber hammer beam roof trusses supporting tiled roof cladding. The floors feature timber joists and tongue and groove flooring. The front Unreinforced Masonry (URM) portion was founded on rubble basalt footings. The foundations of the rear RC portion include a cast-in-situ slab on grade, shallow pads and stepped strip footings. Figure 1 shows the plan layout of the original URM church and rear RC extension linked by a timber floor structure.
Figure 1 – Plan layout of building portions
What did Engineer A do?
To upgrade the seismic performance and rating of the front URM church, the conventional approach would be to discount the strength of unreinforced masonry down to almost nothing, necessitating a larger and more intrusive engineering intervention. Think whacking great steel frames marching down the aisles – that was exactly what Engineer A proposed to the client. However, the proposed solution was rejected by the client due to potential alteration to important heritage features of the church.
How did Engineer B improve the work?
Engineer B completed a Detailed Seismic Assessment (DSA) of both the front URM church and the rear RC extension in accordance with the latest relevant guidelines and standards. The seismic ratings of both portions were less than 34%NBS, meaning that the building was earthquake prone.
It was found by Engineer B that the critical structural weakness for the front church was the internal URM arches. For the rear extension, the main structural weaknesses included the shallow foundations and RC frames (beams and columns).
The retrofit solution developed by Engineer B utilised the existing materials to their own advantage. This is important – very important – for heritage buildings. For the front URM church, there were many significant retrofit features, but the most interesting one is the controlled-rocking URM arches in the nave area. The arches consist of brick spandrels and limestone piers. The brick spandrels would be retrofitted with Near-Surface Mounted (NSM) Fibre Reinforced Polymer (FRP) strips, which were intended to improve both the shear and tensile capacities of the spandrels and minimise visual impacts on the heritage features of the church.
The horizontal NSM FRP strips would be recessed into the existing mortar joints, which would be repointed subsequently. The vertical NSM FRP strips would be installed into very thin pre-cut slots / grooves, which span across several brick units, with anchorage provided by NSM steel rods at the top and bottom.
The lower portion of the limestone piers would be confined using an FRP wrap. During a major earthquake, devastating energy would be dissipated by allowing the piers to rock in a stable fashion as controlled by altering the height of the FRP wrap and hence the rocking height (Hr), and consequently, earthquake forces to be resisted by the remaining structure would be lowered substantially. Figure 2 shows the controlled-rocking URM arch solution.
To complete the DSA and retrofit solution, a series of non-linear pushover analyses were carried out to obtain both force and deformation demands. Material testing was also required to verify the toe crushing strength of masonry and define non-linear properties, which were needed for various hinges as illustrated in a typical perforated URM wall in Figure 3. For the flexural (rocking) hinges of piers, the effect of axial loads was also considered based on a series of axial force-moment (P-M) interaction envelopes. The generalized backbone curves for the flexural (rocking) and shear behaviours are shown in Figure 4, Figure 5 and Figure 6.
Figure 3 – In-plane failure modes of URM walls
Figure 4 – Generalized force-deformation relationship for flexural (rocking) behaviour of pier
Figure 5 – Generalized force-deformation relationship for sliding shear behaviour of piers
Figure 6 – Generalized force-deformation relationship for flexural / shear behaviour of spandrels
For the rear RC extension, only the exterior features would need to be preserved for heritage requirements. Therefore, Engineer B developed a scheme to retain the existing RC frame/infill facades and rebuild the interior portion including the floors, ceilings and walls with a series of new multi-storey steel Eccentric-Braced Frames (EBFs) with removable link beams. The existing perimeter RC beams would be selectively weakened to force a lateral beam-sway mechanism to develop during a major earthquake, and the clay brick infill panels would be isolated from the bounding columns to allow relative movement. Seismic demands would be transferred to the steel frames with damages concentrated to the link beams that would “die” to save the other members of the structure and keep the earthquake forces low to the foundations. After the earthquake, the link beams could be replaced easily. The building could be restored quickly to the original positions. Figure 7 shows the new multi-storey steel EBFs with K and D-braces and removable link beams.
Figure 7 – New multi-storey steel EBFs with removable links
Conclusion
In conclusion, the subject building was assessed and rated at less than 34%NBS in its original condition. This meant an earthquake prone building. A retrofit solution developed by Engineer B would upgrade the seismic rating to 67%NBS at minimum with minimal alteration to both the exterior and interior heritage features of the building – a positive outcome for both the client and local community who greatly value this Category 1 Heritage Church.