New Zealand Society of Earthquake Engineers executive officer Win Clark talks to Helen Frances about how to approach a strengthening programme.

Progressive Building: How important is a thorough preliminary investigation of an unreinforced masonry building prior to embarking on a strengthening programme?

Win Clark: Getting to know a heritage building well (heritage values, construction materials and workmanship, structural form and maintenance requirements) before deciding on a course of action to improve its seismic rating is well worth the time and cost involved. Tackling strengthening work with a whole-of-building approach is likely to reduce future costs and allow for more certainty when developing a works budget. Thorough investigation also helps owners to decide whether or not any maintenance or retrofit solutions should, or could, be undertaken concurrently, or develop a programme where the works are spread over many years to fit within budget constraints. 

PB: What options exist for preliminary investigations?

WC: Like the strengthening work itself, there are various non-invasive and invasive options available. Non-invasive options include ascertaining specific details about materials, mortar mixes and timber types from original drawings and specifications. A conservation plan will indicate past uses, and checking other buildings designed by the same architect will reveal likely construction techniques and materials.

PB: What are the invasive options?

WC: A condition report will provide an overall picture of the building. It describes the materials, the structural condition, and any deterioration evident. Invasive techniques are often required to gain this type of information. Specialist analysis may involve taking cores and testing brick walls to ascertain the distribution of shear strength. 

PB: Do unreinforced brick masonry buildings generally have any seismic capacity?

WC: In general, unreinforced brick masonry buildings were originally built for form and function, with little engineering. The earthquake resistance of many unreinforced masonry buildings is principally provided by the in-plane capacity of the brickwork. Essentially, the brickwork in its own plane has quite a high load-carrying capacity so a building’s ability to resist the effects of earthquakes was a consequential capability rather than a design capability. But if high-quality materials were used in the original building, good workmanship prevailed, and the building has been maintained, it is likely to be able to survive quite a significant seismic event. However, every building is individual with its own response characteristics to earthquake effects. 

PB: Is base isolation alone a viable strengthening option for these types of buildings?

WC: Base isolation has the potential to provide a high level of protection for an existing brick masonry building, but it is generally very expensive where a basement must be formed to accommodate the base isolation bearings. While base isolation reduces the intensity of acceleration generated within the building in a seismic event, you can’t get it down to zero. Therefore, some strengthening of the superstructure may be required. This could require for example, tying the floor diaphragms in to the masonry perimeter walls.

PB: Is there a ‘common’ strengthening option being used for these buildings?

WC: Given the pressure engineers are under at the moment to get jobs out the door, they don’t necessarily have time to do detailed investigations and fully understand the existing building and its capabilities. Where they have experience with putting in steel frames, that is what you get. The design can be knocked out pretty smartly. However, this type of retrofit solution may not be very sympathetic to the building’s heritage values. A more sympathetic solution does take time and require a more detailed investigation and engineering innovation.

PB: Can anything be used in place of steel?

WC: Laminated veneer lumber (LVL) timber can be used in place of steel depending on the architectural requirements. 

PB: What sort of strengthening solution would be used for the most at-risk buildings?

WC: When the engineer feels the brick masonry doesn’t have enough capacity and it would be very difficult to enhance its capacity up to a required level, the most common scenario would involve a new structure being built within the existing building. It could be a series of steel portal frames that are fixed to the inner face of the masonry and project out into the space, or reinforced concrete shear walls could be put in - sprayed, poured or pre-cast concrete, on the inner face, to which the original brickwork is then fixed to.

PB: There is a lot of research underway globally about seismic strengthening.  Have you come across, or used, any innovative solutions for strengthening unreinforced brick masonry buildings?

WC: Results of experimental tests by engineers at the University of Auckland recently confirmed that carbon fibre-reinforced polymer strips are a minimally invasive method of improving the out-of-plane performance of brick walls. This seismic retrofit solution strengthens the walls to resist the lateral forces in an earthquake and it is being used in art deco buildings in Napier and elsewhere. Coring down through a masonry pier to install vertical stressing strands has been shown to enhance the shear and overturning capacity of the pier. This method was used recently to strengthen the Great Hall at the Arts Centre in Christchurch. The engineers ‘tool box’ of retrofit solutions is increasing all the time due to research and practice developments.

PB: How difficult is it, generally, to develop a seismic solution?

WC: There are some hard questions to be asked about what is an appropriate solution for an existing building. The economic value of the work is always a factor. But the social value is not quantifiable. Heritage buildings and monuments are central to a community’s sense of place. It is about identifying those beautiful buildings that society appreciates and ensuring they are strengthened and protected.

PB: What are the implications for training and skill development in the area of heritage building strengthening and maintenance? 

WC:There needs to be collaboration between the academic research, the technical institutes, engineering practitioners and contractors so new people who are coming through are trained in the proper use and application of these retrofit techniques.There is training on site but there needs to be checks and balances so you don’t have a whole lot of cowboys coming in, picking up a material and saying ‘we can do it’, quoting a low price and doing a bad job. We need that quality control.