WE WANT BUILDINGS to be resilient, but what is resilience? On page 44, Suzanne Wilkinson summarises resilience as the ability to absorb shocks – the ability for a building to recover its functionality quickly and continue operating even with partial failure.
Improving existing housing
The costs to make an existing house more resilient are typically those required to bring it up to current Building Code requirements (see Table 1).
Wind resilience
Recent wind events have highlighted that, in some regions, existing fixings are inadequate for the damaging storms that are expected quite frequently. In particular, lightweight roofs on houses built before 1999 in high and very high wind zones need additional fixings. Table 1 shows the costs for:
- fixing the bottom two rows of purlins to the rafters or trusses
- increasing the fixings from rafter or truss to the top plates.
Further details are available in BRANZ Study Report SR187 Retrofitting houses to resist extreme wind events.
Seismic resilience
Old-style heavy masonry chimneys can cause extensive damage to roofs in an earthquake and should be demolished to increase resilience.
In many pre-1970 houses on timber or concrete piles, the lateral restraint and connection of the bearers to the piles is inadequate and can be improved.
Table 1
COMMON MEASURES TO MAKE EXISTING HOUSES MORE RESILIENT
HAZARD | COMPONENT | MITIGATION MEASURE | COST ($) |
---|---|---|---|
Wind | Truss roof | Install fasteners - purlins and top plate | 1,400 |
Rafters | Install fasteners - purlins and top plate | 2,200 | |
Earthquake | Chimney | Demolish and replace with metal flue | 8,000 |
Pile foundations | Fixings to bearers, cross bracing | 8,000 | |
Flooding | 0.5 m above floor level | Raise house at same location | 23,000 |
1.5 m above floor level | Raise house at same location | 30,000 | |
Moisture | Internal moisture | Polythene ground sheet | 1,500 |
Deterioration | Roof and wall cladding | Regular maintenance and painting every 10 years | 7,000 |
Flooding resilence
New housing is very unlikely to be on flood plains, but many older houses are. The costs for raising houses start at about $23,000 and include pile bracing, access steps and service connections.
Resilience to moisture
A low-cost measure that improves the internal environment for all suspended floor houses is to reduce entry of ground moisture by spreading polythene sheeting over the ground and taping it around the piles.
At the same time, installers need to ensure water will not pond under the house.
Positive net benefit
Valuing the benefits of these measures is quite difficult because it depends on the return period of the various hazards. Analysis in BRANZ Study Report SR285 House repair priorities indicates many of these measures are likely to have a positive net benefit under reasonable assumptions.
Measures for new housing
BRANZ has done less work on costs and benefits of resilience measures for new housing, but these could include:
- above-Code insulation to reduce dependence of external energy sources
- using low-maintenance claddings
- installing lifetime design features that make the house easier to use and more flexible for all age groups
- designing for external loadbearing walls only so the interior can be readily changed as occupant needs change.
As new buildings are covered by the Building Code and land zoning requirements to protect against natural hazards such as earthquakes, windstorms, floods and landslips, the life-safety risks from these hazards are covered.
Functionality concerns
However, the building may not be usable for an extended period due to secondary damage, so resilience needs to consider functionality.
Angela Liu (see page 56) discusses this aspect for office buildings, where life safety is preserved but secondary damage to linings causes loss of access during repairs and has major cost effects on businesses.
BRANZ hopes to do more work in the future on quantifying the benefits of these measures.
For more
BRANZ study reports can be downloaded for free from the BRANZ Shop at www.branz.co.nz.
Download the PDF
Articles are correct at the time of publication but may have since become outdated.