Better bracing beckons

By - , Build 170

What homes performed best structurally in the Canterbury earthquakes and why? BRANZ has developed some simple building design advice to help ensure bracing performs as expected.

Figure 1 Under earthquake load, uneven bracing distribution has the potential to cause racking. In this case, the left, top and bottom walls have minimum bracing to accommodate large window openings.

WHILE WE HOPE large earthquakes will never happen, they can provide valuable information about how buildings actually perform. This is extremely useful to supplement theoretical predictions of behaviour in earthquakes.

The Canterbury events gave a real-world insight into the bracing of light timber-framed buildings. By comparing buildings that were badly damaged and those that weren’t, we now have a better understanding of how the bracing in light timber-framed buildings (typically stand-alone dwellings) actually deals with earthquake forces.

Lessons for bracing from Canterbury

What needs to be emphasised is that most modern houses in Christchurch met the life safety objective of New Zealand Building Code clause B1 Structure, as did most other timber-framed houses regardless of age. This is in spite of the Canterbury events generating earthquake loads significantly greater than the design forces used to develop our Building Code and standards, such as NZS 3604 Timber-framed buildings.

Design matters for performance

Buildings seemed to perform better when:

  • the design was based on a number of smaller spaces that connected all parts of the building together – open-plan buildings, while meeting the requirements of NZS 3604 at the time of construction, did not fare as well
  • bracing was very evenly distributed around the building – buildings with greater damage met the distribution rules of NZS 3604 but often had a marked difference in stiffness between parts of buildings
  • the building was simple – complex houses with split levels, vertical irregularity or plan irregularity were more prone to damage
  • for houses with an extension, the addition was well connected to the original structure
  • concrete slabs were reinforced and tied into the perimeter foundation wall.

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Stiffness gained from sheet bracing

Since the introduction of NZS 3604 in 1978, most light timber-framed houses use sheet linings to integrate stiffness (and bracing capacity) into the building design. Sheet claddings and rigid air barriers also contribute.

Even older buildings with nail-fixed sheet linings gained stiffness and strength from the linings. BRANZ Study Report SR327 Structural performance of houses in the Canterbury earthquake series found that the percentage of post-1980 houses with damage was significantly lower than the pre-1980 houses.

Many houses suffered (sometimes significant) damage to their internal linings as earthquake loads were applied to the structure, although no timber-framed houses collapsed due to shaking.

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Plasterboard did well, mixed bracing less so

Observing the damage led to these findings:

  • Plasterboard linings performed as expected. Some owners reported their home became softer as a result of a small zone of hidden damage in the plasterboard at each fixing. Plasterboard has greater stiffness than diagonal bracing, so it tended to sustain cracking damage early, particularly at the sheet junctions as the diagonal bracing took up the load.
  • Bracing elements reacted differently where multiple types of sheet bracing were used. The stiffer material took up the load first, which overloaded the material. The overload was then transferred to the less stiff bracing elements.
  • While compliant, uneven bracing distribution contributed significantly to the damage. BRANZ Study Report SR404 Seismic effects of structural irregularity of light timber-framed buildings shows that plan layouts reaching the limits of permissible bracing irregularity can amplify lateral deflections by 500%, making the building unacceptably flexible under earthquake loads.
Figure 1 Under earthquake load, uneven bracing distribution has the potential to cause racking. In this case, the left, top and bottom walls have minimum bracing to accommodate large window openings.

Rules for good bracing design

A few simple rules help get the best out of bracing:

  • Simplify the layout – use regular shapes, a compact plan, even spacing of smaller openings and fewer large (wide) open-plan areas.
  • Specify a single type of sheet bracing material, such as all plasterboard or all plywood, to provide an even response across all bracing elements when the building is under load.
  • Distribute bracing around the building so that it avoids large variances in capacity along each bracing line, particularly the external lines. The reasons to do this are set out in BRANZ Study Report SR168 The engineering basis of NZS 3604, although the broad engineering principles are to:
    • evenly distribute lateral force-resisting elements to avoid concentrating loads on individual elements and their connections to the structure
    • symmetrically distribute lateral force-resisting elements to reduce torsion loads in winds and earthquakes
    • spread bracing elements to the building extremities, such as the corners of external walls, where they are more effective at resisting torsion loads.
  • Reduce earthquake demand – avoid placing heavyweight building materials high in the building, for example, use a lightweight upper-floor cladding and roofing.
  • Avoid significant variations in mass within a floor plan or between floors.
  • Avoid non-symmetrical arrangements of lateral load-resisting systems in a building structure, both within a floor plan or along an elevation.

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Figure 1 Under earthquake load, uneven bracing distribution has the potential to cause racking. In this case, the left, top and bottom walls have minimum bracing to accommodate large window openings.