70 BUILD 125 August/September 2011

Seismic bracing on hillsidesLight timber-framed houses on hills and those within the scope of NZS 3604 may share many structural design issues but their seismic bracing performance can be very different. A BRANZ research project looked at the seismic bracing design for houses on hillsides.By Angela Liu, BRANZ Senior Structural Engineer

RESEARCH

Residential houses in New Zealand are usually light timber-framed buildings constructed according to NZS 3604, an Acceptable Solution to New

Zealand Building Code clause B1 Structure. Although construction of residential houses to NZS 3604 does not require professional structural engineering, the development of NZS 3604 has an engineering basis, and its application is limited to regular buildings up to 10 m high on relatively flat sites.

Building on hillsides popular

More and more residential houses are being built on hillside sites. Flat land to build on is becoming more scarce, and people want the spectacular views that hill sites often provide.

Hillside light timber-framed houses have many similarities to houses within the scope of NZS 3604 – the building height at any vertical plane is often less than 10 m and the main building is of light timber-framed construction.

Because of this, people tend to use NZS 3604 as much as possible for constructing

hillside houses, but NZS 3604 should not be simply followed for these houses.

Up-slope and down-slope houses differ

Hillside light timber-framed houses fall into two broad categories – up-slope houses and down-slope houses. Up-slope houses are usually built either into the ground or immediately founded on the ground (see Figure 1). The seismic bracing performance of these houses is likely to be similar to that within the scope of NZS 3604.

In contrast, the seismic bracing performance of down-slope houses is likely to be different from NZS 3604 as they are usually separated by substantial subfloor structures (see Figure 2). Because of the sloping ground, down-slope hillside houses usually have: ❚ irregular subfloor structures ❚ short subfloor bracing elements on the uphill side

❚ tall subfloor bracing elements on the downhill side.

Varying stiffness affects behaviour in earthquakes

Stiffness refers to an object’s resistance to deformation. The varying heights in down-slope houses mean varying stiffness, even for similar types of bracing systems such as braced piles. Just as a short squat wall is stiffer than the same kind of tall wall, so short braced piles are stiffer than tall ones.

Stiffer elements absorb more of the earthquake forces than flexible elements when they work in combination. This effect can be seen for hillside houses with a subfloor bracing system of a stepped foundation beam and sheathed timber subfloor walls (see Figure 3).

When the seismic motion is up and down the slope, the stiffest shear panel (sheathed wall panel A) will attract most of the earthquake force. If it is not strong enough, it may fail. Then the next shortest, panel B, becomes the stiffest panel and attracts most of the bracing force until it fails. This continues until the whole structural system has failed and the entire house has collapsed. This is called progressive collapse.

Figure 1: Typical up-slope house. Figure 2: Typical down-slope house.

BUILD 125 August/September 2011 71

Figure 3: Stiffness variation of subfloor bracing system in down-slope house. Failure would occur in panel A first as a result of up- and down-slope seismic action.

Figure 4: Torsional response to along-slope earthquake.

Irrespective of the subfloor bracing systems, hillside houses on steep slopes could potentially experience similar progressive failure because of the nature of stiffness variation on sloped ground.

When the earthquake direction is along the slope, the building tends to rotate because the bracing systems lower down the hillside are usually more flexible than the ones higher up the hillside (see Figure 4).

Seismic bracing design techniques proposed

The BRANZ research project examined the critical engineering issues for seismic bracing of hillside light timber-framed houses. It found that the bracing design of hillside light timber-framed houses varies significantly, depending on whether or not there are adequate bracing mechanisms at floor levels.

If there are adequate tie-back connections from the building to the ground at floor levels, then the bracing design is similar to NZS 3604. Otherwise, the research proposes a bracing design using a direct displacement-based procedure to prevent potential progressive failure.

Due to the complex make-up of subfloor systems, seismic bracing de sign techniques are proposed in principle only for different building categor ies. A structural engineer will need to work through these for each project.

A BRANZ study report with these research findings and design techniques will be available soon from www.branz.co.nz, then BRANZ Shop.

floor rotation away from uphill edge

main floor level

rotation

panel A

panel B

panel C