Shear walls

Shear walls are structural walls designed to resist horizontal force. The term 'shear wall ' originally referred to a wall that had either failed or was expected to fail in shear during a damaging quake. Now that a primary objective for contemporary structure is to avoid shear failure, 'shear wall ' is a somewhat inappropriate description for a modern well-designed wall. However, given the term's international popularity, its on-going usage is justified by appreciating that a shear wall is designed to primarily withstand horizontal shear forces. In the process, of course, it experiences bending moments and a tendency to overturn (see Fig. 5.3).

Of all seismic resistant structural systems, reinforced concrete shear walls have the best track record. During past earthquakes even buildings with walls not specially detailed for seismic performance, but with sufficient well-distributed reinforcement, have saved buildings from collapse. The success of shear walls in resisting strong earthquake shaking has led some leading structural engineers to recommend them, at least for reinforced concrete construction. For example, Mark Fintel a noted US structural engineer who studied the seismic performance of shear wall buildings over a thirty-year period, concludes: 'We cannot afford to build concrete buildings meant to resist severe earthquakes without shear walls.'1

Shear wall buildings are the best choice in earthquake-prone countries. Reinforced concrete shear walls are relatively easy to construct because their reinforcing details are straight-forward, at least when compared to those of moment frames. Their inherent stiffness minimizes horizontal interstorey deflections and consequently earthquake

Wall A

Wall acts as a shear wall in y direction

Wall A

Wall acts as a shear wall in y direction

Force from diaphragm

▲ 5.6 The shear wall sloping in both directions is effective in the y direction but needs support from x direction structure (not shown).

Force from diaphragm

Self-weight of wall increases overtuning moment

Restoring moment (and sliding resistance) provided by foundation Elevation Wall A

▲ 5.6 The shear wall sloping in both directions is effective in the y direction but needs support from x direction structure (not shown).

▲ 5.7 Free-standing shear walls prior to incorporation into surrounding structure highlight their vulnerability to out-of-plane forces. University building, Vancouver.

damage to structural and non-structural elements. The challenge of integrating shear walls with architectural form and planning and their increased cost with respect to inferior masonry walls prevent them being more widely adopted internationally.

Given the excellent seismic credentials of shear walls as a source of seismic resistance, architects should always try to use them in the first instance. Approach each design project with a view to walls resisting seismic forces in both directions. Often walls required primarily for architectural reasons, such as exterior cladding or enclosing stairwells and so on can function as shear walls. However, shear walls need to be long enough in plan so as to attract and resist seismic forces before other structural elements in a building like columns and beams suffer damage.

Where intended to function as a seismic resisting system, a shear wall is usually continuous up the height of a building from its foundations to the uppermost diaphragm at roof level. If a shear wall is discontinuous through a storey or has one or more off-sets in plan up its height it is possibly fatally flawed, as discussed in Chapter 8. But it is permissible to slope a shear wall out-of-plane and even moderately in-plane without significantly affecting its structural performance. The additional gravity bending moments that arise from an in-plane lean require add-itional wall and foundation strength (Fig. 5.6). Although most shear walls are rectangular in plan many other plan shapes such as a gentle curve or C, L and I-shapes are usually structurally feasible.

Finally, it must be reiterated that a shear wall is effective in the direction of its length only. When laden at right angles to its length - that is, when subject to out-of-plane forces - a typical shear wall is very weak and flexible and likely to collapse unless restrained at every one or two storeys up its height by diaphragms. In summary, when acting in-plane shear walls support but where loaded out-of-plane they require support (Fig. 5.7).

▲ 5.8 A reinforced concrete shear wall strengthens an existing building. Note how the number of penetrations per storey increases with height. Telephone exchange, Wellington.

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