Infill walls

Infill walls are non-structural walls constructed between columns. Where located on the exterior of a building as part of the cladding system, infill walls usually are bounded by structure; columns on either side, floor surfaces below and beams above. A beam may not necessarily be present but most infill walls abut columns. The description of most infill walls as ' non-structural' is misleading to say the least.

Shaking

Bare or open frame Infilled frame

▲ 10.3 A comparison of roof-top accelerations of a bare or open frame with an infilled frame. Note the shorter periods of vibration and higher accelerations of the infill frame.

Although they are not designed to resist either gravity or horizontal forces no one has informed them! By virtue of their inherent in-plane strength and stiffness they cannot avoid resisting forces even if they wanted to. Any stiff and strong building elements, whether designed by structural engineers or not, attract forces to themselves. In the process of resisting seismic forces, infill walls can cause serious structural damage to a building. That is why the problems they cause, and solutions to overcome them, require careful consideration.

As discussed in Chapter 5 infill walls can helpfully resist seismic forces in buildings, but only in certain situations. These include where there is no other seismic resisting system provided; the building is low-rise; the masonry panels are continuous from foundation to roof; there are enough panels in each plan orthogonal direction to adequately brace the building; the infills are not heavily penetrated; and finally, where infill walls are placed reasonably symmetrically in plan. Most infill walls do not satisfy these criteria and may introduce configuration deficiencies (Chapter 9).

Particular care is required when adding or modifying infill walls during building alterations. If infills, including those that can be categorized as confined masonry walls (Chapter 5), are to function as shear walls they should not be penetrated nor have existing openings enlarged without engineering advice. A similar cautionary note applies to any insertion of infill walls that might detrimentally affect the seismic performance of the building by, for example, causing torsion.

Infill walls that are capable of causing structural damage to a reinforced concrete or structural steel frame building are usually constructed from solid or hollow masonry bricks or concrete blocks that are usually unre-inforced and plastered. A large concrete panel placed between columns also constitutes an infill. In wood construction, gypsum plasterboard infill walls are also strong and rigid enough to disrupt the primary structure. Infill walls are usually constructed after columns and beams have been cast or erected. They are stiff, strong and brittle when forced parallel to their lengths (in-plane) yet vulnerable to out-of-plane forces.

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