Problems associated with infill walls

So what are the difficulties with infill walls given that they are commonly used in so many countries? Why do they require special attention in seismically active regions?

Firstly, infill walls stiffen a building against horizontal forces. As explained in Chapter 2, additional stiffness reduces the natural period of vibration, which in turn leads to increased accelerations and inertia forces (Fig. 10.3). As the level of seismic force increases, the greater the likelihood

▲ 10.5 Typical infill wall diagonal crack pattern. 1999 Chi-chi, Taiwan earthquake.

(Reproduced with permission from Geoff Sidwell).

▲ 10.5 Typical infill wall diagonal crack pattern. 1999 Chi-chi, Taiwan earthquake.

(Reproduced with permission from Geoff Sidwell).

▲ 10.6 Damage to the tops of several columns due to infill wall compressive strut action. Mexico City, 1985 Mexico earthquake.

(Reproduced with permission from R.B. Shephard).

▲ 10.6 Damage to the tops of several columns due to infill wall compressive strut action. Mexico City, 1985 Mexico earthquake.

(Reproduced with permission from R.B. Shephard).

▲ 10.4 Whereas a bare frame deflects horizontally by columns and beams bending, the stiffness of a masonry infill limits horizontal movement. A diagonal compression strut forms together with a diagonal tension crack caused by elongation along the other diagonal.

of non-structural as well as structural damage. To some degree, the force increase can be compensated for by the strength of the infills provided they are correctly designed to function as structural elements.

Secondly, an infill wall prevents a structural frame from freely deflecting sideways. In the process the infill suffers damage and may damage the surrounding frame. The in-plane stiffness of a masonry infill wall is usually far greater than that of its surrounding moment frame - by up to five to ten times! Without infill walls a bare frame deflects under horizontal forces by bending in its columns and beams. However, a masonry infill dominates the structural behaviour (Fig. 10.4). Rather than seismic forces being resisted by frame members, a diagonal compression strut forms within the plane of the infill, effectively transforming it into a compression bracing member. Simultaneously, a parallel diagonal tension crack opens up between the same two corners of the frame because of the tensile elongation along the opposite diagonal and the low tensile strength of the infill material. The infill panel geometry deforms into a parallelogram. After reversed cycles of earthquake force, 'X' pattern cracking occurs (Fig. 10.5). The strength of the compression strut and the intensity of force it attracts concentrates forces at the junction of frame members. Shear failure may occur at the top of a column just under the beam soffit (Fig. 10.6). Such a failure is brittle and leads to partial building collapse.

During a damaging quake diagonal cracks and others, including those along the interface of infill and columns and the beam above, soften-up the infill. It becomes weaker and more flexible than a less severely damaged infill above it - in effect creating a soft-storey (Chapter 9). Even if infill walls are continuous vertically from the foundations to roof, once ground floor infill walls are damaged a soft storey failure is possible.

Direction of floor acceleration

Direction of floor acceleration

▲ 10.7 A section through two floors and an infill wall. Out-of-plane forces act on the infill which spans vertically between floors.

Another danger facing a heavily cracked infill is its increased vulnerability to out-of-plane forces (Fig. 10.7). The wall may become disconnected from surrounding structural members and collapse under out-of-plane forces. Due to their weight, infill walls pose a potential hazard to people unless intentionally and adequately restrained.

The final problem associated with the seismic performance and influence of infill walls is that of torsion (Chapter 2). Unless infill walls are symmetrically placed in plan their high stiffness against seismic force changes the location of the Centre of Resistance (CoR). In Fig. 10.8(a) the CoR and Centre of Mass (CoM) are coincident; no significant torsion occurs. If infill walls are located as in Fig. 10.8(b), the CoR moves to the right and the subsequent large torsional eccentricity causes the building to twist when forced along the y axis (Fig. 10.8(c)) . As one floor twists about the CoR relative to the floor beneath the columns furthest away from the CoR sustain large interstorey drifts and damage. If the drifts are too large, those columns are unable to continue to support their gravity forces and their damage leads to that area of the building collapsing. In this example, the infill walls cause torsion during y direction shaking only.

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