NON-STRUCTURAL ELEMENTS: THOSE LIKELY TO CAUSE STRUCTURAL DAMAGE161Infill Earthquake force Earthquake force Beam-column region likely to be damaged Compression strut Diagonal crackBare frameInfilled frame鈻�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, 榅�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, softenup 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.鈻�10.5 Typical infill wall diagonal crack pattern. 1999 Chi-chi, Taiwan earthquake.(Reproduced with permission from Geoff Sidwell).鈻�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).
