R EVIEW OF L ITERATURE
2.2 F AILURE M ODES OF S TRUCTURAL W ALL
In this section, the possible failure modes of an RC structural wall are discussed. Although, the RC structural walls are commonly referred to as shear walls because of their large lateral shear capacity, the failure modes of the structural walls are not necessarily dominated by shear alone. Both flexure- dominant and shear-dominant behaviours are possible for the walls. The occurrence of possible failure modes depends on the aspect ratio of the wall.
Lw
Hw
1 ≤ Hw/Lw ≤ 2
Lw
Hw
Hw/Lw <1 Lw
Hw
Hw/Lw >2
2.2.1 Slender Walls
As mentioned earlier, slender walls exhibit flexure-dominant response during strong earthquake shaking. The various possible failure modes for slender walls are discussed below.
(a) Flexural tension failure: It is the principal source of energy dissipation in laterally loaded cantilever walls, characterized by the yielding of the flexural reinforcement in the plastic hinge region (Paulay and Priestley, 1992). The flexural cracks get concentrated at the base of the wall with the occurrence of maximum bending moment. Walls with boundary elements show extensive cracking and yielding of longitudinal steel in the bottom portion of boundary elements. This leads to higher overstrength flexural capacity with higher concentration of steel in boundary region (Elnashai et al., 1990).
(b) Flexural Compression Failure: This brittle mode of failure is characterized by crushing of concrete at the compression toe of wall bottom (Figure 2.3a). For walls with or without boundary elements, compression demand may exceed compression capacity due to inadequate dimensioning of wall base or inadequate detailing of boundary element steel or due to both; in any of these cases, the concrete at the bottom can get crushed. Generally, under increased direct and/or flexural compression demand, crushing of toe concrete takes place if out-of-plane failure modes are suppressed. Confinement of concrete is required at the bottom of the wall in order to avoid such brittle failure.
(c) Diagonal Tension Failure: For a slender wall behaving like a vertical cantilever member, the bottom portion of the wall adjacent to the foundation shows the characteristics behaviour of D- region. This can lead to diagonal tension failure or shear cracking of concrete even under moderate level of earthquake shaking (Figure 2.3b). Web shear cracks and flexure shear cracks occur in the web and at the bottom of the wall respectively. The flexural shear cracks originate at the wall edge and propagate into the web under increased deformation demand. To prevent this mode of failure, web reinforcement consists of smaller diameter bars placed with smaller spacing (Paulay and Priestly, 1992).
(d) Diagonal Compression Failure: In the bottom D-region of a slender RC structural wall, the diagonal compression failure take place at the plastic hinge region with high web shear stress, even when excess shear reinforcement is provided (Figure 2.3c). It has been observed that increasing shear reinforcement beyond the required amount does not improve the post-yield deformational characteristics of such walls (Elnashai et al., 1990). Under reversal of loading, web crushing in the plastic hinge region occurs due to rapid strength degradation of cracked concrete (Paulay and Priestley, 1992).
(e) Sliding Failure: Sliding failure of slender RC wall may occur at the potential plane of weakness which may be generated due to merging of deep flexural cracks under reversal of loading (Figure 2.3d). The planes where excessive sliding displacement can occur, may also be represented by construction joints. Provision of diagonal reinforcement across the potential sliding planes of the plastic hinge zone can prevent this type of failure. A much closer spacing of the vertical reinforcement across sliding planes region is preferable. Barbell-shaped walls have greater resistance against sliding due to dowel action of vertical bars in boundary elements.
(f) Direct Compression Failure:Failure due to concrete crushing can occur towards the bottom of the wall when the direct compression strain in concrete exceeds the failure strain in the compression. Such a failure can occur in slender rectangular walls that have a large percentage of vertical reinforcement and subjected to large axial load. Walls with unsymmetrical cross sections are particularly vulnerable to this failure mode (Medhekar and Jain, 1993). This mode of failure can be avoided by using diagonal web reinforcement. Web crushing usually limits the shear capacity of flanged shaped walls. The compression boundary element in such walls tends to shear through after web crushing.
Figure 2.3: Failure modes in slender walls: (a) flexural compression, (b) diagonal tension, (c) diagonal compression, (d) sliding and (e) rocking. (Dasgupta, 2008).
(g) Out-of-plane Instability: This mode of failure occurs when a thin wall section in plastic hinge region is subjected to high vertical compressive force. During repeated reversals of loading, the modulus of elasticity and the section modulus get reduced. This leads to the possibility of
(a) (b) (c) (d) (e)
Diagonal tension
Diagonal compression
regions of instability between the cracks in concrete, and can cause buckling of vertical steel occur under heavy vertical compression.
(h) Rocking Failure: During reversal of loading, slender walls with isolated shallow footing may get partially uplifted (Figure 2.3e). This also depends on the type of soil and any lateral restraint along the height of the wall. Rocking failure of structural walls is observed during the 1994 Northridge earthquake (EERI, 1996).
2.2.2 Squat Walls
Under lateral loads, the behaviour of squat walls is governed by shear deformation under large shear stresses, diagonal strut-and-tie system develops in the web of the wall (Figure 2.4a). The possible failure modes of such walls are discussed below.
(a) Diagonal Tension Failure: In absence of sufficient reinforcement, a squat wall may fail due to large and extensive diagonal cracks in concrete. Depending on the nature of loading and aspect ratio of the wall, the tension failure plane may be oriented in the exactly diagonal direction or with a steeper angle (Figures 2.4b and 2.4c). A tie beam at the top of the wall can transfer the shear force to the rest of the wall avoiding such type of failure (Paulay and Priestley, 1992).
(b) Diagonal Compression Failure: Under the combined action of vertical and lateral loads, squat walls may also fail due to diagonal compression or web crushing. With high shear demand and adequate horizontal shear reinforcement, the crushing of concrete can rapidly spread over the entire length of the wall (Figure 2.4d). Under reversal of loading, the diagonal compression failure may occur at a level of lower lateral shear force. Some extent of ductile behaviour can be achieved by designing the web reinforcement to yield before the crushing of the web.
(c) Sliding Shear Failure: Under reversed cyclic loading, developed cracks may join to form a continuous horizontal shear path causing sliding shear failure of squat RC walls (Figure 2.4e).
The tendency to fail in sliding shear increases with an increase in the applied shear stress and with a reduction in axial compression and aspect ratio of the wall. Providing diagonal reinforcement in the web of the wall tends to resist the sliding shear failure mode. Also, the provision of well confined boundary elements along the wall edges can prevent this type of failure (Paulay et al., 1982).
Figure 2.4: Failure modes in squat walls: (a) strut action, (b) predominant diagonal tension failure plane, (c) steeper tension failure plane, (d) diagonal compression, and (d) sliding shear.
In all the past studies, failure modes of isolated slender and squat walls have been discussed.
However, investigation of failure modes of a shear wall connected to floor slabs in a multistoried building has not been carried out.