Identification of Cracks

1)

a)

Failure, with concrete crushing and bar buckling at bottom flange next to the column Flexural damage is mostly associated with cracking and spalling of concrete and yielding of the reinforcement. By contrast, flexural failure comes with disintegration of concrete beyond the cover, often with buckling (or even rupture) of bars. Such effects, shown in figure above, happen only at the bottom flange of a beam, because the slab provides the top flange with abundant cross-sectional areas of concrete and steel reinforcement. The vertical crack at the face of the support, across the full depth of the beam, tends to remain open and increase in width for each cycle of deformation; bottom bars may buckle and then rupture under the large cyclic excursions of strain across the open crack.

b)

Pullout of beam bars from narrow corner column, due to short straight anchorage there - Beam bars with insufficient anchorage in a joint may pull out in an earthquake. Such a failure of bond and anchorage shows up at the end section as a crack through the full depth of the beam. A characteristic feature of a pull-out crack is its large width, well in excess of the residual crack width typical of yielding of the steel (which is a fraction of a millimetre or around 1 mm). The impact of this type of bond failure on the global behaviour is not dramatic. The beam cannot develop its full moment resistance at the end section and the force resistance and stiffness of the frame it belongs to drops accordingly. The loss of beam anchorage prevents a proper plastic hinge from forming at the ends of beams. The damage is reparable, although the original deficiency, namely the poor anchorage of beam bars in the joint, cannot be corrected easily.

c)

Wide crack in slab at right angles to the beam at the connection with the columns shows the large participation of the slab as effective flange width in tension - A standard feature of a flexural plastic hinge in a beam is its through-depth crack at the face of the supporting beam or column, with a residual width indicative of yielding of the beam bars; that crack often extends into the slab and travels a good distance at right angles to the beam, sometimes joining up with a similar crack from a parallel beam. The length and the sizeable residual crack width of such an extension show that the slab fully participates in the flexural action with its bars which are parallel to the beam, serving as a very wide tension flange.

2)

a)

Flexural damage - Columns may be damaged or fail in flexure, as shown in a. Flexural damage or failure phenomena are concentrated in horizontal bands at the very top or bottom of a column in a storey (where the bending moments are at maximum). Such regions are the physical manifestation of flexural ‘plastic hinges’, where the plastic rotations take place. It is clear from a that plastic hinging, although essential for the seismic design of the building for ductility and energy dissipation, is not painless: it implies damage, normally reparable, but sometimes not. Flexural damage always includes a visible horizontal crack and loss of concrete cover, often accompanied by bar buckling, opening of stirrups or partial disintegration of the concrete core inside the cage of reinforcement; sometimes one or more vertical bars rupture, or the concrete core completely disintegrates.

b)

Shear failure of columns - A column may fail in shear anywhere between its two ends, the end regions included (since the shear force is essentially constant along the height of the column). The signature of a shear failure is a diagonal crack or failure zone; sometimes such cracks or zones form in both diagonal directions and cross each other. Stirrups intersected by the diagonal failure band may open or break. The concrete may disintegrate all along the diagonal failure zone or across the full core inside the reinforcement cage. Shear failure leads to a very fast degradation of the lateral stiffness and strength of the column, denoting a so-called brittle failure.

c)

Short column effect due to mid-storey constraint by a landing slab supported on the column. Short columns develop very high shear force demands and are very vulnerable to shear; the one in (c) is made shortn unintentionally, as the secondary elements supported by the column between its two ends split its free height into two shorter ones. Such effects must be taken into account in the design stage.