Identification of Cracks

1 a)

It is a kind of rupture failre in beams probabily due to reversal of stresses due to an earthquake. Concrete crushed at the bottom and reinforcments are exposed and they seems yielded. It is a kind of flexural failure due to reversal of stresses. Generally for a framed structure hogging moments at the support causes tension at top and compression at bottom. During the reversal of stresses compression may occur at top and tension at bottom. This tensile stresses caused the failure in bottom side of the beam. Top surface looks safe because of the addition strength given by gtghe flange (T action by the slab). Slab takes the compression while bottom reinforcements couldnt managed to take the excess compressive stresses caused by the external stresses. since the failure occured due to yielding of reinforcements the damage is not repairable. 

1 b)

Given is a beam pull out failure due to insufficient development length provided in beam column joint for lateral strength inside the column support. During the horizontal movement caused by the earthquake we must provide sufficient development or anchorage length to satisfy the lateral stability of the beams. Concrete is not looks like crushed but wide cracks are formed due to pull out reinforcements. It is not localised cracks, it extends entire length of beams from the phhotograph. Providing sufficient anchorage length inside the existing member is very difficult henve the damage can not be repaired.

1 c)

Given picture is a clear evidence of a beam column junction plasdtic hinge failure due to large seismic forces. Stress reversal occured due to earthquake causes damage in the beam to the entire width. cracks can also witness at the bottom side of the slabs which means tensile stresses developed due to reversal of strsses are too large even the flange action of the slabs couldnt handle the excess stresses. Cracks are seems like damaged the structure too much hence with various rehabilitation methods like jacketing at the beam column joints can repair the damages. Various method os rehabilitation methods using fiber reinforced composits (carbon, glass fibers etc.), Shape memory alloys etc can be used for the repair methods. 

2 a)

Failure of column most probablr ground floor or basement floor column due to soft story mechanism. No infill walls are shown in the picture hence we cn assume that the portion between columns are left open to vehicle movement.Only top of the column is failed.Spalling of concrete at the top of columns which extends all around the four faces of the column (three faces can be seen in the pic). Beam seems not damaged. Soft story mechanism arises when upper stories have more stiffness than the lower stories. Soft story mechanism cause damage to the column beacuase it dont follow the weak beam strong column failure mechanism. In this case the plastic hinge formation occured at the column rather than beam due to poor reinforecement detailing.  Stiffness is lower in this case since the infill walls are absent . It can provide the strut action during earthquake loads and give additional stiffness to the columns. In the picture the damage is not looks severe so it can be repaired by various retrofitting methods. These damages are not repairable when damage to the colums are severe.

2 b)

Crushed pattern in the column is diagonal, hence we can conclude the it is a possible case of shear failue. Longitudinal bars look not damaged . No bucling of bars or crushing of concrete is seen hence it is not due to compression failre. Shear failure arises may be due to lack of adequate lateral ties and its spacing. In siesmic design confinement of concrete due to ties are very important which seems like missing in this case, Spacing od shear reinforecement looks more than any codal provisions. Damage can be repairable by using Fiber reinforced polymer retrofitting methods. Carbon fibers are very strong in shear and tension. CFRP wrapping, Near surface mounting techniques using fiber reinforced composits can be effectively repair and regain the lost strength in shear due to siesmic forces.

2.c)

Column is severely damaged especially at top where it connected to beam or slab. This is probabily due to Captive column effect or short column effect. No contact between column to beam and upper storey column. load transfer is completely stopped. The props we see in the picture may be used  to avoid the collapse of the structure as a temporary support. Short column effect is mainly due to the exposure of small length of column in a storey. These parts of columns (captive columns) attracts large amount of Shear (lateral loads) during earthquake since the effective length of column decreases and lateral load increases. This effect is arises when any member intervenes the colum height like landing slabs, intermediate stories, partial infill walls etc. Most of the times these members are not included in the design process. failure given in the picture is not at all repairable since it is severly damaged