Identification of Cracks

Q1 What is the mode of failure or damage of the beams in the figure above? Would you characterize the case as damage or as a failure? Are the damages repairable? Elaborate

Ans.

1 a)

The mode of failure is due to flexural failure in beams.As we can see that damage is caused more to the beam than the column.Damage is caused due to insufficient of reinforcement.It may also be caused due to insufficient vibration of concrete as there will be hinderence of the reinforcement while vibration.The vertical crack at the face of the support,across the full depth of the beam,tends to remain open and increse in width for each cycle of deformation.To overcome this failure steel must be provided at top and bottom of beam.

This damage is repairable and can be done by providing proper cover and reinforcementat at the joint.

1 b)

The failure is due to insuffiecient bond strength and improper curing.The beam cannot develop its full moment resistance at the end section and the force resistance and stiffness of the frame it belongs to drop accordingly.The loss of beam anchorage prevents a proper plastic hinge from forming at the end of beams.These types of column failure can be avoided if attended with much care.Cavities inside the column that were not filled properly with construction ground reduce the compressive area of the column.As a result its axial capacity will be reduced.

The damage is repairable although the original deficiency,namely the poor anchorage of beam bars in the joint,cannot be corrected easily.

1 c)

The failure is due to beam-column joint connection failure in the reinforced concrete frame.In the strong column-weak beam design,beams are expected to form plastic hinges at their ends and develop flexural overstrength beyong design strength.The high internal forces developed at plastic hinges cause critical bond conditions in the longitudnal reinforcing bars passing through the joint and also impose high shear resisting mechanism to a significant extent.Such elements are usually subjected to very high shear forces during seismic activity and if inadequately reinforced,result in an excessive loss in strength and stiffness of the frame and even collapse.

Q 2. What is the mode of failure or damage of the beams in the figure above? Would you characterize the case as damage or as a failure? Are the damages repairable? Elaborate

Ans.

a) The failure is due to the flexural damage.Columns may be damaged or fail in flexure as shown in the fig a.Flexural damage 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 rotation takes place.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.

Adequate development length for the longitudnal bar is to be ensured within the joint taking yield penetration into consideration.

b) This kind of defect occurs due to Shear failures of columns.Lateral loads in a structure are carried by vertical elements such as columns and shear walls.When there are no shear walls,columns carry these lateral loads.Lateral loads are generated by winds,earthquake loads,from reataining structures.Shear links are provided to columns bases on the shear forces excreted on them.Further ,the size of columns is increased in the direction of shear to increase the shear capacity.In general,failure of columns due to shear can be identified as the failure of design.Designers should have provided adequate shear links or sections to carry shear forces.

c) This kind of defect can occure due to Short column failure.Short Columns,failure takes place due to yielding and we define this phenomenon as crushing.Short columns are classified based upon Slenderness ratio which the ratio of the effective length(length between points of zero moments) and least lateral dimension of the column.To know whether column is short or long,we find its critical slenderness ratio using Euler's formula.Short column develop very high shear force demands and are very vulnerable to shear.In the figure it is made short unintentionally,as the secondary elements supported by the column between its two ends split its free height into two shorte ones.Such effect must be taken into account in the design stage.

In existing buildings with short columns,different retrofit solutions can be employed to avoid damage in future earthquakes.Where walls of partial height are present,the simplest solution is to close the openings by building a wall of full height which will eliminate the short column effect.