Identification of Cracks
1.
a)
Answer:
Failure with concrete crushing and bar buckling at the bottom flange next to the column Flexural damage is usually associated with cracking and spalling of concrete and yielding of reinforcement. In contrast, flexural failure comes with the disintegration of the concrete behind the cover, often with buckling (or even cracking) of the bars. Such effects, shown in the figure above, occur only at the bottom flange of the beam because the slab provides the top flange with a rich cross-sectional area of concrete and steel reinforcement. A vertical crack at the support face, through the full depth of the beam, tends to remain open and increase in width for each cycle of deformation; the lower bars may buckle and then crack under large cyclic stress fluctuations across the open crack.
b).
Answer:
Pullout of beam members from a narrow corner column due to short straight anchorage - Beam members with insufficient anchorage at the joint can pull out during an earthquake. Such failure of bond and anchoring will manifest itself at the end section as a crack in the entire depth of the beam. A characteristic feature of a pullout crack is its large width, which greatly exceeds the residual crack width typical of yielding steel (which is a fraction of a millimeter, or about 1 mm). The impact of this type of bond failure on the overall behavior is not dramatic: the beam cannot develop its full moment resistance at the end section and the strength and stiffness of the frame to which it belongs decreases accordingly. The loss of beam anchorage prevents the formation of a proper plastic hinge at the ends of the beams. The damage is repairable, although the original deficiency, namely the poor anchoring of the beam rods in the joint, cannot be easily corrected.
C).
Answer:
The image above shows the failure of a beam-to-column connection in a reinforced concrete frame, the connection should not have been the weak link between the four components. Such members are usually subjected to very high shear forces during seismic activity and, if insufficiently reinforced, lead to excessive loss of frame strength and stiffness and even collapse.
2).
a)
Answer:
Failure due to flexural compression in the column. This type of failure occurs due to very high axial loads in multi-storey buildings and also due to bending moments. This type of failure can be avoided by providing concrete with high compressive strength and by providing proper longitudinal and lateral bonds throughout the column.
b).
Answer:
Shear forces increase during an earthquake, especially at columns and beam-column connections. As a result, special attention must be paid to the construction and design of beam-to-column connections and columns. Seismic design requires increasing the ductility of structures for a performance-based approach. In particular, building columns may have insufficient transverse reinforcement in the area of the plastic hinge. Therefore, structural elements that have such details show low performance against dynamic loads and have lost their load-bearing capacity in shear and axial loads. This failure shows below.
C).
Answer:
This type of mechanism can be developed through structural modifications and/or continuous openings in the upper part of the infill walls between the columns. The lateral forces that occurred during the earthquake are transferred by the columns and shear walls. Column length is an important factor in dispersing these loads. As the column length decreases, the column becomes stiffer and more brittle than other columns, and this column attracts more shear forces. These columns thus experience shear failure, which is a critical type of damage to a concrete column.