P. Varun Chowdary, Asst. Prof. SRKIT, Vijayawada.
IntroductionThe design of structures should satisfy three fundamental requirements:
- Stability: The structure should be stable under the action of loads.
- Strength: The structure should resist safely stresses induced by the loads.
- Serviceability: The structure should perform satisfactorily under service loads.
It appears that the collapse is due to three causes:
- The over lap length of steel rods at the junction of pile & column is inadequate.
- Structure is unstable due to number of columns used (two only)
- Unsymmetrical wall load on the structure.
Collapse of a Foot–BridgeA Footbridge in Vijayawada collapsed one day before opening & fell into the canal below killing two painters. The footbridge has a span of 40m. It consisted of two welded trusses, cross girders & precast RCC slabs. On inspection, it was found that the welding is defective.
PreventionFollowing the provisions in IS code for structural steel fabrication would have prevented this failure.
Collapse of a Residential Building
On inspection, it was found that one of the building columns was constructed on filled up well. The old well is filled with sand and an Isolated footing was constructed on the well. As the construction progressed, load on column increased. The foundation has settled, leading to the failure of the building.
PreventionIsolated footing is a wrong choice for the foundation on the old well. For that column a well foundation should have been a better choice.
Vibration of SlabOwner of a college building complained that a floor slab in one of the rooms is vibrating, when students were walking or jumping on the slab. On inspection, no structural cracks were found in the slab. The size of the slab is 8m X 9m and it is supported on four sides by beams. It is continuous on three sides. The thickness of slab was 150mm. It appears that deflection check was not done in the slab design.
Generally, slab thickness is calculated based on deflection criteria. A proper design of slab may have given a safe design. The slab had failed in deflection check, however it is safe in flexure.
Structural Cracks in the SlabA dining hall of size 18m X 48m (Fig 3.) was constructed with two bays in east west direction and six bays in north south direction. Size of each slab is 8m X 9m and the slabs were designed as two way slabs. 1mm wide cracks developed in the slab at the location of maximum negative bending moment.
It is apparent that the rod bender fabricated the steel for 150mm thick slab and the site engineer did not understand the importance of top steel bars.
The slab became two way simply supported slab instead of two way continuous slab. The dining hall is still functional. But before another floor is added the slab should be checked.
PreventionInspection of steel by structural engineering before placing concrete would have prevented this failure.
Cracks in Walls
PreventionDeflection of frame should be checked for safe design.
ConclusionVarious structural failures that have reached limit state of collapse & serviceability were presented. Following are the failures presented here:
- Limit state of collapse due to stability
- Limit state of collapse due to welding
- Limit state of collapse due to unsatisfactory filling of old well
- Limit state of serviceability due to excess deflections
- Limit state of serviceability due to incorrect fabrication of steel
- Limit state of serviceability due to orientation of columns