Concept on positioning of Columns
Question 1:
Comment on the layout of the framing plan concerning earthquake resistance in the two horizontal directions X or Y (dots are columns, lines depict beams). Look into aspects of symmetry, redundancy and bi-directional stiffness and ductility.
Ans-
Aspects of Symmetric -
Investigation of the above frame plan with respect to X-Direction – If we split the Plan vertically half it will look mirrored, so we conclude the given plan is symmetric in X-direction.
Investigation of the above frame plan with respect to Y-Direction – If we split horizontal half, it will look mirrored, so we conclude the given plan is symmetric in Y-direction.
Aspects of Redundancy-
X-Direction – The column in X-direction are comparatively more and evenly spaced, so the structure is more redundant in this direction, except the top & bottom column rows are having quite longer spans.
Y-Direction- The Column in Y-direction are comparatively lesser and even space, so the structure is redundant but lesser than X-direction.
Aspects of Bi-directional Stiffness- The column in x-direction are connected with straight to straight beams , this means any lateral load passing from left end of building will have its path straight till reaches right end of the building, so no or less possibility of force divorcing take place. The column in vertical direction are not connected with straight to straight beams , so lateral load passing from south side of building will get diverted to east & west to reach next bay. There will be major force diversion take place with seismic force in Y-direction, so frame plan is weak with respect to bi-directional stiffness in this direction.
Aspects of ductility – Firstly the aspect ratio of the plan clearly hints more column in X-direction compare to y-direction, its will result stiffness of building in x-direction reduce ductility. The force will easily pass from one bay to its next due to beam alignment whereas the y-force will struggle to pass through. The north south beams are arranged in zig-zag which load to a discontinuous .so we can conclude that the given plan is more ductile in y-direction than x-direction.
Question 2: What are the possible sources of irregularity in the building shown here? Explain how, as an engineer, each of these irregularities should be dealt with.
Ans-
Vertical irregularity- The building plan area is large in bottom stories and floor plan areas are getting reduced with increased height, these building offsets has to cross checked with reference to Is-1893-2016 table 6
Soft Storey- The building has uncovered opining space in 6th floor, as there is no occupancy or a partition wall. This will lead plastic hinge formation at the column with reference to IS-1893-2016. This has to be checked & sufficiently designed.
Slope base- The building is constructed on a sloping ground at one side; this will lead significant variations in the stiffness with reference IS-1893-2016. Few levels may be subjected to soft storey mechanism.
Question 3: A 4-storey hotel building has an open ground floor for the restaurant. Story 2 to 4 have one row of rooms along each long side in plan, separated by a corridor. The two short sides of the perimeter are fully in-filled in all storeys, except for certain openings at the ends of the corridor at storeys 2 to 4 and along the right-hand side of the ground floor. There is a staircase near the upper left-hand corner, with straight flights between landings at floor levels and in-between floors. Cross-section dimensions are written next to the member no. in meters.
Comment on the features of the structural design and of the layout of infills which are important for earthquake resistance and seismic performance. How do they relate to the almost full collapse of this building (the extreme left-hand bay with the staircase survived, as well as one long-side façade in the rear end, and the frame along the right-hand side in plan).
Ans-
Open ground floor – This denotes soft storey mechanism acting in the building.
Room along 2 bays separated an open corridor- The bays filled with room will have higher mass compared to the middle bay left open for corridor, this induces mass irregularity along height uniformly.
Stair case portion at left corner- Usually staircase and lift portions are stiffer than other parts because of concentrated concrete elements.
Two Short sides covered with infill wall- This means the building’ s left and right side periphery is more stiffer during seismic forces.
No framing along y-direction- We could see no structural beams provided along these vertical direction between columns, this makes building more vulnerable during north south seismic force.
Beams connected along X-direction- The column are having longer dimension in y- direction that means they are more stiffer along y-direction but the beam are connected only x-direction which could attract much lesser lateral forces and hardly transfer it.
Variation in column size- The interior column are having higher size compare exterior column , but this will lead to significant difference in lateral drift of column & so the concrete floor might have suffered heavy plane shear which would be collapse.