Concept on positioning of Columns
Answer to Q.No:1:
The given plan has symmetry in both X and Y directions which means there is no chance for the torsional mode to governing for at least more than three modes. If the plan is drawn with two mutually perpendicular axes then it gives an identical mirrored image on either side of any axis.
Investigation of the framing plan with respect to redundancy in X-direction:
The columns in the x-Direction are comparatively more and evenly spaced. So the structure is more redundant in the x-direction. Except for the top and bottom columns, rows are having quite longer spans.
Investigation of the framing plan with respect to redundancy in the Y-direction:
The columns in Y-direction are comparatively lesser but evenly spaced. So the structure is redundant but lesser than that in the x-direction.
Investigation with respect to Bi-directional stiffness:
The columns in the horizontal direction are connected with straight to straight beams. This means any lateral load(Seismic force in X-direction) passing from the left end of the building will have its path straight till it reaches the right end of the building. So no or less possibility of force diversion taking place.
The columns in the vertical direction are not connected with straight to straight beams. So a lateral load passing from the south side of the building will get diverted to the east & west direction to reach the next bay. There will be major force diversion taking place with seismic force in Y-direction. The framing plan is weak with respect to bi-directional stiffness in Y-direction.
Investigation of the framing plan with respect to ductility:
Prima facie, the aspect ratio of the plan clearly hints at more columns in the x-direction compared to the y-direction. It will result in higher stiffness of the building in the x-direction, which would reduce ductility on the x-axis.
Moreover, the x-force will easily pass from one bay to the next due to beam alignment. Whereas, the y-force will struggle to pass through. The north-south beams are arranged in a zig-zag which leads to a discontinuous bracing.
We can conclude that the given graming plan is more ductile in the y-direction than that in the x-direction.
Answer to Q.No:2:
Vertical Irregularity- The building plan area is large in the bottom stories. The floor plan areas are getting reduced with an increase in height. These building offsets have to be crosschecked with reference to IS-1893-2016 Table -6 serial number (iii).
Soft story- THe building has uncovered an opening space on the 6th floor. As there is no occupancy or a partition wall, this will lead to plastic hinge formation at the columns(with reference to IS-1893-2016 Figure). This has to be checked and sufficiently designed.
Story base- The building is constructed on sloping ground at one side. This will lead to significant variations in the stiffness (with reference to IS-18893-2016 Figure-5E). Few levels may be subjected to a soft story mechanism.
Answer to Q.No:3:
Open ground floor - This denotes a soft story mechanism acting in the building
Rooms along 2 bays separated by an open corridor-The bays filled with rooms will have higher mass compared to the middle bay left open for the corridor. This induces mass irregularity along height uniformly.
Two short sides covered with infill walls- This means the building's left and right sides periphery is stiffer during seismic forces(especially along North-South)
Staircase portion at the left top corner- Usually staircase and lift partitions are stiffer than other parts because of concentrated concrete elements. So the staircase portion contains more stiffness than the provided building plan.
No framing connection along Y-direction- We could see no structural beams provided along the vertical direction between columns. This makes the building more vulnerable to the North-South seismic force.
Beams connected along column minor axes- The columns are having longer dimensions in the y-direction. This means they are stiffer along the y-direction
Variation in column sizes- The internal columns are having higher sizes compared to exterior columns(This is common because of higher load demands) But this will lead to a significant difference in the lateral drift of columns. So the concrete floor might have suffered heavy out-of-plane shear which would lead to collapse.