Concept on positioning of Columns

ANSWERS

1. a) Symmetry

• If we split the plan vertically and horizontally half, it will look mirrored.
• So the plan is symmetrical about the X and Y axes.

b) Redundancy

• Columns in X-direction are comparatively more and are evenly spaced.
• The redundancy at the top and bottom portion columns in the X-direction is less compared to the middle portion.
• Columns in Y-direction are comparatively lesser than X-direction, but evenly spaced.

c) Bi-directional stiffness

• The columns in X-direction are connected with straight-to-straight beams. Any lateral load passing from the left end of the building has its path straight from the left end to the right end.
• The columns in Y-direction aren't connected with straight-to-straight beams. The lateral load passing from the south end of the building gets divert into east & west directions to reach the next bay. There will be a major diversion of seismic forces in the Y-direction.
• The framing plan is weak in Y-direction concerning bi-directional stiffness.

d) Ductility

• The columns in X-direction are higher than Y-direction. So, there's high stiffness in X-direction and increases Ductility in X-direction.
• In the X-direction, the load passes from one bay to another straight by beam alignment. Whereas in Y-direction, it struggles to pass from one end to other. North to South beams are arranged in zig-zag which leads to discontinuous bracing.
• These conclude that the given framing has more ductility in the Y direction than the X-direction.

2. a) Vertical Irregularity

The building plan area is large in bottom stories. The floor plan areas are reduced with increase in height. The building has to be crosschecked with IS-1893-2016 Table 6.

The first 5 stories has more area than top stories.

b) Soft storey

The building has uncovered opening space in the 6^th floor. As there's no occupancy or a partition wall, this will lead to plastic hinge formation at the columns (As per IS: 1893-2016 Fig. 4A). This has to be checked and sufficiently designed.

c) Sloped base

The building is constructed on a sloping ground on one side. This will lead to significant variations in the stiffness (As per IS: 1893-2016 Fig. 5E). Few levels may be subjected to a soft storey mechanism.

3.

• Open Ground floor - This denotes Soft storey 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 corridor. This induces mass irregularity along height uniformly.
• Two short sides covered with infill walls - This means the building's left and right side's periphery is more stiffer during seismic forces (especially along North-South).
• Staircase portion at Left top corner - Usually staircase and lift portions are stiffer than other parts because of concentrated concrete elements. So the staircase portion contains more stiffness in the provided building plan.
• No framing connection along Y-direction - There are no structural beams provided along the vertical direction between columns. This makes the builing more vulnerable during the North-South seismic force.
• Beams connected along column minor axis - The columns are having longer dimension in Y-direction. This means they're more stiffer along Y-direction. But the beams are running only along X-direction which would attract much lesser lateral forces and hardly transfers it.
• Variation in column sizes - The internal columns are having higher sizes compared to exterior colums (This is common because of higher load demands). But this will lead to significant difference in lateral drift of columns. So the concrete floor might have suffered heavy out-of-plane shear which would led to collapse.