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.

OBJECTIVE:

• To understand the impact of vertical and horizontal members on the seismic behaviour of a structure.
• To understand the different irregularities, present in a building.

The investigation of the framing plan with respect to symmetry in X direction:

• If we split the plan vertically half so, the plan looks mirror with each other. That means the given plan is symmetric in the X direction.

The investigation of the framing plan with respect to symmetry in the Y direction:

• Again, if we split the plan horizontally, the plan looks mirror with each other. That means the given plan is symmetric in the Y direction also.

The investigation of the framing plan with respect to redundancy in X direction:

• Redundancy is a behaviour of the structural arrangement in a building, that has a sufficient number of supports which are arranged in order
• The spacing of column in the X direction are evenly placed and the number of columns is more than Y direction, so the structure is more redundant in the X direction
• except for the top and bottom column rows, the number of columns in the top and bottom direction is less. So, the redundancy of the top and bottom rows is less than other rows.
• In the X direction, the number of columns in the top and bottom rows is less comparatively to the other rows, so the redundancy will be less in the top and bottom rows

The investigation of the framing plan with respect to redundancy in the Y direction:

• In the Y direction, the spacing of the column is evenly placed. So, the structure is redundant.
• The number of columns is less than the X direction. So, the redundancy is less in the y direction compared to the x direction
• The load distribution between columns in the X direction is less than in the Y direction.

The investigation concerning bi-directional stiffness:

• In X direction, the beam is connected straight to straight columns, 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 takes 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 a major force diversion taking place with Seismic force in Y-direction. The framing plan is weak concerning bi-directional stiffness in the Y-direction.

Investigation of the framing plan with respect to ductility:

• Ductility is inversely proportional to stiffness. It is Prima facie The aspect ratio of the plan hints more columns in the X direction compared to the Y direction.
• It will result in higher stiffness of the building in X direction. If the stiffness is higher in the X direction it would reduce ductility in the X axis.
• Moreover, the X-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 a zig-zag which leads to a discontinuous bracing. So, we can conclude that the given framing plan is more ductile in the Y direction than in the 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.

1-Vertical Irregularity:

The building plan area is large in the bottom stories, and the floor plan areas are getting reduced with an increase in height. These building offsets have to be cross-checked concerning IS-1893-2016 Table 6 serial number 3. In this picture, we can see the bottom 5 storey has a larger area when compared to the upper levels. This is called vertical irregularity.

2-Soft Storey:

In the building, there is an uncovered opening space on the 6th floor there are no partition walls so, it’s a soft storey, and there is no bracing, no mass, and no occupancies. This will lead to plastic hinge formation of the columns. So, concerning IS-1893-2016 Figure 4A. This has to be checked and sufficiently designed to avoid the plastic hinge.

3-Sloppy Base:

On the right side of the building, there is an embankment. It means the building is constructed over a sloping ground and this will lead to a significant variation in the stiffness (concerning IS-1893-2016 Figure 5E). So, a few levels may be subjected to the 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 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)?

Answer :

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• We have to Observe 3 pictures, 1st picture is a structural plan, 2nd picture is before the earthquake and 3rd picture is after the earthquake of the hotel building
• As mentioned in the question, the 4-storey hotel building has an open ground floor for the restaurant. This denotes soft storey mechanism acting in the building
• Mass irregularity:
• As mentioned in question, rooms are separated by the corridor along the 2 bays, the bays filled with room, will have higher mass compared to the middle bay left open for the corridor. This induces mass irregularity along height uniformly.
• Stiffness irregularity:
• Two short sides covered with infill walls at the right side and left side of the building in the restaurant which means the building periphery is more stiffer during seismic forces. Usually, staircase and lift portions are stiffer than other parts because of concentrated concrete elements. concrete is more stiffer and Stiffness will give us more resistance to the lateral force so, the staircase contained more stiffness, and it could survive after the earthquake.
• No frame connection along Y direction:
• Beams connected in the horizontal direction there is no beam along the vertical direction. If there is an earthquake in the vertical direction, the vertical forces cannot be transferred, for transferring the force we have only 2 options, one RCC beam on the left side and one on the right side with no beams connected in between. This makes the building more vulnerable during the north-south seismic force.
• Beam connect an alarm column minor axis:
• columns have a longer dimension in the Y direction which means they are stiffer along the Y direction. But the beams are running only in X direction which would attract much lesser lateral forces and hardly transfer it.
• Variation in column size:
• The internal columns have 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 led to collapse.