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 in 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’s means given plan is symmetric in X direction.

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

• Again, if we split the plan horizontally half so, the plan looks mirror with each other. That’s means given plan is symmetric in 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, has sufficient number of supports which are arranged in order
• The spacing of column in X direction are evenly placed and the number of columns is more than Y direction, so the structure is more redundant in X direction
• except the top and bottom column rows, the number of columns in top and bottom direction are less. So, the redundancy of top and bottom row is less than other rows.
• In the X direction, the number of columns in top and bottom rows has less comparatively 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 Y direction:

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

The investigation with respect to bi-directional stiffness:

• In X direction, beam is connected straight to straight in columns, this means, any lateral load (Seismic force in X-direction) passing from left end of the building will have its path straight till it reaches right end of the building. So no or less possibility of force diversion takes place.
• The columns in vertical direction are not connected with straight-to-straight beams. So, a lateral load passing from south side of building will get diverted to east & west direction to reach next bay. There will be major force diversion take 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:

• Ductility is inversely proportional to stiffness. It is Prima facie The aspect ratio of the plan clearly hints more column in X direction compared to Y direction.
• It will result in higher stiffness of the building in X direction. If the stiffness is higher in X direction which 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 zig-zag which leads to a discontinuous bracing. So, we can conclude that given framing plan is more ductile in Y direction than that of in 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: Building plan area is large in bottom stories, the floor plan areas are getting reduced with increase in height. These building offsets has to be cross checked with reference to IS-1893-2016 Table 6 serial number 3. In this picture we can see bottom 5 storey has larger area when compared to the upper levels. This is called vertical irregularity.

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

3-Sloppy Base: in 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 (with reference to IS-1893-2016 Figure 5E). So, 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).

Answer-

• We have Observe in 3 pictures, 1^st picture is a structural plan, 2^nd picture is before earthquake and 3^rd picture is after earthquake of the hotel building
• As mentioned in question, in 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 with the corridor along the 2 bays, the bays filled with room, will have higher mass compare to the middle bay left open for the corridor. This induces mass irregularity along hight uniformly.
• Stiffness irregularity: Two short sides covered with infills walls at the right side and left side of the building in the restaurant it 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 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 at the left side and one night right side 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 are having longer dimension in Y direction this means they are stiffer along Y direction. But the beams are running only X direction which would attract much lesser lateral forces and hardly transferred it.
• Variation in column size: The internal columns are having higher sizes compared to exterior columns (This is a 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.