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-

Investing of the given framing plan on aspects of symmetry

• In x direction, the given framing plan is having the mirror image of one part if we take vertically plane at the middle of the plan, that’s mean the given framing plan is symmetry in x direction.

• In y direction, the given framing plan is having the mirror image of one part if we take horizontal plane at the middle of the plan, that’s mean the given framing plan is symmetry in y direction.

Investing of the given framing plan on aspects of redundancy

• In x direction, number of columns is comparatively more and evenly spaced. So the given structure is more redundant in x direction expect the top and bottom column rows is less in number and having longer span.
• In y direction, the number of columns is less as compared to x direction but columns is evenly spaced. Hence the structure is redundant but less as compared to redundant in x direction.

Investing of the given framing plan on aspects of Bi-directional stiffness

• In x direction, the columns are connected straight to straight beams and evenly spaced which means, when lateral loads i.e. earthquake force in x direction is passing then the stress in structure will be passed directly through the beams from left to right or right to left without stress concentration. So no or less possibility of force diversion take place.

• In y direction, the columns are not connected straight to straight beams, which will cause the diversion of force in the structure. If any lateral load for example an earthquake load in y direction is considered, the will be major force diversion take place with earthquake force in y direction.

• The framing plan is weak with respect to bi-directional stiffness in y direction.

Investing of the given framing plan on aspects of Ductility

• The aspect ratio of the framing plan clearly shows that more columns in x direction compared to columns in y direction which means higher stiffness of the building in x direction and its results in reduction in ductility in x axis.
• Moreover force in x direction will easily pass from one bay to its next due to beam alignment whereas force in y direction will struggle to pass through. North south beams are arranged in zig zag which leads to discontinuous bracing.
• For the given farming structure, the structure is more ductile in y direction as compared to x direction.

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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-

From the given picture, all the irregularity has been given below,

• Vertical Irregularity

The floor plan of the building is large in bottom stories up to fourth stories and the floor plans are getting reduced in upward as the height is increasing. These building offsets has to be crosschecked with reference to IS 1893-2016 part 1, figure 4, 4C vertical geometric irregularity, page no-18 which is given below,

• Soft storey

This building has uncovered opening space in 6^th floor, as there is no partition wall or any occupancy, this will lead to the plastic hinged formation at the column as per the is 1893-2016 figure4, 4A stiffness irregularity (page no-17), 4B mass irregularity (page no-17), 4E strength irregularity (page no-18). This has to be checked and correctly designed.

• Sloppy base

This building is constructed on a sloping ground at one side which leads to significant variation of stiffness as per is 1893-2016 part 1 figure 5, i.e. 5E definition height and base width of the building (Figure has been given below). Few level may be subjected to soft storey mechanism.

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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).

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ANS-

• As mentioned in the question that the ground floor is open which may cause the soft storey mechanism acting on the building.
• Rooms along the 2 bays separated by an open corridor that means this bay filled with rooms having higher mass compared to the middle bay left open for corridor, this may cause the mass irregularity along the height of the building.
• This is clearly seen that the two short side of building is covered with infill walls which means the left and right side of the building periphery is stiffer during seismic forces.
• In the building, the lift and staircase is placed on the left side top corner which makes the structure stiffer on the one side of building as compared to the other side of building and this leads mass instability or mass irregularity in the structure.
• It is clearly seem from the picture that no structure beams is provided along y direction between columns this causes building more vulnerable during the north south seismic force.
• The beams are connected along minor axis in between columns only, but not in major axis i.e. in y direction which makes the structure stiffer along one side. The beams are running only in x direction which could attract much lesser lateral forces and hardly transfer it.
• Size of the columns are varying, internal columns having higher size compared to exterior columns which leads to significant difference in lateral drift of columns. So the concrete floor may have suffered heavy out of plane shear which will lead to collapse.

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