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:

Symmertry:

It is observed that the distribution of mass,stiffness is across the plan has good symmetrical aspects.It is symmentrical in both x and y direction.

This tends to reduce the horizontal displacements in both of the two horizontal displacements in both of the two Orthogonal Horizontal Directions.

It also has a very good uniformity also.

Redundancy:

It is more reduntant in x direction than in y direction.

There are missing structural elements in the Periphery of the plan.This causes the uneven distribution of earthquake action effects in non linear range across the plan.

The energy dissipation across the structure is very low which results in non -reliable structure.

Bi-directional Stifness:

The building plan may not be able to resist the horizontal seismic actions in the shorter direction (Y-axis) and also the resistance and stiffness in the shorter direction (Y-axis) is less when compared to longer direction (X-axis).This is due to column in vertical direction not arranged straight to straight to beams.

Ductility:

The building may not be able resist the elastic earthquake forces in shorter direction (Y-axis).So it may result in inelastic Response.

The reduction in design force in the shorter direction (Y-axis) is to be compensated by Ductility.It is more ductile in y direction than 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.

Ans: There are various irregularities in this building which are as follows:

• Vertical irregularity: Building floor plan is more strectched in bottom than the top.This type of irregularity need to be crosscheck according to codal provision of IS 1893:2016 Table 6 serial no.(iii)
• Soft storey: The building has uncovered opening space in 6th floor.As there is no occupancy ,this will lead to formation of plastic hinge formation at columns.This has to be checked accordingly with the codal provision IS 1893:2016 Fig.4A and need to be redesigned.
• Slopy base: The above building is constructed on slopy backfill.This will lead to significant variations in the stiffness and thus need to check according to code IS 1893:2016 Fig.5E. Fewer levels would 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).

Ans:

From the above given images we should deduce the following:

• It has open ground floor which leads to soft storey mechanism
• The bays which are filled with rooms will have higher mass comared to middle bay open for corridor.This induces mass irregularity along height uniformly.
• Two short sides are filled with masonary infill walls which lead to more stiffer in left and right side periphery during seismic force action.
• Staircase portion is more stiffer usually in this building plan.
• No framing in y direction i.e. no structural beams in vertical direction in column.This makes the building more vulnerable during north -south seismic forces.
• Columns are having more dimension in y direction so that makes it more stiffer in this direction
• Interior columns are larger in dimension compared to exterior columns.This will lead to lateral drift in columns. This will lead to collapse due to suffering from heavy out of plane shear.