Concept on positioning of Columns

CHALLENGE-4

Concept on the 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, and lines depict beams). Look into aspects of symmetry, redundancy, and bi-directional stiffness and ductility.

ANSWER:

Investigation of the framing plan with respect to Symmetry:

X-direction:

• If we split the plan vertically in half, it will look mirrored. So we conclude the given plan is symmetric in X-direction.

Y-direction:

• If we split the plan horizontally in half, it will look mirrored. So we conclude the given plan is symmetric in Y-direction.

Investigation of the framing plan with respect to redundancy:

X-direction:

• The columns in the x-direction are comparatively more and evenly spaced So the structure is more redundant in the x-direction. Except the top and bottom column rows are having quite longer spans

Y-direction:

• The columns in the y-direction are comparatively lesser but evenly spaced. So the structure is redundant but lesser than that of in the x-direction.

Investigation with respect to Bi-directional stiffness:

• The columns in a horizontal direction are connected with straight-to-straight beams. This means any lateral load (Seismic force in X-direction) passing from the left end of the building will have its path straight until it reaches the right end. So no or less possibility of force diversion taking 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 directions to reach the next bay. Major force diversion will occur with the Seismic force in the Y-direction. The framing plan is weak with respect to bi-directional stiffness in the Y-direction.

Investigation of the framing plan with respect to ductility:

• Prima facie, the aspect ratio of the plan clearly hints at more columns in the x-direction compared to the y-direction. It will result in higher stiffness of the building in the x-direction, which would reduce ductility in the x-axis
• Moreover, the x-force will easily pass from one bay to the 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 discontinuous bracing.
• 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.

ANSWER:

Vertical irregularity:

• The building plan area is large in the bottom stories. The floor plan areas are getting reduced with an increase in height. These building offsets have to be crosschecked concerning IS-1893-2016 Table-6 serial number (iii)

Soft storey:

• The building has an uncovered opening space on the 6th floor. As there is no occupancy or a partition wall, this will lead to plastic hinge formation at the columns (concerning IS-1893-2016 Figure 4A). This has to be checked and sufficiently designed

Sloppy base:

• The building is constructed on sloping ground at one side. This will lead to significant variations in the stiffness (concerning IS.1893-2016 Figure 5E). Few levels may be subjected to soft storey mechanisms.

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:

Open ground floor:

• This denotes a 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 the corridor. This induces mass irregularity along height uniformly.

Two short sides covered with infill walls:

• This means the building's left and right sides periphery is more stiffer during seismic forces (especially along North-south)

Staircase portion at Left top corner:

• Usually, staircase and lift potions 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 the Y-direction:

• We could see no structural beams provided along the vertical direction between columns. This makes the building more vulnerable during the North-South Seismic force.

Beams connected along column minor axes:

• The columns have a longer dimension in the Y-direction. This means they are stiffer along the y-direction. But the beams are running only along the x-direction which would attract much lesser lateral forces and hardly transfer it.

Variations in column sizes:

• The internal columns are having 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 lead to the collapse

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