Concept on positioning of Columns

CHALLENGE 4

Q.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:

Symmetry: -

• We observe that the mass distribution, stiffness is across the plane has good symmetric aspects.
• This tends to reduce horizontal displacements within both horizontal displacements in both orthogonal horizontal directions.
• It is also very uniform.
• Can we divide the plan vertically half, its look like mirrored copy. So, we conclude the given plan is symmetric in X-direction.
• Can we divide the plan horizontally half, its look like mirrored copy. So, we conclude the given plan is symmetric in Y-direction.

Redundancy: -

• Structural features are missing from the periphery of the plan. This causes the uneven distribution of the effects of earthquakes within a non-linear range throughout the plane.
• The energy dissipation through the structure is very small, which leads to an unreliable structure.
• The column is in x-direction they are comparatively more and evenly spaced. So, that the structure is more redundant in x-direction. Except the top and bottom column rows are having quite longer spans.
• The columns are in y-direction they are comparatively lesser but evenly spaced. So, that the structure is redundant but lesser than of in x-direction.

Bi-directional Stiffness: -

• 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).
• The columns are in horizontal direction are connected to straight-to-straight beams. That means, any lateral load 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 are in vertical direction are not connected with straight-to-straight beams. So, the lateral loads 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.

Ductility: -

• The building may not be capable of withstanding elastic seismic forces in a shorter direction (Y axis). This can result in an inelastic response.
• The reduction in design force in the shortest direction (Y-axis) must be compensated by ductility.
• Prime 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, which would reduce ductility in 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.
• We can conclude that the given framing plan is more ductile in y-direction than that of in x-direction.

Q.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:

Vertical irregularity - The building plan area is large in bottom stories. The floor plan areas are getting reduced with increase in height. The weight of the building from ground floor should decrease towards top. The columns which are provided on the structure should be of same size and height throughout the building

Soft storey - The building has uncovered opening space in 6th floor. As there is no occupancy or a partition wall, this will lead to plastic hinge formation at the columns. Stiff and strong upper floor due to masonry fill. The column in one store is longer than the above. This has to be checked and sufficiently designed. The structure should be built in a well symmetric manner. The mass irregularity is avoided by providing light weight concrete/ masonry or by providing structure with more wood materials.

Slopy base - The building is constructed on a sloping ground at one side. This will lead to significant variations in the stiffness. Few levels may be subjected to soft storey mechanism.

Q.3 [12:28 PM, 4/15/2023] Prakash: A 4-storey hotel building has an open ground floor for the restaurant. Story 2 to 4 has 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.

[12:28 PM, 4/15/2023] Prakash: 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:

Open ground floor - This denotes 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 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 portions 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 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 are having longer dimension in Y-direction. This means they are stiffer along y-direction. But the beams are running only along x-direction which would attract much lesser lateral forces and hardly transfers it.

Variation 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 significant difference in lateral drift of columns. So, the concrete floor might have suffered heavy out-of-plane shear which would led to collapse.

Structure is not symmetric and larger strength is provided hence low ductility. In this building, there is no proper placement of masonry infill walls and Redundancy in the vertical direction is not provided properly.