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.

Answer:

Symmetry:

• The framing plan looks like it is symmetric in nature
• The load resistance framing in this symmetrical structure is distributed evenly throughout the structure
• If the reflection of the structure about the axis is identical in geometry, supports, and material properties to the structure itself, the structure is symmetric with respect to an axis of symmetry in its plane.
• This structure has an exact matching of form and arrangement of parts on opposite sides of the boundary at X and Y same as mirror reflection.

Redundancy:

• Columns in X-direction are comparatively more and evenly spaced, so the structure is more redundant in x-direction.
• But the top and bottom columns are less redundant due to less columns compared to in between in x-direction.
• Columns in Y-direction are alsoevenly spaced so this also redundant.
• But the number of columns in y-direction are less compared with x-direction so it is less redundant than x-direction.

Bi-directional stiffness:

• Horizontal seismic motion will cause the horizontal axis of X and Y to change direction throughout the layout's framing.
• Because this structure can distribute seismic forces due to its bi=directional nature it can withstand and resist any horizontal motion of forces.
• The given plan has an orthogonal layout in both the X and Y directions. So, it can transfer forces evenly and its structure will be able to withstand direct action from seismic forces.

Ductility:

• The aspect ratio of the plan shows no. of columns in x-direction when compared to y-direction are more.
• This shows more stiffness and less ductility in x-direction.
• Due to zig-zag arrangement leads to discontinous bracing.
• We can see that the given framing plan is more ductile in y-direction than 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: The possible sources of irregularity in the building are:

1. Vertical irregularity: The building plan is getting reduced as the number of stories gets increases, the bottom stories have a large area. This irregularities are to be studied by crosschecked with the usage of code book, IS 1893-2016 Table-6 (serial no: iii)
2. Soft Storey: The building has a opening space at the 6th floor, due to this opening the building may experience soft storey mechanism, which may also lead to plastic hinge formation. Refering to IS 1893-2016 Fig 4A this needs to be checked and designed accordingly.
3. Slopy base: Looking at the figure it shows the building is constructed on a slopy ground area, so this can be leading to variations in the stiffness, this part is to be studied taking IS 1893-2016 Fig 5E. This can also cause in 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:

1. As per the details building consists of a open ground floor which makes the building have soft storey mechanism.
2. As the rooms along the bay sides are separated by an open corridor, this can lead to mass irregularity along height uniformly.
3. The short sides are covered using infill walls so they are stiffer in case of seismic forces.
4. Staircases are positioned at left top corner and also are stiffer due to concentrated concrete elements. This concludes that staircase portion in this plan are more stiffer.
5. In the vertical direction, the beams are not provided, in case of seismic force in up and down or north-south direction the building can be vulnerable.
6. Usage of different column sizes in the building plan, as we can see internal columns are higher than that of external columns, this can lead to lateral drift of the columns which inturn can cause collapse of concrete floor.
7. As we can notice the columns in Y-direction are longer, which makes them stiffer in y-direction. As beams run along x-direction which may attract less forces and hardly transfer them.