Concept on positioning of Columns

Solution.1)

Aim: To 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.

Procedure:

• Observation of the framing plan with respect to Symmetry in X-direction:
  • If we split the plan vertically half, it will look mirrored. So we conclude the given plan is symmetric in X- direction.

• Observation of the framing plan with respect to Symmetry in Y-direction:
  • If we split the plan horizontally half, it will look mirrored. So we conclude the given plan in symmetric in Y-direction.

• Comment on the Framing Plan with respect to the redundancy in X-direction:
  • The columns in X-direction are comparatively more and evenly spaced. So the structure is more redundant in X-direction. Except the top and bottom column rows having quite longer spans.
• Comment on the Framing Plan with respect to redundancy in Y-direction:
  • The columns in Y-direction are comparitively lesser but evenly spaced, so the structure is redundant but lesser than that of in X-direction.
• Comment on Framing Plan with respect to Bi-directional stiffness:
  • The Colunmns in horizontal direction are connected with straight to straight beams. This means,any lateral load(Seismic Force in X-direction) 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 in vertical direction are not connectted with straight to straight beams, So a lateral load passing from South side of building will get diverted to east and 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.
• Comment on Framing Plan with respect to Ductility:
  • The ductility is always, inversely proportional to stiffness. If the structure is more stiffer, it will be less ductile and vice versa. Here, the structure is more stiffer in the X direction, i.e it will be less ductile in X direction whereas in Y direction, the structure is less stiffer, hence it will be more ductile in Y direction.

Result: Observation and Comment on the framing plan of the building with respect to the Symmetry, Redundancy, Bi-directional Stiffness and Ductility has been done with justification.

Solution.2)

Aim: To point out the possible sources of irregularity in the building shown in the below figure. Also, explain how, as an engineer, each of these irregularities should be dealt with.

Procedure:

The possible sources of irregularities are:

• Vertical Irregularities.
• Soft Storey Irregularities.
• Slopy Base Irregularities.

Vertical Irregularity: The building plan area is large in bottom stories. The floor plan areas are getting reduced with increase in height. A structure is said to be irregular when its bottom edge is 25% more than the top edge. Hence, this building is subjected to vertical irregularity. These building offsets has to be crosschecked with reference to IS-1893:2016 table-6 serial number (iii).

Soft-Storey Irregularities: A Soft storey is the storey whose lateral stiffness less than that of storey above. 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 (with reference to IS 1893:2016 Figure 4A). This has to be checked and sufficiently designed.

Slopy Base Irregularities: The building is constructed on a sloping ground at one side. This will lead to significant variations in the stiffness (with reference to IS-1893:2016 figure 5E). Few levels may be subjected to soft storey mechanism.

Result: All the possible irregularities that can occur and the solutions that a engineer should use to dealt with these irregularities has been discussed succesfully.

Solution.3)

Given: 

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).

Aim: To comment on the features of the structural design and of the layout of infilles which are important for earthquake resistance and seismic performance.

Procedure:

• Open Ground Floor: This denotes soft storey mechanism acting in the building.
• Rooms along two bays seperated by an open corridor: The bays filled with rooms will have higher mass compared to the middle bay left open for corridor. . So, on the two sides, i.e the top and bottom portion of the plan will have heavier mass and hence more stiffness compared to  middle portion which is left unoccupied. This leads to irregularity in mass. During earthquake excitation, the middle portion will get collapsed due to lower stiffness whereas the other two sides may sustain.This includes mass irregularity along height uniformly.
• Two Shorts sides covered with infill walls: This means the building's left and right sides periphery is ore stiffer during seismic forces(especially along north-south).Since, the lateral force resisting systems, i.e the walls are continous from the foundation this impart greater strength and stiffness to the structure in these locations. That is the reason why the left and right edges remain intact after the earthquake excitation.
• Staircase portion at left top corner: Usually staircase and lift portions are stiffer than others parts because of  concentrated concrete elements. So the staircase portion stiffness in the provided building plan.There is also redundancy in the staircase portion due to increased number of columns. This is the reason why the left-hand bay survived after the excitation.
• From the plan it is clear that there is no beam along the Y direction i.e, there is no structural member to tarnsfer the load from the floor to columns. This may cause the floor to become flexible and due to this the column will be subjected to irregular loading and hence, increases the chances of collapse, especially when the direction of earthquake is along Y-direction.

Result: Comment on the features of the structural design and of the layout of infills which are important for earthquake has been made with proper justification.