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 :
Symmetry
By observing the given plan if we cut the symmetry at the x axis at the middle of the plan it will look exactly mirrored so that the plan is symmetry along the x axis.
By observing the given plan if we cut the symmetry at the y axis at the middle of the plan it will look exactly mirrored so that the plan is symmetry along the y axis.
Redundancy
The columns in the x directions are comparatively more and evenly placed when compared to y direction. Hence the structure is more redundant in x direction, except the top and bottom row of the column. The columns in the y directions are comparatively less but evenly placed when compared to x direction. Hence the structure is redundant in y direction but lesser than x direction.
The mass carried by the x direction is more when compared to the mass carried by the y direction because the columns present in x direction is 7 while columns present in y direction are 6.
Bi directional stiffness
By observing the given plan we can see that the columns in the horizontal directions are connected with straight to straight beam. This means the seismic force in x direction passing from left end of the building will have its path straight till it reaches the right end of the building. So, there will be no or less possibility of force takes place.
By observing the given plan, we can see that the columns in the vertical directions are not connected with straight to straight beam. So the seismic force from south side of the building will gets diverted into east and west direction to reach the next bay. There will be major force diversion which take place due to seismic forces in the y direction. The framing plan is weak with respect to bi directional stiffness in the y direction.
Ductility
The aspect ratio of the plan clearly hints more column in x direction when compared to y direction. It will result in higher stiffness of the building in x direction which would reduce ductility in x direction of the building.
The force from the x direction will pass easily from one bay to the other due to the straight to straight beam alignment. While the y force will struggle to pass through from one bay to the other.
The north south beams are arranged in zig-zag way which leads to discontinuous bracing.
We can conclude that the given framing plan is more ductile in y direction than that of 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 :
By observing the given image we can get to know that the plan in the bottom is large when compared to the top storeys. While increase in the height of the of the building the floor plan of the building is getting reduced. It has to be significantly checked with reference to IS 1893 2016 table 6 serial number (iii)
We can see that the 6th floor of the building has open space which has no walls which leads to plastic hinge formation in the column, it is a soft storey. (with reference to IS 1893 2016 figure 4A). This has to be significantly designed and checked.
We can also see that the one side of the building has sloping ground This will lead to significant variation in the stiffness (with reference to IS 1893 2016 figure 5E) and also few levels may be subjected to soft storey.
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 :
As per the question the building has open ground floor which denotes that the soft storey mechanism is acting on the building.
The bays filled with rooms will have higher mass when compared to the open corridor at the middle, this induces the mass irregularity along the height uniformity.
As per the question there are two short sides covered with the infill walls, this means the building along north and south sides is more stiffer during seismic force.
As per the give structural plan the staircase is situated at the left top corner of the building, in usual the staircase and lift portion are stiffer when compared to the other parts of the building because of its concentrated concrete elements. So the staircase portion contains more stiffness in the provided building plan.
While observing the given structural plan we can see that there is no beam to column connection in the y direction of the building which leads the building vulnerable during the seismic forces.
By observing the given plan we can see that the columns have longer dimensions along the y direction of the building but the beams are running along the x direction which would attract much less seismic forces and hard to transfer it. This means the building is more stiffer in the y direction.
The internal columns are having higher dimensions when compared to the exterior column, this will lead to significant difference in lateral drifts in the column.