Concept on positioning of Columns
Question 1:
AIM:
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.
SOLUTION:
Symmetry with respect to x & y direction:
If we split the plan vertically half ,its look like mirrored ,so the given plan is symmetric in X direction.
If we split the plan horizontally half,its look like mirrored,so the given plan is symmteric in Y direction.
Redundancy with respect to x & y direction:
Expect top & bottom rows the columns in others X direction rows are more & evenly spaced so the structure is more redundant in X direction
The columns in Y direction is lesser than but evenly spaced .so the structure is redundant but lesser in X direction
Bi-directional stiffness with respect to x & y direction:
The columns in horizontal directions are connected to straight to straight beams .so the lateral load passing from left end will have straight path till its reaches the right end of the building.so less possibility of force diversion takes place.
The columns in vertical direction are not connected to straight to straight beams,so the lateral load passing from south direction are diverted to east & west direction.so more force diversion with seismic force takes place in Y direction.The framing plan is weak with respect to Bi-directional stiffness in Y direction.
Ductility with respect to x & y direction:
The aspect ratio of the plan clearly shows that more columns are in X direction when compared to Y direction.so it results stiffness in X direction more whereas the ductility will be reduced.
X force will pass easily from bay to next easily due to beam alignment ,Y force face struggle to pass through.The north south beams are arranged in zig zag manner which leads to discontinous bracing.
From the given plan we can conclude that ductile will be more in Y direction compared to 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 bottom stories.The floore plan area are getting reduced withb respect to height.This building offsets has to be cross checked with reference ti IS 1893-2016 Table 6 serial no(iii).
Soft storey:
The building has uncovered opening in the 6th floor.As their is no occupancy or paartition walls this will lead to plastic hinge formation at the columns (with reference to IS 1893-2016 figure 4A).This has to be checked & sufficiently designed.
Slopy base:
The building is to be constrcuted on a sloping ground at one side.This will lead to significant variation in the stiffness(with reference to IS 1893-2016 fig5E)Few levels may subject to a 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).
Open ground floor - This denotes soft storey mechanism acting on the building.
Room along 2 bay separated by a open corridor-Th bays filled with room have higher mass compared to the middle bay lleft open for corridor.This induces mass irregularity along height uniformly.
Two short sides covered with infill walls-this means the buildings left & right periphery is more stiffer during seismic forces(especialy along north south)
Staircase portion at Left top corner-Usually staircase & lift portions stiffer than other paerts because of concentrated concrete elements.so the staircase contains more stiffness in the provided building plan
No framing connection along Y direction-Structural beams are provided along vertical direction column.This m,akes the building more vulnerable during north south seismic force.
Beams connected along column minor axes-The columns are having longer dimension on Y direction so they are more stiffer in the Y direction.But the beams are running only along X direction which would attract much lesser lateral forces & hardely transfers it.
Variation in column sizes-The interior columnd are higher in sizes compared to exterior columns.(This is common because of higher load demands).But this will lead to significant difference ibn lateral drift of columns.so the concrete floor might have suffered heavy out of plane shear which will lead to collapse..