Concept on positioning of Columns
*Objective of this challenge is to know the behaviour of horizontal and vertical members in the given frame & know the irregularities in the the building, irregularaties in beam connection at nodal joints.
1) Symetric: In the framing plan let us split it in two direction i.e X & Y. Splitting the vertical in the half along X direction we get the other half as a mirror component. Similarly Splitting the horizontal along the Y direction we get the other half also as a mirror component. Thus it can be concluded that the frame is symetric.
Redundancy: In the X direction the number of columns are more and distance between the fixed end are evenly spaced. The end portion at x direction the span of frame are more. Hence the frame is more redundant.
Similary in the Y direction the number of columns are less and distance between the fixed end are of shorte span but evenly spaced and there are few nodes that are not linked to all the beams and nodes in the frame. Thus it is less redundant than X direction.
Bi-Directional Stiffness: In the X direction all the members are connected straight to staright beams in the nodes. Hence the seismic force can directly pass along the members and easily transfer to the nodes than to the columns. Force diversion is not possible in case of X direction. But in the Y direction the beam members are not directly connected to the node to node. Hence the seismic force diversion will take place and it will behave as a weak frame. hence it is not stiffer.
Ductility: Since in the X direction there are more number of columns so it will be more stiffer. And hence the ductility will be less. And in the Y direction there are less number of columns so it will be less stiffer and greater in ductility.
2) a) Vertical Irregularity: The plan of the building is larger at the bottom stories as compared to top stories. As the height of building is incresing the floor area is getting reduced. As per Is 1893-2016 table 6 sl.no. (iii) it can be state that the building is irregular in shape.
b) Soft Storey: The building has noe cover with walls at the 6th floor and also the floo height is varrying this will create plastic hinge formation and the vertical stifness will also change at the 6th floor. The modal mass perticipation factor will also reduce. Thus as per IS 1893-2016 fig 4A it can state as a soft storey building.
c) Slopy Base: The building has been constructed in slopy land which can be seen in the front area of the building this will lead to the variation in stiffness at front as compared to the back portion in filled area.
3) From the above figure we can state the following reasons for the collapse during earthquake:
a) Open ground floor area as said in the question has caused the soft storey mechanism which acted on the building.
b) There is mass irregularity in the building since the middle bay is left open which cause the change of mass. Thus effective mass chnages during the seismic force.
c) There are two short sides in left and right side thus only those sides were seems to be stiffer.
d) The Stair case portion are stiffer as compared to the other parts because it has concentrated concrete elements and shorter beam spans. Thus it was not damaged during earthquake.
e) There are irregular framing connection along y-direction, cantiliver slab along x direction in the front portion. There is also no framing connection in Y direction, so the siesmic force could not be transfered along Y direction.
f) Variation in column sizes, no symetric behaviour of the structure leading to sifnificant difference in lateral drift, variation in deflection. Floors were subjected to heavy shear force which lead to collapse.
g) Columns are longer along Y direction thus more stiffer, but beams are placed only along x direction, thus less lateral forces have been transfered and excess force had hit the x direction only. Thus lead to collapse of the structure.