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 - The plan is symmetric in about X-axis as well as Y-axis.
Redundancy - In X-axis,the columns are evenly spaced except for top and bottom rows. In Y-axis too,the columns are evenly spaced but the number of columns in Y-axis is lesser than that of X-axis and hence is less redundant than X direction.
Bi-directional stiffness - The columns in the X direction are connected with straight to straight beams.This means any lateral load passing from the left end of the building will have its path straight till it reaches the other end of the building.So no or less possibility of force diversion takes place.
The columns in the Y direction are not connected with straight to straight beams.Hence,it will take longer time for the lateral load to travel from the one end to the other.There will be major force diversion taking place in this direction.
Hence,the plan is weak in bi-directional stiffness at Y direction.
Ductility - There are more number of columns in X-direction which means greater stiffness in X direction than Y direction.Greater stiffness results in less ductility than that of Y direction.On the other hand,force will pass more easily in X direction from one bay to another due to beam alignment.
Thus,plan is more ductile in Y direction than in 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:
a)Vertical Irregularity - The floor area of the lower floors are greater than that of the upper ones.This leads to vertical irregularity.
b)Soft-Storey - The building has uncovered open space in 6th floor.As there is no occupation or a partition wall,this will lead to the plastic hinged formation at the columns.
c)Slopy Base - The building is constructed on sloppy ground at one side.This will lead to the significant variations in the stiffness.A few levels may be subjected to 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).
Ans:
Open ground floor - This denotes soft storey mechanism acting in the building.
Rooms along two bays separated by open corridor - The bays filled with rooms will have higher mass compared to the middle bay left open for corridor.This includes mass irregularity along height uniformly.
Two short sides covered with infill walls - This means that the building's left and right side of the building is more stiffer during seismic forces.
Staircase portion at top left corner - Usually staircase portion is more stiffer than the other areas of the building because of concentrated concrete elements.
No framing connection along Y direction - The columns in Y direction are not connected to the structural beams.This makes the building more vulnerable during North South seismic forces.
Beams connceted along column minor axis - The longer side of the beam posses more stiffness than the shorter side.
Variation in column sizes - The interior column sizes are larger than the exterior ones which leads to significant difference in the lateral drift of the columns.