Concept on positioning of Columns
QUESTION 1:
Comment on the layout of the framing plan concering earthquake resistance in the two horizontal directions X or Y (dots are column, lines depict beams). Look into aspects of symmetry, redundancy and bi-directional stiffness and ductility.
Answer:
1. Symmerty:
Having a look over the aspects of symmetry of the framing plan, it is observed that the distribution of mass, stiffness is across the plan. It is considered for one of the good symmetrical aspects. This tends to reduce the horizontal displacementsin both of the two horizontal displacement in both of the orthiogonal horizontal directions.
It also has a good uniformity.
2. Redundancy:
There are missing structural elements in the periphery of the plan. This causes the uneven distribution of earthquake action effects in non linear range across the plan. The energy dissipation across the structure is very low which results in non-reliable structure.
3. Bi-directional stifness:
The building plan may not be able to resist the horizontal seismic action in the shorter direction (Y-axis) is less when compared to longer direction (X-axis).
4. Ductility:
The building may not be able resist the elastic earthquake forces in shorter direction (Y-axis) so it may result in inelastic response. The reduction in design in the shorter direction (Y-axis) is to be compensated by ductility.
QUESTION 2:
What are the possible sources of irregularity in the building shown here? Explian how, as an engineer, each of these irregularities should be deal with.
Answer:
Soft storeys
Flexible structure in torsion
1. Stiffness Irregularity:
The lateral stiffness of the story at the mid height which is open is very less when compared to the above the level because the main reason is the structure is discontinued at the mid height.
It is observed that the open storey causes reduction in the stiffness in the building and attracts lateral seismic exicitation more in that floor which causes sever damage to the upper floors.
The irregularity can be solved by providing the shear walls to this high seismic action prone floor.
2. Maas Irregularity:
The open story also causes mass irregularity in the building because the high mass concentration is present above the open floor. That is the upper floors carry more mass than the open floor.
3. Vertical Geometrical Irregularity:
There is a change with a horizontal direction of the lateral face resisiting due to the open storey in mid height and also the vertical geometry of the building which means the length of the top storey are varying along the building unit .
This causes torsional effects in the building which need to be decoupled by providing shear walls or decoulped moments.
The flexibility provided due the variation need to be compensated by providing ductility.
QUESTION 3:
A 4-storey building has an open ground floor for the restaurant. Storey 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 coner, with straight flights between landings at floor levels and in between floors. Cross section dimension are written next to the member no. in meter.
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 facade in the rear eend and the frame along the right-hand side in plan).
Answer:
The building collapse occured due to the ground excitation in both orthogonal horizontal directions and also due to the torsional effects. Inspite of being having the earthquake resistance for the mentioned actions (above),
The following are the responsible factors:
1.Open ground floor.
2.Two short side of the perimeter in-filled.
3.Extreme left hand bay staircase filled with the concrete.
4.Columns not in the same line.
1.Open ground floor:
The ground floor of the building is open, which may be a softstorey (having long columns) due to this the ground floor excitation the columns could have been unable to resist the earthquake and form collapse.
This is called as soft storey mechanism.
This can be avoided by having strong column-weak beam mechanism.
2.Two short sides of the perometer in-filled:
The mass has been undistributed in all the sides of the building in all storeys because it is given that the two sides of the perimeter of the building is in-filled (may be with the soil) which causes the irregular distribution of mass called as mass irregularity.
This can be avoided by providing the shear walls in the directions of two short sides of the perimeter which can resist the lateral loads (soil due to infill) by providing stiffness in the building frame and helps to resist the earthquake actions.
Also it adjust the mass irregularity for the plan.
3.Extreme left hand bay staircase filled with the concrete:
The extreme left hand bay has staircases (made of concrete) due to this the mass got concentrated in the staircase area due to which torsional effects are formed having the base of the concentration of mass in the staircase.
To avoid this type of failure the staircases, should be kept in the center of the building which makes the torsinal effect as zero by uncoupling the moment, It also makes the implications of the irregularity in plan of the building.
4. Columns not in the same line:
flexibility.
To avoid this the eccentricity should be maintained for the deviation to be with in the limits of beam size and the columns sizes.
Also the sizes of the beams and columns in the plan should be redesigned in such a way that the line of the columns doesn't deviate even in the staircase area.