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 distribution of the mass, stiffness is accross the plan has good symmetrical aspects which tends to reduce the horizontal displacements in both 2 horizontal displacements in both 2 orthogonal horizontal displacements. It's uniformit is also good/
-Redundancy:
The structural elements in the periphery plan are missing. This may cause the uneven distrubution of earthquake action effect in non linear range accross the plan. The eneregy dissipated accross the structure is verr low that cause non-reliable structure resulting.
-Bi directional Stiffness:
The building play may not be able to resist the horizontal seismic actions in the shorter direction and also the resistance and stiffnesss in the shorter direction is less when compared to longer direction.
-Ductility:
The building may not be able to resist the elastic earthquake forces in shorter direction, so it may result in inelastic response. The reduction in design force in the shorter direction is to be compesated by the ductility.
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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:
Soft Storeys
Flexible strucures in Torsion
- Stiffness Irregularity:
The lateral stiffness of the storey at the mid height which is open is very less when compared to the above level bexause, which is the main reason the structure is discontinued at the midheight.
It is observed that the open storey causes the reduction in the stiffness in the Building and attracts the lateral seismic excitation more in that floor which causes severe damage to the upper floors.
The irregularity can be solved by providing the shear walls to this high seismic action prone floor.
- Mass Irregularity:
The open storey causes mass irregularity in the building because the high mass concentration is present above the open floor. That is the upper floors has more mass than the floor.
Also number of floors are higher above this open floor
-Vertical Geometry Irregularity:
There is a change in with the horizontal dimension of the lateral fore resisting due to the open storey in mid height and also the vertical geometry of the building which means the length of the top storeys are varying building unit which causes torsional effects in the building which need to be decoupled by providing shear walls or decoupled moments
The flexibility provided due to the varioation need to be compensated by providing ductility.
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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:
The collapse may have occured due to the ground excitation in both orthogonal horizontal directions and also due to torsional effects. Inspite of being having the earthquake resistance for the two mentioned actions above. The Following are responsible factors.
1. Open Ground Floor:
The ground floor of the building is open , which may be a soft storey (long columned) due to which the ground floor excitation the columns couldn;t resist the earthquake and collapse. This is called storey mechanism. This can be avoided by having strong column and weak beam mechanism.
2. Two Short Sides of the Perimeter In-Filled
The mass is undistributed in all sides in all storeys because it is given that the two sides of the perimeter of the building is in filled which causes the irregular distribution of the mass called mass irregularity. This can be avoided by providing the shear walls in the directio of two short sides of the perimeter which can resist the lateral loads by providing stiffness in the building frame and helps to resist the earthquake.
3. Extreme left hand bay Staircase filled with Concrete
The extreme left hand bay has stairs due to this the mass got concentrated in the staircase area due to which the torisional effects are formed having the base of the concentration of mass in staircase. To avoid this, staircases should be kept in the centre of the builsing which makes the torsional effect as zero by uncoupling the moment. It also implicates irregularity in plan of the building.
4. Columns Not in the Same Line
It is observed that the columns in the extreme left hand middle line columns has the deviation in the line of the construction at the staircases which again a reason. To avoid this the eccentricity should be maintained for the deviation to be with the limits of the beam size and the column size. Also the size of the beams and columns in the plan should be redesigned in such a way that the columns do not deviate even in the stair areas.
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