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:
Symmertry:
It is observed that the distribution of mass, stiffness is across the plan has good symmetrical aspects.
This tends to reduce the horizontal displacements in both of the two horizontal displacements in both of the two Orthogonal Horizontal Directions.
It also has a very good uniformity also.
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.
Bi-directional Stiffness:
The building plan may not be able to resist the horizontal seismic actions in the shorter direction (Y-axis) and also the resistance and stiffness in the shorter direction (Y-axis) is less when compared to longer direction (X-axis).
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 force 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? Explain how, as an engineer, each of these irregularities should be dealt with.
Ans:
The possible type of irregularities for the given structure are:
Soft story irregularity:
- Stiff and strong upper floor due to masonry fill
- The column in one story is longer than the above.
Some provisions to avoid the irregularities:
The building is torsional irregular. So the building framing must be revised.
The mass irregularity is avoided by providing light weight concrete/ Low weight masonary or by providing structure with more wood materials.
The structure should be built in a well symmetric manner.
The columns which are provided on the structure should be of same size and height through out the building.
The weight of the building from ground floor should decrease towards top.
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:
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 perimeter 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 wih 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 adjusts 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 centre of the Building which makes the Torsional 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
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 staircase which again a reason for the Irregularity in plan having the Torsional 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.