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.

Answer :-

Symmetry -

• Having a look over aspects of symmetry of the plan, it is observed that distribution of mass, stiffness is across the plan.
• It is considered for one of the good symmetrical aspects.
• If we split plan vertically half, it will look mirrored.
• So we conclude the given plan is symmetric in X-direction.
• If we split plan vertically half, it will look mirrored.
• So we conclude the given plan is symmetric in Y-direction.

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 pla.
• The column in X-direction are comparitively more and evenly spaced. So structure is more redundant in X-direction. Except the top and bottom column rows are having longer spans.
• The column in X-direction are comparitively lesser but evenly spaced.
• So the structure is but lesser than in X-direction.

Bi-directional Stiffness -

• The column in horizontal direction are connected with straight to straight beams.
• This means any lateral load passing from left end of the building will have its path straight till it reaches right end of the building.
• So no or less possibility of force division takes place.

• The columns in vertical direction are not connected with straight straight beams.
• So a lateral load passing from South side of the building will get diverted to East and West direction to reach next bay.
• There will be major force divergent takes place with systemic force in Y direction.
• The framing plan is weak with respect to bi-directional stiffness in Y direction.

Ductility -

• The building may not be able to resist the elastic Earthquake forces in shorter plan.
• So it may result in inelastic response.
• The reduction in design force in shorter direction 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.

Answer :-

Vertical Irregularity -

• There is a change with the horizontal dimension of the leteral force was resisting due to the open storey in mid height and also the vertical geometry of the building which means the length of the top stories are varying along the building unit.
• This causes torsional effect in the building which need to be decoupled by providing shear was a decoupled moments.
• The flexibility provided due to variation need to be compensated by providing ductility.
• The building plan area is large in bottom stories.
• The floor plan areas are getting reduced with increase in height.
• These building offsets has to be crosschecked with reference to IS-1893:2016 serial number.

Mass Irregularity -

• The open storey also causes mass irregularity in the building because the high mass concentration is present above the open floor.
• That is the upper floor carry more mass than the open floor.
• Also the number of floors are higher about is open floor.

Stiffness Irregularity -

• The lateral stiffness of the storey at the mid height which is open is very less when compared to the above levels because the main reason is a structure is discontinued at the mid height.
• It is observed that the open storey causes reduction in stiffness in the building and attracts lateral seismic excitation more in that floor which causes severe damage to the upper floor.
• The irregularity can be solved by providing shear walls to this high seismic action prone floor.

Soft Storey -

• The building has uncovered opening space in six Floor.
• As there is no occupancy or partition wall, this will lead to plastic hinge formation at the columns.
• This has to be checked and sufficiently design.

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).

Answer :-

The building collaps occured due to the ground excitation in both orthogonal horizontal directions and also due to the Torsional effects. Inspite of being even the earthquake resistance for the two mentioned actions.

The Following are the Responsible Factors :-

1. Open Ground Floor

2. No framing connection along Y-direction

3. Beams connected along column mirror axis

4. Variation in Column Sizes

5. Extreme left hand bay staircase filled with the concrete

1. Open Ground Floor

• The ground floor of the building is open, which may be a soft storey due to this the ground floor excitation the columns could have been unable to resist the earthquake and the form collapse.
• This is called a soft storey mechanism.
• This can be avoided by having strong column-weak Beam Mechanism.

2. No framing connection along Y-direction

• We could see No structural beams provided along the vertical direction between columns.
• This makes the building was unable during the north-south seismic force.

3. Beams connected along column mirror axis

• The columns are having longer dimension in Y-direction.
• This means they are more Stiffer along Y-direction.
• But the beams are running only along X-direction which would attract much lesser lateral forces and hardly transfer it.

4. Variation in Column Sizes

• The internal columns are having higher sizing compared to its exterior column.
• But this will lead to significant difference in lateral drift of columns.
• So the concrete floor might have suffered heavy out-of-plane shear which would led to collapse.

5. Extreme left hand bay staircase filled with the concrete

• The extreme hand bay has staircases due to this the mass got concentrated in the package area due to which torsional effects are formed having the base of the concentration of mass in staircases.
• This type of failures that should be kept in the centre of the building which makes the torsional effect is zero by decoupling the moment.
• It also makes implications of irregularity in plan of building.