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 Stifness:

• 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.

1-Vertical Irregularity: 

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 cross checked with reference to IS-1893-2016 Table 6 serial number 3. In this picture we can see bottom 5 storey has larger area when compared to the upper levels. This is called vertical irregularity.

 

2-Soft Storey:

 In the building there are an uncovered opening space in 6^th floor there is no partition walls so, it’s a soft storey, and there is no bracing, no mass, no occupancies. This will lead to plastic hinge formation of the columns. So, with reference to IS-1893-2016 figure 4A. This has to be checked and sufficiently designed to avoid the plastic hinge.

 

3-Sloppy Base: 

in the right side of the building, there is an embankment. It means the building is constructed over a sloping ground and this will lead to a significant variation in the stiffness (with reference to IS-1893-2016 Figure 5E). So, few levels may be subjected to soft Storey mechanism.

Question 3:

Aim: To investigate the given building structural plan & its causes of collapse after an earthquake.

Open ground floor :  This denotes soft story mechanism acting on the building.

Rooms along 2 bay separated by an open corridor:  The bay filled with rooms will have higher mass compared to the middle bay left open for the corridor. This include mass irregularity along height uniformly

Two short side sides covered with infill wall:  This means the building left and right side periphery is more stiffer during seismic force

Staircase portion at left top corner :   Usually the staircase and lift portion are stiffer then other part because of concentrated concrete elements. So the staircase portion contains more stiffness in the provided building plan.

No frame connection along Y direction :  We could see no structural beams provided along the vertical direction between the columns. This makes the building more vulnerable during the north south seismic force

Beams connected along column minor axes: The columns having longer direction in the y axis. This means they are more stiffer in y direction but beams are running along x direction which could attract more lesser lateral forces and hardly transfer it.

Variation in column size: The internal column having higher size compared to exterior column this is common because of higher load demand. But this will load to significant difference in the lateral drift of the column. So the concert floor might have suffered heavy out of plan shear which would led to collapse.