Concept on positioning of Columns

Answers:

Question 1:

(a)Symmetry:-

1] Investigation of the framing plan concerning Symmetry in X-direction:

If we split the plan vertically in half, it will look mirrored. So we conclude that given plan is symmetric in the X-direction.

2] Ivestigation of the framing plan concerning Symmetry in the Y-direction:

If we split the plan horizontally in half, it will look mirrored. So we conclude that given plan is symmetric in Y-direction.

(b)Redudancy:-

1] Investigation of the framing plan concerning redundancy in X-direction:

The columns in the x-direction are comparatively more and evenly spaced. So the structure is more redundant in the x-direction. Except for the top and bottom columns, rows are having quite longer spans.

2] Investigation of the framing plan concerning redundancy in the Y-direction:

The columns in the y-direction are comparatively lesser but evenly spaced. So the structure is redundant but lesser than in the x-direction.

(c)Bi-directional Stiffness:-

1] Investigation with respect to bi-directional stiffness:

The columns in the horizontal directions are connected with straight beams. This means that any lateral load (Seismic force in X-direction) passing from the left end of the building will have its path straight till it reaches the right end of the building. So no or less possibility of force diversion taking place.
The columns in the vertical directions are not connected with straight beams. So a lateral load passing from the south side of the building will get diverted to east & west direction to reach the next bay. There will be major force diversion taking place with Seismic force in Y-direction. The framing plan is weak in bi-directional stiffness in Y-direction.

(e)Ductility:-

1] Investigation of framing plan concering ductility:

Prima facie, the aspect ratio of the plan hints at more columns in x-direction direction compared to the y-direction. It will result in higher stiffness of the building in the x-direction, which would reduce ductility on the x-axis.

Moreover, the x-force will easily pass from one bay to the next due to beam alignment. Whereas, the y-force will struggle to pass through.

The north-south beams are arranged in a zig-zag which leads to discontinuous bracing.

We can conclude that the given framing plan is more ductile in the y-direction than that in the x-direction.

Question 2:

The possible sources of irregularity in the building are

1.Vertical Irregularity:

The building plan area is large in the bottom stories, and the floor plan areas are getting reduced with an increase in height. These building offsets have to be crosschecked with IS-1893-2016 Table 6 serial number 3. In this picture, we can see the bottom 5 storey has a larger area when compared to the upper levels. This is called vertical irregularity.

2.Soft Storey:

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

3.Slopy Base:

On the right side of building there is embankment, so the building is constructed on sloping ground at one side. This will lead to significant variation in the stiffness (with reference to IS 1893-2016 figure 5E). Few levels may be subjected to soft storey mechanism.

Question 3:

1. Open ground floor:

The ground floor of the building is open. This denotes a soft storey mechanism acting in the building, which can be a soft floor (with long columns), due to the ground floor excitation, the columns would not be able to withstand the earthquake and collapse.

This can be avoided by using the strong post and weak beam mechanisms.

2.Story 2 to 4 have one row of rooms along each long side in plan, separated by a corridor:

Rows filled with rooms have higher mass compared to the mass at the middle for the corridor.

Mass irregularity is an important factor which affects the response of the structure under seismic loads.

3.The two short sides of the perimeter are fully in-filled in all storeys:

Comparatively, the two short sides have more stiffness.

This leads to Stiffness variation in 2 directions.

The Collapse or damage could happen in the weaker stiffness direction.

4.A staircase near the upper left-hand corner:

Comparatively, the staircase and Lift parts have more concentrated concrete elements.

This leads to more stiffness in the staircase and lift parts than other parts in the building.

So, there is a stiffness variation in the left-hand corner and in the other corner of the building.

5.No Framing connections along Y-direction:

No structural beams are provided along the vertical direction to take up forces.

This makes building more vulnerable during north-south seismic force.

6.Beams connected to the Column shorter direction:

Columns with longer dimension have more stiffness than with shorter dimension.

In our case, the longer dimension is in Y-direction, that means more stiffness in the Y-direction.

But no framing connections in the Y-direction. Beams are provided only in the X-direction.

Longer dimensions attract more lateral forces but no arrangement to tranfer the forces,

Shorter dimension attracts lesser lateral forces and hardly transfers it.

7.Variation in Column sizes:

The transfer of seismic forces varies with the column sizes.

Variation in column sizes might have led to significant difference in lateral drift of columns.

The concrete floor might have suffered heavy out of plane shear.

This would have led to collapse of the building.