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.

Symmetry:

Symmetry in X-direction:

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

Symmetry in Y-direction:

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

Redundancy:

Redundancy in X-direction:

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

Redundancy in Y-direction:

• The columns in Y-direction are comparatively lesser but evenly spaced, so the structure is redundant but lesser than that of in X-direction.

Bi-directional stiffness:

Bi-directional stiffness X-direction:

• The Colunmns in horizontal direction are connected with straight-to-straight beams. This means, any lateral load (Seismic Force in X-direction) 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 diversion takes place.

Bi-directional stiffness Y-direction:

• The columns in vertical direction are not connected with straight-to-straight beams, so a lateral load passing from South side of building will get diverted to east and west direction to reach next bay. There will be major force diversion take place with seismic force in Y-direction. The framing plan is weak with respect to bi-directional stiffness in Y-direction.

Ductility:

• The ductility is always, inversely proportional to stiffness.
• If the structure is more stiffer, it will be less ductile and vice versa. Here, the structure is more stiffer in the X direction, i.e. it will be less ductile in X direction whereas in Y direction, the structure is less stiffer, hence it will be more ductile in Y direction.

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.

The possible sources of irregularities are:

1. Soft Storey Irregularities.
2. Vertical Irregularities.
3. Slopy Base Irregularities.

Soft Storey Irregularities:

• As we can see in the above given image that this building at the 6th floor has a soft Storey because it has uncovered opening.
• As we can see that there is no partition wall or no occupancy or any other shear wall or any other type of support elements etc. nothing is there to support the upper blocks of the building during the earthquake and this will also cause plastic hinge at the columns.
• According to the reference IS 1983-2016 Figure 4A, this has to be checked and we have to design the building accordingly. So that there will be no mistake in designing the building.
• Below are the images showing the stiffness irregularities (soft Storey).
• 

Vertical Irregularities:

• The building plan area is large in bottom stories. The floor plan areas are getting reduced with increase in height. A structure is said to be irregular when its bottom edge is 25% more than the top edge. Hence, this building is subjected to vertical irregularity. These building offsets has to be crosschecked with reference to IS-1893:2016 table-6 serial number (iii).

Slopy Base Irregularities:

• If we observe properly then we can see that at the right-hand side of the image there is sloping ground and this building is constructed at that sloping area.
• Which means that there are risks that this building may collapse during the earthquake.
• And this will lead to the variation in the stiffness as we can conclude properly.
• So according to the IS 1893-2016 FIG 5E we have some standards in order to design a building in such slopy areas and if we follow these standards then there is high possibility that we can save the structure from collapse during the earthquake attack.

• 

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).

Open Ground Floor:

• This denotes soft Storey mechanism acting in the building.
• As it is already mentioned in the question that the hotel building has an open ground floor for the restaurant, it means that the restaurant has a soft Storey mechanism acting on the building that cause the damage to the building. And this is one of the reasons why this restaurant building collapse.

Rooms along two bays separated by an open corridor:

• The bays filled with rooms will have higher mass compared to the middle bay left open for corridor. . So, on the two sides, i.e. the top and bottom portion of the plan will have heavier mass and hence more stiffness compared to middle portion which is left unoccupied. This leads to irregularity in mass. During earthquake excitation, the middle portion will get collapsed due to lower stiffness whereas the other two sides may sustain. This includes mass irregularity along height uniformly.

Two Shorts sides covered with infill walls:

• This means the building's left and right sides periphery is ore stiffer during seismic forces (especially along north-south). Since, the lateral force resisting systems, i.e. the walls are continuous from the foundation this impart greater strength and stiffness to the structure in these locations. That is the reason why the left and right edges remain intact after the earthquake excitation.

Staircase portion at left top corner:

• Usually, staircase and lift portions are stiffer than others parts because of concentrated concrete elements. So, the staircase portion stiffness in the provided building plan. There is also redundancy in the staircase portion due to increased number of columns. This is the reason why the left-hand bay survived after the excitation.

No frame connection along the Y-direction:

• As we can see in the above image that there are no structural beams that are provided along the Y-direction between the columns.
• This means that the building is more vulnerable during the earthquake because of the no frame connection along the Y-direction.
• Thats why we have to be more careful while designing the structures because a little mistake can lead to a very serious problem.

Beams connected along column minor axis:

• As we can see in the above image that the columns have the longer direction in the Y-direction.
• But there is no direct connection or frame in the Y-direction which may connects the beam in the Y-direction.
• There are columns that are connected in the X-direction but the large side of the column is in the Y-direction which means that they are more stiffer in that direction which can attract more lesser lateral force and this is one of the reasons for the collapse of this building.

Variation in column sizes:

• As we can see in the above given image that the sizes of the columns are different.
• We can say that the size of the interior column is big in size and the size of the exterior column is smaller as we can see in the plan.
• And because of the difference in the size of the columns the structure collapse because it may suffer heavy out of plan shear.