Concept on positioning of Columns

Aim:

• To understand the impact of horizontal and vertical members in seismic behavior of the structure.
• To understand the different irregularities of the building.

1.Question: 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.

Given Data:

Answer:

• Point of view on the layout of the framing plan regarding symmetry:

Symmetry helps bind various elements of a structure together into a single, unified whole. 

Example of symmetric and asymmetric behavior

In X-Direction:

• If we spit the structural plan in to half vertically, it looks the same on each half with respect the number of columns, beams and its placement.
• Hence it is symmetric in X-direction.

In Y-Direction:

• If we spit the structural plan in to half horizontally, it looks the same on each half with respect the number of columns, beams and its placement in y- direction.
• Hence it is symmetric in Y-direction.
• Point of view on the layout of the framing plan regarding redundancy:
• Redundancy therefore reduces the risk of failure and increases the factor of safety.
• Structural redundancy is defined as redundancy that exists as a result of the continuity within the load path.
• 

In X-Direction:

• The columns in X- direction are equally spaced in the middle portion of the layout with comparative better redundancy.
• Except the top and bottom rows where the space between the column is more.

In Y-Direction:

The number of columns is less in Y- direction but evenly spaced so it has less redundancy when compared with X- direction.

• Point of view on the layout of the framing plan regarding bi- directional stiffness:
  • The ability of a body to resist lateral deflection when a lateral force is applied in both x and Y direction.

In X-Direction:

The columns in X- direction are connected in straight-to-straight beams. When the external force is applied the passage of force from one end to the another is clear so it is less likely to affected by the seismic force.

In Y-Direction:

The columns in Y- direction are not connected in straight-to-straight beams. Dis continuity between the load path will lead to the force diversion which will affect the structure when lateral force is applied.

• Point of view on the layout of the framing plan regarding ductility:

The number of columns in X- direction is more when compared with Y-direction which contributes more stiffness and result in low ductility in X- direction and more ductile in Y- direction, which is contrast to it.

2. Question: 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:
• The area of the building plan is larger in bottom stories when compared with the top stories which has less floor area.
• This building offset has to be checked with the IS 1893- 2016, table 6 serial number iii., which is shown below.
• 
• 
• Soft storey:
• The building has uncovered opening space in the 6^th floor with out any walls, partition walls. This will lead to the plastic hinge formation at the column.
• This has to be referred with the IS 1893- 2016 figure 4A, which is shown below.
• 
• Slopy base:
• The building is constructed at the sloping ground at one side. This will lead in variation in stiffness.
• This has to be referred with IS 1893- 2016 figure 5E, which is shown below.
• 
• Few levels may be subjected to soft storey mechanism.

3. Question: 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).

Structural plan:

Before earthquake:

After earthquake:

Answer:

Open ground floor:

This resulted in soft storey mechanism in the ground floor.

Rooms along 2 bays are separated by an open corridor:

The bays on both sides have higher mass when compared with the corridor which was left open. This formed mass irregularity along height uniformity.

Two short sides covered with infill walls:

The building’s left and right-side peripheries were stiffer during seismic forces.

Staircase portion at left top corner:

The staircase portions are generally stiffer as it is concrete built and has greater mass when compared with rest of the building elements. Hence it survived.

No framing connection along the Y- direction:

There is no beams connection in the Y- direction except the left and right side of the structure. Hence there is no redundancy along north- south direction which resulted in main reason for the collapse.

Beams connected columns have minor axes:

The columns are having longer dimension in Y-direction which results in more stiffness along that direction. But the beams are connected along X- direction. There is no load path to transfer the forces.

Variation in column sizes:

The internal columns are having bigger size than the outer column which will lead to difference in lateral drift of columns. So the concrete floor might have suffered heavy out- of- plane shear which would have led to collapse.