Concept on positioning of Columns

Q1. 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. 

SYMMETRY:

• The framing layout is symmetric in nature
• The load resistance framing in this symmetrical structure is distributed evenly throughout the structure
• If the reflection of the structure about the axis is identical in geometry, supports, and material properties to the structure itself, the structure is symmetric with respect to an axis of symmetry in its plane.
• This structure has an exact matching of  form and arrangement of  parts on opposite sides of the boundary at X and Y

 REDUNDANCY:

• Because this structure has perfect redundant, the structure becomes more reliable in resisting any lateral forces acting on the structure.
• Furthermore it tends to prevent any extent of degradation that the structure may suffer without losing some specified elements of its functionality.

Bi-directional stiffness.

• Horizontal seismic motion will cause the horizontal axis of X and Y to change direction throughout the layout's framing.
• Because this structure can distribute seismic forces due to its bidirectional nature it can withstand and resist any horizontal motion of forces.
• The given plan has an orthogonal layout in both the X and Y directions so it can transfer forces even Y and its structure will be able to withstand direct action from seismic forces. 

Ductility:

• Prima facie, the aspect ratio of the plan clearly hints more column in X-direction compared to Y- direction.
• It will result in higher stiffness of the building in X- direction, which would reduce ductility in X-axis.
• Moreover, the x-force will easily pass from one bay to its next due to beam alignment.
• Whereas, the y force will struggle to pass through.
• The north-south beams are arranged in zig-zag which leads to a discontinuous bracing.
• We can conclude that the given framing plan is more ductile in y-direction than that of in X-direction.

Q2. 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.

Ans:

• This pitcher represents multistoreyed building rests on hill slope.Various floors of building has step back & set back configuration.
• It consists of unequal column height which causes constructional problems.The various floors of the building are supported on two type of column.
• (1) columns resting on the floor & (2) columns resting on sloping ground.
• These building are highly irregular & asymmetric.The centre of mass & center of stiffness lies on different vertical axis.Due to irregular soil profile structure may also results in unequal settlement of foundation.
• Sometimes plastic hinges are also formed on the periphery of the building & columns.

Vertical irregularity: 

• 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 Table-6 serial numbe(3).

Soft storey:

• The building has uncovered opening space in 6th floor.
• As there is no occupancy or a partition wall, this will lead to plastic hinge formation at the columns (with reference to IS-1893-2016 Figure 4A).
• This has to be checked & sufficiently designed.

Slopy base:

• The building is constructed on a sloping ground at one side.
• This will lead to significant variations in the stiffness ( with reference to IS-1893-2016 Figure5E).
• Few levels may be subjected to soft storey mechanism.

Vertical irregularity Is 1893-2016 Table 6 serial no(3)

Soft storey IS 1893-2016 Figure 4A

Slopy base IS 1893-2016 5E

Q3. 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).

Ans: 

• In buildings,floors play a very important role in the overall seismic response of the structure.They act as horizontal diaphagms.
• That collect & transmit the inertial forces to the vertical structural systems & ensure that those system act together in resisting the horizontal seismic action.
• A soft story at ground level is certainly the whole building.

Open ground floor:

• This denotes soft storey mechanism acting in the building.

Room along 2 bays separated by an open corridor:

• The bays filled with rooms will have higher mass compared to the middle bay left open for corridor. This  induces mass irregularity along height uniformly.

Two short sides covered with infill walls:

• This means the building's left & right sides periphery is more stiffer during seismic forces (especially along North-south).

Staircase portion at left top corner:

• Usually staircase & lift portions are stiffer than other parts because of concentrated concrete elements.So the staircase portion contains more stiffness in the provided building plan.

No framing connection along Y-direction:

• We could see no structural beams provided along the vertical direction between columns. This makes the building more vulneable during the North-South seismic force.

Beams connected along column minor axis:

• The columns are having longer dimension in Y-direction. This means they are stiffer along Y-direction. But the beams are running only along X-direction which would atttract much lesser lateral forces & hardly transfers it.

Variation in column sizes:

• The internal columns are having higher sizes compared to exterior columns (This is common beacause of higher load demands). 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.