Concept on positioning of Columns

AIM: TO understand the 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.

To describe the symmetry, redundancy, bi-directional stiffness, and ductility aspects of a given framing plan layout.

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 evenly, and its structure will be able to withstand direct action from seismic forces.

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:

Irregularity due to diaphragm flexibility
Irregularity due to stiffness-soft storey (stiffness irregularity)
Irregularity due to mass-soft storey (mass irregularity)
Vertical geometry irregularity
- Diaphragm flexibility:

Here, the aspect ratio of the building will be very large, as the larger plan dimension is more than 3 times the smaller plan dimension and this can induce flexibility to the floors during seismic action, which is not desirable. This is as per rigid diaphragm assumption.

This can be avoided by maintaining the aspect ratio within 3. Also, for the diaphragm to be considered rigid, it should be free of large openings, especially in the vicinity of main vertical structural elements.

- Stiffness irrregularity-soft storey:

The shaded portion represents an opening. This opening, due to the absence of the walls, will result in considerable reduction in the stiffness of the floor compared to other floors and will result in soft storey mechanism.

So care must be taken to avoid such opening and to provide walls wherever possible without affecting the aesthetics of the structure.

- Mass irregularity:

Mass irregularity can be considered to exist, when the seismic weight of any floor is more than 150% of floor of that of the floors above.

Here, since there is an opening in this structure it can lead to considerable decrease in the mass of that floor compared to other floors and also in the upper floors only part is retained in the structure for architectural purpose and this causes the lower floor mass to be higher than that of the upper floors. This can lead to soft storey mechanism.

The solution for this is to make the mass of the lower and upper floors approximately same.

- Vertical geometry irregularity:

In the elevation of the structure there is change in the horizontal dimension after a particular height ( in front elevation).The top width of the building is smaller than the bottom width. 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 geometry irregularity.

This irrregularity can be minimised by keeping both the widths same as far as possible with consent of the Architect.

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).

• The structure's frame is not symmetric.
• If the columns in this structure are not properly aligned in a symmetrical plane, then any lateral forces may cause damage.
• There is no proper masonry infill wall placement.
• In this structure. Redundancy in the vertical direction is inadequate.
• If the framed structure is capable of transferring the loaf from one direction to another of the foundation's structure.
• The columns in this layout are closely spaced, resulting in the formation of short paths for the transfer of stresses, which is also an important factor in earthquake resistance.
• The interaction of the staircase with other parts of the three-dimensional structure, specifically the interaction with the beam, column, and shear wall, results in patterns of deformation that are not typically studied during the design process.
• The increased strength and stiffness of the structure in the stairway region, for example, will influence the seismic torsional distress of the structure and may lead to shear failure of the staircase or other structural elements.