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.

ANS:

AIM: To explain the aspects of symmetry, redundancy, and bi-directional stiffness and ductility.

Symmetry: This structure is symmetric in nature. This structure has a good distribution of load resistance framing throughout the entire structure.

In symmetric structure with respect to an axis of symmetry in its plane if the reflection of the structure about the axis is identical in geometry, supports, and material properties to the structure itself. 

It is also having very good uniformity.

Redundance: This system is redundant, as the structural elements of the framing layout given are placed at equal intervals. The placing of equally distributed structural elements increases the redundancy of the structure. The structure is desirable to resist earthquakes as there is redundancy in the structure.

Bi-directional stiffness: The system may not be able to resist the horizontal seismic action in the shorter direction and also the resistance and stiffness in the shorter direction i.e Y-axis is less when compared to the longer direction i.e X-axis.

Ductility: The building may not able to resist the elastic earthquake forces in a shorter direction (Y-axis) so it may result in an inelastic response. The reduction in the shorter direction (Y-axis) is to be compensated by ductility.

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.

ANS:

Stiffness irregularities and Mass irregularities are present in the structure. The soft storey is the cause of irregularities present in the structure.

Soft storey:-  Soft storey is generally characterized by the vertical discontinuity in stiffness. Any storey for which lateral stiffness is less than 60% of the storey which is immediately above or less than 70% of the combined stiffness is classified as soft storey. It may be at the ground level or upper level. During an earthquake, most of the energy is dissipated by the column of soft storeys. Mass irregularities may be considered to exist when the seismic weight of any floor is more than 150% of the floor of that of floors above.

Solutions to avoid soft storey effect-

• Increase lateral strength and stiffness of soft storey.
• Open bays of the building must be braced.
• The number and stiffness of ground floor columns may be increased.

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).

ANS: The given building structural plan is consisted of a one-way slab and beam. The RC slabs are vertically one-way, and the RC beams are in X-direction which transfers all floor loads to columns. Also, small cantilever RC slabs are provided for the balcony.

Causes for failure of the structure:

• We can see the structure is not symmetric and larger strength is provided hence low ductility.
• In this given Structure, Redundance in the vertical direction is not provided properly.
• Then the loads are not distributed evenly in the unsymmetric structure.
• Bi-directional stiffness and ductility.
• The increased strength and stiffness of a structure in the stairway region will influence the seismic torsional distress of the structure and may lead to shear failure of the staircase itself or of other elements of the structure.