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:

• Symmetry- It is observed that the mass and stiffness distributions in the plane have symmetrical sides. This tends to reduce the horizontal displacement in both of the two orthogonal horizontal directions. Uniformity is also very good.
• Redundancy-There are no structural elements around the plan. This leads to non-uniform distribution of seismic activity in planned non-linear regions. Very low energy dissipation throughout the structure, making the structure less reliable.
• Bi-Directional stiffness- The design may not be able to withstand horizontal seismic action in the short direction (Y-axis), and the resistance and stiffness in the short direction (Y-axis) are lower than in the long direction (X-axis).
• Ductility- Buildings may not be able to withstand lateral (Y-axis) elastic seismic forces. Therefore, inelastic reactions can occur. Reduced design force in the transverse direction (Y-axis) must be compensated for 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: The irregularities for the given structure are:

• Soft storey
• The main reason is that the structure is suspended at mid-height. Open floors have been observed to reduce building stiffness, attracting more lateral seismic motion on this floor and causing severe damage to sensitive upper floors.
• Elimination of abnormalities can be done by installing earthquake-resistant walls in the ground where earthquakes occur frequently.
• Open floors also cause mass irregularities in buildings due to the high concentration of mass above the open floor. In other words, upper floors support more mass than open floors. There are more floors than this open floor.
• Mid-height open floors change the horizontal dimensions of shear absorption and the vertical geometry of the building. This means that the length of the top floor varies along the building unit. This creates a twisting effect on buildings that need to be separated by providing a shear wall or separation moment. Flexibility due to variation should be compensated for by providing ductility.

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 stories, except for certain openings at the ends of the corridor at stories 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 building collapse was caused by ground excitation and torsional effects in both orthogonal horizontal directions, although it was resistant to the two aforementioned actions.

The responsible factors for the collapse are as follows:

• Open Ground floor.
• Perimeter in-filled at the two short sides.
• Staircase filled with concrete at the hand bay in extreme left.
• Columns not in the same line.

Open Ground floor:

• The ground floor of the building is open plan, possibly with soft floors (long columns). Due to this excitement on the first floor, the pillars could not withstand the earthquake and collapsed. This is called the soft layer mechanism. This can be avoided with the Strong Column Weak Beam Mechanism.

Perimeter in-filled at the two short sides.

• Two sides of the perimeter of the building were filled (possibly with dirt), so the mass was not distributed on all sides of the building on all floors. This can be circumvented by providing shear walls in the direction of the two short sides of the perimeter that can withstand lateral loads (soil from infill), giving the building frame rigidity and resistance to seismic action help. Adjusted the mass unevenness of the plan.

Staircase filled with concrete at the hand bay in extreme left.

• The leftmost bay has a stairwell that concentrates mass in the stairwell area, creating a twisting effect that underlies the stairwell mass concentration. To avoid this type of obstacle, the stairs should be placed in the middle of the building and the torsional effect due to decoupling should now be zero. This also explains the impact of irregularities in building floor plans.

Columns not in the same line.

• The leftmost centerline pillar is observed to indicate a deviation in the construction line of the stairs. This is the reason for the irregularity of the floor plan with torsional flexibility. To avoid this, the eccentricity must be maintained so that the deviation is within the bar size and column size limits. It is also necessary to review the size of the beams and supports so that the line of the support does not shift even in the stairs.