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:

To assess the framing plan for earthquake resistance in the X and Y directions, consider the following aspects:

1. Symmetry:
• A symmetrical layout ensures uniform distribution of seismic forces, reducing torsional effects.
• If the framing plan is irregular (e.g., L-shaped, U-shaped, or with large asymmetries), torsional moments will develop, increasing the risk of localized failure.
2. Redundancy:
• Multiple lateral load-resisting elements (shear walls, moment frames, or braced frames) enhance system reliability.
• Lack of redundancy can lead to load concentration and potential collapse if a key element fails.
3. Bi-Directional Stiffness and Ductility:
• The structure should have comparable stiffness in both X and Y directions to avoid excessive deformation in one direction.
• If one direction is significantly stiffer than the other, seismic forces may concentrate in weaker regions, causing failure.
• Ductility ensures energy dissipation, preventing brittle failure. Moment frames or ductile detailing in RCC elements can improve this.

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:

Possible Sources of Irregularity in the Building:

1. Plan Irregularities:
• Torsional Irregularity: Uneven mass or stiffness distribution causes the building to rotate under seismic loads.
  Solution: Place shear walls symmetrically, ensure uniform stiffness, and use dual systems (frames + shear walls).
• Re-entrant Corners: L-, U-, or T-shaped buildings create stress concentration points.
  Solution: Provide expansion joints or stiffen these regions using additional bracing or shear walls.
2. Vertical Irregularities:
• Soft Story: Weak lower levels (e.g., parking floors) lead to excessive drift and potential collapse.
  Solution: Use moment frames, bracings, or infill walls to enhance stiffness and strength.
• Mass Irregularity: Sudden mass changes (e.g., heavy roof slabs or equipment floors) alter dynamic behaviour.
  Solution: Gradually transition mass changes and reinforce supporting elements.
• Stiffness Irregularity: Significant variation in stiffness between floors causes stress concentration.
  Solution: Avoid sudden stiffness changes by using uniform column and beam sizes or adding bracing.
3. Discontinuities in Load Paths:
• Weak Column-Strong Beam: If columns are weaker than beams, they fail first, leading to progressive collapse.
  Solution: Ensure column strength is at least 1.2–1.5 times beam strength (capacity design approach).
• Discontinuous Shear Walls: If shear walls stop at an intermediate floor, forces redistribute unpredictably.
  Solution: Extend shear walls to foundations or provide alternate load paths.

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:

Analysis of Structural Design and Infills Affecting Seismic Performance

The observed partial collapse of the hotel building can be attributed to several seismic vulnerabilities arising from its structural configuration and infill layout:

1. Soft Story Effect (Weak Open Ground Floor)

• Issue: The open ground floor (restaurant space) lacks infill walls, while upper floors have continuous infill along the perimeter, creating a soft story effect. This causes excessive drift concentration at the ground level, leading to collapse under seismic forces.
• Relation to Collapse: The building likely failed at the ground floor due to inadequate lateral stiffness and strength, leading to a pancaking effect of the upper floors.

→ Solution: Increase stiffness at the ground level by adding shear walls, moment-resisting frames, or diagonal bracing.

2. Infill Irregularities and Load Path Disruptions

• Issue: The short sides of the building are fully infilled at all floors except for corridor openings, making these sides significantly stiffer than the long sides. This stiffness contrast concentrates seismic forces, leading to differential movement and failure of weaker frame elements.
• Relation to Collapse: The extreme left-hand bay with the staircase survived because it had load-bearing components (possibly reinforced by stairwell walls), while the rest of the structure suffered excessive deformation and failure.

→ Solution: Ensure more uniform infill distribution across all facades or reinforce the flexible sides with structural bracing or shear walls.

3. Staircase Location and Structural Integrity

• Issue: The staircase is in the upper left-hand corner, likely enclosed with infill or reinforced walls. Stairwells often provide additional lateral resistance, protecting that part of the structure.
• Relation to Collapse: Since this bay survived, it likely acted as a rigid core, while the rest of the building was vulnerable to lateral forces due to irregular stiffness distribution.

→ Solution: Design stairwells as intentional seismic cores, integrating them into the lateral load-resisting system.

4. Torsional Effects Due to Infill Openings

• Issue: Openings at corridor ends and on the right-hand side of the ground floor cause asymmetrical stiffness, leading to torsional rotation under seismic forces.
• Relation to Collapse: The frame along the right-hand side survived, possibly because it was stiffer than other frames due to remaining infill. The rest of the structure collapsed due to excessive torsional movement.

→ Solution: Avoid significant asymmetries in stiffness by ensuring more evenly distributed infills or using moment frames to counteract imbalances.

Conclusion

• The soft story effect, uneven infill distribution, torsional irregularities, and differential stiffness contributed to collapse.
• The stairwell bay and right-hand frame survived due to additional stiffness and reinforcement.
• To prevent such failures, ensure continuous lateral load paths, uniform stiffness distribution, and reinforcement of soft stories.