COLLAPSE OF AND DAMAGE TO STAIRS AND RAMPS DURING EARTHQUAKES IN NEW ZEALAND

Clause 5.5.2c of IS 4326 recommends providing sliding joints in staircases to prevent them from acting as diagonal bracing, which can attract unwanted seismic forces. However, the Christchurch earthquake highlighted a critical issue with this design approach.

Debris-Induced Joint Failure

During the earthquakes in New Zealand (on 4 September 2010 at Darfield and on 22 February 2011 at Lyttleton) stairs collapsed in at least four multi-story buildings and in many other cases, the stairs sustained serious damages. The debris accumulation around the staircases prevented the sliding joints from functioning properly. The debris, which included fallen walls, and other objects, effectively "locked" the joints, causing the staircases to behave as rigid diagonal bracing elements. In the Forsyth Barr Building, Christchurch, the provided seismic gaps were reduced by construction tolerances, debris, etc. which resulted in insufficient space to accommodate even small amounts of inter-storey drifts, resulting in compression of the stair slab and subsequent collapse, trapping office workers in the 18-storey building. [see Fig.1(a)]. Where a straight stair or ramp had a mid-landing, the front part of the mid-height landing failed. It is called an 'opening knee' failure- the top of the landing at the first step will be squeezed off, when the top of the stair is compressed (Williams, 2012). Simmons and Bull (2000), based on their research, recommend transverse ties to be placed at the knee [as shown in Fig. 1(b)] to resist the bursting forces and to reduce the buckling of any longitudinal reinforcement, should cracking through the knee occur.

Consequences

As a result, the staircases attracted significant seismic forces, leading to:

1. Increased stress concentrations: The rigid behavior of the staircases caused stress concentrations at the connections, which could lead to failure.

2. Reduced structural performance: The sliding joints' inability to function as intended compromised the building's overall seismic performance.

Lessons Learned

The Christchurch earthquake highlighted the importance of considering debris accumulation and joint functionality in seismic design. To mitigate this issue, designers can:

1. Use debris-resistant joint designs: Develop joint designs that can accommodate debris accumulation without compromising their functionality.

2.?Provide clearances and debris management: To minimize joint failure risk, ensure sufficient clearances around staircases and implement debris management strategies.

3. Conduct rigorous seismic analysis: Perform detailed seismic analysis to account for the potential rigid behavior of staircases and the associated seismic forces.

By incorporating these lessons into seismic design, we can improve the resilience of buildings and reduce the risk of damage or collapse during earthquakes.

References

1.????? IS 4326:2013 - "Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings" (Bureau of Indian Standards)

2.?????Li, B., Wang, Z., Mosalam, K.M., Wang, X., and Wei, Z. (2008) "Analysis of Stairwells Performance and Damage During Wenchuan Earthquake", 14th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China, 8 pp.

3.????? Q.W. Su, Finite Element Analysis of Staircase under Earthquake Action. Advanced Materials Research, 163, 2964-2968, 2010.

4.????E. Cosenza, G.M. Verderame, and A. Zambrano (2008). "Seismic performance of stairs in the existing reinforced concrete building" 14th World Cοnference on Earthquake Engineering: Beijing, China, October 12-17, 2008.

5.?????H. Dai and A. Qi, (2009) "Analysis of the performance of reinforced concrete frame structure with staircase based on ETABS", Journal of Earthquake Engineering and Engineering Vibration, 6, 2009.

6.????? Bull, D. K. (2011). "Stairs and Access Ramps between Floors in Multi-storey Buildings. A report of the Canterbury Earthquakes Royal Commission.", Holmes Consulting Group, Christchurch, N.Z., 8.

7.????? New Zealand Society for Earthquake Engineering (NZSEE) - "Christchurch Earthquake: Lessons Learned"

8.????? Simmons, P.W. and D.K. Bull, ?The Safety of Single Storey Straight Stair flights with Mid-Height Landings Under Simulated Seismic Displacements, Research Report 2000-09, Dept. of Civil Engineering, University of Canterbury, 2000, 293 pp

9.????? Williams, S., et al. Expert Panel Report-Structural Performance of Christchurch CBD Buildings in the 22 February 2011 Aftershock, Feb. 2012, 136 pp. (https://www.dbh.govt.nz/UserFiles/File/Reports/quake-final-expert-panel-report.pdf-Accessed Mar. 6th, 2025)

10.? Ahmed, O.H., Hazem, A.-R., and Shawky, A.A. (2022) “Seismic Performance of Staircases in the 3D Analysis of RC Building”, Civil Engineering Journal, Vol.7, pp. 114-123.

11. Wang, X., Wang, P., Zhang, Y. and Liu, B. (2024) Seismic performance impacts of staircases in reinforced concrete frame structures under rarely occurred earthquake, Journal of Computational Methods in Sciences and Engineering 24 (2024) 2447–2467. DOI 10.3233/JCM-247468

12. FEMA (2015). FEMA E-74 Reducing the Risks of Nonstructural Earthquake Damage – A Practical Guide (4th Ed.), Federal Emergency Management Agency, 2015

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