Stairways

Stairways are quite often permanent, rigid and heavy. They usually connect directly to the primary structure in some manner. Due to this, they may cause unanticipated responses from the primary structure itself. “As nonstructural elements, stairway and stair enclosure contributions to the stiffness of the building structure have not always been considered. Local failures of primary structural elements have occurred adjacent to stairways due to the resistance provided by stair flights, landings, and enclosure walls” (Roha, Axley, & Bertero, 1982). Stairways also serve as the primary exit route for high risk buildings and at the same time serve as entry routes for buildings used for tsunami safety. Thus, it is critically important that there be means of egress and access during seismic events of all sorts. In past seismic events, “Components of stairways, including stair enclosures running the height of the building, doorways and lighting, have been damaged while the primary structure has shown little or no distress” [1,2].

One way in which stairways can impact structural soundness is by contributing to stiffness in particular segments of the building which impacts the overall structural response. Stairways can impact building components such as columns, girders, shear walls, and flooring. Local practices often allow stairs to act as weak diagonal braces between floors. Architectural elements such as these often participate in the initial stages of response until they crack, fail, or separate from the structural system [1].

Ideally, in order to prevent stairs from behaving like diagonal struts between adjacent floors, the stairs should be detailed with a fixed connection at one floor and a sliding connection at the other that allows movement parallel to the direction of the stair (FEMA, 2011). Below staircase shows damage although it is found in an evacuation building in Lombok and has yet to experience any significant seismic activity. It is somewhat alarming as this could pose a significant safety risk. It should be noted that the risk is not just in the staircases being able to withhold the weight of anticipated traffic, but also in potential debris which could fall and either injure individuals or significantly block pathways.

Cracking leads to decreased strength:
Cracked-staircase1-300.jpg

Failing concrete may cause falling debris to levels below:
Cracked-staircase2-300.jpg

In above photo the concern is not just with the staircase strength, as it likely contains reinforcing bars within the concrete, but with concrete that may break off and fall to levels underneath. To remain functional stairs must not merely retain their structural integrity, but also must remain clear of debris (Berg &Degenkolb, 1973). In some cases, even while the staircase remains standing, it is not functional due to failures surrounding them which cause debris. “The need for damage control measures is strongly supported by evidence from earthquake damage which clearly indicates that non-structural damage may involve a high degree of life hazard. Examples are failures of exterior pre-cast concrete panels, masonry infilling walls and partitions, particularly around stairs and means of egress” [3].

It is worth emphasizing that these debris hazards do not necessarily come from the staircase structures themselves. If a window near a staircase shatters, for example, glass shards could impede movement in all directions. As another example, the walls surrounding a stairway may be damaged during an earthquake causing debris to fall into the stairwell and rendering the stairs unusable. Brittle materials such as brick, hollow clay tile, or glass are particularly vulnerable. Therefore, “If stair enclosures are built using brittle materials such as unreinforced masonry, hollow clay tile, glass block, or skylights, it is recommended that they be encapsulated or replaced to prevent falling hazards and debris in the stairwell. Provide bracing and anchorage for pipes, lighting, emergency lighting or ducts to prevent falling hazards and debris in the stairwell. Maintaining safe exits is a critical element of earthquake safety” (FEMA, 2011). According to the Federal Emergency Management Agency of the United States, stairs are primarily damaged by inter-story drift, which is the differential movement of the adjacent floors. This forces a stairway to try to act like a diagonal brace. Stair damage is more likely to occur in flexible buildings with larger inter-story drift and less likely to occur in stiffer buildings [4].

Stairway damage includes:

  • Damage to primary structures
  • Damage to stair towers
  • Damage to stair enclosures (stairwell)
  • Damage to stairs
  • Damage to other stairway components

Minimum Standards

Recommended minimum standards:

  • Guardrails on open-ended sides of stairs, ramps or landings
  • Stairways shall not be less than 36 inches (914 mm) in clear width at all points
  • Minimum tread depth shall be 10 inches (254 mm)
  • The minimum headroom in all parts of the stairway shall not be less than 6 feet 8 inches(2032 mm)
  • Riser height 7-7.75”
  • Tread depth 10-11”
  • Handrail height 34-38”
  • Ascent angle 32-33 degrees

For more information, see the performance of stairways in earthquakes at https://nehrpsearch.nist.gov/static/files/NSF/PB83157693.pdf

Proper Reinforcement

For earthquake-ready staircase structures, stairways should be made of durable materials. In multi-story engineered buildings, common construction materials include structural steel, reinforced concrete, or reinforced masonry. They also need to be properly supported. Stair flights may be supported continuously, at their sides, or at their ends by the floors and landing platforms.

Proper reinforcement is important for strength and durability. In the case of concrete, which is commonly used in Indonesia, non-reinforced concrete is far too brittle as well as less strong. Due to flexure in the staircase, additional rebar reinforcement is necessary on the top and bottom.

Bowing of staircase due to lack of support on end points:
Stairs-bent-200.jpg

Staircase reinforced at top and bottom:
Stair-rebar-200.jpg

Visit http://www.oas.org/cdmp/document/codedraw/sectionb.htm for recommended reinforcement for concrete. Notice the significant amount of rebar within the stair framework. Notice, as well, the controlled spacing.

Stairs should also be well secured. This entails anchoring the stairs at the base to solid materials. The anchors must not be susceptible to shearing (breaking off).
Visit https://www.fema.gov/sites/default/files/orig/plan/prevent/earthquake/fema74/pdf/chapter6_3_8/chapter6_3_8_1.pdf to view information on anchoring stairs.

Multi-run Staircases

For multi-run (segments) staircases, there should be flexibility between each run as well. As Roho, Alxey & Bertero point out, “Damage has occurred to structures due to the behavior of stairways and to stairways due to the seismic response of the structural systems. Structures have been affected by (a) the creation of "short columns" due to landings connected at midheight resulting in brittle shear failure, (b) the creation of "short beams," leading to a significant increase of shear, (c) the creation of high local shear stresses in floor diaphragms due to the restraint offered by stairs and enclosure walls and (d) the introduction of torsional eccentricities. Damage to stairways has included (a) the failure of brittle enclosure materials (b) cracking and spalling of concrete at the stair flight-landing junction, (c) jamming of exit doors (d) dislocation of nonstructural components such as light fixtures or seismic joint coverplates, and (e) disruption of building services. These conditions seriously affect the use of stairways for emergency egress and could become extremely dangerous should panic or fire ensue” (1982) [5].

For more information stairway recommendations for earthquake preparedness, see https://www.fema.gov/sites/default/files/orig/plan/prevent/earthquake/fema74/pdf/chapter6_3_8/chapter6_3_8_1.pdf

Learn more

References

[1] Roha, C, Axley, J, and Bertero, V. 1982. The performance of stairways in earthquakes. Retrieved July 21, 2017 from https://nehrpsearch.nist.gov/static/files/NSF/PB83157693.pdf
[2] Bertero, 1982
[3] Glogau, a.A. "Separation of Non-Structural Components in Buildings." Bulletin of the New Zealand National Society for Earthquake Engineering, Vol. 9, No.3, September 1976, pp. 141-158.
[4] FEMA, 2011. https://www.fema.gov/sites/default/files/orig/plan/prevent/earthquake/fema74/pdf/chapter6_3_8/chapter6_3_8_1.pdf
5https://nehrpsearch.nist.gov/static/files/NSF/PB83157693.pdf

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