Guardrail Deflection Limits
The article Holding On (Modern Steel Construction, February, 2009) by Benjamin R. Baer, P.E., S.E., provided a great overview of handrail and guardrail strength requirements. As stated by Mr. Baer, “There is no single handrail or guardrail design that will work for all situations.” In this article, we intend to follow-up on Mr. Baer’s article, and review and discuss the subject of guardrail deflection limits.
Guardrails (also called guards or railings) that do not meet code requirements, or those that have excessive deflection, present a danger to the public and will result in a multitude of insurance and construction-related claims. As forensic engineers, we typically see common areas of railing design that are deficient to code or in serviceability, both in construction and design.
Mr. Baer’s article addresses the structural strength design criteria and construction considerations for steel handrails and guardrails. Many industry articles and publications have little to no discussion regarding allowable deflection of railings. There are, however, articles provided by construction and design industries regarding deck railing connections and the need to provide connections that are required to resist the required lateral loads.
The International Building Code (IBC) and the International Residential Code (IRC) do not prescriptively address allowable or tolerable guardrail deflections. In fact, the IBC and IRC provide no deflection limits for handrails and guards. However, good design practice would limit the amount of deflection a handrail or guardrail would experience in any direction under the loading requirements. Regardless of the fact that the structural needs of such design and construction in both the IBC and IRC are silent on this matter, it remains a very important aspect in the construction and design of railings, and thus must be considered.
When a railing has a significant amount of deflection such that it results in a person to feel unsafe or gives a sense that the railing is not structurally sound, the railing can be considered improperly constructed or designed. Although the railing can adequately withstand the designed structural loads, a significant amount of deflection creates the perception that an individual is not safe. Additionally, repeated deflections will weaken the structural members through fatigue or overstress, creating the potential for failure that could result in injury.
In order to establish the design criteria for rails, the minimum standards of the building code include the design loads for railings. These are addressed in the 2006 IBC Section 1607.7 Loads on handrails, guards, grab bars and vehicle barriers. According to Section 1607.7.1 Handrails and guards, “Handrail assemblies and guards shall be designed to resist a load of 50 plf (0.73kN/m) applied in any direction at the top and to transfer this load through the supports to the structure.”
The IRC also prescribes that only the single concentrated load as required by Section 1607.7.1.1 shall be applied. According to Section 1607.7.1.1 Concentrated load, “Handrail assemblies and guards shall be able to resist a single concentrated load of 200 pounds (0.89 kN), applied in any direction at any point along the top, and have attachment devices and supporting structure to transfer this loading to appropriate structural elements of the building.” These standards have existed throughout previous versions of the IRC and in the American Society of Civil Engineers (ASCE/SEI) 7 Minimum Design Loads for Buildings and Other Structures.
Beyond strength resistance requirements, the 2006 IBC, Section 1604.3 addresses the serviceability requirements of structural members. According to Section 1604.3 Serviceability, “Structural systems and members thereof shall be designed to have adequate stiffness to limit deflections and lateral drift.” According to Section 1604.3.6 Limits, “Deflection of structural members over span, L, shall not exceed that permitted by Table 1604.3.”
Deck and stair railings are typically exposed only to live loads, such as a person leaning or pushing against the railing. As far as serviceability requirements of the codes, the 2006 IBC Table 1604.3 for floors, roofs, and walls applies: floor member deflection is limited to a span (L) of L/360 under live loads; roof members that do not support ceilings are limited to L/180 under live loads; and exterior walls and interior partitions with flexible finishes are limited to L/120 when loaded with snow or wind. Another important element is that for cantilevered members, the span shall be taken as twice the length of the cantilever. When standard deck railings are cantilevered from the deck surface, twice the length of the cantilever is considered for the drift calculation when utilizing the IBC table. Based on this analysis of the IBC table, the deflection limits could be calculated utilizing h/60, h/90, and h/180 deflection limits.
Chapter 35 of the 2006 IBC also adopts the ASCE Standard ASCE/SEI 7-05 Minimum Design Loads for Buildings and Other Structures by reference. Section 4.4 of the ASCE/SEI 7-05 has similar load requirements to the IBC, including the 200-pound point load, the 50-pound-force per linear foot load, and the infill load of 50-pounds over a 1-foot square area. According to the ASCE 7-05 Section 1.3.2 Serviceability, “Structural systems, and members thereof, shall be designed to have adequate stiffness to limit deflections, lateral drift, vibration, or any other deformations that adversely affect the intended use and performance of buildings and other structures.”
According to ASCE 7-02 Section C1.3.2 Serviceability, “The fact that serviceability limit states are usually not codified should not diminish their importance. Exceeding a serviceability limit state in a building or other structure usually means that its function is disrupted or impaired because of local minor damage or deterioration or because of occupant discomfort or annoyance.” Deflections on railings must be considered and deflection limits must be set based on the amount of allowable deflection developed under lateral loading requirements.
Consider a typical cantilevered deck railing that extends 36-inches above the deck surface. When calculating deflection, the height of the rail post (h), as defined by the equation in IBC Table 1604.3, is actually two times the height, or 72-inches. Similarly, for a railing that extends 42-inches above the deck surface, the height (h) for the design deflection calculation would be 84-inches. The following table outlines the design deflection to be used in order to size the rail for minimal deflection:
Another source that aids in the review of standards available for the forensic and original design/construction is the American Standards and Testing of Materials (ASTM) E 985, Standard Specification for Permanent Metal Railing Systems and Rails for Buildings. According to Section 7.2.2 of the standard, “When the load is applied at the line of vertical support, the horizontal deflection shall not exceed the rail height (h) divided by 12, or h/12, with h being the distance between the surface of the post anchorage and the top of the top rail.” Per section 7.2.3, “When the load is applied at the midspan of the rail, the horizontal deflection shall not exceed the sum of the rail height (h) divided by 24 plus the rail length (l) between the vertical supports divided by 96, or h/24 + l/96.” Based on this ASTM standard, a 36-inch high rail would be allowed to have a maximum deflection of 3-inches, and a 42-inch high rail would have an allowed deflection of 3.5-inches. These would be perceived as large deflections by the user when compared with a stiffer system.
The available literature on the construction of hand rails includes the 1914 Universal Safety Standards, which outlines that a “standard railing” should be constructed such that the: “Top railing to be not less than 2×4 inches; center railing to be not less than 1×4 inches, of straight-grained lumber, dressed on four (4) sides, supported on 4×4 inch posts, dressed on four (4) sides, spaced not more than eight (8) feet center; or of built up construction of equal strength.” These railings were to be a minimum of 3.5-feet in height. Based on a 4×4 railing at an 8-foot on-center spacing, and provided with a 3.5-foot height, the deflection of a standard railing would be 0.38-inches (h/111) under a design point load of 200-pounds placed at the top of the support post. When considering the design load of 50-pounds per linear foot load, the deflection of a “standard railing” would be .75-inches (h/56).
These deflection calculations do not include the contributing deflection of the top railing under the load applied across the rail span. Thus, when utilizing the current code required loads, this “standard railing” as specified in the Universal Safety Standards resulted in a railing with deflections between h/56 and h/111.
The International Code Council (ICC) Evaluation Service, Inc. prepared an Acceptance Criteria for Handrails and Guards, published in February of 2007. Under the heading “Structural Tests,” a concentrated load test should include testing a rail with 500-pounds per foot at the midspan of the top rail and at the top of a single post. When the applied load reaches 200-pounds, the deflection at the point of the loading is to be measured. The ICC document has the same deflection limits as the ASTM E 985 standard that relates to metal rails. According to the ICC document, the allowable deflection at the point of loading shall not exceed the following:
- a. “The sum of the rail (guard) height, h (in inches/mm), divided by 24 plus the effective rail length, l (in inches/mm), divided by 96 or (h/24 + l/96). Where the effective rail length is the distance between the edges of the posts, the deflection at the midspan of the rail (guard) is measured relative to the center of the two posts (i.e., it does not include post deflection).”
- b. “The effective newel post height (vertical support) divided by 12, or (h/12), where the effective newel post (vertical support) height is the distance from the top of the top rail to the first point of fastener connection to the supporting construction.”
The following is a summary of our findings regarding deflection limits (shown in the table as the least stringent (most deflection) to the most stringent (least deflection):
Mr. Baer’s article suggests the use of HSS 1-1/2 x 1-1/2 posts to simplify the fabrication of the guardrails. Assuming the posts are spaced at 4-feet on-center, they are required to resist a 200-pound point load at the top. Based on a fixed condition at the bottom, our calculations indicate the following deflections utilizing the proposed HSS:
Based on our review of the 2006 IBC requirements and other reviewed documents, it is our opinion that deck and stair railing posts should be designed for a maximum allowable deflection of L/120 (h/60). That deflection limit is also the largest allowable deflection shown in IBC Table 1604.3 for other components of the building. In fact, the IBC states that all structural members shall not deflect more than the limits specified in the table.
With those minimum standards, the maximum allowable railing deflection is 0.60-inches for a 36-inch railing height and 0.70-inches for a 42-inch railing height. Another aspect of design and construction is the consideration of complete load paths. The deck railing should be analyzed as part of a system that includes posts and guards, and the total deflection should not exceed those minimums code requirements. The fixity and stiffness of the posts at their bottoms is critical to the performance of guardrail systems.
Providing this level of deflection in the design and construction of guardrails would meet the intent of the codes for serviceability in addition to providing a comfort level to users regarding the structural stability of railings. For critical service facilities, tighter design criteria should be considered.
American Society for Testing and Materials, ASTM Standards in Building Codes, Thirty-Seventh Edition, 2000.
Institute for Product Safety, Universal Safety Standards, Workmen’s Compensation Service Bureau, Carl M. Hansen, M.E., Copyright 1913 and 1914.
ICC Evaluation Service, Inc., Acceptance Criteria For Handrails and Guards, AC273, effective November 1, 2004, editorially corrected February 2007.
International Code Council, International Building Code, 2006.
American Society of Civil Engineers/Structural Engineering Institute, ASCE/SEI 7-05, Minimum Design Loads for Buildings and Other Structures, 2005.
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