Deflections in Deck and Balcony Railings

Deck and stair railings that do not meet code requirements or have excessive deflection present a danger to the public and 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 serviceability.  In order to understand the construction and design of such systems, thorough research of the topic is required.

The construction and design industries have provided numerous articles regarding deck railing connections and the need to provide connections that will resist the required lateral loads and perform in service.  An example of a trade article regarding these issues can be found in The Journal of Light Construction article, Strong Rail-Post Connections for Wooden Decks, which provides an overview of the deck railing connections utilized in residential deck construction.  That article also includes the testing of several connections.  However, many industry articles and publications have little to no discussion regarding allowable deflection of the stair and deck railings.

The International Building Code does not prescriptively address allowable or tolerable rail deflections.  According to a staff member of the International Code Council, the code has no deflection limits for handrails and guards.  Good design practice would limit how much a handrail or guard could be pushed outward or downward.  Regardless of the IBC silence 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 causes 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, the significant amount of deflection will exceed an individual’s perception of safety.  Also, the repeated deflections will weaken the structural members through fatigue or overstress, creating the potential for failure resulting 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 International Building Code (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 International Residential Code (IRC) for one- and two-family dwellings also indicates 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 Code and in ASCE 7 Structural requirements for Engineering.

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 to live loads only, such as a person leaning or pushing against the rail.  If one turns to serviceability requirements of the codes, the 2006 IBC Table 1604.3 for floors, roofs, and walls applies:  floor member deflection is limited to L/360 under live loads; roof members that do not support ceilings are limited to L/180 under live loads; 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 (L) 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, the deflection limits could be calculated utilizing h/60, h/90 and h/180 deflection limits, based on the IBC table.

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 ASCE/SEI 7-05 has similar load requirements as the IBC including the 200-pound point load, the 50 pound-force per linear foot 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.”  Thus, it is our opinion that deflections on railings must be considered and that the deflection limits must be set for 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.  The height of the rail post (h) as defined by IBC Table 1604.3 is two times the height, or 72 inches (L).  The following table indicates the design deflection to be used to size the rail for minimizing the deflection:

balcony1

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 the 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.”  According to 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 deflections would be perceived by the user to be large when compared with a stiff system.

In review of the available literature of construction of the hand rails, one could turn to the 1914 Universal Safety Standards, a ‘standard railing’ was constructed with:  “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 ½ feet in height.  Based on a 4×4 railing at an 8 foot on-center spacing and provided with a 3 ½ 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 0.75 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, the ‘standard railing’ as specified in the Universal Safety Standards resulted in a railing with deflections between h/56 and h/111, when utilizing the current code required loads.

The International Code Council (ICC) Evaluation Service, Inc. prepared an Acceptance Criteria For Handrails and Guards, as published in February 2007.  Under the heading of “Structural Tests,” a concentrated load test includes 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 stated 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:

  1. 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 to rail (guard) is measured relative to the center of the two posts (i.e., it does not include post deflection).
  2. 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 in regards to deflection limits (shown in the table as the least stringent (most deflection) to most stringent (least deflection)):

balcony2

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 the 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.”  According to 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 deflections would be perceived by the user to be large when compared with a stiff system.

In review of the available literature of construction of the hand rails, one could turn to the 1914 Universal Safety Standards, a ‘standard railing’ was constructed with:  “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 ½ feet in height.  Based on a 4×4 railing at an 8 foot on-center spacing and provided with a 3 ½ 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 0.75 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, the ‘standard railing’ as specified in the Universal Safety Standards resulted in a railing with deflections between h/56 and h/111, when utilizing the current code required loads.

The International Code Council (ICC) Evaluation Service, Inc. prepared an Acceptance Criteria For Handrails and Guards, as published in February 2007.  Under the heading of “Structural Tests,” a concentrated load test includes 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 stated 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:

  1. 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 to rail (guard) is measured relative to the center of the two posts (i.e., it does not include post deflection).
  2. 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 in regards to deflection limits (shown in the table as the least stringent (most deflection) to most stringent (least deflection)):

Matthew T. Blackmer and Edward L. Fronapfel are Principals at Pie Consulting & Engineering.  They can be contacted at www.pieforensic.com or 1-866-552-5246.

 


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.

 


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.


Journal of Light Construction, Strong Rail-Post Connections for Wooden Decks, Joseph Loferski and Frank Woeste, P.E., with Dustin Albright and Ricky Caudill, February 2005.

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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