Freestanding roof guard woes

by | April 29, 2015 10:35 am

Regulations specify a freestanding rooftop guardrail shall be constructed to meet certain structural requirements.[1]
Regulations specify a freestanding rooftop guardrail shall be constructed to meet certain structural requirements. Photo courtesy BigStockPhoto

Understanding staged structural failure analyses
By Bill Yao, P.Eng.
HVAC equipment is generally installed on top of the flat roofs of industrial, commercial, and institutional buildings. The maintenance personnel sometimes are exposed to danger of falling if the units are close to the roof’s edges. In this case, a guardrail system is required by Ontario Ministry of Labour Regulation 851, the Occupational Health and Safety Act. The regulation also specifies a guardrail shall be constructed to meet the structural requirements for guards as set out in Ontario Building Code (OBC), which has been adopted from the National Building Code of Canada (NBC) 4.1.5.15.

Set up at the edge of the roof, these guardrail systems are laid directly on a flat roof without any anchors penetrating through. This means the stability to resist a concentrated load specified in the province’s code fully depends on its integrity and the balance weight of the base plates.

A guardrail system generally consists of three components:

Design standards for a guardrail system are specified in the 2012 OBC under 4.1.5.14. (“Loads on Guards”):

1. The minimum specified horizontal load applied inward or outward at the top of every required guard shall be,

B. A concentrated load of 1.0 KN applied at point for access ways to equipment platforms, contiguous stairs, and similar areas where the gathering of   many people is improbable.

The question is, how can one be certain a guardrail system will protect those on a roof? This article suggests an analysis option to determine structural success.

The setup
A typical three-bay guardrail system is set up on a gravel protected roof. The vertical posts of the system are spaced at 2.4 mm (8 ft). The vertical posts are installed to the base plates, and a 1219-mm (48-in.) long outrigger is attached at each end to increase the system’s stability. The plan view of the guardrail system is shown below.Fig1[2] Fig1b[3]

In this scenario, there are the following assumptions:

For the analysis, the initial calculations were as follows:

At the beginning, the guardrail system stands there with all posts fixed. When a point load is applied and slowly increased, the first post would reach its breaking point in which the bending moment is 0.222 KN-m, or lateral shear force is 0.22 KN, then the base plate will rotate or slide. If the load increases, it will be transferred through the rails to the other base plates, which will rotate or slide—either the whole system stably stands or completely collapses.

The guardrail system will change its stability mode during the loading process. Or, in other words, the analysis model will change. Therefore, the general structural analysis is unsuitable for this situation.

Staged structural failure analysis
A special procedure for this situation is called ‘staged structural failure analysis.’ The term derives from the concept of staged construction created by Computers & Structures Inc. This is a static modelling, analysis, and design application that enables the definition of a sequence of construction stages in which structural systems and load patterns are added or removed, and time-dependent behaviours are evaluated, including creep, shrinkage, aging, and tendon relaxation.

For the example discussed in this article, one model can simulate the whole restraints that are fixed, while another simulates one restraint that is hinged or rolled. However, one should know when the first model would switch to the second model—a trial-and-error method could be adopted to determine the maximum load withstood by the specific model.

The main procedures are described below:

Fig2[4]

The detailed calculations are shown below to demonstrate the trial-and-error calculation methods step by step. The guardrail system’s three-dimensional structural analysis is conducted with SAP2000 v.15.

Fig3[5]From this calculation table, one can obtain some helpful conclusions:

  1. Staged structural failure analysis can provide enough clues for how the system would fail step by step. From that, one knows where the system can be used.
  2. Model 1’s Element 1 shear force would reach its critical condition first—V3= −0.232 KN great than 0.222 KN—so the base plate would slide. If the base plate kept sliding, the whole system would collapse. Therefore, the guardrail system can only be used where a roof curb or low parapet on the leading edge of the roof is present. The base plates should be placed right next to the vertical rise of the said structures to eliminate possible base plate slide due to friction failure.
  3. Model 1 would change to Model 2 when the turn-over moment reaches its limit.
  4. One should always use an outrigger assembly at the beginning and end of a continuous run. (The outrigger is a strong frame which can significantly increase the stability of the systems.)
  5. Model 2’s Element 4 is subject to tension, which would reduce the friction force the base plate provides. The critical condition is the tensional force is big enough the horizontal shear force is equal to friction force. In this case, the base plate would slide.
  6. The guardrail system analyzed in this article is adequate to withstand a concentrated load of 1.0 KN specified in the Ontario Building Code, although the deflections and displacements might be much greater than general requirements.

Conclusion
Staged structural failure analysis is a practical method that can be applied to many cases where the structural systems and loads change. When engineers sometimes have to handle this kind of problem, the so-called staged structural analysis might provide an ideal solution.

Bill Yao, P.Eng., has been a civil/structural designer for 20 years. He currently works as a structural engineer in an engineering firm in Markham, Ont. He can be reached via e-mail at wyao03@yahoo.com[6].

Endnotes:
  1. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/04/dreamstime_s_41336655.jpg
  2. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/04/Fig1.jpg
  3. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/04/Fig1b.jpg
  4. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/04/Fig2.jpg
  5. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/04/Fig3.jpg
  6. wyao03@yahoo.com: mailto:wyao03@yahoo.com

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