One of the greatest challenges is co-ordination of the roof design and getting mechanical, electrical, and architectural information supplied to the structural designer. Often, co-ordination is seen as how all the different trades geometrically fit together. Co-ordination is not thought of as the cost implications of what is being designed by the different trades.
In the case of roof design, the impact of hanging loads have a significant impact on the structure if the clear spans of the roof structural members exceed 9 m (29.5 ft). The author recommends the project’s co-ordinating professional receive line drawings showing weights for mechanical ductwork, fire sprinkler lines, lighting, and architectural finishes. Transferring this information is best done during design development but can also be part of the submittal process during construction. The coordinating professional can identify the impact of these weights on the structure and optimize the layout of the suspended loads. In the author’s experience, there is significant structural savings if the loads are evened out across the roof structure.
Conclusion
Design of roofs require co-ordination and cooperation of all the design disciplines to ensure no design details are overlooked. Each separate discipline impacts the other often more than designers recognize; therefore, communication is critical.
Considerations, all of which must be recorded in the project manual, include extreme weather systems such as wind gusts, tornadoes, hail, rain, snow, and both occupancy loads and dead loads on roofs.
Resources
Codes
- Minimum Design Loads and Associated Criteria for Buildings and Other Structures–Provisions.
- Minimum Design Loads and Associated Criteria for Buildings and Other Structures–Commentary.
- ASCE Standard, ASCE/SEI 7-22
- National Building Code of Canada (NBCC) 2020, 2015
- Structural Commentaries of the NBCC 2015
- Safety and Limit States Design for Reinforced Concrete, MacGregor, J.G. Canadian Journal of Civil Engineering, V. 3, No. 4, Dec. 1976, pp. 484-513.
Rain
- Rain Loads–Guide to the Rain Load Provisions of ASCE-7 -16, Dr. Michael O’Rourke, Aron Lewis, ASCE Press.
- Do Structural Engineers Design for Rain Loads? Dr. Michael O’Rourke Anthony Longobard April 2019, Structures Magazine.
Ponding
- Structural Design of Steel Joist Roofs to Resist Ponding Loads Technical, Digest 3, Steel Joist Institute.
- Strength and Reliability of Structural Steel Roofs Subjected to Ponding Loads, Mark D. Denavit and Michael H. Scott, Journal of Structural Engineering, American Society of Civil Engineering.
Wind
- Wind Loading of Structures, 4th Edition by John D. Holmes.
- Designing for Tornados, Dr. Ray Denoon, July 2017, Structures Magazine
Snow
- Snow and Rain loads in ASCE 7-22 Part 1 & 2, Dr. Michael O’Rourke, John F. Duntermann, January & February 2022 Structures Magazine.
- Ground Snow Loads for ASCE 7-22 – What Has Changed and Why? Marc Maguire, Brennan Bean, James Harris, Abbie Liel, Scott Russell, 2021 Digital Communications Utah State University, https://doi.org/10.26077/rxhj-cb29.
- The 2020 National Snow Load Study, Brennan Bean, Marc Maguire (A.M.ASCE), Yan Sun, Jadon Wagstaff, Salam Al-Rubaye, Jesse Wheeler, Scout Jarman, and Miranda Rogers, 2021 Digital Communications Utah State University, https://doi.org/10.26077/200k-pr86.
- Roof snow loads in Canada D.A Taylor, Canadian Journal of Civil Engineering, Volume 7, March 1980.
- Snow Thermal Factors for Structural Renovations, Dr. Michael O’Rourke, Scott Russel; June 2019 Structures Magazine.
Future Weather
- Climate-Resilient Buildings and Core Public Infrastructure 2020, Alex J. Cannon, Dae Il Jeong, Xuebin Zhang, and Francis W. Zwiers, Environment Canada.
Author’s note: I would like to thank the following people who reviewed this article, provided advice and emendations on the details, Jan Dale P.Eng., technical director, principal with RWDI; Keith Robinson FCSC, RSW, LEED AP, associate, specifier with DIALOG; and Dr. Brennan Bean, associate professor, Utah State University.
Discussion on structural considerations in roof design in the U.S. has also been included as products produced in the U.S. are often proposed for use in Canadian projects. Ensuring that technical information is interpreted correctly can prevent costly mistakes.
Author
David Thompson is a principal at KTA Structural Engineers Ltd. of Calgary, Alberta. He has been a professional engineer for more than 35 years. He has been involved in the design of hospitals, student residences, office buildings, film studios, and sports facilities. Thompson specializes in the design of tension membrane structures and has dealt with projects in 55 countries. He was a member of the Canadian Standards Association (CSA) committee for CAN/CSA S157, Strength Design in Aluminum/Commentary on CSA S157-05, Strength Design in Aluminum Design of Aluminum Structures, and he serves as a member of ASCE Standard Committee for ACSE-55 Tension Membrane Structures. Thompson has been a member of CSC since 1990 during which he served on the Canadian Construction Documents Committee (CCDC) for 10 years representing the Association of Consulting Engineering Companies. He can be reached at dpt@kta-eng.com.
