Connection details for hanging loads have been identified as an area structural engineers should be aware of and not left to a sub-trade to do. The engineer for the sub-trade should know the type of detail that is acceptable to the structural engineer of record.
Roof design and its systems and components involve all the design disciplines, for example, roofing Division 7, structure Divisions 3, 5, or 6, reflected ceiling plans and bulkheads gypsum board assemblies Division 9, lighting Division 26, and HVAC ductwork Division 23.
Roof design can be handled in many ways: by the owner’s design team, delegated designers, or a combination of both. The same considerations also apply if the owner’s design team (professionals of record) do all or part of the design work. The transfer of design information is the same for all these approaches.
The author’s preferred approach of interacting and sharing design information is to supply it through drawings and specifications. In the author’s case, they need to supply the loads the roof components must resist and how they should attach to the roof structure. The load information that needs to be conveyed is shown below:
- Dead load
■ Self-weight, mechanical units
■ Mechanical ductwork
■ Architectural finishes (ceilings, bulkheads)
- Live loads
o Minimum live load
- Wind loads
o Load on roof membrane
o Projections off the roof (mechanical units and screens)
- Snow loads
o Basic snow load
o Snow buildup due to
■ items on the roof (mechanical units and ductwork), screens
■ roof geometry
- Rain load
o Distribution due to roof geometry and deflection of members
In Section 1603.1 of the UBC, the following information should be shown in the construction documents:
- a) sufficient detail for the dead loads to be determined
- b) all effects and loads, other than dead loads, used for the design of the structural members and exterior cladding
- c) the dimensions, location, and size of all structural members in sufficient detail to enable the design to be checked
The author puts the main portion of the design loads on the roof structural drawings and adds the required information in the general notes (Figure 8).
Mechanical ducting or gypsum assemblies are often supported off the roof structure. This can be done in two ways: typical details on the structural drawings with the specifications cross referencing the details, or sketches included in the specification section of the sub trade. Often, the latter are missed and catch the sub trades by surprise. Commonly, architectural sub trades do not look at, or are not given, the structural documents by the general contractor.
One of the greatest challenges is coordination of the roof design and getting mechanical, electrical, and architectural information supplied to the structural designer. Often, coordination is seen as how all the different trades geometrically fit together, and 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 (30 ft). The author recommends the project’s coordinating 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.
Roof design requires coordination 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.
Author’s note: I would like to thank the following people who reviewed this article, provided advice, and emendations on the details, Jan Dale PE, technical director, principal with RWDI; Keith Robinson FCSC, FCSI, RSW, LEED AP, associate, specifier with DIALOG; and Dr. Brennan Bean, associate professor, Utah State University.
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 CSA Group committee for CAN/CSA S157, Strength Design in Aluminum/Commentary on CSA S157-05, Strength Design in Aluminum Design of Aluminum Structures, and 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 (ACEC). He can be reached at email@example.com.