Wind Loading on Rooftop Equipment by Robb Davis P.E.

I recently attended a continuing-education conference for civil/structural engineers that discussed changes in the 2012 International Building Code (IBC) and the referenced ASCE 7-10 “Minimum Design Loads for Buildings and Other Structures”. During the seminar, the question was asked: “Who is responsible for the design of wind loading to rooftop equipment as defined in the IBC and Chapter 29 of ASCE 7-10?” The most accepted response was to add a section in the structural general notes that wind design on rooftop equipment is to be designed “by others”.

The design requirements for wind loading on rooftop equipment have been included in previous editions of the IBC and ASCE 7, but significant changes have been included in ASCE 7-10. The increased attention is in part because of more severe wind events in recent years. While it is not the primary responsibility of the roofing consultant or contractor to evaluate the systems being placed on the roof, it is good to understand the code’s requirements for loading to rooftop equipment, how the load is determined and applied, and how the load is transferred to the building structure.

CODE REQUIREMENTS

The primary focus of the roofing professional in the IBC is concentrated on Chapter 15 (Roof Assemblies). While there are requirements in Chapter 15 addressing rooftop structures, these requirements, particularly in relation to wind loading, extend beyond Chapter 15. It is therefore imperative to be familiar with other sections of the code.

For instance, Section 1504 (Performance Requirements) refers the user multiple times to Chapter 16 (Structural Design) for wind-loading-design requirements. While roof manufacturers typically prequalify their systems based on various industry standards (ASTM, FM, ANSI, etc.), rooftop equipment supports are not typically prequalified because of the variability of placement and conditions. Similarly, new to this code cycle, Section 1509.7.1 includes the requirement for wind resistance for rooftop-mounted photovoltaic systems per Chapter 16 of the IBC. Other industries or trades have similar requirements. Section 301.15 of the 2012 International Mechanical Code and Section 301.10 of the 2012 Fuel and Gas Code require “equipment and supports that are exposed to wind shall be designed to resist the wind pressures in accordance with the IBC”.

Section 1609 of Chapter 16 (Wind Loads) applies to wind loading on every building or structure. Section 1609.1.1 provides two design options. The designer can use chapters 26 to 30 of ASCE 7-10 or Section 1609.6 of the IBC. Note however that Section 1609.6 is based on the design procedures used in Chapter 27 of ASCE 7-10, which does not address wind loading on rooftop equipment and thus is not applicable. Chapter 29 of ASCE 7-10 (Wind Loading on Other Structure and Building Appurtenances) contains the procedures used to determine wind loading on rooftop structures and equipment.

DETERMINING AND APPLYING WIND LOADING ON ROOFTOP EQUIPMENT

To determine wind loading on rooftop equipment, the first step is to identify the building Risk Category (formerly the Occupancy Category) and the building location. The Risk Category is determined from Section 1604.5 and Table 1604.5 of the IBC or Table 1.5-1 of ASCE 7-10. There are slight variations in the two codes but typically each will produce the same Risk Category.

The Risk Category and the location are then used to determine the design wind speed based on published wind-speed maps, available in Section 1609.3, figures 1609 A to C of the IBC, or Section 26.5.1, figures 26.5-1 A to C of ASCE 7-10. It can be difficult to read these maps to select the appropriate wind contour line, specifically along the East Coast. The Redwood City, Calif.-based Applied Technology Council (ATC), a non-profit that advances engineering applications for hazard mitigation, has digitized the maps providing a valuable resource for determining design wind speeds by GPS coordinates or the building’s address. Visit ATC’s wind-speed website. Note however that it is always advisable to cross check this design wind speed with the maps in the adopted code or with the local building authority.

For rooftop equipment, there are two classifications: buildings over 60 feet (h>60 ft) and buildings under 60 feet. Research has shown that design wind loading on low-rise buildings (h≤60 ft) has been significantly lower than what is being observed. ASCE 7-10 applies a specified gust-effect coefficient factor of 1.9 for lateral loading and 1.5 for vertical uplift loading. There are provisions that allow this factor to be reduced but is rarely if ever applicable to rooftop equipment. For buildings over 60 feet, a gust-effect factor and a force coefficient factor are determined based on the parameters associated with the equipment and the structure.

Equations in ASCE 7-10 Section 29.5 are then used to determine the appropriate wind force for the corresponding building height classification.

LOAD TRANSFER TO THE STRUCTURE

Once the appropriate force is determined, a design can be completed to resist the forces or transfer the load to the building structure. For wind loading, this can be accomplished by ballasting or by mechanically anchoring the equipment to the structure. (Seismic loading must also be considered, which, under certain circumstances, can prohibit the use of ballasting.)

Ballasted systems can be a very attractive option to address wind loading. No roof penetrations are required with ballasting. However, care must be taken to provide sufficient safety factors and balance the additional dead load required with the capacity of the roof system (membrane and insulation load capacity) and the structural capacity of the roof framing.

Mechanically anchored systems can include structurally designed frames or cable ties. Each of these options requires a physical attachment to the structure with penetrations through the roof surface. This type of system can be designed for almost any roof system and provides a positive load path to transfer forces down through the structure. Coordination of these anchors amongst trades is critical to avoid costly damage to the integrity of the roof envelope.

Selecting the proper system must be done on a project-specific basis because each project has its own strengths and limitations. This is particularly true for existing buildings where additional weight is not an option when the structure is likely at its load capacity. In this situation there are various options available, including reinforcing the roof structure to carry more load or updating the entire roofing system to a lighter system. Each of these options can be costly and time-consuming. Aligning the supports over the existing structure can also provide a viable option but, again, coordination between professions is essential. On new construction, loading to rooftop equipment can be incorporated in the design phase when appropriate structural framing, anchor points and materials can be specified to allow either option.

The roof of a building is a critical defense against the elements. Failure to address external loading on rooftop equipment can lead to significant rooftop damage or a catastrophic failure. It is imperative for roofing professionals to be aware of code requirements to ensure compliance, maintain roofing warranties and limit liability. How the load is determined and transferred to the structure should be completed by a design professional and is typically not the responsibility of the roofing professional. However, an understanding of the process can aid in better serving clients’ best interests, protecting the integrity of the roofing system.

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robbRobb Davis, P.E., is a senior engineer for MIRO Industries Inc., Heber City, Utah. In his role, Davis evaluates job-specific applications to ensure rooftop product components are sufficient for designed wind and seismic loading.