Wind Loading on Rooftop Equipment

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”.

A structural engineer designed the metal support system and load transfer from the new HVAC unit down through the structure.

A structural engineer designed the metal support system and load transfer from the new
HVAC unit down through the structure.

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

Properly specified ballasting blocks are designed and formed to better address the freeze/thaw cycle.

Properly specified ballasting blocks are designed and formed to better address the freeze/thaw cycle.


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.

PHOTOS: MIRO INDUSTRIES INC.

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Against the Wind

The city of Moore, Okla., recognizes it cannot keep doing things the way they’ve always been done. You may recall on May 20, 2013, an EF5 tornado did extensive damage to the town. The new residential construction codes are based on research and damage evaluation by Chris Ramseyer and Lisa Holliday, civil engineers who were part of the National Science Foundation Rapid Response team that evaluated residential structural damage after the May 2013 tornado.

“A home is deconstructed by a tornado, starting with the breaching of the garage door,” Ramseyer explains. “The uplift generated by the wind causes the roof to collapse until the pressure pulls the building apart. These new residential building codes could possibly prevent that in the future.”

The new codes require roof sheathing, hurricane clips or framing anchors, continuous plywood bracing and windresistant garage doors. Moore’s new homes are required to withstand winds up to 135 mph rather than the standard 90 mph.

Although the city of Moore deserves to be commended for passing a more stringent building code less than one year after the 2013 tornado, this wasn’t the first damaging tornadic event Moore had experienced. The town also made national headlines in 1999 when it was hit by what was then considered the deadliest tornado since 1971. Moore also was damaged by tornadoes in 1998, 2003 and 2010. In my opinion, it was time for the Moore City Council to do the right thing by its citizens.

As extreme weather events occur more frequently, more emphasis is being placed on commercial roof wind resistance, as well. Robb Davis, P.E., recently attended a continuing-education conference for civil/structural engineers that discussed changes in the 2012 International Building Code and the referenced ASCE 7-10 “Minimum Design Loads for Buildings and Other Structures”. During the seminar, it became clear to Davis that nobody is specifically responsible for the design of wind loading to rooftop equipment as defined in the IBC and Chapter 29 of ASCE 7-10. Therefore, Davis reached out to Roofing because he believes it’s important roofing professionals understand the code requirements for wind loading to rooftop equipment, how the load is determined and applied, and how the load is transferred to the building structure. Davis shares his insight in “Tech Point”.

As Davis points out in his article, by better understanding wind loads on rooftop equipment, roofing professionals will be even better positioned to lead the design and construction industry in creating more resilient roofs and, ultimately, strengthening the structure and protecting the people underneath.