ASCE 7-10 Wind Loads On Roof Mounted Equipment

Evolving Design Wind Forces On Retail RTUs
Wind forces on Roof Mounted Equipment for typical retail and restaurant structures are receiving, at long last, a lot of attention by the engineering community, with somewhat surprising results. Not entirely surprising though, because there has been a growing body of evidence, primarily from post hurricane field surveys, that the standard roof top unit (RTU) anchorage has been insufficient.

1970s
In the old days, we'd apply a wind pressure against the face of an RTU, likely the same used on the windward wall of the building, design overturning anchorage if the weight of the equipment seemed insufficient to hold it down, and call it good.

1990s
The late 1990s saw the first significant change in wind design in the past twenty years, noted in, what are now called, our legacy codes.

2002 ASCE & 2003 IBC, current code in a few states
The 2002 edition of ASCE 7, which became the basis of design by reference for IBC 2003, was the first code reference to specifically address roof mounted equipment as requiring special attention for wind design.

The lateral force applied to rooftop equipment is given by: F = (qz)x(G)x(Cf)x(Af) (lb) where qz is the velocity pressure evaluated at height z of the centroid of area Af using the appropriate exposure category. G is the gust-effect factor, Cf is the force coefficient, and Af is the projected area normal to the wind except where Cf is specified for the actual surface area.

A typical G would be .85, a typical Cf would be 1.3 or 1.4, yielding a product around 1.0 or 1.1 typically. We'll use this product, (G)x(Cf), as the mode for comparison with subsequent codes. All else being equal, design forces can be seen to change in direct proportion to the changes in this product (G)x(Cf).

The ASCE 7 committee was vague about providing guidance for dealing with the possibility of increased loads exerted on the RTUs because at that time they felt there was no basis to make a recommendation. Uplift forces due to wind across RTUs received no consideration in the methodology at all.

2005 ASCE, current code in more than 40 States
The 2005 edition, ASCE 7-05, is the basis of IBC 2006 and IBC 2009 which are the current codes in the vast majority of the US at the time of this writing, late 2012. ASCE 7-05 used the same equation but provided more specific guidance in consideration of the increased loads on RTUs, and specifically for roofs less than 60 feet high, which fits the majority or our restaurants and retail stores. The familiar equation is F = (qz)x(G)x(Cf)x(Af) (lb) but with added requirement that the product of the factors G and Cf for roof mounted equipment shall be adjusted from 1.0 to 1.9 based upon certain geometric factors. But in most cases for typical packaged RTUs, the correct number to be used is (G)x(Cf) = 1.9, which is almost double what it was under ASCE 7-02. The net effect is that design wind loads upon RTUs developed under ASCE 7-05 will be almost double the same loads developed under ASCE 7-02.

Regarding wind induced uplift effects upon RTUs, ASCE 7-05 does not address it in the body of the code, but buried in the commentary pages is this observation, "The designer should design for uplift."

2010 ASCE, The Upcoming Building Code
The 2010 edition, ASCE 7-10, the basis of IBC 2012 and not widely adopted as of this publication, but obviously soon to become law, uses the same equations as its predecessor but adds a specific uplift force requirement. The lateral wind pressure on an RTU is determined from the equation, F = (qz)x(G)x(Cf)x(Af) (lb) This is the same equation with (G)x(Cf) = 1.9, generally, as before.

But then ASCE 7-10 adds that the uplift wind pressure on an RTU shall be considered to act simultaneously with the lateral pressure and shall be determined from the following equation, F = (qz)x(G)x(Cf)x(Ar) (lb) with (G)x(Cf) = 1.5, generally based again upon certain geometric conditions. This uplift force essentially cancels out the "holding down" effect of the RTU weight.

The net result is that, not only will the lateral design wind loads upon RTUs developed under ASCE 7-10 will be almost double the same loads developed under ASCE 7-02, but much of the holding down effect we might consider from the sheer weight of the equipment, has been eliminated from our design. RTU anchorages must increase in capacity yet again.

The Current Florida Building Code, Harbinger of Things To Come?
Florida has taken it a step further with its March 2012 adoption of the FBC Section 1609.8. The design methodology is the same as under ASCE 7-10, requiring (G)x(Cf) of 1.5 for RTU uplift design, but adjusting (G)x(Cf) for RTU lateral design to 3.1. Yes, the word would be "triple".

We're obviously moving in a direction to eliminate the problem of detaching RTUs. And this is a good thing. Buildings that otherwise have performed rather well under hurricane conditions have nevertheless turned in huge insurance claims due to the water damage ensuing upon the consequences of gaping holes in roofs after RTUs have been detached.

Major Texas Retailer Receives Engineering Report From TR2

TR2, The Roof Reinforcer, completed and submitted its final evaluation report for this eastern Texas major retailer today. Tim McCarthy P.E. was consulted to investigate a deflected 20T RTU mounted on the roof. "We performed two site visits altogether," said Mr. McCarthy, principal with The Roof Reinforcer. The first visit yielded measurements of deflections at the top of the RTU and the surrounding roof membrane. The second visit was necessary to measure structural element elevations inaccessible without proper equipment."

The project required two weeks to investigate the structure, analyze the acquired data and develop conclusions.

"The deflection in the roof mounted equipment was obvious," noted Paul, Southern Region Project Manager for TR2. "And there were elevation divergences in the roofing measurements and the structural supporting elements too. But what was unusual was that the RTU measurements and the roof top and structural measurements were telling two different stories. The RTU measurements indicated an unacceptably large deflection to one side, unacceptable from the viewpoint of HVAC performance. The structural measurements corresponded in direction but were much less in magnitude, a magnitude that was well within design standards." The expected solution, an inadequate structural roof system for the support of the HVAC, was not indicated by these measurements. Further investigation was indicated.

Tim McCarthy P.E.: "We performed calculations of the center of gravity of the 20 ton RTU in relation to the supporting structure and found it to be significantly off center, about 50%. We further discovered that the mounting system was significantly smaller than the HVAC base. The combination of these two geometries renders this particular RTU significantly vulnerable to laterally imposed loading conditions." Further investigation revealed that this store was within the current code prescribed hurricane region and likely subject to significant wind gusting.

"This appears to be a classic case study for the significant acceleration in recent codes of wind loads on roof mounted equipment," noted Tim McCarthy. "It's only been since 2000 that industry research and the building codes have directly addressed wind gust loads on roof mounted equipment. And the results have been surprising. Current wind codes mandate design loads frequently double or even triple what many current buildings have been designed for."

The report recommends a contemporary wind analysis be performed for this deflected RTU, that a proper load distributing curb be designed, and anchorages installed in accordance with the wind design.

Calvin Klein Store Roof Strengthening Engineering By TR2

Calvin Klein's Massachusetts store engineering is to be performed by TR2. TR2, The Roof Reinforcer, supplies comprehensive roof reinforcement engineering services nationwide.

This local Calvin Klein store is replacing and upgrading its HVAC system for increased customer and employee comfort. "We have handled a number of Calvin Klein's stores," said Cris, engineering manager for TR2. "We know what they need, who their national suppliers are, and there national standards, and are able to provide what they need economically and quickly." It is expected that the work will be completed within two weeks.

TR2, The Roof Reinforcer, headquartered in Oxnard California, is the nation's leading supplier of turnkey roof structure strengthening services.

Roof Strengthening Engineering Contracted With The Roof Reinforcer

TR2, The Roof Reinforcer, is being consulted by the national brand IZOD for engineering services on a local Massachusetts store. The local store is engaged in an HVAC replacement and improvement project. TR2 will provide roof strengthening analysis and recommendations. "This is the second one in the last two months," said Cris, eastern region engineering manager for TR2. "I guess they're happy! I know we turned the first store around in a week, and it was pretty tricky. That store was right on the coast in the heart of hurricane territory. The engineering was critical."
The Roof Reinforcer, TR2, specializes in roof structure engineering for retail stores nationwide.

Catherine's Delaware Selects The Roof Reinforcer, Inc.

The Roof Reinforcer, TR2, is pleased to announce an agreement to provide consulting engineering services for this prominent North Eastern retailer. "The snow load in this region is not insignificant," noted Cris, TR2 project manager for the region. "The Building Department is being cautious, which is prudent considering the life and safety issues involved in this type of engineering." The Roof Reinforcer, TR2, will provide a structural analysis of the ramifications of the proposed new roof mounted HVAC equipment that Catherine's is planning. "We will perform the analysis and generate the report," Cris continued. "If reinforcements are required, we will recommend them, and design them, if the client requests it. Our company is equipped to provide all services needed for retailers for their HVAC replacement projects, up to and including the installation of roof structure reinforcements."
The analysis is expected to be completed within one week.

Oklahoma City Retailer Consults The Roof Reinforcer, Inc. For Roof Structure Reinforcement

The summer may be winding down, but the heat is still high in this midwest city. The downtown retail center project manager says it's time to ungrade the HVAC equipment. "What's important to us and to our customers will be the speed with which the project can be done. We need the team to be in and out and done with little or no disruption to our clientele."

"We performed a preliminary review based upon available site photographs," said a TR2 project manager. "The roof structure consists of steel joists and metal deck, a very common roof system west of the Rockies. Our company has developed sophisticated modeling and analysis techniques and proceedures to analyze these elements swiftly and economically. We'll definitely be done ahead of their schedule."

TR2, The Roof Reinforcer, Inc., provides truss reinforcements, beam and girder reinforcements nationwide for every kind of manufactured steel and wood structural element.

National Pharmacy Contracts Roof Reinforcement Services With The Roof Reinforcer, TR2 Inc.

The Roof Reinforcer, TR2, Inc., is proud to announce the reception of a contract to provide consulting engineering services for this Southern California retail pharmacy. The store is planning a complete HVAC replacement project this Fall.
"There are seven roof mounted units dating back to 1991", said Cris, national accounts Project Manager for The Roof Reinforcer. "The Building Codes have evolved significantly since that time," noted Cris. "The Northridge earthquake was the catalyst for seismic engineering modifications. The last decade's hurricanes have been the catalyst for wind engineering modifications. We now know far more than we knew in 1991."
A site survey will be conducted within the next week. Engineering will be concluded a week later.
TR2, The Roof Reinforcer, is a leading supplier of turnkey roof structure support services for retailers nationwide.

Massachusetts Retailer Roof Reinforcement Engineering Completed

The Roof Reinforcer submitted engineering to support the planned replacement of roof mounted HVAC equipment at this location yesterday. "A quick turnaround was needed because of the time of year," noted Chris, regional project manager for The Roof Reinforcer, Inc., "so we fast tracked it, and completed it ahead of the customer's wish date." The roof structure was analyzed for dead loads, the significant local snow loads as well as the new HVAC load. "The customer is not using an adaptor curb, per the mall standards, which saved some weight. This likely saved the customer the need for any reinforcements," noted Cris. HVAC installation is scheduled for the second week of October. The Roof Reinforcer, Inc., TR2, is a leading supplier of turnkey roof reinforcement services nationwide.

Roof Reinforcement Services For Germantown, Maryland Retailer Contracted To TR2

The downtown Germantown location of this national retailer is upgrading its store wide HVAC system this autumn. The plan is to replace existing equipment, to redesign the system for the proper HVAC loads, and to install new HVACs in the most suitable locations. TR2 sent a surveyor team to the site this week, "to determine the existing structural framing plan, as well as capture certain as-built information the mechanical engineering team is needing," said Cris, project manger in charge of the Northeast. There is an urgency on this project, having commenced it after the summer's record heat wave and needing to complete it before the holiday season. "The schedule is not a surprise to us," said Cris, "most of our clients are retailers and restaurateurs, and tight schedules come with the territory. We will deliver on time, no question."

The Roof Reinforcer, Inc., TR2, is a leading supplier of turnkey roof reinforcement services nationwide.

Lane Bryant Store Utilizes The Roof Reinforcer

The Roof Reinforcer, TR2, is pleased to announce the completion of its analysis for this western Massachusetts retail store. "Between the snow load and the significant increase in equipment weight, there was a significant structural consideration," said Cris, project manager for TR2. The work is complete and the new equipment is scheduled for installation within two weeks.