Adaptor Curbs and the Balance of Forces

A Situation
“Think teeter totter”, the final report said. “It’s not hard to imagine a sudden wind gust upsetting the fine balance of this HVAC curb system.”

The engineer was called in because the left end of this RTU HVAC was discovered to be 4” lower than the right end, 2 months after installation.
Was the roof structure OK? Was the roof sagging?

A site inspection, measurements and calculations determined that the structure was indeed OK, but the curb connections were deficient.

An adaptor curb will elevate and often shift the Center of Gravity.

Three rules of thumb to guide you to structural stability.
1. Vertical Changes. The ratio of the change in center of gravity heights is an indicator of the ratio of the increase in connection strength needed. If it used to be 3’ from roof to HVAC Center of Gravity, (say 12” curb plus 24” to HVAC c.o.g.) and now it is 4’ (say 12” curb plus 12” adaptor curb plus 24” to HVAC c.o.g.), the increase in connection requirements is around 4/3. A 33% stronger connection is in order.

2. Horizontal Changes. The ratio of the change in distances of the HVAC center of gravity from the edge of the existing curb is an indicator of the ratio of the increase in connection strength needed. If it used to be 2’ from the curb edge to the COG for a 4’ wide curb and now it is 1’, the increase in connection strength requirements is around 2/1. A 100% stronger connection is in order.

3. Time Changes / Code Changes. The ratio of the GC factor from the older code to the current code is another indicator of the ratio of the increase in connection strength needed. Codes before 2006 had a GC generally around 1.0 whereas codes since 2006 generally have a GC around 1.9. (In Florida it is 3.1). The increase in connection strength requirement is around 1.9/1. A 90% stronger connection is in order.

And yes, these percentages are cumulative. The numbers above translate to a connection that is 33% + 100% + 90% stronger. In other words, what use to be screws at 24” o.c. will now be screws at 4” o.c.

But this is the job of your structural engineer. It should be a routine part of his design package for your team. If it isn’t, ask him “Where’s the other half of your package?”

Pop Quiz. Using these Rules of Thumb, Vertical Change, Horizontal Change and Time/Code Change, what is the effect on the connections for the adaptor curb configurations below?

TR2 Completes AMC Theater Structural Roof Survey

A red-eye from Los Angeles to Atlanta got me to the airport at 6:00 am. A hotel catnap and my local surveyor's car got us to the AMC theater 15 minutes ahead of schedule. I was looking forward to this.

The projection room was vacant, but the theaters were occupied with a healthy weekday afternoon crowd, spending time together, enjoying the air conditioning and watching the movies. The old film reel tables were there, as well as the conveyors for carrying the film reel from theater 3 to theater 9 for the next showing in 20 minutes. The serpentine corridor-like room weaving between the back-to-back theaters, once traversed back and forth, back and forth, by a couple of projectionists with eyes glued - one to the movie schedule and one to the clock, is now stacked with satellite-connected computer servers and digital projectors.

What a "cool" room (not as in AC cooled) these projectionists had as their secret domain! There are a number of current articles lauding these technicians and lamenting their passing. For more from a projectionist, see "The Last of the Real Live Projectionists". For a slide show of images from projector rooms, try this one from NPR (and don't miss the comments from authentic projectionists). For observations on the current transition from celluloid to digital, see "Where Have All The Projectionists Gone?" And lastly, for a nerdy blog among projectionists that might awaken your inner projectionist, try this. As I write this, I can almost imagine their happy ghosts lingering, longingly, lovingly, in these back rooms of theaters, their secret place, facilitating the entertainment of millions. I stepped closer to peek through the little glass pane through which the projector was throwing the current film onto the theater screen. Quickly stepping back I said, "Digital or film, those projector bulbs still get hot!" "In the Summer, it gets unbearable in here," the Facilities Manager said. "It's not good for the equipment and it's not good for the service technicians. That's why we're installing the new HVACs. Come on, let me show you where."

Through a nondescript door we were now in the backside, underbelly shell of the mall. Down a flight of stairs and then another flight. Back out into the mall space filled with customers. The original mall was built in 1961 with a major expansion in 1991 including two additional wings. The logistics of interlinking the service corridors were obviously challenged by the expansion. "Sorry for the long walk and climb," the manager offered. After the 5 hour plane ride, this was good exercise. "Not a problem," I said. Through another door and up a flight of stairs. Down along a winding corridor with painted doors every 50 or 75 feet. "These doors open into the backside of the mall stores." Up two more flights of stairs and though a door. The door opened onto a brilliant sunshiny day, made almost blinding by the combined facts of the exit from the long walk in darkened corridors and the pure white roofing material recently installed. I squinted through my sunglasses and we all took a moment to let our eyes adjust.

"Here, and....." he walked over approximately 70 feet, "Here. This is where where we want to place the new RTUs." I nodded, scanned the accumulation of other equipment scattered across the roof, and asked, "Do you have flexibility to adjust the locations plus or minus 10 feet if that suits the structure better?" "Sure, no problem." "Alright", I said, "I think we'll be able to work out a solution for you."

Van Heusen Outlet, Orlando, Roof Structure Submittal Completed

The Roof Reinforce, TR2 Inc, completed its submittal for the building deparment for this national retailer. "The client is upgrading the HVAC equipment mounted on the roof," said Tim McCarthy P.E., chief engineer for TR2. It's a routine project for us. We were able to turn it around rapidly for the client."
TR2 supplies structural services for Retailers and HVAC contractors nationwide.

Sears, Texas, Roof Structure Engineering Report Completed

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.

Banana Republic, Wichita, OK, Roof Structure Reinforcements Installed On Schedule

TR2, The Roof Reinforcer, is please to announce the completion of design and installation services for this Kansas area Retail store. "The client upgraded its roof mounted HVAC equipment," said Cris, Midwest project manager for TR2. "The schedule was very tight and reinforcements were determined by analysis to be required. We researched four different alternatives, but in the end, the client's cost benefit analysis opted for the installation of HVAC supports." TR2 crews arrived after business hours and worked through the night to complete the installation of speciality HVAC bracing prior to the installation contractor's crane arrival in the morning. "The schedule and the conditions both pointed to a customized specialty installation," Cris continued. "The ceiling permitted only a 2‘ x 2’ access, so we had to customize our standard HVAC support kit to accommodate this limited access and still maintain the aggressive schedule." The HVACs were installed on schedule.

TR2, The Roof Reinforcer, is the nation's leading supplier of turnkey HVAC roof reinforcement support services.

Wrentham, MA, Mall Structural Services Provided

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.

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.