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.