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?