RTU Forces and Diaphragm Forces

[structeng2]

Hello,

I have a question about rooftop unit nonstructural component forces and how they relate to building diaphragm forces.

Say I have an RTU that weighs 1000lb. It is spring isolated, resulting in an Fp ~= 2500lbs. I design the connections to the roof utilizing the Fp force (ASCE7 Ch. 13), but when I include the RTU in my building diaphragm design, do I lump the 1000lbs into the overall diaphragm weight (and then calculate diaph forces per Ch 12), or do I use the 2500lbs directly as an addition to the calculated diaphragm forces?

Thanks.

 

[driftLimiter]

From what I see, for unit's of that size the diaphragm is designed as normal (udl). Generally I see some additional psf in the roof loading for "misc" that could be shown to include the weight of RTU. I have ever seen anyone load the diaphragm with a concentrated load at RTU locations. That being said another thing to be concerned with is how the opening effects diaphragm load path.

 

[structeng2]

Thanks, driftlimiter.

I guess the question is more conceptual than the actual weight of the unit.

Basically, you have an Fp force that is 2x the weight of the unit. So if you just lump the weight of the unit into the diaphragm mass, are you underestimating the true load that the diaphragm needs to resist? Or... after the Fp force gets into the diaphragm, it just becomes part of the mass (at the true weight of the unit).

 

[phamENG]

The connection needs to be designed for F[sub]P[/sub] - including any distribution elements to develop the load in the diaphragm and to handle any shear increases around openings. But after that, the energy dissipation usually lets you make a qualitative assumption that the mass just gets lumped in to the rest of the LFRS. That works fine on a large roof where that 1,000lb unit is a very small portion of the overall load. If this is a 1,000sf building and that 1,000lb starts to make up a considerable portion of the mass, you may want to handle it a little more specifically.

 

[structeng2]

Thanks, phamENG. This is how I understood it but wanted to make sure I wasn't missing something.

The building is a single story, ~20,000sf office/lab and I have 2x 9000lb units on the roof. The units result in about a ~6% increase in the overall diaphragm weight. I've added columns below the units to support the added vertical load and added nailing/strapping to develop the load into the diaphragm using the Fp force.

The concern was if I was required to use the Fp force directly added to the diaphragm shear force at the shear walls - it would have been a significant increase, and greatly exceeded the IEBC 10% rule. But it sounds like this is not the case.

Thank you all for the input.

 

[phamENG]

if I was required to use the Fp force directly added to the diaphragm shear force at the shear walls - it would have been a significant increase

starts to make up a considerable portion of the mass, you may want to handle it a little more specifically.

I think you need to follow the load path more discreetly, then. The whole point of "qualitatively" assuming it doesn't have a significant impact is because, if you run the numbers, it doesn't. If you run the numbers and quantitatively show that it is significant, you cannot "engineering judgement" that quantitative assessment away.

 

[structeng2]

I guess the way I think about it is like this...

Case 1:
The total diaphragm weight is around 300k. If you do a ch. 12 building analysis, with a Cs = 0.33, your total diaphragm force is 100k. If you add your Fp forces directly to this, (about 20k each unit), this is nearly a 50% increase in diaphragm force.

Case 2:
The total diaphragm weight with the units added is 300 + 2*9 = 318k. Doing the same calc with Cs = 0.33, the diaphragm force is now 105k. This is only a 5% increase.

Which is correct?

 

[phamENG]

So you may not need to do anything to the diaphragm writ large, but you have to look at the whole load path and see how much it increases. If it's sitting on top of a previously lightly loaded shear wall, that shear wall may now require reinforcement.

 

[structeng2]

right, and that is sort of where I get hung up. At what point does the Fp force end, and the 'lumped mass' begin?

Obviously, I design the anchors and transfer of load into the plywood using the Fp force. And the units are not directly over an existing wall, so I would imagine I can still assume the lump sum case and still meet the 10% rule. (Case 2 in my previous post)

 

[driftLimiter]

This isn't a smoking gun but the commentary does mention this in ASCE7-16 C13.4 it is saying...

The effective seismic weight used in design of the seismic
force-resisting system must include the weight of supported
components. To satisfy the load path requirements of this section,
localized component demand must also be considered. This
satisfaction may be accomplished by checking the capacity of
the first structural element in the load path (for example, a floor
beam directly under a component) for combined dead, live,
operating, and seismic loads, using the horizontal and vertical
loads from Section 13.3.1 for the seismic demand, and repeating
this procedure for each structural element or connection in the
load path until the load case, including horizontal and vertical
loads from Section 13.3.1, no longer governs design of the
element. The load path includes housekeeping slabs and curbs,
which must be adequately reinforced and positively fastened to
the supporting structure.

Would want to look at the shear in the diaphragm at the location of the units and at the location of the supports and see if it really pans out as increased load. I think if your going to go down this road, you want to be clear what loads were used to come up with the Diaphragm Weight, and identify any mass there that could reasonably attributed to these units. If you do this then at least when you apply the Fp loads your being fair to the diaphragm