RTU Holddown

[RFreund]

How do you typically handle RTU anchorage for uplift/overturning? I normally there is some sort of support for the curb but what bout the actual attachment of the RTU to the curb or curb to substrate?
Do you typically provide an RTU holddown or specify a required hold down force on your drawings?

EIT

http://www.HowToEngineer.com

 

[SteelPE]

I have a hard enough time trying to get the building design complete. It's hare enough to get the weights of the units out of the contractors where they usually give us the weights 6 months after the steel is fabricated. Once I give them a surface to attach their RTU I generally leave the attachment to the RTU contractor specifier.

I would think, depending on the size of the unit and the location of the building, that the weight of the unit would be sufficient to resist the forces from the wind..... but I may be wrong.

 

[EngineeringEric]

I agree with SteelPE, never enough budget/time and the unit changes five times before one is installed. I believe there are code standards depending on seismic zone for loads and often times i leave it to the GC/sub/supplier to specify based on the loading i lay out in general notes...

So my guess would be it is typically a combination of self-weight, standard straps, and fasteners in the curb unit itself... (crossing my fingers)

 

[a2mfk]

In Florida even residential AC compressors on the ground are required to be attached to a weighted pad.

We have typical details using Simpson flat straps (from a coil) or similar material with screws into the curb. You get in the 110+ mph range (allowable), in my experience, most RTUs will need to be strapped. There is a lot of empty space in those units and they keep making them lighter, so the overturning moment can be substantial.

I learned to make sure this was on our drawings a long time ago from my boss, who learned the hard way. They had a project in the Caribbean where a hurricane took off several units, and water damage to the retail box store was extensive. They were involved in a lawsuit where the mechanical engineer, contractor and my boss all did a lot of finger-pointing. I used to seem some details (usually not very good) on mechanical drawings until I think they started to defer to the structural engineer.

 

[RFreund]

Ha, kinda interesting responses in that in my experiences, the connections are handled similar to how SteelPE and Eric describe and my guess is that is because we are in non-hurricane territory. However this job happens to be in a high wind zone and thus my curiosity (I probably should have prefaced the question with this information but I was curious about other situations as well). In most cases outside of a high wind region, uplift and overtuning are probably not so much on an issue.

A2mfk - So you show anchorage from curb to structure? The connection of the RTU to the curb is typically submitted by the contractor then?

Thanks all !

EIT

http://www.HowToEngineer.com

 

[XR250]

Seems like a job for the ME, not the SE.

 

[SteelPE]

Well, my buildings are typically in hurricane zones but not as bad as they are in Florida..... The wind speeds I typically work with max out at 120 mph (in accordance is ASCE7-05). The units I work with typically weigh max out at 3,000 lbs. I like to put a C6 on top of the deck toed down for curb support but some of my clients hate that and specifically request frames be placed under the deck. The C6 is a nice detail which can be welded to the structure below through the deck. The angle frames are not as easy to install and don't provide as good of an attachment as the channels.

I'm not saying my method is correct.... but I have a hard time keep up with the code changes to be too worried about RTU attachment.

 

[Archie264]

Fyi, if you do calculate the wind forces be aware that ASCE 7-05 requires almost doubling the calculated wind on rooftop structures in certain circumstances. See Section 6.5.15.1 for further info. I don't know how ASCE 7-10 handles it.

 

[Triangled]

ASCE 7-05 practically doubled the wind loads on RTUs by adding GCr = 1.9 for horizontal loads as Archie264 pointed out above, and adds in the commentary "The design should design for uplift" but nothing else on uplift.
ASCE 7-10 retains the GCf = 1.9 for horizontal loads and adds for uplift loads GCf = 1.5.
Florida, at least portions of it if not all, is using GCf = 3.1
in most cases for me, these are significant forces.

 

[Triangled]

GC_r in the above post

 

[Archie264]

Triangled, thanks for the update.

 

[Once20036]

Like SteelPE, I typically use typical details to show a frame made up of angles or channel below the RTUs. Lots of notes like, "Coordinate with MEP".
But I`m not sure that gets to the heart of the question. I have no idea how the curbs are attached to the frame or how the units are attached to the curb. There are lots of joints in the system that should be looked at, and I've never known anybody to look at them.

I`ve wondered about this in the past as it seems like a pretty (dark) gray area.
Once I specifed all of these attachments and dealt with several angry calls from the contractor about how what I spec'd was so far above and beyond what he's always installed. I`m ended up using his details rather than mine.

 

[Archie264]

Once,

Once (heh, heh) you multiply the wind force by the 1.9 and 1.5 factors as listed above you might be able to show the contractor that what "he's always installed" isn't technically adequate. Bear in mind that this factor is in addition to the other load factors. Or multiplied by those factors, rather. It can add up.

 

[Triangled]

With the elevated emphasis in asce 7-05 and 7-10, it seems to be plainly a structural item. I don't thnk I'd expect my mechanical friends to calc those forces and design those attachments. And I don't really want to calc their outside air requirements.

 

[Jerehmy]

Oh the horror of retrofitting 100+ RTU's for seismic and wind restraint .

Don't forget 2*Fp for components on vibration isolators.(seismic)

If you're design RTU anchorage for structures taller than 60ft, the Cf factor can get quite large.(wind)

I swear there's a 1.5 times overturning factor depending on the unit weight but I can't seem to find it in the code...(don't remember if its for seismic or wind either)

 

[RFreund]

@Triangled -
what sort of details do you typically show?
How far do you take the design (provide a frame, provide holddowns to curb, curb to rtu)?

Thanks

EIT

http://www.HowToEngineer.com