RTU Holddown
RTU Holddown
(OP)
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?
Do you typically provide an RTU holddown or specify a required hold down force on your drawings?






RE: RTU Holddown
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.
RE: RTU Holddown
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)
RE: RTU Holddown
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.
RE: RTU Holddown
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
www.HowToEngineer.com
RE: RTU Holddown
RE: RTU Holddown
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.
RE: RTU Holddown
RE: RTU Holddown
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.
RE: RTU Holddown
RE: RTU Holddown
RE: RTU Holddown
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.
RE: RTU Holddown
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.
RE: RTU Holddown
RE: RTU Holddown
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)
RE: RTU Holddown
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
www.HowToEngineer.com