Strength of Existing Metal Roof Deck
Strength of Existing Metal Roof Deck
(OP)
As part of a project to support a 4,000+ lb mechanical unit on an existing building, I will need to check the metal roof deck.
The unit sits on 1 1/2" curbs that will be attached to the existing wide-rib 1 1/2" x 20GA metal deck. In turn, the curb load will be supported by new beams.
My question is: how can I check that the metal deck won't crush under the curb loading running perpendicular to the flutes (275 lb/ft in this case)?
I am a recent graduate looking to gain experience from this group.
The unit sits on 1 1/2" curbs that will be attached to the existing wide-rib 1 1/2" x 20GA metal deck. In turn, the curb load will be supported by new beams.
My question is: how can I check that the metal deck won't crush under the curb loading running perpendicular to the flutes (275 lb/ft in this case)?
I am a recent graduate looking to gain experience from this group.






RE: Strength of Existing Metal Roof Deck
Provide additional members underneath the frame and a curb to transmit the loads to those members. Either the manufacturer of the unit or a curb company can design a curb to carry the weight of the equipment.
RE: Strength of Existing Metal Roof Deck
RE: Strength of Existing Metal Roof Deck
I have even seen new plates/angles fabricated that actually matched the deck profile and installed. Seems like a pain to me!
RE: Strength of Existing Metal Roof Deck
The new mechanical unit will be supported by two new W16 beams, installed below the metal roof deck.
The mechanical unit will sit on top of a curb that will transfer the load to the new beams.
The issue is the connection of the curb to the metal deck and if the deck can transfer the load effectively to the beams below without crushing. Or will I need to specify openings in the deck to attach the curb directly to the beams?
I appreciate the advice.
RE: Strength of Existing Metal Roof Deck
It seems like the overall opinion is that the deck is not strong enough to transfer the load effectively. I will design stand-offs to penetrate the deck and attach to the beams.
RE: Strength of Existing Metal Roof Deck
Structural Design Engineer
New York, NY
RE: Strength of Existing Metal Roof Deck
RE: Strength of Existing Metal Roof Deck
RE: Strength of Existing Metal Roof Deck
flash3780: AISI actually has a direct analysis method that gets better strength predictions than using FEA alone. I don't think it's really worth the time to do the analysis though for one piece of equipment.
Structural Design Engineer
New York, NY
RE: Strength of Existing Metal Roof Deck
Not difficult at all if you can get at it. Access to the underside of the deck is often restricted.
RE: Strength of Existing Metal Roof Deck
Guys got you covered on this, bypass the deck to get to the WF beams. You also have to design for overturning and uplift due to wind/seismic, and if you are in snow country this will cause drifting around the unit.
RE: Strength of Existing Metal Roof Deck
RE: Strength of Existing Metal Roof Deck
RE: Strength of Existing Metal Roof Deck
RE: Strength of Existing Metal Roof Deck
RE: Strength of Existing Metal Roof Deck
Hokie66- if you graduated in '66, then all these other hokies owe you an Aussie nickel every time they post. Official ACC message board rules.
RE: Strength of Existing Metal Roof Deck
Sounds fair to me, a2mfk.
RE: Strength of Existing Metal Roof Deck
Loading perpendicular to the flutes is not as much of an issue as loading parallel to the flutes.
Support from bottom as others have noted. If you are using a pre-fab curb section, most will span the deck flutes without issue and the load distribution is such that the deck corrugations are not deformed much, if at all.
As hokie66 mentioned, flute packing can be done just make sure you don't create a condensate dam in the process.
RE: Strength of Existing Metal Roof Deck
Sometimes a roof top unit can make a metal deck vibrate like a drum and sound accordingly.
Bob G.
RE: Strength of Existing Metal Roof Deck