steel deck diaphragms
steel deck diaphragms
(OP)
I am getting back into designing steel buildings...I understand the methodology/theory behing the diaphragm transfering the external wind/EQ loads out to the bracing through, mainly, shear.
I seem to have forgotten how the roof diaphragm works when wind/EQ is acting in direction 2 (ie the short direction) shown in my sketch. The deck would be orientated spanning across the OWSJ, thus when wind acts on the short side of the bldg, there would be less in plane stiffness from the deck. I realize the deck will be fastened down (through some means) to the framing.
My question: how do you check the diaphragm in for wind/EQ in the short direction?
I seem to have forgotten how the roof diaphragm works when wind/EQ is acting in direction 2 (ie the short direction) shown in my sketch. The deck would be orientated spanning across the OWSJ, thus when wind acts on the short side of the bldg, there would be less in plane stiffness from the deck. I realize the deck will be fastened down (through some means) to the framing.
My question: how do you check the diaphragm in for wind/EQ in the short direction?






RE: steel deck diaphragms
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: steel deck diaphragms
Metal roof deck is perfectly capable of functioning as a diaphragm in either direction. You'll just need to design the collector to drag your forces into the bracing from the end bays. Am I missing something?
RE: steel deck diaphragms
I dont think roof bracing is required either, I am just trying to figure out what to check/design in the short direction
thanks
RE: steel deck diaphragms
What frv is indicating is that metal deck diaphragm shear capacity is independent of the wind direction (across or with the flutes).
Shear is a multi-directional effect (remember your mechanics of materials classes with the little square turning into a parallelogram).
The Steel Deck Institute in the US (and I'm sure other agencies in other countries) provides diaphragm capacity tables for various roof purlin/joist spacings as well as various types and frequency of fasteners and side laps connections.
The deck "spans" from side wall to side wall where the brace lines are in both directions in the same manner - just with different force results.
The collector is a roof element/beam that "collects" the diaphragm shear from the metal deck along its depth and delivers it into a discreet brace location.
Example - your diaphragm spans across a 100 ft. wide building with the building being 150 feet deep (say three bays of 50 feet) in the direction parallel to the wind.
You have on one side of the building a single bay X brace in one of the three 50 ft. bays. The collector would typically be the roof beam running over those three bays, collecting your deck shear and delivering it to the single X brace. It takes roof bending as well as axial wind load.
RE: steel deck diaphragms
I have tried to forget about the parallelograms, at my own peril, apparently.
I had been thinking of the deck being more rigid in the spanning direction; however, this is for axial load only. when in shear, it makes sense that stiffness is independent of the loading direction.
thanks to all...happy deck designing!
RE: steel deck diaphragms
Dik
RE: steel deck diaphragms
The reason I suggested X-Bracing is that in PEMB's (steel buildings) the roof diaphragm is frequently 28 or 29 gage metal deck, way too thin for any structural diaphragm action.
If the decking you are talking about is 24 gage or heavier, then X-bracing need not be used.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: steel deck diaphragms
RE: steel deck diaphragms
For example, the wind load acts on the short walls. Before your roof diaph. works,
you need to transfer the wind from the walls into the roof diaph. Make sure
you have adequate load path.
Just thinking out load, your bldg. may require torsional wind loads.
This will require the deck to work in both directions simultaneously. Similarly,u
wind load could act on blgd. at an angle. Check your wind loading (ASCE 7?). Like I said, thinking out load. Just make sure you've covered all load cases (wind or seismic)
and you have adequate load path. The collector won't help you if the load can't
get there.
Good luck.