steel deck diaphragms 2

[StructFton]

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?

 

[msquared48]

Use X rod bracing in the plane of the roof diaphragm.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[frv]

M^2.. why?

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?

 

[StructFton]

thanks frv, what do you mean by collector? The questions I had is what do I need to check for the diaphragm in the 'short direction'

I dont think roof bracing is required either, I am just trying to figure out what to check/design in the short direction

thanks

 

[JAE]

StructFton,

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.

 

[StructFton]

ok, thanks JAE

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!

 

[dik]

JAE... ya did good! Missed out on providing him the shear/joist seat connections and the attachment of the deck to the beams parallel to the span <G>. Also the manner of putting the roof drains at the high points...

Dik

 

[msquared48]

StructFton:

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

 

[frv]

M^2.. That is true, but I've never really heard of what the PEMB guys use referred to as "deck".. I've usually heard of it as "light gage roof"

 

[humanengr]

Be sure to address all items on your "check list".
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.