Metal Deck Diaphragm
Metal Deck Diaphragm
(OP)
Folks,
Please excuse my ignorance on the behavior of metal deck diaphragms. I have a few basic things that I am trying to get my head around.
Assume that I have a 1.5" metal deck spanning 5'-0" o.c between roof joists. The building is 100' wide and 50' long. There are shear walls at each end (one at 0' and one at 100'). The diaphragm spans 100' between these shear walls. The shear is perpendicular to the flutes of the deck.
For the deck to behave as a deep beam, sufficient side lap connectors must be provided. Are these side lap fasteners designed based on shear flow with highest number needed at mid depth of diaphragm? I am thinking of 3' wide pieces of deck connected by side lap fasteners that help to make it a 50' deep beam. I have searched online quite a bit but not found any answers that explain it.
The interior joist connections are required mainly to transmit uplift and no wind shear. However, the end connection to the shear walls will need to transfer a lot of shear and uplift.
The deck to chord connection will need to transfer diaphragm tension/compression to the chord member. This will be attained through side lap connections in the above case. Do you vary the number of side lap connections based on how much the tension/compression drops away near the supports?
I may have more questions based on your answers.
Please excuse my ignorance on the behavior of metal deck diaphragms. I have a few basic things that I am trying to get my head around.
Assume that I have a 1.5" metal deck spanning 5'-0" o.c between roof joists. The building is 100' wide and 50' long. There are shear walls at each end (one at 0' and one at 100'). The diaphragm spans 100' between these shear walls. The shear is perpendicular to the flutes of the deck.
For the deck to behave as a deep beam, sufficient side lap connectors must be provided. Are these side lap fasteners designed based on shear flow with highest number needed at mid depth of diaphragm? I am thinking of 3' wide pieces of deck connected by side lap fasteners that help to make it a 50' deep beam. I have searched online quite a bit but not found any answers that explain it.
The interior joist connections are required mainly to transmit uplift and no wind shear. However, the end connection to the shear walls will need to transfer a lot of shear and uplift.
The deck to chord connection will need to transfer diaphragm tension/compression to the chord member. This will be attained through side lap connections in the above case. Do you vary the number of side lap connections based on how much the tension/compression drops away near the supports?
I may have more questions based on your answers.






RE: Metal Deck Diaphragm
For your 100 ft wide building (wind perpendicular to the 100 ft length) your diaphragm spans 100 feet as you say. The wind is usually calculated as a lbs/ft lateral force along the diaphragm.
With that lateral force, w, you would then determine a shear envelope across the diaphragm (bow-tie diagram) with shear at a maximum along the 0 and 100 ft. locations and shear at zero in the middle of the diaphragm.
The Steel Deck Institute has analyzed and tested decks to take lateral shear in the plane of the deck. This research is pretty well described in the Steel Deck Institutes Diaphragm Design manual:
(found here )
This provides values of shear capacity for decks based on various parameters:
1. Deck depth (i.e. 1 1/2")
2. Deck thickness/gage
3. Joist spacing
4. Support fastener type and spacing
5. Side lap fastener type and spacing.
The SDI also provides the stiffness of your deck (Shear stiffness G')
Your design deck shear at each end of the diaphram is (w x L) / (2B) where w is your wind force on the 100 ft length of deck in plf, L is the width of the diaphragm (100 ft) and B is the depth of the diaphragm (50 ft).
This value of shear is in plf and only occurs along the far ends of the diaphragm - again - max at ends and zero at center.
For practical purposes, it is usually a good idea to keep the number of deck types to a minimum in any particular project. We sometimes will have a deck type (fastening spacing, sidelap spacing) for the first end bays (where shear is maximum) and then use a lighter design in the center bays of the building where shear is lower.
Having a different design for every bay can be confusing and get installed incorrectly in the field.
For uplift, I've seen some literature dealing with the combined planar shear and uplift but I'm not sure how many engineers worry about the combined effect. I'd be interested in other's take on that subject.
RE: Metal Deck Diaphragm
The one thing I am still not understanding properly is how/why the side lap spacing would affect the shear strength of the diaphragm. Is it some sort of a buckling mode that is controlling the strength based on fastener spacing?
RE: Metal Deck Diaphragm
RE: Metal Deck Diaphragm
If you get a copy of the SDI diaphragm manual, you will see the testing and theory of these deck "plates" under shear. Keep in mind that shear is not really directional (i.e. shear takes a rectagular shape and changes it into a parallelogram where all sides of the rectangle change their angular orientation).
RE: Metal Deck Diaphragm
To consider it just a beam, the web continuity must be implemented.
RE: Metal Deck Diaphragm
RE: Metal Deck Diaphragm
Hope that is right, and helps... :)
RE: Metal Deck Diaphragm
It is perfectly permissible to not even have sidelap fasteners, and the more fasteners to the supporting structure, the stronger the diaphragm gets.
RE: Metal Deck Diaphragm
The end reaction in a rectangular roof is wL/2. This is presumed to be distributed uniformly across width B so that the shear normal to the flutes is wL/2B as JAE stated. It is not a parabolic distribution as you would find in a rectangular beam.
The shear parallel to the flutes is also wL/2B at the shearwall tapering down to zero at midspan. At each seam, this is resisted by a combination of welds to the joist chords and sidelaps. The perimeter framing members span L and have a compression or tension of wL^2/8B at midspan of the diaphragm. This force is built up by attachment to the deck between shearwall and midspan which imparts an average shear of wL/4B to the perimeter member and wL/4B * L/2 = wL^2/8B.
If the sidelaps are omitted, the only fastenings acting to carry shear parallel to the seam would be the attachment to the chords which are usually either puddle welds or screws. I do not believe it is generally acceptable to omit sidelap connections except perhaps in very small roofs.
BA
RE: Metal Deck Diaphragm
.............the only fasteners acting to carry shear parallel to the seam....
I presume you are referring to the case of shear parallel to L. Right?
RE: Metal Deck Diaphragm
BA
RE: Metal Deck Diaphragm
Are there any research documents or testing done on double decks? What kind of attachments are required?
For example, if a 3" deck does not have adequate capacity based on a certain fastening pattern, then can doubling up the deck and keeping the same fastener pattern double the capacity or should the fastening spacing be also halved?
RE: Metal Deck Diaphragm
RE: Metal Deck Diaphragm
RE: Metal Deck Diaphragm
RE: Metal Deck Diaphragm
Puddle welds at supports and pins at sidelaps.
Bare metal roof deck. No concrete.
RE: Metal Deck Diaphragm
I have used two thicknesses of deck to adress snow loads, but never uplift or shear.
RE: Metal Deck Diaphragm
Slick- if I remember right you are in S FL? Which would explain a lot.
The issue with doubling up deck is that the shear and tension in the puddle weld is not decreased, so you'd have to know somehow which failure mechanism controls, deck or fastener.
RE: Metal Deck Diaphragm
To add to a2mfk's post above, you also need to keep in mind the practicality of putting two fluted pieces of steel one on top of the other and getting them to fully nest or seat properly to make a solid connection. I personally don't think it's viable.
RE: Metal Deck Diaphragm
Another option is to ignore the metal diaphragm and use a horizontal roof truss made out of 3/8" thick plate. The 3/8" plate can rest on top of the joists and can easily fit in-between the joists TC and the decking. However, you can't go much thicker than 3/8". The 3/8" straps act as tension only members and you must include the drag forces in the joist TC.
This is covered in the Fisher book Designing with Steel Joits, Joist Girders and Steel Deck. I just attended a seminar with Fisher where he covered the detailing and design procedure for this case.
Are you requiring a single 18ga or 16ga deck? or would it be a double 18ga or 16ga deck? I can't say that I have ever doubled up the deck to gain diaphragm strength. If you are looking at a double 22ga vs a sing 16 ga, I would just use the single 16ga decking.