Metal Deck Diaphragm 1

[slickdeals]

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.

 

[JAE]

slickdeals,

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.

 

[slickdeals]

JAE...Thanks for the detailed post.

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?

 

[slickdeals]

If side lap fasteners are omitted completely in my case above, will the diaphragm still be 50' deep or only as deep as the width of the deck (say 36")?

 

[JAE]

Sidelap fasteners help to improve deck shear strength. They do not affect your global diaphragm width/depth.

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).

 

[ishvaaag]

If you omit the sidelaps you have some kind of notional Vierendeel with the chords.

To consider it just a beam, the web continuity must be implemented.

 

[JAE]

Without sidelaps you only have diminished shear strength and diminished shear stiffness. That is the only practical difference relative to basic design of a flexible diaphragm.

 

[a2mfk]

It'd be the difference between a wood framed diaphragm with blocked and nailed edges vs unblocked edges. All shear forces for a steel diaphragm with no sidelaps must be transferred through the joist fasteners, usually puddle welds. Also with no sidelaps, you obviously are going to have more deflection/diaph. movement, because of the lesser stiffness of the entire system like JAE said. You can probably imagine the difference when the shear is parallel to the deck flutes, with no sidelap fasteners, you can see the shear flow is dependent only on the deck-fastener-joist interaction....

Hope that is right, and helps...

 

[spats]

You really need to look at the SDI Diaphragm Design Manual. Your assumption about sidelap fasteners doing all the work is incorrect. Page 2-4 of the Manual shows FBDs of the deck, and how all fasteners participate in resisting the shear.

It is perfectly permissible to not even have sidelap fasteners, and the more fasteners to the supporting structure, the stronger the diaphragm gets.

 

[BAretired]

I do not have the SDI Diaphragm Design Manual, so I may be a little off in my explanation.

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

 

[slickdeals]

@BA:
.............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?

 

[BAretired]

Yup!

BA

 

[slickdeals]

A sidebar question:
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?

 

[slickdeals]

Question above is regarding shear strength of metal deck diaphragm.

 

[a2mfk]

slick- is this for a new project or renovation? You've played with all the fastener options and its still not working (puddle welds...)? Is lightweight concrete an option?

 

[ash060]

I guess you could do the combined capacity, but the connector may control design. You may exceed the weld strength, so you would not get double the capacity. I would check the limit states for the connection using the AISI code. For shear buckling I would just use double the capacity, but this would probably not be your control limit state. There would be an extra check because of the sheet-to-sheet connection as well.

 

[slickdeals]

No, this is not for a current project. This has been discussed in our office in the past. Sometimes due to high shear/uplift values in corner zones of a roof, doubling up metal deck instead of using a much heavier gage was discussed. You may need a 20 ga deck in a majority of areas, but need a 18 or 16 ga at certain regions.

Puddle welds at supports and pins at sidelaps.

Bare metal roof deck. No concrete.

 

[OHIOMatt]

In my experience, the easisest way to adress the situation described was to reduce the joist spacing. It adds a little to the joist cost. In high shear or uplift regions, I have gone to joist spacings of 3' or so. This allows for twice the number of welds. In addition, you get a high capacity deck in the areas were it is needed.

I have used two thicknesses of deck to adress snow loads, but never uplift or shear.

 

[a2mfk]

My first boss did as Ohiomatt said on all jobs as a standard.

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.

 

[justsometallguy]

Slick-

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.