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Composite Floor conventions

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I've been out of the office for awhile, so this is an old issue, but I
figure some may be interested in the answer.

According to the Specification the minimum stud spacing is 6 stud
diameters (which for a 3/4" diam. stud gives 4.5" minimum spacing). The
12" spacing value is therefore not a Code requirement, however it is a
practical limit for beams since the spacing of ribs in the metal deck is
12" o.c. for most decks. If you specify studs with a spacing closer than
12" for beams where the deck is perpendicular to the beam, the
contractor will have to cut (or smash) the deck in order to accommodate
the closer spacing of studs. Note that if the deck is skewed with
respect to the beam the spacing between ribs as they cross the beam
becomes even greater than 12" o.c., so this also needs to be considered.
Some of the 1.5" decks - for example VERCO B Formlok and VULCRAFT 1.5VL
- have 6" rib spacing, so for those decks you could specify a 6" stud
spacing if necessary (when the deck is perpendicular), although those
decks aren't commonly used since they generally require closer beam
spacing. For girders, since the deck is parallel, the rib spacing
doesn't impact the placement of the studs so the studs can be placed as
close as the Code permits, 6 stud diameters o.c.

Some engineers specify, as a rule of thumb, a maximum spacing of 12"
o.c. In my opinion this is unnecessary (and somewhat uneconomical), but
it does eliminate the need for puddle welds. The code requires that,
when the deck is perpendicular to the beam, the deck be attached to the
beam with either puddle welds or studs at a maximum spacing of 18" o.c.;
since the ribs are 12" o.c., this effectively requires that the deck be
attached to the beam at every 'trough'. It also probably simplifies
inspection of stud placement to require a stud in every 'trough'.

Also, as has been indicated, the RAM Structural System calls out the
total number of studs required on the beam (and it also considers the
rib spacing as discussed above).

Allen Adams, S.E.
Chief Structural Engineer
RAM International
(760) 431-3610 x113

From: "Jordan Truesdell, PE" <seaint1(--nospam--at)>
To:  seaint(--nospam--at)
Subject: Re: Composite Floor conventions

Thanks for the replies - it confirmed what I thought I knew.  The 12" 
was probably just a reference by the "professor" (actually a working SE 
who taught one or two classes in the grad program) and his personal rule

of thumb.  I'm still not sure if it was a max (to avoid having to puddle

weld the deck in addition to studding it) or a min, but it sounds like 
it really doesn't matter.

I read through the 2005 spec., and didn't see the 25% limit, which is 
why I asked. I don't have an ASD book anymore, or I would have checked.

Somehow I missed the 6xslab thickness in I3.2d(6).

RAM confirmed that the # studs = total end to end, that you can set the 
limit for composite action below which the beam will be designed as 
non-composite, and that the newer versions of the program now optimize 
studs as pairs on uniformly loaded spans (it just looks wrong to see a 
uniform span with an odd number of studs).  A big thanks to Seth in tech

support at RAM (Bentley) for answering my questions very quickly, 
especially since I don't own the program.

Stay tuned for a followup question...


Carter, Charlie wrote:

>The number of studs on the drawings *should* be the total number
>distributed over the segment, as the person who has to build it likely
>does not know we calculate half the number. I'd get verification of
>from RAM, too, though.
>There is a good discussion about lower percentage composite action on
>page 16.1-311 in the 2005 Specification. There is no codified limit, so
>you can use your engineering judgment.
>Maximum spacing of studs is given in Section I3.2d(6) on page 16.1-88
>6 times the total slab thickness or 36 in. Maximum spacing of deck
>attachment (studs or puddle welds) to the beams is 18 in. per Section
>Not sure where the 12" rule is from.

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