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Re: Joists-girders-beams

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Hi members,
A little late but I guess the issue is still hot so i'll drag on with this a little more.
Before, I have observed from various designs that:
- 'ceiling and timber decking' definitions normally stick to joists,
- roofing definitions normally stick to purlins, tie-beams etc
- concrete structures stick to (concrete)beams,
- bridge structures sometimes include (concrete)box-girders, headstocks etc.
- steel structures stick to Girders and Steel I-beams/T-beams.
Maybe this is how Engineering has slowly unified concepts.
So I always use the wording above to the appropriate structure in concern because that's how I have been taught.
Otherwise, on a serious note, a structural engineer still designs a [joist, underpurlin, girder] as a structural member spanning horizontally carrying load over it and transfering it to support(s)...which in simple terms is a 'beam' designed according to those simple beam theories (free body diagrams, sum of forces/reactions...).
Thus why aren't they [joist, underpurlin, girder] all simply 'types of beams'?
Possibly the classification of the required beam jets in when it comes to detailed beam designing which includes characteristics of 'shape and material properties' in addition to the 'stresses and loading properties/range'.
in that learneth me.
a thing that drags on shows there was no definitive catch on it yet.
From: John Riley <jpriley485(--nospam--at)>
Subject: RE: SEMANTICS: Def. of "Joist" vs. "Beam"
To: seaint(--nospam--at)
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: 8bit
A joist, in architecture and engineering, is one of the horizontal
supporting members that run from wall to wall, wall to beam or beam
to beam, to support a ceiling, roof (or floor). It may be made of
wood, steel or concrete. Typically a beam is bigger than a joist and
thus is distinguished from a joist. Joists will often be supported by
beams. Joists support the sub-floor (floor deck) directly.
A girder is a large iron or steel support beam used in construction.
They often have an I beam cross section for strength. Girder is the
term used to denote the main horizontal support of a structure.
Joe Grill <jgrill(--nospam--at)> wrote: The way I was taught:
Plywood (or decking wood or metal) load joists
Joists load purlins
Purlins load beams
Beams load girders
Girders to columns
Right or wrong that is what has been in my pea brain for many years but,
still open for change.
Joseph R. Grill, P.E. (Structural)
Shephard - Wesnitzer, Inc.
Civil Engineering and Surveying
P.O. Box 3924
Sedona, AZ  86340
PHONE (928) 282-1061
FAX (928) 282-2058
-----Original Message-----
From: Polhemus, Bill [mailto:BPolhemus(--nospam--at)]
Sent: Monday, May 01, 2006 4:07 PM
To: seaint(--nospam--at)
Subject: SEMANTICS: Def. of "Joist" vs. "Beam"
Just a little item here:
I am in a little dispute with a colleague over the use of the term in a
What is the definition of a "joist" (generically speaking) as distinct
from a "beam"? I say that what I'm looking at are "joists": They are
flexural members that directly accept the service loads, are laid
parallel to one another and are closely spaced.
He says they are "beams."  But maybe I'm putting too fine a point on it.
What say you?
William L. Polhemus, Jr. P.E.
Senior Associate
Wiss, Janney, Elstner Associates, Inc.
Engineers I Architects I Material Scientists
10235 West Little York, Suite 245
Houston, Texas 77040
P: 832-467-2177 F: 832-467-2178


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