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Definitions from The Contractor's Dictionary of Equipment, Tools, and

Beam: Structural member supported at two or more points, but not
throughout its full length, transversely supporting a load, subjected to
axial load and flexure but primarily flexure.

Girder: Main horizontal support beam, usually supporting other beams.

Rafter: Roof timber sloping up from the wall plate to the ridge and
extending down to the eaves.

Definitions from Dictionary of Architecture and Construction:

Beam: A structural member whose prime function is to carry transverse
loads, as a joist, girder, rafter, or purlin.

Girder: A large or principal beam of steel, reinforce concrete, or timber;
used to support concentrated loads at isolated points along its length.

Rafter: One of a series of inclined members to which a roof covering is

My interpretation:

Beam: A structural element that carries a transverse load primarily in
flexure.  Typically, the load will be a uniform load.

Girder: A beam that typically supports other beams.  Thus, will typically
carry concentrated loads.

Rafter: An inclined roof beam.

Purlin: A roof beam.

Joist: In wood construction, typically a wood floor or roof beam, such as 
a 2x8 or 2x10 or an "engineered" shape (i.e. wood I beams). In steel
construction, typically would be refered to as an "open web joist".  In
other words, a manufactured truss-like member.  In concrete construction,
a floor or roof beam that would be rather closely spaced (typically about
2'-0" on center).

Hope that helps.


On Wed, 21 Feb 2001, [iso-8859-1] Juan Josť Treff De la Mora wrote:

> Can anyone explain me the difference between a beam, a rafter and a girder?
> THX,
> Juan Jose
> _________________________________________________________________________
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