×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Beam formulas.
5

Beam formulas.

Beam formulas.

(OP)
I have not calculated a beam in 20 years and all of my books are out of reach.  I am looking for the formulas to calculate bending moment on a simple beam supported at both ends with a uniform load combined with a point load in the center.  This is to get preliminary sizing for a glulam beam spanning 25 feet, supporting the uniform load of the floor above and a point load from a column placed in the center of the span, which supports the ridge beam of the roof above that floor.  My memeory says PL/4 & WL(SQUARED)/8

Also what is a source for moment of inertia tables for glulam beams?  many tables size the beams by spacing, combined with span, for various floor loads/sq.ft., which is no help when there is a point load to be considered also.

Thanks

RE: Beam formulas.

Your memory serves you well for the bending moment formulae -- it is indeed P*L/4 for a point load at midspan and w*L2/8 for a uniformly distributed load on a single-span simply supported beam.

I don't work with timber though so can't help you with the beam properties.

RE: Beam formulas.

Go to WWPA.org, and download their free Excell-based beam and post design suite.

RE: Beam formulas.

OOPS-
Nothing for Glulams at WWPA.org
sorry.

RE: Beam formulas.

You don't need to memorize a formula for those two loading conditions- just take half the beam as the freebody, and sum moments about the center point.

RE: Beam formulas.

Moment of Inertia of a glued lam beam is no different from a moment of inertia of any rectangular element.

I = bd^3/12

Section modulus, likewise.

S = bd^2/6

Modulus of Elasticity of a glued lam member depends on the species of wood used and combination of laminations.

All of these can be found in tabular form in the 1997 NDS supplement.

RE: Beam formulas.

g'day

not knowing much about the structural strength of wood products, what would Abelgus use as his allowable ? ... it doesn't sound like it's at WWPA.org ?

RE: Beam formulas.

Allowable stresses depend on the combinations of laminations and species of wood.  Using 24F-V8 combination using Douglas Fir, allowable bending stress (both tension size and compression side) is 2400 psi and allowable shear stress is 190 psi.

24F-V4 compression side allowable bending stress is 1200 psi.

I am referencing from 1997 NDS.  The latest edition should have the same values (I think).

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources