Beam formulas. 5

[AbelGus]

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

 

[dbuzz]

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.

 

[SacreBleu]

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

 

[JedClampett]

Or go to:

http://www.apawood.org/level_c.cfm?content=pub_glu_libmain

and get one of the excellent downloads. Note that registration is required.

 

[SacreBleu]

OOPS-
Nothing for Glulams at WWPA.org
sorry.

 

[JStephen]

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.

 

[whyun]

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.

 

[rb1957]

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 ?

 

[whyun]

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).