# Beam strenght

1. Jan 14, 2009

### ganathius

G'day

I was wondering if anyone could give me some information to do with the max load a given beam can support. I have done some study in statics but it was along time ago.

I am considering a 6 metre distance, spanned by a rectangular timber beam. I was wondering what information other than the length, width, depth and modulus of elasticity of the timber was required? Also is there a formula to work it out?

Any information of this would be great.

cheers

PS This is my first post, i hope this is in the right section

2. Jan 14, 2009

### minger

Strength is a term used to describe the stress loads that a material can handle. Some values that I seen on a quick google gave between 72-90MPa for spruce and pine respectively.

With this number you then need to determine what stress you have in your problem. This will heavily involve how the beam is held in place, and how the beam is loaded. Typically, for pure bending, we can say that the maximum stress is
$$\sigma = \frac{My}{I}$$
Where M is the maximum moment, y is the maximum distance from the centroid to the cross section tip, and I is the area moment of inertia.

3. Jan 14, 2009

### nvn

ganathius: (1) What type of room is above (supported by) this beam? (2) How is the applied loading distributed along this beam? Is the load uniformly distributed along the beam length? Or is a concentrated load applied at midspan? (3) What is the tributary width (perpendicular to the beam length) supported by this beam? (4) What are the cross-sectional dimensions of lumber available for you to purchase, and the wood species and strength grade? The grade is often stamped on the lumber, and also lists the species. This is called a grade stamp. Does your available lumber have cross-sectional dimensions 38 x 286 mm?

4. Jan 15, 2009

### FredGarvin

You need to be careful when it comes to lumber strengths. The properties are directional and vary drastically between wood species and moisture content. Because of the variations in strengths it is always best to look for building codes in your area that cover this.

If you really want to look for some comprehensive lumber data, look at this publication by the US Dep't. of Agriculture:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm

The entire PDF is here:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.pdf