I have been contacted by a friend that is building a log cabin. He has approved and stamped drawings, but the designer has designed timbers 8-3/4 x6 x 15'-4"long span for the floor joists 36" on ctr. He would like to use logs in liew of timber. he has asked me what size logs to use. does anyone know of any sizing charts for logs, or any available formulas so we can try to figure the log diameters. any help would be appreciated, thanks,John..

If you don't won't to see any sacking in that timber it should be designed not to sack more than a 1/300 of it's span ( in barns 1/200 or US 1/240)

Areal Momentum 2. Degree in a square timber is I=bhhh/12 (i equals d times h to the third divided by 12)

for a circle it is : i equals pi times d to the forth divided by 64

To calculate the right # depends now on the structural system the beam is in ( if the weight is being distributed evenly or concentrated in certain spots on the beam.)

For an evenly distributed weight the forces in each post A=B=force times span divided by 2

Max bending momentum M = force times span to the second divided by 8

Bening = force times span to the forth divided by 77 times elastisity# times areal momentum 2. degree

hope this helps

John,

The International Log Buidling Association has published a booklet called "Log Span Tables for Floor Joists, Beams, and Roof Support Systems" written by B. Allan Mackie, Norman A. Read, and Thomas M. Hahney. You can order it from the International Log Building Association at www.logassociation.org or from Schroeder Log Home Supply's online bookstore at www.lhgic.com

David

John:
If you came to me at the sawmill and asked me to make you a beam that was 8 3/4" x 6" x 15'4" it would take a log that was at least 10 5/8" inside the bark at the narrow end to produce such a beam.
I would probably look for one at least 11" inside the bark narrow end to make sure it would cut out without any wane on the edges.
What I use to determine log size is a chart I created myself. It's a spreadsheet program that takes the size of the timber and generates the diameter needed to mill out the needed timber from a round log.
Basically the old a squared + b squared = c squared system where c is the diameter of the log. a is one leg of the right triangle or one dimension of the beam (8 3/4") b is the other leg/dimension (6") and c is the corner to corner measurement or the hypotenuse of the right triangle, and the diameter of a log necessary to make the beam. To solve the answer you need to re-arrange the formula to find c by saying what is the square root of a squared + b squared? The answer to your size is 10.609” which is just under 10 5/8”.
I hope this helps, good luck with your project, Jim