3/4"x24" wood, Allowable Fb?

[ARLORD]

I am an experienced structural engineer, however, I am a novice to wood design, I do have the current NDS set. I am building a home bar, and would like to check how long I can span 24"x3/4" maple or cherry wood, flat use, simply supported. How do I determine or where can I find the allowable Fb, prior to multiplying by the applicable adjustment factors. I plan to use 10psf total load.

 

[nutte]

Is your bar so big that you're concerned with saving a few intermediate supports?

My 1999 NDS set (ASD) came with a supplement for structural use panels. It has the information you're looking for, as well as examples showing all of the design checks.

 

[jike]

If you are using it as a shelf, I would think long term deflection may control.

 

[JStructsteel]

this is somewhat a paradox, an engineer building a bar, yet wanting to make sure it meets code.

to be honest, use some plywood substrait or MDF, and put the wood over it, it will be alot more solid.

 

[jike]

If indeed it is a shelf, you can stretch the span by supporting along 3 sides and providing a small edge board (glued and screwed to the shelf) along the unsupported edge for stiffening.

 

[ARLORD]

Not a shelf, just a standard 42" high, wet bar. I want to verify if it can span 6 to 7 feet without interior supports. I looked thru 1999 NDS Structural-Use Panel supplement, I didn't see anything regarding the allowable stresses, Fb, for different types of wood, specifically maple or cherry.

 

[RARWOOD]

It is unlikely that the 3/4" material you are using is stress rated material. If the material is not grade to assure that it meets a structural grade, then you will not find any design values for it. Most 3/4" materila is graded for appearance not strength.

 

[DaveVikingPE]

If it's a "bar" that will support drunken co-eds, something along the lines of what they do at Coyote Ugly, then go with a 100 psf live load. Self-weight dead load.

here's some info on Hard Maple:

http://www.hardwoodinfo.com/species_guide/display_species.asp?species=hardmaple#

here's some on cherry:

http://www.hardwoodinfo.com/species_guide/display_species.asp?species=cherry

The values given need to be reduced by a safety factor; I think the NDS (don't have it at my desk...) builds-in a safety factor of 2.6 to the values published in the supplement.

Apply wet use factor, too.

You should have too much of a problem with building the bar at all.

 

[structuralaggie]

Mixed Maple beams and stringers No. 1 has Fb=975psi without any adjusment factors. CF=.74

6 or 7 feet sounds too far. You don't want a bouncy bar. I am thinking 2' is best, 3' most.

 

[structuralaggie]

Another table has 725 for 2" wide. I don't think these values offically apply.

 

[SlideRuleEra]

NDS-2001 has values for "Mixed Maple" and "Red Maple" in table 4A. Allowable bending stress varies from 225 PSI to 1300 PSI, depending on grade.

NDS takes this information directly from the "Northeastern Lumber Manufacturer's Association"; here is their download

http://www.nelma.org/files/File/Chapter_8-2006_Grade_Rule_Book.pdf

Cherry is a little trickier, it does not seem to be rated by any organization. However is you download the USDA Forest Products Lab's "Hardwoods of North America"

http://www2.fpl.fs.fed.us/publications/fplgtr83.pdf

and compare the values for cherry (page 97) with maple (page 8) the properties are similar.

The most direct comparison for bending strength is the "Modulus or Rupture" (MOR).

You can also get the Modulus of Elasticity (MOE) for cherry from this document.

The "flat use" factor, and many of the others apply only under very specific conditions. 3/4" inch thick lumber is too, thin to take advantage of "flat use".

http://www.SlideRuleEra.net

 

[DaveVikingPE]

"You should have too much of a problem with building the bar at all."

Oops, should have been "You should not have too much of a problem with building the bar at all."

Post a photo of the bar when it's complete -or, better yet, invite all of us over for a beer.

 

[apsix]

As noted above it's deflection and bounce which will limit the span. Six feet is way too long, try about half of that.

A carpenter/joiner should be able to tell from experience what maximum span to use.