Large timber design values

[JStructsteel]

Im putting together a chart for bending stresses on large timbers for emergency use - Think of a building collapse, shoring etc for USAR work. Figure 7 days max duration.

For timbers 8x8 and larger, I was using a Fb=1000, and Fv=165 as generic values. How do you folks feel about those to be conservative? Cant get species specific, since in a emergency situation, will take what can be found.

Thanks

 

[phamENG]

These are new timbers to be installed temporarily? That may be reasonable. Are you taking your Fb to include an adjustment for duration? Or are you going to increase from 1000psi with an appropriate C_d? You may want to do one for each region it may be used in. Determine the lowest strength wood likely to be used in that region and base it on that. There are plenty of timbers with F_b values closer to 500psi. So I'd say either use a common species and ignore your duration bump to make up for the possibility that you could end up with some really weak stuff, or give yourself some options to you can make an informed decision based on what you're presented with.

 

[JStructsteel]

These would be used at a building collapse, and be used for less than 7 days. I might give the option to increase by 25% for Cd. After that, they would be replaced with a permanent support, or until demolished.

I would figure SPF, SP, or DF type wood. Im trying not to get specific, and also not be too conservative. What woods would you expect a 500psi Fb?

 

[phamENG]

Some lower grade spruce and SPF, certain species of pine and oak, maple is 650, cedar, and lots of others. Take a look at Table 4D of the NDS.

 

[JStructsteel]

Thanks, looking at those, most in that range if #2 timbers, which is realistic. I think I will use Fb 500psi, and Fv 125 to be conservative for 8x8 and larger.

 

[Celt83]

Is this lumber the emergency crew is going to have a stock of or would this be a run flash a card at the local Lowe’s/Home Depot and grab some stock 8x8 and 6x6’s off the shelf and go.

If the latter I’d check what kind of heavy timber those outlets stock and use that as the base line.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[JStructsteel]

Celt83,
It would be whatever is available. Chances are HD or Lowes would not have over a 6x6.

Its down and dirty so the rescue types can get an idea of how much they can lift w/o consulting the engineer.