Wood Post Embedded in a Foundation Shaft? 1

[abusementpark]

I have a job where the Architect would like to embed wood posts in an unreinforced concrete shaft, maybe 5 or 6 feet deep, and have that act as the foundation. Is an acceptable method of transferring vertical loads? I've about 8 kips of load in these 8x8 wood column, as well as about 4 kips of uplift. I guess you have to rely on friction between wood column and the concrete to transfer the load?

Any guidance would be greatly appreciated.

 

[theonlynamenottaken]

This type of embedment is traditionally not recommended. You create a pocket where moisture can be trapped, and wood subject to cycles of wetting/drying is the first to rot. Rotting could occur within the embedment where it wouldn't be seen. Also, if you somehow flash or waterproof the posts so that they stay dry, they'll shrink as they dry, pulling away from the concrete and killing off any friction you counted on.

A similar detail that I've used is to place a bed of crushed stone at the base of the post, and then run rods through the post that extend out into your concrete shaft. You'd then be looking at the dowel bearing on holes through your post, and bending/shear of the rods extending into the concrete for resisting your downforce and uplift loads.

 

[Archie264]

Also, a quick note on resisting uplift: a relatively small cylindrical footing, anchored, perhaps, as mentioned above, might be better at resisting uplift than filling the whole shaft with concrete. While perhaps counterintuitive, the cylinder engages a cone of soil above it while the pier only relies on skin friction. BUT, I'm anything but a soils specialist, so if a foundation engineer chimes in pay attention to what he says.

 

[msquared48]

I agree with the moisture problem here, but is there anything at the bottom of the shaft to bear on, as an old footing? If there is, whn not just fill the shaft with concrete and bear the new wood posts on top of the new concrete?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Splitrings]

Use the dowel equations in NDS to calculate the number of required dowels to resist uplift.

I would recommend you specify the AWPA Use Category for the pressure treatment you best think meets the needs. I would say UC4C as a minimum. If the design life is long and the components are critical, you might consider UC5x. This is over 4 times more treatment than any PT post you'll get at your local lumber yard.

 

[Splitrings]

Oilborne preservatives typically stabilize members against shrink and swell better than waterborne preservatives. CCA is a waterborne type with a proven history that may be acceptable.

 

[abusementpark]

Is there any issue with only pressure-treating the portion to be embedded in the concrete?

 

[abusementpark]

A similar detail that I've used is to place a bed of crushed stone at the base of the post, and then run rods through the post that extend out into your concrete shaft. You'd then be looking at the dowel bearing on holes through your post, and bending/shear of the rods extending into the concrete for resisting your downforce and uplift loads.

Does this alleviate the rotting concerns?

 

[Splitrings]

I don't believe you will be able to pressure treat only a portion of a timber. The whole timber must be in the pressure vessel. I have seen laminated posts constructed this way. They are tyically 2x's either nail or bolt laminated. The bottom laminations are PT lumber and the rest are KD. If there is an architect involved, I doubt they will go for this!

 

[ztengguy]

why you letting the architect dictate structural details? Can you not use a Simpson plate or something that is embedded in teh top of the footing for bearing and uplift? Eliminates all water concerns. Just seems a bad detail and a waste of 5-6 feet of lumber at each post.

 

[Splitrings]

I assumed there must be moment at the base of the post to be resisted. If not, then there is no reason to embed the post.