Wooden post embedded in concrete

[psychedomination]

Hi there,

I have a colleague that did an original design for a wood post to be embedded about 4' into concrete. The contractor found on site that they ran into some very hard rock that will be difficult to remove. So they won't be able to go the full 4' down; to save time they proposed the detail below.

My colleague is asking my thoughts.

I am in an area of high winds.

I'm not too familiar with embedded wooden posts but my first thought is that I think the use of dowels into the hard rock is fine to hold down the concrete itself, but from that sketch, I am not sure about what is holding the wooden post down in the ground during wind uplift?

How can I check that the wooden post doesn't just pull out of the ground? Would it be a skin friction calculation?

 

[JoelTXCive]

I'm not sure of the overall capacity, but I do have some ideas:

1) your goal is to get the wooden post securely connected to the concrete; and then have the concrete securely connected to the rock.

2) I like the stirrups and the embedded rebar, but my gut says you need more than 6" embed into the rock. You can calculate the moment at the base of the concrete section; then resolve the moment into the tension/compression components. Then use Hilti's Profis rebar to approximate the connection capacity.

3) I do not know how to quantify the results, but you could improve the wood to concrete connection by drilling some large diameter (~5/8")lag screws 1/2-way into the wood post. They will turn the bottom into a 'porcupine' and increase the composite action between the wood and concrete.

 

[SlideRuleEra]

psychedomination - Keep it simple, you know (KISS). I would omit the rock anchorage, all the rebar, and "small" neatly sized hole and go with unreinforced concrete, in an (approximately) cylindrical hole... say 18" diameter. Rely on mass (about 500 lb). Drive nails, say 20d common into all four sides of the embedded portion of the 4x4, leaving the nail heads protruding 1 1/2", or so. This is unlikely to be pulled out of the ground by high winds (150 mph) .

http://www.SlideRuleEra.net

 

[JoelTXCive]

I change my answer and defer to Slide Rule Era.

Could you do an +/-18" diameter hole down to the hard rock; and then do a 6" diameter hole cored into the rock?

 

[IFRs]

Without knowing the desigh loads (particularly the uplift reactions, if any), the extent of the hard rock and the soil capacity below the hard rock, in my opinion it is inappropriate to suggest anything.

 

[MotorCity]

Unless this is for a residential fence post, I don't like embedding wood into concrete because it will trap moisture near the bottom of the post (which is probably untreated). I would prefer a detail where the bottom of post is attached to concrete above grade using a Simpson or similar post fastener. Assuming the condition of the rock is suitable for anchoring to, you could pour a block of concrete to make up the height difference and anchor the concrete/post to it.

 

[psychedomination]

Thanks all, sry for just getting back it's been hectic in the office!

This issue got resolved as the contractor had their own engineer that stamped of on their detail. Still out of interest wanted to get opinions.

@SRE yea I tend to overthink things a bit sometimes lol. I was thinking of a similar connection to what you were proposing but just wasn't sure how to quantify it (I guess it could've just been a shear flow exercise). I did something similar with a steel tube tower in a concrete pad footing for a past project (although used rebar instead of nails).

@Motorcity, yea I don't tend to embed wood in concrete like that. Although most wood used in the construction where I'm from is wolmanized.