Wood post-Wood truss connection

[JoeBaseplate]

I am working on a party pavilion with wood trusses, plywood roof, gyp ceiling and wood posts. Posts are 14’ oc and trusses are 2’ oc. The structure is open to the sides.

I am having a very hard time getting the posts to work for unbalanced uplift wind load (fig 16-b of ASCE 7) and the posts are already very large (8x8). The only way I can think this could work is if I could somehow fix the top of my posts at the truss connection.

Is it possible to fix that and have the truss manufacturer account for the fixity, or is it a complete no no? Any input would be great.

 

[msquared48]

Joe:

A couple of things here:

1. In the detail you have shown, due to the spread footing at the bottom of the concrete pedistal, since the material around the pedistal will be disturbed souil (backfill), you will not be able to use the tabularized lateral bearing values in the IBC. You will need the recommendations of a geotech for that, unless you are just trying to resist the moment with P + M on the footing.

2. If you do not want to use either base fixity or fixity at the top of the wood column, why not go to plywood exterior shear walls with a plywood roof diaphragm?

Toad:

That is the first I have heard of that. Can you post a link to an article on that please? If so, there are a lot of buildings here being built as pol;e barns that would be affected. Personally, though, the treated wood posts that I embedded in concrete 30 years ago are still as good as the day I installed them.

I know I'm sorry. I'm sounding like a contractor now.

Mike McCann
MMC Engineering

 

[ToadJones]

Mike-
I may have made a blanket statement there.
During my nailing bending contractor days, there were counties where placing posts in concrete was a code violation.
I did it sometimes anyway.
However, my comment is a direct reference to stickers I remember reading on the treated lumber stating that placing posts in concrete below grade v (fence, decks, pole barns) voided the warranties.

IMO, placing the posts in concrete is a waste of time and money. I always thought it just made DIY guys feel better.

 

[msquared48]

Interesting. Never noticed that - I'll have to check. But I'll be d@#$%^ if I'm going to dig the posts out of the ground to find out.

Thanks.

Mike McCann
MMC Engineering

 

[ToadJones]

Mike-
Im doing some internet reading....
Sounds like the key is "below grade". In other words, if you set the pole in concrete, make sure the concrete extends above grade and slopes away from the pole so as to avoid water pooling in the inevitable void between the concrete and the pole.

 

[msquared48]

That's why I put gravel at the bottom of the post to bear on and then fill it with concrete. The gravel allows any water that might get between the wood and concrete to drain out. Water will still run down the post and penetrate any void whether the top[ is sloped or not. But minimizing any ponding can only help.

Mike McCann
MMC Engineering

 

[ToadJones]

yep...precisely how i set posts....#57 washed gravel tamped in base of hole. Sometimes I encsed in 'crete, other times not. I pretty much only ever used concrete on fences. I saw no need for decks unless they were very tall and in that case I usually knee-braced them and made the knee bracing to look architecturally pleasing (hate that word)

 

[JoeBaseplate]

msquared,

this is a fancy schmancy golf course so I do have a soils report and the soil is pretty good, 4000 psf. And yes I am designing the footing to take the moment. I am relying on the weight of concrete and soil above the footing when I have uplift to drive down my eccentricity [q= 4P/(3L(B-2e)]

 

[RFreund]

Mike/Toad,

You say to have the wood post bear on the crushed rock, however how do you maintain the allowable bearing capacity. Any time I analyze a deck foundation I always end up with piers bigger than your used to seeing. I think part of that is due to the required live loads (sometimes), but even so there is not much bearing area for a 6x6 post.

Also if you are not trying to achieve a fixed base do you still run the post down to the bottom of the hole?

EIT

 

[msquared48]

If you are not trying to achieve a fixed base, there is no need to run the post to the bottom of the hole.

As for the vertical bearing, that is a problem sometimes for pole structures. You can e3nlarge the diameter of the concrete footing, and you can increaswe the allowable soil pressure with depth.

Mike McCann
MMC Engineering

 

[JoeBaseplate]

msquared48 , I actually am trying to fix the base.

 

[msquared48]

Sorry - I had to run a quick errand.

A third way to limit the soil pressure would be to shorten the bay spacing (speaking again of pole structues here).

For decks, I usually use pier blocks or spread footings, with the size suiting the load. I use the augered hole footings only for pole structures.

Mike McCann
MMC Engineering