×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Wood post-Wood truss connection

Wood post-Wood truss connection

Wood post-Wood truss connection

(OP)
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.
 

RE: Wood post-Wood truss connection

What span do you have.

Yes - it can be a problem and NO you will not be able to get a true fixed connection.  Two bolts will come somewhat close but they will move, the holes will expand, the holes in the lumber will crush, etc, etc.

Most of the time - I bury the posts in the ground and assume those are fixed.

That will usually work.

And picnic pavilions will "move" around a bit - just live with it.

RE: Wood post-Wood truss connection

(OP)
Mike,

The truss clear span is about 36' and columns are 14' oc. Trusses are 2' oc, thus I would need to "fix" the trusses only at the columns.

Are there any special provisions for burying the post in the ground? Can the regular treated wood for exterior use be buried into concrete?

RE: Wood post-Wood truss connection

I supposed you could make the frame like a mill building and make the trusses like 3 ft deep at the ends and run the column to the top of the 3 ft tall end.
Essentially makes a moment frame.  

RE: Wood post-Wood truss connection

No. The truss to post can not be a moment connection. You would need to use a brace. See attach.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Wood post-Wood truss connection

I'm essentially saying the same thing as Woodman....only I think mine would look a little nicer :)  

RE: Wood post-Wood truss connection

Use treated lumber - bury down about 4'-5' depending on soil.

Good for at least 20 years.

Look up "Sturdi-bracket".  Not sure I completely agree with their numbers - but you will get the idea.

Many post frame (pole barns) builders use a multi-ply column consisting of say 3-2x6's with treated lumber at the bottom connected by a truss plate to above ground un-treated member.  The whole thing is then nailed together...

Seems to work quite well.

RE: Wood post-Wood truss connection

This a pole structure Toad, and there are provisions in the code to determine the burial depth.  This is done all the time.

However, the Bay spacing for a 36 foot span is normally 8 to 12 feet, not 14.  That is what is driving your column size.   

Mike McCann
MMC Engineering

 

RE: Wood post-Wood truss connection

While I won't disagree w/ Woodman - putting those braces in can way overload the truss.  It needs to be designed for that...  Just don't do it without thinking it thru..

RE: Wood post-Wood truss connection

I knew it was just a pavilion, it sounded like he didn't want to bury the poles.
What I was describing was essentially the same as Woodman, except rather there would be no knee-brace.
the OP asked how to get fixity at the top of the column, so I offered some idea, however weird it may have been.
(personally I think it would look nice)  

RE: Wood post-Wood truss connection

(OP)
Thanks for all the responses, and Woodman..thanks for the sketch.

Sounds like burying the post would be the way to go. We are going to have large concrete piers (18"x24") to carry the 3'-6" tall brick pier around the post. The piers go down 3' and then the formed footing is 3x3.

I also need some clarification on the load combos. I am using D+L+W for the post design and use duration factor (1.6) in post analysis. When it comes to foundation design, I am using D+.75L+.75W. Makes sense to me.

RE: Wood post-Wood truss connection

Joe:

The fixity you describe would be impossible to attain with a shallow end truss without knee braces.  

If the end of the truss was 3 to 4 feet in depth with the column running to the top chord, maybe some fixity could be achieved, but only a limited amount, and the truss manufacturer would have to be a big part if the design effort to maintain the truss warranty.   Do not do this without their involvement.

Not considering any Architectural concerns, knee braces would still be the best option here, or reduce the bay spacing.

Mike McCann
MMC Engineering

 

RE: Wood post-Wood truss connection

You also need .6D + W/S which WILL govern in uplift.

Mike McCann
MMC Engineering

 

RE: Wood post-Wood truss connection

(OP)
Yup Mike, I am using that too. 06D+W

RE: Wood post-Wood truss connection

(OP)
BTW I was hoping to avoid the base fixity, Teguci create a great sketch on one of previous posts for creating a fixed base.

msquared48, I understand your concern and that's why I did not want to just simply "fix" the top of the column to truss and show it on the drawings to let the poor truss guy deal with it (it would have eventually come back to bite me in the *** anyway). Putting kickers would be an architectural concern plus not truss mnfr at this point since they job may not even build (though we are supposed to do present a full design). I think its down to fixing the column base.

RE: Wood post-Wood truss connection

Really, with a deep truss at the end, you get a couple between the top and bottom chords, which can equate to a lot of fixity.  

RE: Wood post-Wood truss connection

You might have another problem.  If the trusses are 2'-0'' and posts are 14'-0''oc - what are you using for a header between columns and how are you attaching that to the columns??

I often find that can be a problem...depending on spans and loads.

RE: Wood post-Wood truss connection

(OP)
Mike, I am using a large PSL (5 1/4x 11 1/4). I am thinking of using some sort of Simpson header for connection, haven't worked out the cap yet.

RE: Wood post-Wood truss connection

(OP)
Attached is the sketch (rough and not to scale) I have decided to go with. I like the idea of embedding post into the concrete to create fixity but am not comfortable with concrete in contact with wood, even though its going to be treated. The thru bolt near the bottom will do the job for uplift loads.

RE: Wood post-Wood truss connection

you cannot bury treated lumber in concrete.
It voids the warranty...at least it did 5 yrs ago before this new garbage came out.  

RE: Wood post-Wood truss connection

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.  bigsmile

Mike McCann
MMC Engineering

 

RE: Wood post-Wood truss connection

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.  

RE: Wood post-Wood truss connection

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.  bigsmile

Thanks.

Mike McCann
MMC Engineering

 

RE: Wood post-Wood truss connection

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.  

RE: Wood post-Wood truss connection

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

 

RE: Wood post-Wood truss connection

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)

RE: Wood post-Wood truss connection

(OP)
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)]

RE: Wood post-Wood truss connection

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

RE: Wood post-Wood truss connection

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

 

RE: Wood post-Wood truss connection

(OP)
msquared48 , I actually am trying to fix the base.

RE: Wood post-Wood truss connection

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

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources