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.

 

[MiketheEngineer]

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.

 

[JoeBaseplate]

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?

 

[ToadJones]

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.

 

[woodman88]

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.

 

[ToadJones]

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

 

[MiketheEngineer]

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.

 

[msquared48]

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

 

[MiketheEngineer]

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..

 

[ToadJones]

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)

 

[JoeBaseplate]

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.

 

[msquared48]

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

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[JoeBaseplate]

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

 

[JoeBaseplate]

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.

 

[ToadJones]

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.

 

[MiketheEngineer]

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.

 

[JoeBaseplate]

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.

 

[JoeBaseplate]

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.

 

[ToadJones]

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