Large Truss Question

[woodhick]

My church is wanting to build a large picnic shelter.  I have been given the task of cost analysis. .  There are a couple of members (myself included) that own woodmizer mills.  We would like to have at least a 40' clear span and would prefer a 50' span.  Have access to lots of yellow poplar that could be cut into timbers for project.  What would be the best approach to build trusses this size.  They would not have to mortise and tenon but could be steel plated and bolted.  Any opinions?  Thanks.

[thurlow]

A quick search turned up this......
Click here for link to truss plan site
and there are many others. 

[slowzuki]

50 ft is long span, you will certainly need an engineer involved especially since many people will be under the roof.

[Jim_Rogers]

If you download the plan in the link above be sure to read the second page......
Any wide design like this will have to be reviewed by a local structural engineer for using local woods.

Jim Rogers

PS. I cleaned up the link to make it work smoother......

[Jayson]

Hey Woodhick,
                    That 50' is a doozie of a span,but I would not let that spook me.You may start buy looking for some of Jack Sobon's material on historic american joinery.I clearly remember reading about some trusses with huge spans and the joinery that was used.I think that The History of Traditional American Joinery by Jack is available through the Timber Framers Guild,but you need to make sure that the info on trusses is in there.I think it was published as parts in the Timberframing Journal.After scanning that stuff you might be able to at least come up with preliminary stuff to talk about with a qualified engineer.And secondly I'm not sure that poplar is going to be the species for those type of trusses.

[Larry]

There is a lumber yard in Clifty Arkansas that built such a structure.  They made steel plates with through bolts to hold the timbers together to avoid joinery.  The owner told me they had a local truss manufacture come up with the design to meet building codes.

I bought some trusses from the manufacture and they showed the software to me.  Seemed quite simple to me...plug in spans, rise, and wood type...the PC kicks out a design and the engineer signs off on it.  Might be an option to explore.

[woodhick]

I am not against using metal plates and through bolts to assemble truss.  Just wandering if anyone here had a design to save me some time.  I have access to plenty of poplar,  but may be able to come up with enough oak for the trusses if needed.  I have seen long span trusses built out of 2x10's or 2x12's that were laminated and bolted through, but it's been a long time ago.  Would like to see some pics of some it anyone has any.   thanks.

[Jim_Rogers]

It can be done with wood, but you'll need to consult an engineer.
There are truss designs and stories that can be downloaded off the Timber Framer Guild web site, in pdf format.

If your building is 50' long how wide is it?
Can you run your truss the other way and make it shorter?

Some guys have been using a short (in height) lattice truss to hold up roof loads.
What is the pitch of the roof?
What is the snow load of the area?
You'll need lots of information in order to design one.....

Jim Rogers

[Don P]

I don't know enough to design one, I've been reading up on it.
One example is using a 6/12,  48' truss on a 12' bent spacing as an example. The loads are 15 dead, 35 live. The bottom joint in the example has about 25,000 lbs of force in the connection. That is definitely where I would want steel connections and an engineer is a must, that's quite a sail too.

[Thehardway]

Our church uses a picnic shelter on a neighbors ranch that is built from peeled logs that are staked to the ground at the butt end and leaned together and thoughbolted at the top.  A ridgepole is placed across them and then grommeted canvas tarps laced with cable forms the cover.  It is extremely cheap and quickly built.  This structure could be adapted for a lightweight steel roofing quite simply.  If you have long straight poplars you could cut them tapered and anchor the bottoms on concrete abutments.  Steel cables with eyebolts could be used to limit outward thrust.  This homebrew solution requires no joinery, no steel plating and is very cheap.  It does require some area around the site to accommodate the angled supports sticking out. (just based on angles, a 50' usable span with 32 degree angles would be roughly 76' W and 24' H with a 8' eave.  The timbers would need to be 45' long but could be tapered )

More elegant but similarly simple solutions would be an arch or a cruck.  In my opinion the trick to the large span is avoiding vertical supports and carrying the outward thrust all the way to the ground.

I will try to get a picture of the picnic shelters