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wood truss design

wood truss design

wood truss design

(OP)
Someone added a second floor to a 24' wide garage using a hip roof with no ceiling joists. A 12' 1 3/4"x 12" lvl ridge beam was used with posts at each end. Roof pitch is 3/12 and 2x12 rafters 16" o.c. were used. Snow load is 42 psf.
Problem is that the front post rests on an inadequate garage ceiling beam which rests on a 16' garage door header. There is however, a 24' long 1 3/4" x12" lvl connecting the rafter ends and the 4x4 post, creating a 'simple' truss.
We would like to take out the post, and create a permanent truss. Seems as though the lvl is more than adequate as a bottom chord, and possibly the 2x12 as top chords.
This 'truss' would support approximately 7500 pounds roof load from the ridge beam and hip rafters.
I'm looking for some references/software(freeware hopefully) to check the adequacy -even hand calculations. Secondly, are the connections. The rafters are plumb cut and nailed to the ridge beam. Seems as though we would need to attach the rafters/top chord more securely, possibly with metal joist/rafter connectors. Thirdly, maybe add some webs, although there's not much room with the roof pitch and lumber sizes. Lastly, members are about 6 inches deep above the top plates. 2x12's have horizontal cuts even with the top plate toward the interior, and the lvl has been cut on the top to match the roof sheathing, until full depth is realized.
Hopefully I've explained ok - thanks for comments or suggestions.

RE: wood truss design

If I understood your question, it seems that you want to remove the post below one end of the LVL ridge beam and then create a simple truss at the same location by using the 24' LVL as the bottom chord and 2x12's for the top chords.  This "truss" would support a 7500# point load at the peak.

Please don't take offense, but I think you are in over your head on this one.  The analysis of the truss forces, and more importantly the connections, are critical to get right, and you really shouldn't attempt the design without assistance or close review of a structural engineer.  I investigated a 10-yr old cabin a few years ago that had non-engineered roof trusses - I got to see it after the chord connection failed, the walls were pushed out, and the roof collapsed onto the floor.  Luckily no one was home at the time.

Don't put your Civil PE license at risk on this.  Either leave the post where it is and strengthen the beam and door header, or else send this one out to a structural PE.

RE: wood truss design

At 3:12 pitch, I would expect the thrust between the top and bottom chords to be quite large. This would be difficuly to resist with conventional fasteners unless a large number of fasteners is used. PE trusses can do it because the allowable values are based on testing and the great controls you have in manufacturing. Field connections are another story.

RE: wood truss design

(OP)
Thanks Houseguy-
That's why I was looking for some information on connections. However, there is alot of overlap area between the top & bottom chords.
Even if I assumed a 10,000 pound center load, the axial forces are about 20,000 pounds. So as far as the members go, I'd double up the 2x12 doug-fir rafters (1350psi allowable compression parallel to grain), and also re-use a 12" LVL from the ceiling below and add to the existing 12" lvl bottom chord. Thus 2 rafters sandwiched between 2 lvl's. I'm looking for the allowable tension stress for the lvl's. Size of the members now should be plenty sufficient.
Next, like you say, is the connection betweem the top and bottom chords. Preliminary, I'm looking at a sufficient number of through bolts (and checking shear capacity of the bolts). As I mentioned, there's alot of overlap area.
Timber trusses have been around for awhile, but it seeme as though all trusses are manufactured now. I have a book on timber design coming, so I'll see if it has anything regarding bolted connections.
We'll also be seeking the advice of a structural engineer - I'm curious to see what he suggests. The contractor has already come up with some crazy and expensive solutions like installing ceiling joists throughout the second floor, and putting a permanent post right in the middle of the garage(right where a car can run into it).   

RE: wood truss design

Even with alot of lap, you have to consider edge and end distances with the bolts (7xD for end dist and 4xD for edge distance). If you are setting up a "sandwich" I'd expect one LVL bottom chord and a single 2x12 rafter on each side would be most efficient. 1/2" bolt capacity might be in the range of 1100 to 1300lbs per bolt I'd say (for double shear) so your talking 16 to 19 bolts. If you could go with 3/4" bolts you might be in the range of 1500 to 1800 lbs per bolt so (again assuming 20,000 lbs is right) your talking 12 to 14 bolts each side.
There's probably more that should be considered with such a large bolt group but this simple analysis is a good start I think. Often, I find steel plates are needed to make such a connection.
Good luck

RE: wood truss design

(OP)
Thanks again hg-
I was also thinking of another option by using the other 12" lvl from the garage ceiling, and attaching it to the bottom of the 2 lvl's, as you mentioned; thus creating a box beam. And then add a 2x4 spacer between the 2 lvl on top. Don't need to rip the lvl down to a 5" plate, we could use 7" and trim the whole member out to look like a barn beam. Thus we have additional cross-sectional area for tension.
I know there's a limiting span to using box beams. Our span would be 23', with a concentated load of 7500 lbs. in the middle.
I also just found a test report on nail resistance. They did some other destructive tasts as well. Take a look. Also shows a similar situation on page 26. (except my span is wider). http://www.huduser.org/publications/pdf/roof_connections.pdf

RE: wood truss design

In regard to analysis of loads on the truss, a few years ago while perusing the shelves of a used book store, I came across a copy of Harry Parker's book on roof truss design published in 1949.

It was originally designed to help architects understand the statics involved the design, but what floored me when I read it is that it utilized a method that used graphic representations of all the loads in the truss.  I vaguely remembered it from my days in school back in the sliderule Age, but never gave it much thought afterward.

But fastforward to a world with ACAD and lo' and behold, the system turns out to be the simplest, fastest, most accurate method of determining truss loads that I have ever run across. I can analyze every member in any truss nearly as fast as I can draw the load diagram without utilizing a single mathematical calculation. I doubt the book is still in print, but perhaps an INET search might turn up a copy.

RE: wood truss design

One common thing I have done in these cases (typically 2 car garages) is to run parallel roof girders to support the floor loading and roof loading. Typically they are 3 or 4 PLY girders with 2x10 BC's to facilitate hangers for the floor system.

Problems arise when you are focusing the loading on the interior walls and over garage openings. At 16" with a common 16' garage door you will have problems getting a 2-PLY 16" LVL to support that loading. If you have a center post support the garage header you might be able to get away with the 16" LVL header, however the problem then arises that the heavy focus'd loading on the interior bearing wall is going to require more bearing area than the original 2x4 wall will provide, creating the need for a 2x6 wall or the use of bearing enhancers if you are comfortable with them.

You also need to make sure there is adequate support at the foundation for that large of a point load.

RE: wood truss design

I would follow HeberPE's advice.  The connections will probably be the critical element.  I would not attempt what you are proposing with out guidance from some one who is very  experienced in the design of wood connections.

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