site-built simple trusses for 12x16 cabin with loft?

[tekhan]

Hi,

We need advice making site-built "trusses" for a small 12x16 gable roof cabin with a loft (12x8) that we will be living in temporarily.

We live in an earthquake zone and are leaning towards making simple trusses for the roof instead of rafters, because the full triangle of the truss seems stronger (though we haven't seen any data either way).

We are trying to find out if a simple triangle "truss" with just 2 rafters ond one joist (so technically not a truss?) would be structurally sufficient for this purpose. We were simply going to cut the "ceiling joist" at 12' (the span) and attach "rafters" with plywood gusset plates to the ceiling joist and to themselves at the top, all in the same plane.

We have seen this method of simple trusses used in framing a small garage where the ceiling joist was not intended for human occupation (Taunton's "Building a Shed" p.154), so we assumed that it would be structurally sufficient so long as we upgraded to the correct lumber dimensions to accomodate human occupation of the loft.

Any suggestions as to how to find out if our planned site-built trusses are OK would be appreciated.

Thanks!
Tonya & Cory

[Richard A Hetzel]

Instead of homebrewing "trusses" and hoping they might work, why not instead use a structural ridge? That will work. The ridge would be a simple beam spanning the 16-foot dimension, carrying 6 feet of roof. Probably a single LVL would do it, but don't quote me. Have your lumberyard get some engineering for the beam from its manufacturer.

[Don_P]

The connection details of the truss are the only difference from a typical rafter/ ceiling joist roof. Why not just use the charts in the codebook and build prescriptively? In your area is prescriptive code allowed (using the tables and charts in the codebook for simple construction) or are all buildings engineered?

[tekhan]

Thank you for your suggestions. We chose the joist and rafters based on local code and figured that since it is just their method of attachment to each other that differs from the usual ridge board method of contructiing such a small dwelling, that we should have a fairly sound structure. But we wanted to be sure, either by running a mathematical formula or inputing data into a program. Unfortunaely we haven't been able to find either.

We will be building in a fairly remote location and most homes are built by the owners. We decided against a ridge board because we thought that a truss being a triangle would be stronger in an earthquake than a ridgeboard & rafter, which is a bit more of a rectilinear structure. We do get quite a few small and even medium earthquakes and it's just a matter of time before a bigger one hits (hopefully a long time).

We called a truss company yesterday and after asking us four questions(span: 12'4"; pitch: 12 in 12; snow load: minimal; joists 2x8 & rafters 2&6) they were able to use THEIR program and tell us within a few seconds that it was a structurally sound truss and did not need additional web members. Unfortunately their engineer is on holiday and we'll have to wait for his return to get a printed version including plywood gusset manufacture.

If anyone knows if this program is available online to the general public, we'd love to know how to find it so we can start building the roof.

Thanks again,
Tonya & Cory

[Richard A Hetzel]

The truss manufacturer was probably using "Gang-Nail" plates to make the truss connections.

I may be all wet here because we in the northeast do not have to concern ourselves with earthquakes, but my understanding of general seismic design is that the stiffer the structure is, the more likely it is to suffer damage in a quake. That would tend to indicate that a truss structure may not be the best idea. Perhaps someone who does know what he or she is talking about will weigh in with some seismic advice.

[Don_P]

I've never been able to find the truss programs online. Mitek, alpine etc do have websites and do sell license their software and train designers in its use. Probavbly not what you want. Most of the software originates from the PPSA software Perdue Plane Structures Analyzer (think I'm remembering that right). I've written them and its no longer available, it used to be some years ago.

You can check rafters and cj's on the awc.org span calculator

I've written this little calc to check rafters, thrust and connections, it will output the necessary forces but doesn't give the ply shear numbers you need ... use at your own risk;
http://www.windyhilllogworks.com/Calcs/Sloped%20Rafter%20Design%20for%20Bending1.htm

The NDS at the AWC has those numbers in it, there may be some other free pubs there in the download library with what you need.

Like Richard my ground hasn't really moved in 300 million years so don't necessarily take any of this as applying to there.

[tekhan]

Thanks, everyone, for your ideas and suggestions. The truss company did provide us with an approved model for our simple truss (the exact same that we had drawn up ourselves - two rafters and a joist joined by gusset plates).

It is a great experience building our own first little home. So far we have been able to find all the information we needed in books, the internet or talking to people - with the exception of designing this little truss. Too bad someone doesn't make a program available online for owner-builders to design their own simple trusses.

Thanks again for taking the time to help us out!

Cheers,
Tonya & Cory

[Russell]

Take my advise, Buy your trusses from a manufacturer. They can produce a much more uniform truss that will save you a lot of hassle in the long run and chances are they can purchase lumber cheaper and build the trusses for less money than you will have invested in them with lumber prices and your labor.

I am speaking from experience. I have done what you are doing both ways, (built my own trusses and purchased) and it was much better to buy pre-built trusses for their better structural integrity and consistency.

Russell