[PLUG-TALK] OS Software for Architectural Design

[John Jason Jordan]

On Sat, 12 Sep 2009 20:31:57 -0700
Larry Williams <larryw at holbrookmasons.org> dijo:

> On Sat, Sep 12, 2009 at 19:53, John Jason Jordan <johnxj at comcast.net> wrote:
> 
> > I probably don't even need software. All I want to do is design a
> > one-in-four 14 foot roof truss. Just your basic W truss. You'd think
> > I'd be able to find free designs aplenty on the web, but every site is
> > intent on selling me a full set of plans complete with list of
> > materials.
> >
> > It's just for a shed, so I could probably wing it. But not being an
> > architect or construction engineer I am unfamiliar with things like the
> > load bearing capacity of a 7' 2x4. And I can't even find that on the
> > web.

> Wouldn't Home Depot or Lowe's have any information?

I looked at all the how-to books and came up empty.

But I finally did discover the secret. And it's so simple I'm surprised
at myself for not figuring it out.

If the truss is 14 feet you start with a 14 foot 2x4 that will be the
ceiling/eave. Calculate the height of a right triangle that is 7' on
its base, assuming you want a one in four pitch (whatever height a one
in four pitch comes to at the top of the ridge line). And then you
calculate the hypotenuse, which will be the top 2x4 of one side. Find
the midpoint of this 2x4. Place a pin there down to the bottom 14' 2x4
at a point midway between the center point of the top rafter and the
midpoint of the 14' 2x4. And where that pin hits the bottom 2x4, place
another pin up to the ridge line. There you have your basic W truss.
The bottom 14' 2x4 can span great lengths because the stresses on it
are at self-canceling angles; that is, the pin coming down from the
midpoint of the top rafter and the pin going up to the ridge line are
at the same angle on the bottom 2x4.

I can't even remember the name of my high school geometry teacher. But
I do remember what he taught me.