Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Ridge beam/joist analysis

[Subject Prev][Subject Next][Thread Prev][Thread Next]
 
----- Original Message -----
Sent: Monday, June 28, 2004 5:43 PM
Subject: RE: Ridge beam/joist analysis

I sure appreciate all the input...thanks to everyone...this is a great
resource for a guy working solo. In sum I have a vertical reaction that
induces axial load (due to pitch) in the joist. I will carry all the axial
load as tension at the ridge connection, which should eliminate lateral
thrust (besides ridge deflection).

    Don't be so quick to conclude all of the above.
 
The hump I can't seem to get over is a statics problem of analysing the
joist tension load....
 
No reply ok, you can still come visit me at the Northern Michigan Looney
Farm. I'll be the guy scratching his head with a confused look on his face
mumbling something about "vector...mmmm...span...lateral...".

Craig
    Keep at your statics. use it faithfully, on free-bodies. Do not let intuition intrude, except to pose questions.
 
And feel free to pick analogous framing elements as models to clarify things. One that comes to mind is a stair stringer, the sloped support for stair treads, that can bear on a flat surface at the bottom, and be carried by whatever connection at the top. Fancy all the treads bearing on horizontal surfaces notched into the top edge of the stringer. People stand on the treads. Gravity acts straight down. Take entire stringer as free-body, show each tread load on it. Statics compels end reactions to be vertical. Vertical end reaction at each end can be separated into components parallel and perpendicular to stringer, but resultant of them is still the vert reaction already found. You can chase those non-vertical components back up the stringer (and down from the top) as axial and shear forces-- do it.
 
Custom says that you can merely model the member as having a span equal to the slope length's horizontal projection. Do it both ways to see how they compare. Is the custom adequate?
 
Another model is a ladder. Applied load is still purely vertical, but this time assume a no-vertical-friction contact with the wall at the top ends of the stringers. Statics  will compel all the vert load to be resisted at the bottom. Since the ladder leans, statics (ie, sum of moments) compels the wall to offer a horizontal reaction. Thus the base connection has to push back equally, horizontally. The resultant at the base once again may have its components parallel and perpendicular to the stringer axis examined and chased up the ladder.
 
Now you can consider the rafter (joist) in the real situation, as to its upper support connection's nature and the vert capacity and stiffness of the ridge element as a beam. If the ridge element can't span, and there is no diaphragm, then the ladder model tends to apply, and cross-ties are needed. If there is a diaphragm instead of cross-ties, then you have a shear panel situation for in-plane components of load to the ridge line. Statics done to completion will always tell the tale.
 
It's mixing intuitive assumptions with incomplete analysis that will make you loony. That, plus a cocksure attitude, is what happened to state legislators in California. There aren't enough guards yet to keep the crazies confined inside 24-7 and undergo treatment.
 
C. O. Greenlaw, SE   Sacramento Cal