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Gable dia. was Ridge beam/joist analysis
- To: seaint(--nospam--at)seaint.org
- Subject: Gable dia. was Ridge beam/joist analysis
- From: Jason Atwood <atwoodjason(--nospam--at)yahoo.com>
- Date: Thu, 1 Jul 2004 15:01:47 -0700 (PDT)
Think I'll beat this one to death a little bit too. Now a rafter with horiz. seat cuts at bearings has no thrust- period ask Newton. Now as far as the collar tie thing-as I understand it a "collar tie" is located in the lower third of a site built truss. The tie near the ridge is a "wind beam". As the name would imply this member is provided to provide a tension connection to resists wind suction in an otherwise compression framed joint. I have never seen a site built truss with only a "wind beam" and no bottom chord or collar tie, and I have only seen this type of site built trusses in older residential buildings.
As far as diaphragm action on a gable roof:
For a cut and stack roof we specify a stl. strap across the ridge (note these rafters bear on a ridge beam) tying opposing rafters together- this does 2 things creates a "wind beam" and ties the two halvs of the roof dia. together for forces parallel to rafter span. Like many I have been troubled by continuity of a gabled diaphragm. However recently I have had some of my troubles allayed by a seemingly disparate source. While flipping through the PCI design manual I came across figure 3.6.1. which indicates that at a discontiunty in a diaphragm, equilibrium is maintained by designing for horiz. shear flow (i.e. f=VQ/I or at a centered ridge f=3V/b ) across discontinuity. I thought no sh**! Therefore it would seem b.n. at ridge needs to be provided on each side o/ p.w.d. butt jt. designed for VQ/I. Now for forces perp. to rafter span obviously the shear is zero at the ridge (assuming ridge is centered), but chord forces are max. Per ATC-7 the
design of the compression chord splice will need include consideration of uplift forces.
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