Subject: Failure of Connection between Rafter and Ridge Bea
Date: Fri, 30 Apr 1999 02:16:56 -0400
SE>This post relates to sloped wood roofs framed with sawn lumber or
SE>manufactured I-joist rafters supported by a ridge beam. Does anyone know of
SE>a case where the connection between the rafter and the ridge beam failed due
SE>to axial load in the rafter?
SE>The axial load, which equals the vertical load times the sin(slope), can
SE>exceed 2,000 pounds for I-joist rafters. The typical details provide by the
SE>I-joist manufacturer calls for a Simpson LSSU variable angle seat hanger,
SE>which has an allowable tension of zero pounds. The staff engineer at the
SE>I-joist manufacturer believes the roof diaphragm carries the axial load from
SE>all the rafters into the roof diaphragm and over to the end walls.
SE>I would like to know if other engineers size the tie strap at the ridge beam
SE>based on the calculated rafter axial load and if anyone knows of a failure
SE>resulting from an inadequate connection. If you would like a spreadsheet for
SE>determining this axial load request it from the e-mail address shown below.
SE>Mark Oakford, P.E., oakfordm(--nospam--at)RSEC.com
SE>RSE Consulting, Federal Way, WA 98093-1417
SE>T 253-927-6169 F 253-838-3823
I believe that a pair of rafters act very much like the legs of a step
Once the horizontal component of the axial force is restrained, the
vertical component is transmitted axially to the lower rafter support.No
vertical load is therefore carried by the ridge beam unless the angle of
inclination of the rafter is zero.