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Failure of Connection between Rafter and

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Is this a situation that defies statics?

Assuming that the ridge beam is a beam (supported by columns or walls and 
capable of supporting gravity loads from the rafters) and is not a ridge 
plate, and the wall is incapable of resisting horizontal loads, how do you 
get horizontal loads at the ridge beam?  (Wind loads that act normal to the 
roof structure are a different subject.)

Now, if you have a ridge *plate* that is not capable of supporting gravity 
loads from the rafters, then the only reaction possible at the ridge plate is 
a horizontal load which has to be resisted by some other horizontal 
restraint.  If the wall is incapable of resisting horizontal forces, then 
collar or ceiling joist ties are necessary to provide the horizontal 
restraint.

For the broader question of why some of these roofs don't collapse since some 
of the collar ties are seriously inadequate and ceiling joist ties are not 
always connected to the rafters, it seems to me that the roof begins to 
perform as a folded plate structure, i.e., a deep inverted "V" beam, rather 
than a series of rafters supporting sheathing, etc.  This is also a possible 
explanation for bow string truss supported roofs not collapsing when the 
tension chord fails --- the roof starts acting as a curved shell and holds up 
the failed trusses.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Mark Oakford wrote:

. > This post relates to sloped wood roofs framed with sawn lumber or
. > manufactured I-joist rafters supported by a ridge beam. Does anyone know 
. > of a case where the connection between the rafter and the ridge beam 
. > failed due to axial load in the rafter?

. > The axial load, which equals the vertical load times the sin(slope), can
. > exceed 2,000 pounds for I-joist rafters. The typical details provide by 
. > the I-joist manufacturer calls for a Simpson LSSU variable angle seat 
. > hanger, which has an allowable tension of zero pounds. The staff engineer 
. > at the I-joist manufacturer believes the roof diaphragm carries the axial 
. > load from all the rafters into the roof diaphragm and over to the end 
. > walls.

. > I would like to know if other engineers size the tie strap at the ridge 
. > beam based on the calculated rafter axial load and if anyone knows of a 
. > failure resulting from an inadequate connection. If you would like a
. > spreadsheet for determining this axial load request it from the e-mail 
. > address shown below.

. > Thanks

. > Mark Oakford, P.E., oakfordm(--nospam--at)RSEC.com