Convetional Light Frame Limitations
Convetional Light Frame Limitations
(OP)
Hello All.
I had a question from an architect recently that made me think. The limitations for specifying wall bracing under the conventional light frame guidelines in the 2009 IBC state that the truss spans shall be less than 40' (section 2308.2).
A year or so ago, I went to an ASCE seminar about designing single family residences using the IRC. There are limitations to using that method of wall bracing which include winds over 100 mph, heavy roofs, tall walls, etc. But, I don't recall seeing a truss length limitation.
1) Does anyone know if there is a limitation to using the IRC wall bracing based on truss spans?
2) If conventional light frame provisions in the IBC are limited by the truss spans, Does this negate our use of other conventional light frame provisions on that project?
3) More importantly, does anyone know what is the basis for limiting conventional light framing to truss spans under 40'?
(I suspect that the limitation is based on the test setup for qualification of the conventional light frame provisions and not engineering)
I had a question from an architect recently that made me think. The limitations for specifying wall bracing under the conventional light frame guidelines in the 2009 IBC state that the truss spans shall be less than 40' (section 2308.2).
A year or so ago, I went to an ASCE seminar about designing single family residences using the IRC. There are limitations to using that method of wall bracing which include winds over 100 mph, heavy roofs, tall walls, etc. But, I don't recall seeing a truss length limitation.
1) Does anyone know if there is a limitation to using the IRC wall bracing based on truss spans?
2) If conventional light frame provisions in the IBC are limited by the truss spans, Does this negate our use of other conventional light frame provisions on that project?
3) More importantly, does anyone know what is the basis for limiting conventional light framing to truss spans under 40'?
(I suspect that the limitation is based on the test setup for qualification of the conventional light frame provisions and not engineering)






RE: Convetional Light Frame Limitations
RE: Convetional Light Frame Limitations
RE: Convetional Light Frame Limitations
The limitation is under section 2308.2 #5 "Roof trusses and rafters shall not span more than 40 feet between points of vertical support."
AELLC,
I went back through prior editions of the building code and this limitation was added in the 2000 IBC. Your thought about the plate strength is interesting. I imagine that with the various allowable stud spacing (24" or 16"), variable stud heights and the 50 psf snow load, the code council had to put some limitation.
Do you think that partial engineering (like adding the blocking under the double top plate and checking stud capacity) would be all that is required to fit the house back into the conventional parameters?
RE: Convetional Light Frame Limitations
RE: Convetional Light Frame Limitations
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Convetional Light Frame Limitations
This is especially true is areas where there are local severe conditions, such as hurricanes and tornadoes. Any local seminar will probably reflect the local requirements that do not exactly mirror the model codes. One other example is the preferred insurance requirements for lower rates in hurricane subjected areas where history has shown problems with weak areas such as garage doors and gable end bracing that lead to uplift.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.