Convetional Light Frame Limitations

[zdude]

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)

 

[MiketheEngineer]

Not heard of that limitation. Can you recite chapter and verse where this is found??

 

[AELLC]

You can increase the truss span over 40' if you install blocking under the double top plate, ie the double top plate may break for spans > 40' for non-engineered structures.

 

[zdude]

Mike,

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?

 

[MiketheEngineer]

BTW - I would also block between trusses at the end walls to get a good shear flow into the walls from the roof diaphragm

 

[woodman88]

The 2009 IBC section 2308.1.1 states that portions exceeding the limits in section 2308 are allowed, but they shall be designed per accepted engineering practive and the code. So just design the wall to support all loads being applied to it.

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.

 

[concretemasonry]

the model codes can always be superseded by local requirements as long as they are more rigid than the model code. The model codes are just minimums and local conditions may be more critical. Model codes and standards like ASCE, ACI and ASTM are just minimums that are adopted locally and can be altered within bounds.

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.