stick frame roof "bracing applied to rafters above collar ties"

[Materofact]

STRUCALC design software gives the option of "bracing applied bottom of rafters above collar tie" which significantly increases the strength of the stick frame roof. in this particular (as built) scenario i am looking at, from 80% inadequate to 20% adequate for the loading with a ceiling joist system. But what technically is "bracing", what is minimum bracing to qualify? plywood full length on the underside? a 2x4 midspan? Cant seem to find any info.

 

[msquared48]

If the collar tie is placed high in the rafters, I cannot see any advantage putting more reinforcing above the tie to help the rafters.

However, if they mean rafter ties, which also serve as ceiling joists and are placed low on the rafters, then yes, adding in horizontal meebers (collar ties) above the rafter tis will strengthen the rafters significantly.

Mike McCann, PE, SE (WA)

 

[jayrod12]

I wonder if it's intended to brace the compression flange, assuming they're getting negative bending above the collar tie.

But generally the unbraced length above a collar tie is so short it shouldn't make that much of a difference.

 

[Materofact]

yes goes from 90% undersized to 20% adequate when "bracing is applied to bottom of rafter" above a rafter a rafter tie (with ceiling joist at plate hieght). the software can place the "tie" anywhere from on the plate to the peak as part of the design. It is interesting that "bracing" is not specifically detailed or described or modeled. I would agree that collar ties above the rafter ties placed at midspan of the rafter is probably what is needed. I always thought that collar ties in addition ceiling ties were mainly for spreading of the rafters under uplift conditions. For inferior sized stick framing, to go from significantly understrength to ok is a big boost from just collar ties.

 

[msquared48]

Seems to me that it would be a lot simpler to just double up the rafters at the collar tie locations.

Mike McCann, PE, SE (WA)