Heel/Eave blocking for shear transfer

[Robert216]

I have used this detail as a mechanism to transfer shear from a roof diaphragm to shear walls. Most contractors tell me that they have never done this. They state that they have never seen this detail. Even when I thumb through "typical wood framing details" found in published literature, I find more often than not that this issue is not addressed (there is no heel blocking shown). Are other engineers not specifying this in their drawings? I thought it was a fairly typical detail. If it's not, how would one transfer shear from a roof diaphragm to a shear wall without putting the rafters in cross grain bending? Do most engineers ignore cross grain bending in the rafters? Thanks in advance for any insight.

 

[JAE]

I think for smaller, one or two family dwellings, the use of blocking isn't too common.

However, for larger apartment buildings and commercial buildings I think this is more necessary.

In the US, the IBC has two methods that framed buildings can be designed to. One is the "light framed construction" where the IBC presents empirical details and requirements that allow a designer to avoid doing a direct wind/seismic design and simply "follow the rules". I don't think the light framed requirements include blocking.

The other way is for an engineer to directly design the building based on lateral wind and seismic loads. Here, I would always use some type of shear transfer.

Other threads here have discussed different ways of doing this - blocking every other truss space, blocking with holes for ventilation, etc.

Lastly, the contractors don't have to take responsibility if the roof blows off.

 

[ash060]

Down south blocking does not exist. Everything is transferred thru the truss seat connector, I guess.

There should be blocking at the heel, but it doesn't happen.

The thing is roofs still tend to perform ok without.

I tend to get lucky with low heel heights, but i will put it in at the heel if it is too large

 

[Robert216]

Being in a high wind zone and seismic design category D all of the time, I have never used the emperical (or prescriptive) design. It sounds like most everyone has been ignoring the cross grain bending. JAE, one point that you brought up is one that I have never understood. Why would a shear transfer mechanism (or any structural design/detail) be less for a one or two family dwellings? I have even had a building official tell me that in a certain municipality the design wind speed was 130 mph for commercial structures and 120 mph for residential. I would think that in a design wind event, more people would be at home and not at work... Just a thought. Thanks.

 

[DaveAtkins]

Sometimes we structural engineers should step back, away from our calculations, and think about failure modes.

I have never heard of a wood frame roof racking sideways due to a lack of blocking.

In a hurricane, most roofs fail in uplift. Hence, most jurisdictions (even away from hurricane zones) are now insisting on hurricane clips at all trusses (i.e., no toenailing allowed).

This is just my philosophical two cents--I don't disagree that calculations indicate a need for blocking, and I use blocking and/or sheathing at high heels.

DaveAtkins

 

[Robert216]

Good point DaveAtkins. Thanks

 

[woodman88]

I have always detailed the blocking. But under the "Structural engineering other technical topics" forum, A thread "Blocking at end of wood trusses" talks about this. One statement is that wood trusses will transfer 50plf if under 6" and spaced no more than 24" oc. If so I would get the truss engineer to state this on the truss designs.

Garth Dreger PE
AZ Phoenix area