Blocking at end of wood trusses 3

[NFExp]

I'm looking for comments/information on blocking required at end of wood trusses. We generally design our diaphragms as unblocked however we specify 2x blocking at the ends of trusses with a heel height greater than 4". For trusses with a heel height less than 4" we do not require blocking. We justify using the blocking to prevent truss rotation and to help distribute shear.

If diaphragm shear is transfered by truss connectors, is it possible to have a truss heel height of 6" without blocking. If so how do I justify this?

Thx

 

[BAretired]

Blocking would seem to be a sensible precaution, even if only provided every fourth or fifth truss space. What is the big deal? It costs almost nothing and is not worth arguing about.

BA

 

[icfbunt]

On trusses with say a 24" heel, typically I see the trusses stubbed 1/2" and the sheathing continues from the wall up the heel of the truss. Is this sufficient to transfer the shear in most cases? Or do most of you prefer actual blocking? I thinkg in most cases sheathing works (depending on geographic location). I have seen some jobs with thall heels that actually have flat trusses between for blocking.

Larry

 

[DaveAtkins]

The sheathing can certainly resist lateral loads. The difficult part is justifying how lateral load gets out of the roof diaphragm and into the sheathing, because there is no blocking at the top of the sheathing between the trusses.

DaveAtkins

 

[BAretired]

I agree with DaveAtkins. Also, blocking panels give the framers a better opportunity to square up the trusses than simply nailing sheathing from the exterior. For 24" deep heels, blocking panels should be used in every joist space.

BA

 

[icfbunt]

Here is a decent article on heel blocting from WTCA.

Larry

Larry

 

[icfbunt]

I'll try again.

Larry

 

[abusementpark]

If the truss top chord and the truss bottom chord meet at the eave/overhang on the building, couldn't the bottom chord of the truss bring the shear load back to the shear wall in weak-axis bending (assuming it was designed to take that load)?

 

[DaveAtkins]

abusementpark,

Yes, that works well also. Many times, there is a continuous ribbon board out at the tip of the trusses which can get the load into the truss bottom chords.

DaveAtkins

 

[BAretired]

If the underside of the overhang is sheeted, some diaphragm action would stiffen up the bottom chords considerably.

BA

 

[vandede427]

that was a pretty good article. i didn't read it thorougly (sp) but it seemed to show different cases.h

my only issue with putting moment into the truss heel or the partial height blocking is that you've still got to collect the linear diaphragm force and dump it into each truss. you're basically taking the edge nailing pattern, which is already tight, and squeezing it even closer.

let's say that you've got 8d @ 6" o.c. as an edge nailing. to put all that into the truss top chord, you've have no nails between trusses and 4-5 nails into the truss. this could probably split the wood all up.

even if you ran some continous metal strapping to catch the edge nailing, you'd still have to attach the strapping to the top chord to transfer 24" of diaphram load.

let's also not forget that the diaphragm and shear wall values for nail spacings are tested values based on a specified assembly. there's some internal effects going on that make the strength of the system stronger than each individual part (the whole is actually greater than the sum of it's parts, they don't correlate to shear values of a single nail).