wood roof diaphragm to wall connection at eave 1

[bjb]

My first question: Considering a roof framed with wood trusses, with the roof diaphragm shears transfered into the walls through properly designed light gauge steel anchors, and an unblocked wood diaphragm. In IBC Table 2306.3.1 for unblocked diaphragms, the maximum nail spacing is given at supported edges. At the eaves, is it correct to say that no full depth truss blocking is required, even though this is a diaphragm boundary?

My second question: When a blocked diaphragm is necessary and you need continuos blocking at the eaves, how do you deal with ventillation requirements at the eaves? I have been told by architects that drilling holes in the blocking won't provide enough area, and that V shaped notches will probably be obstructed by attic insulation. Apparently they are not allowed by the energy code to squash the insulation at the truss heel. For a similar reason, using a short piece of blocking with an air gap at the bottom in conjunction with full depth blocking in alternating truss spaces won't work.

 

[jjeng2]

Second Question: Use a different ventilation system. Ridge Vents, Mechanical Ventilators, Etc.

First Question: Not totally sure what you are saying but....
Im assuming that the blocking would only be required for the transfer of diagphragm forces and not as part of the truss bracing. If your light gauge connectors carry load directly from the diagphragm to the wall I dont think you would need blocking. The tables in IBC account for the maximum diaghphragm loads which would be at the transfer point. I think I just confused myself more!

 

[JAE]

Some type of blocking or transfer member IS required at the eaves where the diaphragm ends and the shearwall occurs below.

In most cases, solid blocking between trusses is required directly above the shearwall/support wall and this allows edge nailing at the sheathing/blocking interface. Also, the blocking must then be properly connected to the double top plate of the wall framing to transfer the load from the roof to the walls.

This is all thought of in terms of load path...follow the load and at each interface there must be a connection design to transmit the lateral forces to the resisting elements....whether the diaphragm is blocked or not, you do need edge nailing around the perimeter of the diaphragm.

 

[jjeng2]

I agree with JAE but I think bjb is saying the load from the diagphragm is completely transferred by steel anchors. If he didnt say this I would think there would be a gap between the bottom of the diagphragm and the shear wall and this would have to be filled so the entire edge of the diagphragm was connected.

 

[bjb]

In my first question I am saying that the diaphragm load is completely transfered by steel anchors. This is for a very small building with low lateral loads. I agree that full depth blocking is the best way to go. However, I run into conflicts with architects because they want ventillation at the eaves. They point to Section 1202.2 of the IBC 2000 that requires a minimum of 1" clear space between the insulation and roof sheathing.

Providing full depth blocking in every other or every third truss space can work to transfer the loads from the diaphragm into the walls while allowing ventilation. For an unblocked diaphragm, is it OK to ONLY have full depth blocking every other or every third truss space, with nothing in the other truss spaces?

Table 2306.3.1 of the IBC 2000 requires fasteners at 6" max for supported edges. It seems that only providing blocking every other or every third truss space is acceptable for an unblocked diaphragm because while the partial blocking provides load transfer from roof to walls, the allowable loads in Table 2306.3.1 for an unblocked diaphragm take into account the unsupported panel edge.

For conditions where you do need continuous blocking at the eave, what can you specifically do to ventilate the attic? It seems that Section 1202.2 requires eave vents no matter what, but this conflicts with structural requirements.

 

[JAE]

bjb:

The ventilation at the eaves can be solved by specifying holes (like 3 or 4 inch dia.) drilled throught he eave blocking. Many times you, the engineer, have some control over the height of the rafters/trusses at the eave/wall location so the blocking between trusses at the wall can be specified as 6, 8, 10, or 12 inch lumber - and the holes then can be drilled in the mid-height to allow the ventilation.

Another solution is to use full depth blocking in every other truss space and then use partial height blocking (say 2x4's) in the other spaces where the tops of the full height blocking and the tops of the partial 2x4's are at the underside of the sheathing. This gives you a continuous edge nailing along the eave, but allows space for ventilation as well.

In terms of unblocked/blocked diaphragms - I believe that for unblocked diaphragms, each sheet of sheathing is presumed to be fully nailed along two opposite edges and not nailed along edges that span across truss spaces. But I also believe that all unblocked diaphragms are intended to be fully edge-nailed around their perimeter.

I don't think you can confuse unblocked edge nailing with perimeter edge nailing. Now this idea of nailing the perimeter of a diaphragm isn't specifically in the IBC, its just my own preference and interpretation.

 

[bjb]

Thanks for the reply JAE. Here in upstate NY (Albany area) we usually need 9" insulation in the attic for R30, and we're not allowed to squash it at the truss heel. The truss heels that I usually see from our architects are 12" and greater. I just ran some numbers, and if we use a 14" heel depth this allows 9" of insulation, 3" dia holes, and 1.5" between the holes and the top of the block. The 1.5" allows enough penetration for 8d nails according to table 2306.3.1. Two 3" dia holes should be sufficient for ventilation with my trusses at 24" centers. I could use Timberstrand Rim Board by Truss Joist to do this. I can't drill at midheight because the insulation will obstruct air flow, and because the insulation can't be squashed I'm stuck with engineered lumber for my blocking.

Having run these numbers, I see that providing full depth blocking with holes does work with the truss heels and insulation that are typical for my area. Thanks for pointing me in the right direction.

 

[FalsePrecision]

in a nutshell...

Simpson H1

 

[JAE]

I guess I wouldn't go with the H1 anchors (2 3/4" tall) to resist a lateral force from my diaphragm - the anchors in Simpson are shown to have a 415 lb lateral capacity. But this is straight shear at the base of the truss...how do you account for the moment developed with your force occurring 12" above the H1? Simpson is silent on this - assuming that there is no overturning moment.

 

[bjb]

Another thing to consider is that when the wind blows, you will have forces perpendicular to the wall, in the plane of the wall, and possibly a net uplift. Simpson requires a unity equation check for the interaction of these loads.

 

[FalsePrecision]

JAE - oops! - I meant Simpson H14, and partial depth blocking (2x8), such that the top of blocking is snug to the roof sheathing? Also - there are cardboard insulation "dams" that may be useful in some applications.

 

[jike]

So the roof sheathing is nailed to the partial depth blocking which transfers its load to trusses (in cross grain bending) and then the H14 resists the overturning and shear. Before using this concept, I suggest that you contact the truss manufacturer to see if the trusses are designed for the cross grain bending. I also suggest that you talk to Simpson about overturning capacities. I would guess that it was not tested for that since no overturning capacities are given.

I would guess that the truss manufacturer and Simpson will not go along with this idea. Let us know what they tell you.