full depth blocking at eaves
full depth blocking at eaves
(OP)
In my area (upstate NY), I see designs by others that do not have any blocking at the eaves where trusses (or rafters) bear on the wall. I know that this topic has previously discussed at some length. I provide these blocks at every other truss space so the attic can be ventilated without drilling holes in the blocking, which the architects that I work with are dead set against. In my area, our wind and seismic loads are low to moderate.
This is not so much a question, but I would like to start a discussion about these blocks because I think that many omit them from their designs. I would like to know how it is possible to transfer roof diaphragm shears into the walls without blocking of some sort.
In my area a lot of insulation is required, and truss heels tend to be very deep, and we are also not allowed to squash the insulation at the eaves. This makes blocking even more important due to the deep truss heels. Also, 1604.4 of the 2000 IBC requires a complete load path capable of transferring loads from their point of origin to the load resisting elements. How else can this code requirement be feasibly met without the blocking? The NY building code is still based on the 2000 IBC, but I guess that the above code requirement is still in the 2003 IBC.
This is not so much a question, but I would like to start a discussion about these blocks because I think that many omit them from their designs. I would like to know how it is possible to transfer roof diaphragm shears into the walls without blocking of some sort.
In my area a lot of insulation is required, and truss heels tend to be very deep, and we are also not allowed to squash the insulation at the eaves. This makes blocking even more important due to the deep truss heels. Also, 1604.4 of the 2000 IBC requires a complete load path capable of transferring loads from their point of origin to the load resisting elements. How else can this code requirement be feasibly met without the blocking? The NY building code is still based on the 2000 IBC, but I guess that the above code requirement is still in the 2003 IBC.





RE: full depth blocking at eaves
As you indicate the forces have to be transfered into the shear walls. I don't know how you do that with out blocking and sheathing. One thing you might think about is using a structural facia on the end of the truss tails to act as your diaphragm chords. I can't remember if I have every actually done that. I know I have thought about using that approach in the past, combined with a plywood soffit on the underside of the truss chords.
Another idea is to see if you can get the press plate truss people to step the truss down over the bearing wall to allow for a double 2x4 plate to pass through. I know the press plate truss people will not like that idea. However sometimes you have to require the supplier to make accomadations they don't like inorder to provide their product for the project.
Another idea is to stop the trusses on the bearing wall allowing for a diaphragm chord to be framed in. If you do that you would have to hand frame the overhang.
RE: full depth blocking at eaves
RE: full depth blocking at eaves
Omitting ever other load tranfer menber is not much better then none.
RE: full depth blocking at eaves
RE: full depth blocking at eaves
Dik
RE: full depth blocking at eaves
Other times we have used partial height blocking to cut down on the vertical height of the truss "seat" - still counting on the trusses being bent sideways, though, so we only use that on smaller structures.
We've also used full height blocking everywhere, but drilled in large holes to allow the ventilation to occur.
We've used this in more critical areas where a lot of load gets pulled down into a shorter shearwall.
But basically, its a structural system that doesn't really lend itself well to a pure load path. The same is somewhat true about steel joists where a lateral deck shear is many times assumed to just flow down through those 2 1/2" deep joist seats...magically I guess.
RE: full depth blocking at eaves
We almost always use full depth blocking, with 3 - 2 inch diameter holes for ventilation.
For steel joists, always either an "inverted" 2.5"x2.5" angle or 2.5" HSS as "blocking".
RE: full depth blocking at eaves
When I have blocking every other truss space, I make sure it is connected to properly transfer the calculated shear load. I don't rely on toe nails, I have used Simpson A35 anchors to transfer the shear. To deal with gaps in the roof sheathing, we put a flat 2x4 up there, so that we can get the perimeter nailing of the sheathing. I know it's not perfect, but it's better than the nothing I usually see, and it does provide a load path for diaphragm shears. The architects that I usually work with so far haven't budged on having full depth blocking everywhere and drilling holes in the blocking for ventilation. It seems to me like there is a conflict between the code requirements for ventillation and providing a load path, which is also required by the code.
For those of you who have used full depth blocking everywhere, have you had problems with the contractor not providing the holes, or any other constructability issues? Have you had problems getting the architects to go along with it?
RE: full depth blocking at eaves
Consider these options:
Formed sheet metal shear strip stapled to the sheathing
Diagonal tension straps (Simpson) to take the shear down to the base of the adjacent truss. But that truss connection has to be sized appropriately for that shear and combined with uplift.
Mini-shear walls at every third or fourth truss, with independent tie downs for OT. Some may argue that the plywood, laid lengthwise across the trusses, will act as the diaphragm shear strut and carry it to that shear wall (which is the way a diaphragm transfers shear, to my understanding). 2x4 blocking and a flat strap at the diaphragm boundary element that directly ties the roof diaphragm (plywood) into the shear walls.
Solid blocking at every second or third truss space (depends on the shear), tied together as mentioned above. The blocking has its own connection to take the shear into the bond beam or top plate. If the shear is not too high, the truss connection itself may handle the shear.
Check:
APA document called "Introduction to Lateral Design"
IRC 2003 TABLE R602.3(1)
footnote I. "... Spacing of fasteners on
roof sheathing panel edges applies to panel edges supported by framing members and at all roof plane perimeters. ..."
The ridge is a roof plane perimeter and so requires fasteners as called out in the table for the IRC.
Of course edge support is required for some sizes of plywood. Refer to Table 503.2.1.1(1) some don't require it. The vast majority of wood truss roofs I've seen have used trusses at 24" with 1/2" or 9/16" sheathing, which doesn't require edge support per the table.
What about the ridge (we use continous ridge vents)?
RE: full depth blocking at eaves
It is certainly correct that the diaphragm (blocked or unblocked) must be connected to the shearwalls to transfer the load into them. In unblocked diaphragms the loads are relatively low and for transferring these loads into the perimeter shearwalls a number of methods may be used. If a conventional truss in conjunction with an unblocked diaphragm is used where the depth of the truss over the walls is about 6" or less, I think a saddle-type connector such as a Simpson H1 or H10 is appropriate even without blocking. As the saddle supports the sides of the shallow framing, I think the tendency to roll is minimized at the loads seen in an unblocked diaphragm. Of course if we are talking about a raised heel truss or deep rafter or deep parallel chord truss, or a highly loaded blocked diaphragm, even when a properly sized Simpson-type saddle anchor is used, blocking would be required to prevent overturning. Just where the "tipping point" occurs, (no pun intended) that point where a saddle-type anchor is sufficient and where blocking is required is a matter of engineering judgment. In most cases the requirement for bird blocks takes the question off the table. Note that if the shear is completely taken out by the anchor, the bird block only has to be minimally attached to the top plate.
RE: full depth blocking at eaves
I think that we are back to full depth blocking in every other joist space.
RE: full depth blocking at eaves
However, I have seen test results where a full scale sloped roof diaphragm was tested for shear transfer between the diaphragm and the shear walls without blocking. The depth of the truss over the wall was only about 6" though. They looked at the transfer capacity without hurricane ties and with hurricane ties. They found that even with strap-type ties (Simpson H2.5) there was an increase inshear transfer capacity. The document that has these tests is "Roof Framing Connections in Conventional Residential Construction" by HUD. You can get it at www.pathnet.org. I must give credit to SLideRuleEra for pointing hooking me up with this document.
In my practice, I provide the load path using full depth blocking without relying on the hurricane ties, especially because in my area we use trusses with energy heels because we can't squash insulation at the heels. Maybe in the future Simpson will develop an anchor that can handle the overturning, providing an option to the use of blocking. When challenged by architects and contractors about the full depth blocking, I am better able to defend the use of full depth blocking by being able to refer to the APA supporting its use. That was one of my primary reasons for contacting them, to be able to have something in writing from an acknowledged source supporting blocking. When I tell architects and contractors that the Code requires a complete load path that transfers load from their point of origin to the resisting elements, their eyes tend to glaze over.
RE: full depth blocking at eaves
I know they use heel straps but I will have to look more closely for what they do to prevent rollover in shear.
RE: full depth blocking at eaves
For venting, you can cut a V shaped notch into the block at the top, or you can drill large holes for venting. You can also have solid full depth blocking every other space assuming the nailing works as a compromise between venting and struct requirements. The engineered wood handbook by APA talks a little bit about this.
If you don't have blocking of some sort, I don't think you have a positive load path that transfers roof diaphragm shears into the shear walls, and the chord also might not be connected to the diaphragm. Our Code, the 2000 IBC requires a complete load path.
Is your area of Canada a high-seismic or wind area? What area of Canada are you in? In upstate NY, we are neither high-wind or high-seismic. Prior to adopting the 2000 IBC in 2003, we had our own unique code that was light on the lateral requirements, and I never saw any full depth blocking. With the IBC requirements for load path, and the raised heel trusses required by our energy code, I don't see how you can have a reliable load path that has a calculated strength unless you have blocking of some sort. I have seen test results where unblocked shallow truss heels were tested, but haven't seen any testing on raised heel trusses. With these test results, I could probably justify some of the older unblocked shallow truss heels in my area. Maybe the answer is "we have never used blocking before and never have had a problem". Raised truss heels are going to much more prone to overturning, and I think this has to be considered.
RE: full depth blocking at eaves
I would agree with the statement it has always been done that way. I am in a low wind and seimic load area, but even higher snowload than yourself. The peak wind gusts experienced here in the last 100 years would be something in the magnitude of 60 mph.
I just read through the local code and there is nothing about blocking, only about the tie down of the joists to the sill.
Maybe we will see some problems some day, who knows.
RE: full depth blocking at eaves
My code (IBC 2000) has load path requirements for transfering a force from its point of origin to a resisting element. I interpret this to be concerned with more than just uplift.
RE: full depth blocking at eaves