full depth blocking at eaves 2

[bjb]

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.

 

[RARWOOD]

I get asked a lot about similar load transfer problems, however I don't have any good answers. From people I've talked with, the use of blocking as you describe is very common. I know architects and builders don't like the blocking however they are not responsible for the structural design. When I worked for an architectural firm, my boss had a easy solution, if the architects would not take no for an answer, he would tell them to go ahead and do what they wanted and sign the plans themselves.

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.

 

[bjb]

Thanks for the reply. Part of the problem here in New York State is that prior to adopting the IBC 2000, the structural requirements of the old NY building code were a bit of a joke when it came to lateral loads. I think that a lot of people around here are not used to seeing them, and then when you say that full depth blocking is required, they fight you on it. Also, there is the problem with getting ventilation, which is the major stumbling block that I have encountered with architects. Fortunately, I have convinced the architects at my company that ventillation is possible even with full depth truss blocking. I also site the code reference in my first reference as part of my reasoning for the full depth blocking.

 

[boo1]

Why not use the truss hardwear to transfer the loads?

Omitting ever other load tranfer menber is not much better then none.

 

[SacreBleu]

Truss hardware typically can't transfer shear loads because the upper chord of truss is in cross-grain bending. Only the Simpson H-1 type of hanger can transfer shear loads.

 

[dik]

Does the full depth blocking interfere with ventilation?

Dik

 

[JAE]

We've used the alternative blocking (every other truss space) idea - but then you have to worry about the gaps in the nailing of the roof sheathing.

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.

 

[Structr]

JAE,
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".

 

[bjb]

for steel joists, rollover capacities are discussed in Vulcrafts "Designing with Steel Joists, Joist Girders, and Steel deck." The capacity is low, but shear collectors like a 2.5x2.5 tube steel work well if you need them.

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?

 

[boo1]

Typically I just use the truss sheets reaction loads to develop my connections. When the shear force exceeds the unblocked shear resistance of the sheathing. (engineered design, not prescriptive conventional construction).

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)?

 

[bjb]

I have been in contact with a senior engineer at the APA, and this is what he said:

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.

 

[jike]

The APA guy might want to talk to Simpson before making those kind of statements. Simpson has told me that the H1 and H10 should not be used for that purpose. There is a torsional (overturning) component that Simpson hasn't tested for in their connection.

I think that we are back to full depth blocking in every other joist space.

 

[bjb]

I agree with you jike. If you have the full depth blocking, even if just in every other space, then you take overturning and shear transfer problems off the table. I have an earlier email from APA where they say that using full depth blocking every other space is commonly used.

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

http://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.

 

[Slowzuki]

I live north of the border in Canada and we have similar light trusses to allow for the full depth insulation at the eaves and there is no blocking used in order to allow venting.

I know they use heel straps but I will have to look more closely for what they do to prevent rollover in shear.

 

[bjb]

Slowzuki,

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.

 

[Slowzuki]

bjb,

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.

 

[bjb]

The problem that I have with "we have always done it that way" is that using raised truss heels is relatively new in my area. I question if there is enough experience using raised truss heels without blocking to justify "we have always done it without blocking and never had a problem".

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.

 

[jike]

In addition to the fact that the raised heels are a relatively new requirement, we also know that many of these buildings have never seen their full design wind load in their lifetime. We still, nevertheless, have a professional obligation to design for these loads. I believe we have no choice but to use full depth blocking.