Wood floor truss bearing details 2

[bjb]

I have a 2 story wood apartment building to design, with wood floor trusses and wood roof trusses. I have questions about the bearings of the floor trusses, especially at the exterior walls. The truss bearings are bottom chord bearing. Right now we are thinking trusses at 24" and wall studs at 16".

In the details from the WTCA, they only show a 2x rim band at the top of the floor truss. Maybe you can transfer the vertical force from the load bearing studs with this band, but can it be considered adequate to provide lateral stability for the trusses during construction? Also, can you transfer your horizontal shears between the floors using just this band? Or would you use a full depth rim board where your shears walls are? I like the idea of using full depth rim board everywhere, but I worry that the cost for this will cause the owner and contractor to complain, especially if a full depth rim is not typical. Thanks in advance.

 

[SperlingPE]

Some kind of blocking at or near the bearing location will be required. Be sure of your load path for horizontal loads. Your horizontal floor diaphragm should be connected to your vertical shear wall (if you are using plywood sheathed wood framed walls as shear walls).

 

[bjb]

I do have one idea that I would like to put out here for comments. I could use the standard WTCA detail and use a 2x6 rim band at the top of the floor truss. I then design a connection for the sill plate of the upper wall into the rim band for the horizontal shear. The sill plate transfers roof diaphragm shears. The rim band collects floor diaphragm shears, so I now have collected both roof and floor diaphragm shears into my rim band. Then I can have a flat 2x between the floor trusses that sits on the double top plates of the lower wall, and I also have a continous band of plywood that goes from the top of the rim band down to the bottom of my flat 2X. I design connections that transfer my shears into the rim band at the top, and also that take these shears into the flat 2x and double top plates at the bottom. Essentially, I am taking my rim board that would be sitting on the double top plates with sawn laumber or I joists and moving it to the exterior face of the wall. The thickness of this matches the typical sheathing thickness. My uplift is transfered between upper and lower chord studs with continuous straps, and my vertical load transfer is done with my 2x rim band which is being bent the strong way for veritcal loads.

With this approach I think that I can transfer shear forces between the upper and lower floors and still use typical WTCA details for the floor truss bearing. Thanks in advance for your comments.

 

[RARWOOD]

The problem you are dealing with is very common. I imagen there are a number of different solutions, some more costly than others.

Basically it is just a load transfer problem. Your solution sounds like a good approach. The key to the problem is establishing a load path. Your solution provides a load path for the forces you need to transfer.

When I was younger my engineering mentor indicated to me he thought that if you designed a load path, that even if the force distibution was not exactly as you assumed your structure would still be ok. His idea was that when you build stiffness into a stucture, as it was loaded that the forces would be redistributed to the stiffer elements as other elements tried to resist loads they were not design for.

I've all ways found this to be a pretty good rule. Structures have a lot of redundancies which we can not account for. I've all ways felt that if my load path was not quiet right that the stiffness I built in along with the redundancies would produce a safe design.

 

[bjb]

RARSWC, thanks for the post. The engineer who mentored me taught me the exact same thing about load paths, it was one of the most useful things that has ever been passed on to me. That viewpoint helps to simplify things, I have seen other (mostly fresh out of college) engineers who try to make a theoretically rigorous and exact analysis of things that could be more easily handled with a few conservative simplifying approximations.

I have a tangent from the intent of my original post. One load path that I have found difficult to handle is transferring sloped roof diapragm shears in a wood building into the wall below. I haven't seen a relaible way to do it without full depth blocking, but I get resistance because of ventillation concerns and the fact that is was not commonly done in my area (upstate NY). To deal with ventillation, we cut a triangular notch in the block above the insullation. Many are still not used to seeing this detail here, and I think it's because berfore NY adopted the 2000 IBC, we used a code that was unique to NY where the lateral force and load path requirements were not well spelled out. We also used to be able to squash insulation at the truss heels, and the truss heels were not very deep at all. I think we gor by with low truss heels without blocking becuase of the very fact that the heels were low, and most of NY has relatively low wind loads. Now, we can't squash insulation anymore, and I see 9 1/2 to 12 " of insulation up there. With a truss heel that deep, I think you need blocking up there.

 

[RARWOOD]

djd

Do a search of your question on this web site. In the last year, if I remember correctly, there was a post on this web site addressing your blocking question. The post may have been in the shear wall and diaphragm section, or the general structural section.

 

[bjb]

RARSWC, I have done this search, and some of the previous questions about blocking were started by me. I have since been in contact with the senior engineer at the APA because this issue with the blocking kept on bothering me. This is what he said regarding unblocked diaphragms, in a nutshell:

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.