Chord Force Blocking Detail 1

[Gopher13]

I have a long and skinny billets building with cmu walls and wood trusses spaced at 2 ft o.c. Please see the attached drawing and details. There is a 3500 pound chord force that I would like to transfer into the bond beam at the top of the cmu shear wall. There is about a 4'-0" elevation difference between the top of the roof sheathing and the top of the shear wall. I have detailed truss blocking panels running between the main roof trusses to transfer the chord force into the shear wall. The question I have is; does each blocking panel and the connections to and from the blocking panels have to be designed to take the 3500 pound force or do all of the blocking panels share in transferring the 3500 pound chord force into the shear wall?

I appreciate any help or guidance you can give me. Please feel free to comment on anything I have detailed too. Thanks!

 

[jayrod12]

My vote is share. Maybe I'd see what each one is capable of when standard connectors and members are used and see how many I needed.

 

[DaveAtkins]

The chord force is not a single, 3500 pound force. It is gradually developed as you move from the end of the diaphragm to the middle of the diaphragm. The force per foot will be maximum at the end of the diaphragm, and will be zero at the middle of the diaphragm. So the blocking nearest the end of the diaphragm will need to be designed for the most force.

DaveAtkins

 

[KootK]

I vote for sharing but not uniform sharing. For example, with a uniform load, your demand at the middle of the diaphragm would be 0 kN/m and your demand at the ends would be [3500 lb x 4 / diaphragm length]. The trick is to remember that your horizontal shear stress is equal to your vertical shear stress at all locations.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Sorry Dave -- didn't see your response before posting.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Some other ideas:

1) Might be cheaper/easier to run the block all the way up and just hang the trusses from the sides of the CMU.

2) We discussed something similar here: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

I would make the ceiling level the primary horizontal brace.

 

[KootK]

I'd considered using the ceiling too, although perhaps differently from hokie. If you sheathed the ceilings of the patios with plywood/osb, you could probably scab together a load path.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

I think I must be missing something. Why don't you add some strapping to the blocking at the diaphragm for the chord forces. Then design the braces between the wall and the diaphragm the for the collector forces?

 

[cal91]

Am I confused as well in thinking that the force per foot of the chord will be zero at the ends, and max in the middle?

I thought that the chord forces behave like the chords in a simple supported truss - Maximum where the moment is maximum

I.E. for a simple truss the max chord force is wl^2/8 / d at midspan, and likewise for the diaphragm chord force?

Unless we're talking about collectors/drags of course...

 

[KootK]

Am I confused as well in thinking that the force per foot of the chord will be zero at the ends, and max in the middle?

That is true of the aggregate chord axial force (kN rather than kN/m) but not the shear being transferred to the chord (kN/m). Continuing with your truss analogy, the forces in your truss webs (max at ends) are analogous to the shear force being transferred to the chords (also max at ends). The forces on the shear panels being considered here are governed by the shear being transferred to the chord (bond beam).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[cal91]

Great explanation. Makes perfect sense. Thanks for clearing it up!

 

[KootK]

You're most welcome. Thanks for the star. wannabeSE might still be on to something here with the overall approach.

Then design the braces between the wall and the diaphragm the for the collector forces?

At the risk of sounding obtuse, what braces? Are you envisioning intermittent diagonal bracing in the vertical plane connecting the strapping and blocking to the top of the block wall? I've no doubt that your approach is sound. I just want to understand the details being proposed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.