Wood Diaphragm Question 1

[zrck99]

The attached sketch shows a large 50' x 75' wood diaphragm with a small bumpout 10' x 50' diaphragm next to it. What are the rules for treating a small side portion like the 10' x 50' bumpout as if it will not act as a beam but rather just transfer its shear directly back to the main diaphragm?

Thanks.

 

[zrck99]

So what about a case where you have an interior bearing wall that the trusses break over. Do you think it would be necessary to connect your truss chords at the break to make them continuous across the full width of the diaphragm? It seems pretty obvious that your truss could easily dump the load back out by the midpoint of your main building but all the textbook examples force you to go full depth of the main building in order to use the main building chord as your indvidual transfer diaphragm chord.

Same question applies at lightly loaded reentrant corners. If you framing is parallel to your chord, you would have to block and strap all the way across the building to "properly" engage the opposite chord of the building. If you have a say 1000lb force at your corner, you could dump the load back out into the diaphragm within a couple feet... It seems pretty overkill to detail the force properly to go all the way across the diaphragm.

 

[KootK]

That doesn't instill the greatest confidence in the industry.

Good. Those guys are all over the map. Some are excellent and would dazzle you with their technical chops. Many are former carpenters or high school graduate family members of whomever owns the truss shop. My wife, who is an accomplished structural engineer in her own right, got started in trusses in exactly the latter way. It's pretty weird that we're both not only SE's but also both got started in the truss industry. Back in her truss days, my wife designed her father's lake home soup to nuts. It's a less sexy version of what you see below. In a storm, that front wall vibrates like a drum and it's terrifying to be eating dinner behind it. I guess my wife wasn't so hot with lateral loads back then either.

Though it feels like there's a story in the last part...

Kind of a sad one. I did trusses evenings and weekends while in university studying childhood education. I like kids and books so it had curb appeal. The truss gig wasn't very schedule friendly but it was the only way that I could find to pay for things. Advancement at the truss plant was crazy fast. To them, I was like a magic Pharaoh because I could extend the carpenter's 3:4:5 squaring rule to other situations. It was fun to be good at something. At the beginning of my 3rd year in teacher's college, I had to work a night shift at the truss plant to help with a schedule emergency. When I showed up for class the next morning at 8AM, straight from the rotary saw, I had sawdust in my hair and a wife-beater on. That day's class was about professionalism in the teaching industry and the professor made a point of singling me out to comment on how unprofessional it was of me to show up to class in that state. I was tired, hurt, and young. And I lost my temper. Now now I do this.

 

[zrck99]

KootK,

From all of your comments on here, It seems that unfortunate circumstance may have been a blessing in disguise. I'm glad you became an engineer.

 

[zrck99]

In the wood code they have allowable values for treating wall sheathing as a tie for uplift while there is a perpendicular wind suction force on the panel. It isn't identical because your sheathing thickness/fastener type/stud vs truss spacing etc. is different, but it is apparent that there have been some sort of studies in using sheathing as a tension tie.

 

[phamENG]

The sadness is relative. I sometimes regret not taking the ROTC spot to go fly helicopters for the Navy back while I was in. But had I done that, I wouldn't have met my wife and we wouldn't have our 2 amazing kids. My life is somewhat less exciting in the death defying thrill department, but I wouldn't trade where I am now for anything.

Hopefully you've had the opportunity to enrich the lives of other youngsters in ways other than as a teacher (or maybe as a teacher - I don't know what you do in your spare time). On a purely selfish note, though, I'll second zrck99's sentiment and say I'm glad you're here to contribute to our knowledge.

 

[KootK]

Do you think it would be necessary to connect your truss chords at the break to make them continuous across the full width of the diaphragm?

Same answer as before for me. I don't think that it's necessary or reasonable but, if you're sticking rigorously too the pure shear panel methodology, it's required. This is a bit of a red herring of an example though. With no connection between the truss halves, you've basically got yourself two unconnected diaphragms. So I'd be wanting to connect the bottom chords or this reason alone lest you get separation at the ridge or something.

Same question applies at lightly loaded reentrant corners.

Same answer yet again. There's usually sort of an out for this case though. Often, your partial depth drag strut will naturally run into a perpendicular framing member that can be said to complete the boundary element system for a true sub-diaphragm. That, even if you don't explicitly design the framing member for that purpose.

It's worth noting that this issue isn't specific to wood diaphragms. It's identical in steel roofs sheathed with metal deck. It's not like every wind column is attached to a diaphragm strut that runs 300', clean back to the other side of the building.

 

[zrck99]

KootK,

What do you think of justifying the use of sheathing as a tension tie by referencing the attached table from SDPWS? My roof sheathing would be 5/8" thick and I would have 10d nails at 6" o.c. at edges. My suction force on my roof diaphragm would likely be similar to the suction force on a wall panel. Thoughts?

I guess it's a little different because you don't address compression in the sheathing for the reverse load case.

 

[KootK]

Thanks for the kind words. In retrospect, it may have been for the best that the teaching gig didn't work out. About that time, in my area, there was a huge rash of male teachers getting sacked as a result of rather dubious sexual harassment charges. Putting a hand on somebody's shoulder etc. I'm pretty absent minded socially and probably would have gotten myself locked up as a pedophile. Although I don't much like touching people.

 

[KootK]

What do you think of justifying the use of sheathing as a tension tie by referencing the attached table from SDPWS?

I see where you're coming form but the limiting factor in our situation would be two nails in the same truss chord pulling the chord apart perpendicular to grain right? I'm on expert at the shear+tension wall panel thing but most of the details that I've seen are decidedly not that. Do you know of other details other than this kind of stuff?

My feel with this stuff is that the answer is just to not let yourself get too bogged down in making it technically perfect. Unlike the case with gravity design, our diaphragms are mostly just cobbled together out of stuff that happened to be there for other reasons anyhow.

 

[KootK]

Along very similar lines, check out this thread where we discussed tension action in wood diaphragms ad nauseum, including these two bitchin' sketches by yours truly: Link. You get the idea.

 

[KootK]

Apparently I imagined RFreund as some manner of insect back in 2015. Rest assured that my opinion of him has only improved since then.

 

[AaronMcD]

I'd put collector at both corners and one more just to make sure the middle doesn't separate.

I assume the big ass diaphragm is broken up by either shear walls/collectors or cross ties? Maybe align the bumpout corner and center with cross ties.

I don't ever use plywood diaphragms in tension except for really small stuff where I'm not worried about the load. But you could take the linear load and maybe get a small load per truss/rafter and connect with smaller hardware.

As for taking the load all the way across, I doubt I'd do that other than at collectors or cross ties. Just calc the excess diaphragm capacity and drag it in far enough for that plus a bit extra. Maybe drag it to the first perpendicular collector/cross tie.

If I have a valley in the middle of my collector I add the vertical load to the analysis, and make sure to not use a strap on top of plywood. If the valley is at the wall I can perhaps take the load into a post or stud. If it's at a diagonal valley with a beam I'd size the beam for the vertical load.

http://www.hellodwell.design

 

[zrck99]

Thanks for all the comments everyone, I really appreciate hearing others thoughts on these different items!