Plywood Diaphragm Attachment to Steel Moment Frame

[AndBre44]

Looking for some guidance from those who have more experience then myself. I have a case for a one story building design where I need a moment frame to pick up an approximately 7k lateral load with 17ft of wall length with a rather large door/glazing opening leaving me with too little full height sheathing for a shear wall. The roof is D.F.L. 2x8 @ 16" o.c. with a plywood diaphragm, and I've sized the moment frame and its able to fit into the wall and be more than adequate. That said, following my load path I'm floundering a bit at the attachment of the plywood diaphragm to the moment frame beam.

Typically on any jobs where I've prescribed moment frames its been with a metal roof deck where I can reference Hilti allowable loads with powder-actuated fasteners, but unsure of what the best option would be here for a plywood connection. I've seen is that in the ESR report for the X-U powder-actuated fasteners that they can be treated as an equivalent to 10d nails; is it then just a matter of using the allowable diaphragm values from the relevant SPDWS Table (4.2) and ensuring enough fasteners go in to meet the allowable shear loads from Hilti along with it? Or would it be more prudent to use something such as APA E825 or E830 and use those values/fastener types to achieve what I need?

Any advice would be greatly appreciated.

As an EIT, I'm open to being wrong now if it means being right when it counts.

 

[phamENG]

Give us a sketch of your section cut through the beam.

Usually, it goes like this:
1) Plywood is connected to the rafters and blocking is added between rafters (diaphragm edge nailing goes to this).
2) Rafters and blocking are connected to a nailer on top of the beam. Connection from blocking to nailer has to meet diaphragm shear transfer demand.
3) Nailer is connected to the beam. Nailer to beam usually fastened with through bolts, wood to steel screws, or PAFs with washers. These need to be designed for the diaphragm shear transfer AND uplift from the rafters (paying close attention to potential cross grain bending - very bad).
4) You're in your beam and good to go.

 

[AndBre44]

Pham,

See attached sketch, a bit crude but should get the point across as to what my initial thoughts were before reading your response; the moment frame beam is parallel to the direction of the ceiling joists, and originally my goal was to have the top of the moment frame (T.O.S.) match the top of joist (T.O.J.) so the sheathing would be able to directly attach to the beam thinking that would be the best means of attachement. In essence, I was planning to essentially add framing to the area to be indicative of a sidelap connection.

Per your response, I can see how using a nailer would be another alternative in this case that may be a better means of connection. Assuming I'm reading your suggested path, rather than having the T.O.S. and T.O.J. match, I should have my T.O.S. match the bottom of my joist (plus another 1.5" for the 2x nailer), and then use the nailer to directly attach wood to wood, correct? The height of the moment frame beam is flexible enough that I could drop it to this level if I'm understanding you correctly.

Thank you for your advice on this.

 

[driftLimiter]

The wood nailer is the simplest option for you and your construction here. You can use it for top flange hangers and diaphragm nailing can be standard. Going with screws or pins is an alternative but then you will have to reconcile the diaphragm strength with these alternate fasteners.

 

[AndBre44]

Pham & driftLimiter,

Thank you for your responses; the thought of using a nailer certainly simplified my approach when it came to my load path and connection analysis; certainly an easier way to go.

I come to you with a different question for the same building, one that I'm sure is just me overthinking the situation...but would rather be safe and ask someone else, rather then sorry and pay the consequences of lack of diligence. I've attached a second & cleaner sketch of the building layout; the short of the project is that the existing SE corner of the building is being taken out and that part of the building reframed, hence the moment frame now designed to pick up the end-shear. The portion of the east wall to remain is the primary entryway into the building, with a large degree of openings and glasswork leaving very little CMU going to the existing foundation wall; they're hoping to leave this area as-is when doing the alterations.

I understand my load to know that when for example wind moves North and hits the South wall, it should transfer through my wall and into my slab on grade & roof diaphragm, and then at my roof diaphragm be able to transfer into the existing West masonry shear wall and the new SE moment frame I've designed. My question however is this; is the lack of a shear element at the existing north wall on the East side a concern at first glance? Or will the diaphragm still be effective at transferring the load into the shear wall at the south side even if the diaphragm ends at this area with the multiple door openings? Frankly, I've been lucky enough that all buildings I've worked on up to this point has not had an issue with providing moment frame or a shear wall at its outermost point; I haven't had an instance like this where, effectively, an entire section of my exterior I'm virtually saying has no shear resistance.

I've considered taking that moment frame and effectively adding a second bay that goes to the existing North wall, but it would certainly require some additional demo & concrete work that would be nice to avoid if I could.

 

[driftLimiter]

You don't nessecarily need to add another bay of moment frame. You could use a drag strut collector.

Then for the region on east of the drag strut you have a three sided diaphragm scenario. Its doable just takes a little more effort.

 

[TORCHMAN]

I am always curious about these situations. A question that comes to mind when I see a system like the one you encountered is did you check the stiffness of the West masonry shear wall vs the moment frame? The stiffest member attracts most load but I wonder if in a flexible diaphragm like the one you have and give the distance, the load would actually be split per the geometry rather than stiffness?

Regarding the section that sticks out on the NE, it is similar to how people build sunroom extensions on their houses - typically with lots of windows on the end w/o shear wall - where the roof acts as a diaphragm and brings all the shear back to the superstructure. I agree with driftLimiter that a collector would be the way to go. Never seen calcs for a "3 sided diaphragm" if I am thinking correctly, all the shear goes to the moment frame, not sure how you would go about checking deflection though.

 

[AndBre44]

driftLimiter,

Yes a drag strut is exactly what I was looking for in this case; I had a feeling there was some fundamental solution to my problem and that I was just missing the piece. That ensures the lateral load essentially treats that portion of the building as a shear plane. My thanks to you!

As for the remaining East Portion (and to comment back to TORCHMAN), the remaining bit of CMU that does exist there (barely) has enough capacity to maintain the small remaining bit of wind load; since the tributary area by that point is so small even the what is only about 5ft of collective CMU pier has enough to resist what is only about 800lb of wind load remaining per the TM 5-809-3 shear wall analysis. I also must say I'm in the same position as Torchman of having never quite seen/heard of a three-sided diaphragm calculation, and after doing some brief research I can see what you mean in terms of being possible but certainly having a few more hoops to jump through.

I do like what you bring up there Torchman in terms of the idea of how items like stiffness may come into play; I've once or twice thought that myself when it comes to having different wall materials in parts of a building; the closest analogy I give is how when doing geotechnical review I'd never want to have half a building on a spread footing and another half on piles because of differential settlement potential; i.e two different materials will never quite act exactly the same. Some part of me has always made sure to within reason beef up my applied loads on any moment frame (and diaphragm attachments to such) I've installed for that very reason...call it a personal safety factor of sorts.

 

[driftLimiter]

For 3-sided diaphragm you will need to satisfy equilibrium. All the shears goes to the collector that is inside, but there is an unresolved rotation of the 3-sided region. That gets taken into a couple of shear forces along the two sides perpendicular to the shear collector.

Check out Malone's text book for a good example. Also there is a structure magazine article that reviews the concepts well.

 

[TORCHMAN]

Check out Malone's text book for a good example.

Good point. Chapter 6 of "The Analysis of Irregular Shaped Structures" goes over a useful example under 6.4 Open Front One Side

 

[lexpatrie]

FAQ entry - Certified fresh as of today. Tackling the 3 sided box diaphragm, Lintz, Structure Magazine, February 2022.