Roof Truss Ceiling Diaphragm

[Ryan H]

Hello, new here but love the topics

I have a 98' wide x 65' deep municipality building with 16.5' wall heights and a 41' tall ridge line. simple gable roof spanning 65 ft with the trusses spliced 2/3 depth over steel beams. I have 20.5' long moment frames located at 1/4 points that are parallel to the trusses and situated in the back third of the 65' deep building.

I want to use the ceiling that attaches to the bottom chord of the roof trusses as a lateral diaphragm and then transfer that lateral load to the 4 moment frames and the two gable ends. the ceiling will be 5/8" drywall. I have about 550 plf of wind horizontally along the top of the stud framed exterior wall.

my questions are:
1. how do I design the 5/8" drywall ceiling as a diaphragm?
2. I know the spacing of my moment frames so I can tell you it is about 13k of lateral per frame which is 20.5' long = roughly 1200 plf that I need to transfer from the diaphragm into the top of the frame. How is the connection accomplished?
3. same thing on the gable end, but with less lateral into the gable end shear wall. how is the roof diaphragm attachment (5/8" drywall ceiling) attached to the gable end wall?

thanks for all the help

 

[skeletron]

Sheath it with plywood so you can count on lateral load transfer via nails or screws (ie. load-rated elements). Not sure I would rely on drywall fasteners as primary load transfer elements.

 

[BAretired]

I agree that a drywall ceiling is marginal as a diaphragm. I don't like it. Plywood over the entire ceiling seems a bit much too. Perhaps horizontal trusses in the plane of the bottom chords of the trusses would be worth considering.


BA

 

[dik]

BART's suggestion is what is normally done... and if wood trusses, the bracing can be provided by the truss supplier. For spans over 60' I generally have additional requirements for wood trusses.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ryan H]

I agree from a coat perspective sheathing the entire ceiling with plywood would be very costly given the size of the building.

If I were to consider the horizontal diaphragm trusses, those would be cross bracing horizontally between each 24" o.c. Truss? How would that then connect to the gable end shear wall? And the top of the moment Frame? Ultimately I need to transfer that load to the shear segments somehow

 

[XR250]

There is no free lunch here. Diaphragm trusses would be a detailing nightmare IMHO.
I feel like plywood is the way to go.

 

[BAretired]

I have a 98' wide x 65' deep municipality building with 16.5' wall heights and a 41' tall ridge line. simple gable roof spanning 65 ft with the trusses spliced 2/3 depth over steel beams. I have 20.5' long moment frames located at 1/4 points that are parallel to the trusses and situated in the back third of the 65' deep building.

I'm not quite clear on the 2/3 splice over steel beams. Doesn't look like steel beams are required, but I may be misunderstanding the geometry. The following is a sketch of what I am suggesting.

BA

 

[Ryan H]

BA, good sketch. Instead of spanning the trusses 65' I have them spanning 44' and splicing over a steel beam line going the 98' dimension. The steel beam line also has column at 1/4 points which doubles as my moment frame columns. I guess ultimately I'm not familiar with bottom chord diaphragm truss bracing. How do you detail that and how does that attach / transfer to the moment frames? And knowing my trusses span 44' to steel and splice with another 25' span to rear of building

 

[BAretired]

Okay, now I understand what you meant. The rise of the roof is 41' less 16.5' = 24.5', which means your roof has a slope of Edit: 24.5/32.5 = 0.75 = about 9/12. That is a slope of 37[sup]o[/sup]. For transportation, you may have to consider piggy back trusses because they are so high.

I like the idea of a steel beam and columns at the interior end of the moment frames. The trusses could be spliced at the 2/3 point as you have suggested, or alternatively, at midpoint of the roof, making all trusses 32.5' long. In the latter case, the rear trusses would cantilever about 65/6 = 10'-10" to pick up the front trusses, not a problem given the depth of truss required.

Members of the horizontal trusses could be timber, and as dik suggested, supplied by the truss supplier or they could be steel sections for ease in field splicing. They would be fastened to the top of the bottom chords of each truss. In that way, they would be laterally braced at every truss, so should be adequate in tension or compression.

The truss fabricator must ensure that web members are positioned so as to miss the truss diagonals. That should not be a problem, but needs to be carefully checked at the shop drawing stage.

BA

 

[hokie66]

In your case, I would stick with your diaphragm approach. Horizontal trusses at the bottom chord of timber roof trusses can be done, but with a lot of detailing, and then a lot of inspection.

Whether or not you provide discrete bracing, the drywall will be by far the stiffer element, thus will take the load, at least until it fails.

Here is one reference:

https://www.buildsite.com/pdf/natio...ling-Assemblies-ICC-ES-Evaluation-1312642.pdf

 

[Ryan H]

Hokie,

Full disclosure this is what I have. 500 plf horizontal diaphragm loading across the top of the Wall / truss bearing elevation. That translates to about 13k per moment Frame. Also that would be 13k per drag truss but the drag truss would have an unbraced bottom chord of 45' before it's connected to the frame, so I don't see that working. I also see gyp ceilings have about 70 plf horizontal leading capacity, far way to go to get the 500 plf needed.


Ultimately, I'm looking for the best way to brace this building. 64'x99' with 16.5' walls and 9/12 roof. Trusses are spliced over steel beam line at roughly the 2/3 depth on the building


Open to any and all suggestions

 

[hokie66]

How do you get the 500 plf down through the wall? Seems to me the ceiling and wall diaphragms would have to develop the same force at that junction.

 

[Ryan H]

If I am using the ceiling as a diaphragm for lateral, that's another question. How to attach the ceiling to the gable end walls and moment frames that ultimately take it to the foundations? Simpson a35 clips?

 

[hokie66]

My understanding is that you are trying to develop 500 plf from the ceiling into the gable end wall, or the side wall. But when it gets into those walls, how do they take it to ground? Is that by diaphragm action or by bracing. Seems to me the same system would work for both ceiling and shear wall.

 

[Ryan H]

Yea I have shear walls and braced frames. Just need to get the lateral into those segments. My collector member (double top plate) is too weak and not continous to transfer the lateral 500 plf to the braced walls and frames. Thought about using the entire ceiling but I don't know if gyp will give me the capacity, and if it does I don't know the exact way to transfer the horizontal resisting ceiling to the vertical resistanting wall

 

[BAretired]

Drywall is not adequate as a diaphragm to resist 550 plf lateral load at the top of wall. The roof system makes an adequate diaphragm if the trusses provide a tension tie across the building. They would do that if they span 65', but handling such a long truss may be a problem. If trusses are spliced at midspan, or at the third point, the splice must provide a tie to prevent the roof from spreading. With the height of truss specified, it will likely be necessary to use a piggy-back system.

BA

 

[Ryan H]

BA,

Okay so how exactly would I convey the need to transfer 500 plf lateral through the trusses to the truss designer? And in what way, if any would my moment frames, as you drew them in your previous response, come into play? I will likely be splicing the trusses. And they will have piggy backs and the underside of the piggy back will have plywood sheathing across the top flat section of the main truss, as required by truss design.

 

[skeletron]

If you know the truss supplier, get them on the horn and ask how they like to see it on the drawings. Maybe get their two cents on whatever it is you are planning too.

 

[BAretired]

Okay so how exactly would I convey the need to transfer 500 plf lateral through the trusses to the truss designer? And in what way, if any would my moment frames, as you drew them in your previous response, come into play? I will likely be splicing the trusses. And they will have piggy backs and the underside of the piggy back will have plywood sheathing across the top flat section of the main truss, as required by truss design.

We are in the process of discussing preliminary ideas. They need to be refined quite a bit. There are probably several workable solutions and we need to kick them around for a few days until you find the best. Let's worry about how to communicate with the truss designer when you have decided what to communicate.

If you are going to splice the trusses, you need a beam and columns, but not necessarily moment frames. My current thought is that the roof diaphragm could easily span 98'-0" and be supported by the end (gable) walls.

The gable walls will be shear walls and, because of their height, will need lateral bracing, possibly lateral support at ceiling level.

BA

 

[Ryan H]

BA, yes I have a 98' beam line split into 4 simple supported beams positioned about 44' back in the overal 65' deep building. Ceiling finishes may be ACT but structurally could be gyp or something if needed. The two gable end walls are 2x8 studs with 15/32" plywood both faces and Simpson holdowns for shear walls. That part I understand

My questions are, what is needed to reinforce the 98' double wall top plate, if anything? Instead of spanning that diaphragm 98' I could / do have those moment frames at 1/4 points going from the center beam / column line towards the rear of the building to cut the diaphragm span down. What is that diaphragm material made of and is it the ceiling of the space? My gyp ceiling I don't think will have enough capacity.