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

 

[BAretired]

BA[/color]]My questions are,what is needed to reinforce the 98' double wall top plate, if anything?
If the trusses are capable of tying the long walls together, the top plate is braced at every truss, which I assume are spaced at 24" o/c.
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.
Moment frames would mean using a plywood diaphragm and a close spacing of nails in order to transfer the shear to each frame.
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.
I agree and, furthermore, I don't believe gypsum board is a reliable structural material to provide lateral stability to the entire structure. But, it is a judgment call.

BA

 

[Ryan H]

BA,

Okay so if I specify that each truss needs to be designed as a collector truss spanning from long wall to long wall, they will splice so I'll need reinforcement at the splice over the steel. They will most likely splice butt to butt so mending plates or plywood reinforcement could be used. But if each truss can drag the 500 plf * 2ft centers = 1000 lbs drag load and hurricane clipped accordingly I guess I need to apply that horizontal load to the top of the Wall studs and look at the stud assembly then as well.

But ultimately the idea is no moment frames, no ceiling diaphragm, gable end shear walls and the trusses are noted each to have an axial load is 1000lbs lateral?

 

[Ryan H]

BA,

Another thing I was thinking about, if I assume the roof trusses connect the front wall to the back and the top plate spans 24" o.c. Between trusses. What is the tributary area on my gable end shear walls? 98'/ 2? That's about 25k of wind load into that gable end

 

[BAretired]

That's right, 49*550 = 26,950# which is 415#/' for a 65' long wall. Pretty high, but doable. Needless to say, there will be some openings, so they will have to be considered.

BA

 

[Ryan H]

BA,
Alright I'll look into what a plywood double sided shear wall gets me. Probably need 3" o.c. Spacing

With the idea that the roof trusses take the load from front to back, do those trusses need to be noted and designed for that 1010 lb bottom chord drag? Do the walls need to be looked at with horizontal plf loading along the top plate?

Or is it all simply the roof takes the weight and the gable ends brace?

 

[BAretired]

Ryan,
I would expect some door openings, and perhaps a few large window openings in a municipal building. It is unlikely that you will have the luxury of a solid wall, especially at the front of the building. If there are too many openings to get a decent shear wall, you may have to consider stiff frames within the wall. Also, the wall is so high that you may decide to provide columns from floor to roof, to resist direct wind pressure, rather than relying on a drywall ceiling.

1. With the idea that the roof trusses take the load from front to back, do those trusses need to be noted and designed for that 1010 lb bottom chord drag? Do the walls need to be looked at with horizontal plf loading along the top plate?

2. Or is it all simply the roof takes the weight and the gable ends brace?

1. The trusses need to be designed for several combinations of load including dead load, wind load, snow load and perhaps other loads. They don't all occur simultaneously; you should review load combination requirements in the code applicable in your area.

The lateral force applied to the top of each long wall comes from wind pressure. Pressure varies according to wind direction and intensity. Usually, but not always, wind pressure is positive on the windward side and negative on the leeward side. Wind pressure is always normal to the slope under consideration. For a 37[sup]o[/sup] slope, with wind running normal to the ridge, the windward slope feels a positive pressure while the leeward slope feels a negative pressure, so the horizontal components are additive. When the wind blows parallel to the ridge, both slopes feel negative pressure.

2. That is a very poor description of the way the structure acts. The trusses carry the dead load and snow load, producing tension in the bottom chord. Wind load may produce either tension or compression in the bottom chords, depending on direction, but maximum snow load does not occur when wind is strong enough to remove it.

If the bottom chords are continuous across the building, the walls are effectively tied together. They cannot separate and they cannot come together, but they can both move in sync.

BA

 

[Ryan H]

BA,
I guess I am going back to the idea that this building is going to want to blow over when the window hits the 100ft long wall perpendicular. what is going to stop that lateral force? I guess the answer is a combination of things:

1. the gable end shear walls
2: the lateral drag trusses each with their respected lateral drag force
3. the moment frames in each the lateral drag trusses attach to

I am having trouble seeing how all of that windward and leeward force which is around 90kips can blow and pull on this building and everything just goes to the gable end walls. if I plywood the flat ceiling then I can develop a diaphragm which is 100 ft long and transfers to the gable end shear walls. but without plywood the ceiling the trusses are going to want to blow right off the walls or blow the walls over with them. am I wrong?

 

[BAretired]

I guess I am going back to the idea that this building is going to want to blow over when the window hits the 100ft long wall perpendicular. what is going to stop that lateral force?
Nothing will stop the lateral force. The force persists as long as the wind is blowing, but hopefully is resisted by the structure if properly designed and built.

I guess the answer is a combination of things:

1. the gable end shear walls yes
2: the lateral drag trusses each with their respected lateral drag force yes
3. the moment frames in each the lateral drag trusses attach to yes
4. The roof structure could span 100' without a ceiling.

I am having trouble seeing how all of that windward and leeward force which is around 90kips can blow and pull on this building and everything just goes to the gable end walls.
Why is that? It is a common type of design in many open buildings.

if I plywood the flat ceiling then I can develop a diaphragm which is 100 ft long and transfers to the gable end shear walls. but without plywood the ceiling the trusses are going to want to blow right off the walls or blow the walls over with them. am I wrong?

Without a ceiling diaphragm, the roof structure would need to be designed to transfer the entire 90k load to the end walls. That is not particularly unusual.

BA

 

[Ryan H]

BA,

I think I'm going to proceed with the idea of moment frames at 1/4 pts and drag trusses over top and clipped to the frame. I can determine the tension and compression chord forces in the wall top plates between each drag truss. I guess I would like to know with those forces, how can I use that to validate the double or maybe triple top plate work?

 

[BAretired]

I don't know how you can "clip trusses to the frame". The trusses are parallel to the frames. You would need a member spanning 25' with a load of 90/4 = 22.5k. The bending moment is 70'k. Triple top plates won't even come close. You could provide a channel cap on the beam designed to resist the lateral load. Trusses each side of the splice would be fastened to the channel to resist their share of the wind load. It would work, but is somewhat unconventional, to say the least.

BA