Light Guage Roof Truss

[bookowski]

Guys,

Sorry for a basic question here - I am doing a small one story retail building, steel frame with light gauge gable roof trusses. I have never used the light gauge roof trusses before (and not much experience with this type of building in general) and I am trying to work out how my diaphragm works with this type of structure.

I have three lines of columns, the truss is supported on all three (2 ends and midpoint), the total span is about 70ft (x ~120' long building). I am using moment frames and I need to engage some of the interior frames in addition to perimeter. The gable roof will have metal decking but it's cut up quite a bit for some flat mep areas and then dormers etc. What I am not seeing is how I create a diaphragm at the structural steel level, the bottom of the truss. To get load to those interior frames I need some kind of diaphragm. I have never worked with these trusses before - do they provide strapping or some other method to create a diaphragm at the bottom of the truss?

Thanks

 

[JedClampett]

The light gage trusses really don't change the design aspects any different from wood trusses or open web joists. These are just different ways to support the roof loads.
When you say you're using moment frames, are these the exterior walls? And why are you supporting the trusses at midspan? It seems with that length to width ratio, the shear forces should be able to be carried by the outside walls alone. If the roof is peaked, that is the strongest, deepest part of the truss.
As far as the diaphragm, unless you have very large loads, you only need a diaphragm at the top of the trusses. Even if they're sloped, they'll work fine. You need blocking of some sort to get the diaphragm forces to the wall top at the exterior. If you need the center wall to act as a shear wall, add braces or some other shear carrying assembly to get the shear forces to the moment frame. Don't forget to add a drag member to get the forces to the braces.
If these terms are unknown or confusing to you, ask a more experienced engineer how to proceed.

 

[bookowski]

Jed, thanks for that condescending finish to your answer. It's nice to know that there are forums where you can ask for advice and be treated like an idiot. I appreciate that.

I need to use moment frames instead of braced frames or shear walls due to architecture, the walls are mostly open. The architect is pushing for w10 columns so there isn't much stiffness which is why I am also engaging interior frames. I need to have a centerline girder because the roof actually has a large flat (not gabled) area for mechanical equipment. Since this girder is required anyway I figured that I would split the truss span to lighten it up. As I said I don't have much experience with this type of building which is why I was asking. Do these trusses typically sit on top of the supporting steel? I saw one detail where the bottom chord framed into the steel beam webs, obviously this is no longer prefab but stick built, is that more common? And in general is the only diaphragm the gabled roof, nothing is done to the bottom of the trusses?

 

[hokie66]

I don't believe Jed meant to be condescending...he isn't like that.

When you say you are using moment frames, perhaps you can give a bit more information. I assume you mean steel moment frames in both directions, with the frames across the building supported by the central frame. How do the horizontal frame members relate to the ceiling level?

If your roof decking is also the roofing, I wouldn't use it as a diaphragm. Steel roofing is normally screwed through the crowns or uses concealed clips which allow slippage, so it doesn't work as a diaphragm. Best to put your diaphragm or bracing at ceiling level.

 

[bookowski]

Thanks Hokie. Yes, steel moment frames in both directions. I have 2 bays by 5 bays (2@~35ft, 5@~25ft). I am forced to stick with w10 cols so it's relatively soft. I am fine with designing moment frames but my experience with them is with high rise so i'm typically doing slab on deck or something where the diaphragm is clear. I don't have a lot of experience with light guage and even less with gable roofs. Since the building is soft I need to use the interior frames in addition to the perimeter for lateral.

I guess my question was what is typical for creating a diaphragm at the bottom of the trusses - is this accomplished with light guage strapping to the trusses? Any suggestions on where to see typical details? I don't want to reinvent the wheel and I also don't want to detail something that is nonsense relative to typical details.

 

[hokie66]

Light gauge steel is not my thing, either. But with moment frames in both directions, you have no need for a diaphragm. You just need to get the load into the moment frames. I think that a perimeter girt at the eave, and diagonal strapping on the bottom chord, would be the way to go. As your moment frames provide a "soft" lateral system, you don't want the ceiling to try to take the load, so a suspended ceiling system with edge isolation would be my choice.

 

[ash060]

Bookowski

What is the spacing of you trusses? If they are 2'0"c/c and you can use plywood sheathing, than AISI has values for plywood diaphragms over light gauge framing. I have used this system many times and it works just fine. If you are sticking with steel deck you can use the values out of a deck catalog, but they will have to be modified for using light gauge if the deck catalog assumes an attachment to rolled steel.

 

[hello12345hello]

if metal deck is attached to to top chord of truss can't we consider that as diaphragm and don't add any bracing at bottom chord. we might need some additional bracing to brace the trusses at bottom chord but it's not a part of diaphragm.

 

[hokie66]

That depends. The OP said there was metal decking, but it had a lot of openings in it. Now if it is structural deck and not roofing, maybe he can make a diaphragm work. But if it is metal roofing, the primary purpose is to shed water. Metal roofing should not be used as a diaphragm on a building of this size.

 

[bookowski]

Yes, the roof is decking (not roofing) but it is very discontinuous. On one side of the ridge there is a very large dormer. On the other side of the ridge the gable is cut out for about ~60% of the area for a flat roof mechanical area. I'm starting to come around to the idea that I don't need a diaphragm if I provide girts all the way around to distribute lateral loads to the frames. It makes sense, it was just something that I haven't done before.