Light Gauge Steel Truss (28m Clear Span)

[TekEngr]

I am designing a shed with light gauge steel which contains the 28m clear span , so i am confused about the LGS truss support condition both supports of the trusses should be pinned support or one pinned and other should be roller however my truss are resting on the light steel wall.
please also suggest is it good idea to make 28m x 72m and 5.5 m height shed in light gauge steel , however i can use maximum section is C150-1.55 (no internal partition walls )
Roof Dead Load = 0.25 KN/m^2
Roof Live Load = 0.60 KN/m^2
Wind Load = 1 KN/m^2 (uplift)

 

[XR250]

One needs to pinned, one roller or you can do both roller and restrain the peak in the X direction. The supporting walls do not provide stability for the truss, the roof diaphragm does.
I have done alot of light gage truss design, and I would recommend going to structural steel for a span of this length. The analysis is difficult and the loads are going to be huge. There is lots of eccentricity in the connections that is hard to qusntify. Alot of web and chord bracing may be required which rarely ever gets done correctly.

 

[TekEngr]

Truss is working @ 1.2 m spacing under these loads with one end roller support but how we can get the actual roller support condition practically both end of the trusses are connecting on the top track of the wall through screws so I think we can’t get the roller support effect practically and the walls will be effect not only axially but also horizontally as well (when the truss will deflect vertically then the horizontal force will generate which push the wall outside at both ends).
Does the LGS wall have capacity to restrain the horizontal pressure this is my confusion part however I am using C150-1.55 back to back @ 600 spacing (wall stud working for axial force).
Unfortunate thing is that we don’t have enough space to provide the strap bracing in the walls (there is no internal partition walls and client don’t want structural steel system even not a portal frames for LGS system)

 

[XR250]

The trusses brace the walls, not the other way around. You analyze it as pinned/roller just to make the model stable but it is the diaphragm that does the overall bracing. I would be very careful designing a light gage structure of this size unless you are very experienced (which, no offense, you do not seem to be). Alot more detailing and analysis is required compared to other building systems.

 

[mike20793]

They have connectors that allow horizontal deflection. They're usually reserved for scissor trusses, but I've used them elsewhere.

 

[TekEngr]

yes you got the rite idea i am not very much experienced....i also don't want to go with Light gauge system for this kind of dimension (forced by my boss..) because individually all the members are working but as a whole structure we cant control the drifting because of the less bracing space.
i think the better way to go with blending structure (Structure Steel and Light gauge steel mixed)
or any other suggestions ??

 

[XR250]

I agree - a hybrid of structural steel and light gage steel.
Its tough to sell the clients on that as they see light gage steel and hear "cheap"
I am an experienced designer and I would not touch a building like that anymore.
There is a reason light gage is cheaper - the subs are cheap and the work typically reflects that.
If you are stuck with light gage, you might want to explore the proprietary trusses by Alpine.

 

[JedClampett]

I've done very few of these, but I thought it's a delegated design type item. You give spacing, loading, roof slope, etc. and someone (Alpine, TrusSteel) does the design. They'd prefer it that way, and I guess, so would you. They're like OWSJ.
I'd call them, get their opinion on such a long span and go from there. If they say they can't do it, you've got some bad news for your client.

 

[XR250]

Alpine leases their software to their suppliers who do the design - just like wood trusses.

 

[jwilki]

That's 60 truss's that need to be welded up and a hell of a lot of connections... how are the purlin's connected, direct fix or via welded cleat? This structure would not be economical in my practicing area (Australia).
I design these kind of structures as "light guage" C sections portal/sway frames with double C35030 or C40030 back to back and frame spacing’s of 4.0-6.0m. Sway is taken out by the frame action.
In your case.... you will need a wind truss/bracing that is 72m long from end wall to end wall which then needs to be adequately connected to the structure (1.5mm steel???). I would follow the Jed's advice and talk to the manufacture that can manufacture and safely erect such a structure. At least then you can convince your boss that such a structure is not economically viable... (I imagine you will need a double C150 top & bottom chord to handle the axial forces…if that works)
Edit -spelling

 

[TekEngr]

if i go with webbed LGS portal frame after every 2m or 3m then what do you think is it possible to stable the structure ???? below are the design criteria
Shed Size 72m x 28m x 5.5m (Clear Height)
DL = 0.35 KN/m^2
RL = 0.60 KN/m^2
WL = 100 MPH (Location UAE)
roof slope can be considered up to 10 degree (but no limitations from client).
Purline will be connect directly on the truss top chord through screw.

 

[jwilki]

I like to think anything is possible But should it be done?

When we tried to design a system of OWSJ trusses for a local fabricator, the cost of bolting or welding C-sections to form the truss was more expensive than the cost of the C-sections used!

Without seeing the final engineering documents I can only guess (or hope?) that the image you attached has some sort of bracing because those columns look inadequate for frame action.
Your attached picture has plates on the ridge that may server to transfer moment, a similar thing can be done at the knee to transfer moments, shear and axial forces and turn it into a portal frame.

Alternatively you can design braced side walls and provide a brace system in the roof plane to take the sway force back to the side walls.

Good luck...

 

[XR250]

if i go with webbed LGS portal frame after every 2m or 3m then what do you think is it possible to stable the structure ???? below are the design criteria

I would be real surprised if you could get the numbers to work out on that scheme.
I think you are playing with fire here. You are much better off walking away from a job that is impractical to design rather than regret it later.