Elevated Conveyor Walkway 1

[little49]

I am curretnly working to redesign the standard conveyor walkway used by a local fabricator. The walkways are meant to span between adjacent grain bins and will hold both a drag or belt conveyor as well as a 2ft pathway for workers. They feel that it is currently so overdesigned that it is no longer economical and would like it streamlined. By streamlining, I mean changing from the current square tubing to angle iron and from the current welded connections to bolted.

This is in an attempt to make it more of a modular site build than the current process. They hope to accomodate anything from 30ft up to 150ft. The problem I see with this is the lack of strength in the top chord if they switch to the angle iron, possibly double angle. Since it must be open to the top (pony truss) and open sides (no panels), I am concerned about what will happen with the top chord. What possibilites are there for bracing? They currently have free spans up to 140ft and plan to keep it that way. Just need to get some ideas and my thinking seems to be a little off.

Thanks in advance... any help is appreciated.

 

[little49]

Sorry, I failed to state that it is a steel truss, approximately 4ft deep and 5.5ft wide open space between the railings. Hope this helps with the mental picture a little more.

 

[jike]

Option #1:

Is it possible to have a beam under the walkway cantilever out on either side and have a diagonal from each end of the beam go up and securely brace the top chord?

Option #2:

Is it possible to design the truss like a highway pony truss? Usually the top chord is a wide flange (with its web horizontal) and the verticals are designed to brace the top chord.

 

[aggman]

What kind of loads will you be seeing? 4' deep seems pretty shallow for a 150' span, at least for conveyor truss. You will have to brace the top chords somehow. You may use stiff uprights (portal frames) to brace the top chords. Bolting is fine, but if they have a borderline design to begin with then the added flexibility in bolted connections will only make matters worse. It will also take a larger angle to carry the same load as the tube. Some things that will require consideration.

A) Lateral torsional buckling will now become a concern.
B) Lateral bracing of the top chord will be more of an issue.
C) If you have a continuous chord or loading between panel points then bending will have much larger effects on the open sections.

Give us some more information and we may be able to help you better.

 

[little49]

jike,

They currently use outriggers to support the upper chord but hoped to get rid of them as part of the refining process. In addition, they want to use all angles since they have an abundance of them. This would rule out the use of a wide flange but there is the posibility of combining the angles somehow to strengthen the upper chord to accept larger loads.

 

[little49]

aggman,
This is primarily used to transport the grain along the conveyor. In addition, there will be live load from workers, wind load, snow loads, seismic loads for central Illinois, thermal forces from the bins on either side as well as the conveyor, any forces that may be exerted on the platform from the conveyor motor. The torsion will be a problem to be addressed as well.

The previous design had 20ft continuous spans connected with gusset plates along both sides and the bottom. These sections would then be combined as necessary to cover the given span. Since the previous truss was made of tubing, I thought it might be possible to combine angles to get a similar effect, even though the strength will not the the same as a uniform tube, I was thinking it could be varied somehow to get ner the same strength.

 

[aggman]

I think it comes down to being able to generate enough section to carry a higher unbraced length for the top chord. You may consider a double angle with short legs vertical and then place your upside down portal frames every 20' or so. You could incorporate the portal frame into the chord splice. It still gets down to providing lateral bracing as required by the methods jike brought to the table.

 

[little49]

I agree, I don't think they are goign to be able to go to angles all over and get rid of the outriggers if they plan on being able to cover such a large span. With the portal frame, I assume you are meaning to include larger members for the side and bottom, since it has to be open on top. Use these as a connecting point between the sections. Any other thought on provided lateral support without the use of outriggers?