One long beam VS several shorter beams

[jsimeroth]

I'm a new mechanical engineer, and one of the projects I’m working on involves a good amount of structural steel for a suspended platform/walkway. It has one main beam that spans the length of the platform, and several cross members connecting that beam to another parallel beam for support. (Imagine your typical horizontal truss/walkway, where the cross members are all perpendicular to the main supports.) My question is this:

Assuming the beam is supported at the same points along its span either way, is there a significant difference/benefit to breaking the beam (which is wide flange by the way) up into shorter pieces bolted to one another? Would this eliminate additional bending moments, or is this already taken care of by the intermediary supports?

Thanks in advance!

 

[KootK]

If this is an existing facility, I'd probably beak up the beams just to make them easier to handle. Would the cross beams tie into the side of the main beams or run over the top?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jsimeroth]

Sorry, I'm new to the term "beak up" the beams. I would be happy to learn though. To answer your question, the cross beams tie into the side. The main beam that spans the length, as well as the cross beams, are all the same beam type/size.

 

[jsimeroth]

Oh I'm sorry. That was almost certainly a typo. You would break up the beams. Gotcha! I was thinking the same, but was wondering if there's a difference, structurally

 

[canwesteng]

Sketch?

 

[KootK]

I'll beak you up smart ass!

There is a difference structurally. If the girders are continuous, they will be more efficient and a lighter section can probably be used. However, this is often offset by the need for bottom flange braces which is why I inquired about the elevation of the cross beams. With the set up you have, the cross beams would brace the girders and additional bracing would be required.

For small stuff like this, I find that ease of assembly is much more important to economy that section weights. Ergo the comment regarding handling. Another consideration may be ability to stuff the pieces in a galvanizing tank if it's to be that kind of structure.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MikeHalloran]

You didn't give us a clue about the scale of the project.

about beam length:
I have seen W sections up to 133 feet long, but they had to be shipped by barge.
I think 60 foot lengths are truckable and usually available, but check with your local steel suppliers before specifying a particular section or a particular length.

Some stuff that's in stock lists is only available if you buy really big lots and can afford interminable delays for ocean shipping or setup of mills.

OTOH, for small projects, you may be able to find a local stock of offcuts that are just what you need. You discover this kind of thing by sending an RFQ to each of the suppliers in your area, including a list of exact lengths and sections that you want, maybe with a note describing any flexibility you may have.

I used to do this with a group email of an Excel .xls file that was set up to look like (and print like) a paper form. Be sure to use an older Excel format like 2003 or earlier; lots of businesses are using very old computers. The really old school guys still use fax.




Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

you can break (or beak) up a long beam into smaller lengths, but you Have to provide moment continuity across the joints. In the civil business, maybe weld a plate to the end of the beam, filling the face, and bolt (or weld?) the two plates (from adjacent beams) together.

another day in paradise, or is paradise one day closer ?

 

[jsimeroth]

Thanks for your comments everyone. It sounds, from the input so far, as though there is a difference, but it is negligible on small scale projects. Let me know if I'm wrong. The reason I didn't include anything related to the scale of the project (it's a small platform by the way, 19' by 8') is because I wanted to know more generally the theory behind whether or not there is a structural difference between one long beam supported exactly in the same way as several shorter beams. It sounds from KootK's comment that there is.

 

[MikeHalloran]

That's small enough to fabricate in a shop and bring in complete on one truck, and place with one crane lift, assuming you don't have to carry it through a building or have some other odd restriction.

Because of the width, you should include a load case where a police car crosses the bridge from time to time. ... and in some locales, a fire truck.

Did you know that there are several outfits that specialize in repurposing old railroad flatcars into short bridges?

Mike Halloran
Pembroke Pines, FL, USA