Deflection discussion 24

[bisandcan]

I was having a discussion with another engineer today, he is an older guy that learned the old ASD method of design. He states that he would never, ever, under any circumstance use a member that didn't follow the depth equals half span rule of thumb. I think that this is more of a guide, and something to use as a good starting point.

I like to design according to LFRD strength design, and then check for L/360 LL deflections and L/240 LL+DL deflections and make adjustments of my depth based on the deflections I am expecting to see.

Am I wrong to do it my way? I understand the rule of thumbs come from somewhere, but don't they eventually get outdated?

Thoughts?

 

[bookowski]

Apparently, there is some reasonable flexibility.

OP asked if "never, ever, under any circumstance" this rule of thumb should be violated - how is that room for flexibility? Seems like people are not reading his question.

standing on the shoulders of giants

Span over two is the work of giants? Come on. Give a new grad an hour to play around and come up with his own rule here and he'd likely come up with span/2. I've worked with a lot of old engineers - some good, some hacks - pretty much the same as young engineers just older.

Seems like the OP walked in just before last call and all the old farts have had a few too many and want to reminisce about the good old days when men were men and beams were always span/2 regardless of loading, beam spacing, criteria.

 

[hokie66]

Watch yourself, bookowski. You may be right about the question at hand, but don't carry it too far. Old farts rule.

 

[KootK]

Firstly, thanks to everyone who corroborated the opinion that I expressed at the top. And thanks to those who expressed dissenting opinions as well. Those are required for meaningful discourse.

koot,In the design that was in discussion, it was simple 3' wide elevated walkways with no equipment or other loads on the beams. I used a HSS3x2 tube, and was told that I was crazy and needed to use a 5" tube, no matter what the deflections were, they came up to be L/1347 for LL deflection. There are several of these platforms on the site and would you really double your steel weight to control deflections more than L/1347?

While I still don't feel that I understand your situation well enough to comment with any certainty, I would be inclined to question your choice of stringer size. I'm imagining your walkway to be HSS stringers with something like bar grate spanning in between. Things that might concern me include:

1) I sketched this system to scale and do find that the proportions draw the eye.

2) If this is an elevated walkway, will there be handrail mounted to the stringers?

3) If it would take 5" to satisfy L/24, that implies that 3" is around L/40. This could put you in vibration issue territory for footfall traffic. If you get three big guys walking along in lockstep on your 1.5ft trib structure, there might be some bounce. I'm feeling industrial structure here, however, so it may well be that nobody would care.

4) It's actually the members with small tributary areas that I often worry about. They will be more susceptible to problems related to unanticipated point loads than members supporting larger tributary areas. Could you end up with shoveled snow piled up somewhere along the walkway? Might a moose attempt to use it as a thoroughfare and leave the thing demolished? Will vehicle be running into the walkways?

5) Given your slenderness, could you get wind induced resonance as wind passes above and below your walkways? I know that sounds pretty extreme. That said, I'm sure that the guy who designed this keeps a tight on things now: Link. Be sure to watch the video.

6) If this is an industrial application, those folks tend to prefer spending a few extra dollar rather than deal with problems later.



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.

 

[sandman21]

Rules of thumbs are guides to follow to approximate sizes and are not actual design. If you never ever go against a rule of thumb you are going to walk right into issues. Garbage in garbage out applies not just to modern computer programs, if you would not blindly accept the computer output you should not blindly accept a rule of thumb. Also the span rules breaks down when checking long span beams in office buildings, running tracks, walkways, gym floors for vibration, just to name a few.

 

[bisandcan]

koot,

This is an elevated walkway, and there will be handrail posts welded to the side of on tube.

That is about right, but it is only for equipment access and not an everyday use platform. This would be used once or twice a year, by a guy lubing some bearings unless there was a problem.

This is inside with no chance for other loads.

This is inside with no wind loads acting on it

It is industrial, they want to spend as little as possible just like the next guy.

 

[KootK]

Fair enough. Depending on what your floor deck is and how it's connected, I still question the viability of the 3x2 stringers for imposed handrail loads.

This is essentially the conversation that you and I would be having if we were colleagues and I were reviewing your work. Hopefully your conversation with your real colleague is going equally well.

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.

 

[TehMightyEngineer]

How far is the fall? I've definitely stepped on some infrequently accessed, flexible catwalks 100+ feet in the air and then decided that zoomed in pictures from a distance were sufficient for what I needed. Pedestrian confidence is just as important as actual deflection/strength in industrial catwalks.

Also, you wouldn't believe what they actually load those catwalks up to. "Yeah, those 10 sheets of plywood can totally lean against that handrail. No problem!"

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[canwesteng]

Have you checked it for code mandated minimum point loads? Typically that controls for small platforms. This is intended to capture the (very likely) case where they store something on the platform.

 

[SethGuthrie]

It's such a popular rule of thumb that we made it an optional design criteria in RAM Steel years ago. Word of caution to any who uses the Span/Depth maximum ratio in the program, 25:1 works better than 24:1 because we consider the exact beam depth and span, not nominal values.

 

[SlideRuleEra]

Consider using an appropriately sized W, M, S, or C shape instead of an HSS. During the typical long useful life of an industrial project, even indoors, undetectable corrosion can compromise the structural integrity of a hollow section from the inside... and there is no reasonable way to clean or paint the inside. Sources of corrosion may be routine wash-down water, chemical spills, etc.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[axiomata]

My little steel cheat sheet has:
Room Bm: d = 0.5*span
Floor Bm: d = 0.6*span
Composite Bm: d = 0.55*span

Everyone has different thumbs.

 

[JLNJ]

When my Grandfather taught Journalism, he used to tell his English Composition 101 students, "You have to demonstrate that you know and understand the rule before you are allowed to break it."

I think this is as true for engineering as it is for grammar.