Deflection discussion
Deflection discussion
(OP)
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 I understand the rule of thumbs come from somewhere, but don't they eventually get outdated?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?
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 I understand the rule of thumbs come from somewhere, but don't they eventually get outdated?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?






RE: Deflection discussion
1) Even members that are not usually checked for vibration from wind and/or portly maintenance personnel can potentially suffer from vibration issues. Span to depth ratios can help with that.
2) Healthy span to depth ratios often lead to member sizes that facilitate straight forward connections.
3) There is, always, a perception game at play. Just wait until you're standing under a "works by the numbers" span/52 roof beam with a contractor standing next to you and looking up at your little miracle skeptically. In that moment, you'll know, with absolute certainty, that anything that goes wrong with that beam will be blamed on you. And you'll deserve that.
4) Following span to depth ratios and other rules of thumb make it easier for your QC reviewers to verify the adequacy of your work.
5) Future renovations.
6) Construction loads. They're real.
7) Inspiring client confidence. Often, a static answer will be just as important to your client as an efficient one. After all, they're going to build their model/drawings around yours. And they desperately want to believe that you're so good at your craft that you can pull rabbits out of hats on the spot. Initial sizes are usually based on rules of thumb. If we're talking about 20 beams that are 50% over designed, will I refine them? You bet. A couple of random beams framing out a stair opening? Probably not.
Nope. This is like saying that wisdom eventually goes out of style. Don't kid yourself about what structural engineering really is. 95% of it is repeating solutions developed by the brilliant engineers that preceded you. The little scraps of innovation that we all introduce into our projects are minor and, frankly, often the stuff that lawsuits are made of.
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.
RE: Deflection discussion
RE: Deflection discussion
Of course there have been some retrofit situations that I had no choice.
RE: Deflection discussion
You have outdone yourself this time. Great reply in the spirit of what Eng-Tips is all about.
RE: Deflection discussion
This article contains a summary of rules of thumb, which includes the aforementioned 1/2 span beam depth approximation. These are well-founded and serve to avoid real problems, as others have mentioned.
One of the other rules of thumb that is very important is the simple span deflection reduction due to inherent shear connection moment rigidity. If you don't employ this rule of thumb for unshored composite design, there is a risk of over-specifying camber on your beams/girders. A composite deck isn't very effective if the concrete is 20% thinner at mid-span than required. Most composite beam design programs have a specific input for reducing the construction dead load to account for this, and is usually defaulted to 80% CDL..........all from the "outdated" rule of thumb.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Deflection discussion
www.SlideRuleEra.net
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RE: Deflection discussion
I'll go against the grain here and agree with OP. If you have a 36ft span and eyeball it to client that it's an 18x35 and they come back and say they need a few inches for head room are you really going to say that no W16 can work? A 16x100? No? What if you tighten the beam spacing, still no? Sure the 18 is most efficient but since when is that the only concern. You check vibration, deflection, blah blah - a w12 could work if you need it to.
What good is engineering if you're glued to some rules that fit on an index card - might as well just sell that index card to the architect and let him do the design.
google 'rule of thumb' = "principle with broad application not to be strictly accurate or reliable in all situations"
RE: Deflection discussion
Just to clarify, the rule of thumb does not require a depth of half the span. If it were so, we would have some mighty deep beams in our buildings. The old rule of thumb is that the depth in inches should be half the span in feet. This means the minimum depth should be L/24 according to the rule of thumb. I mention this because many of the readers are unlikely to be using Imperial measurements.
A uniformly loaded simple span beam with maximum fiber stress of 24,000 psi would require a d/L ratio of 50fb/E = 0.0414 or approximately 1/24 for a deflection of L/240.
I typically used the old rule of thumb without bothering to calculate deflections, but there were times when I reduced the depth slightly in order to satisfy architectural requirements. At such times, deflection had to be calculated and the beam was generally not the most economical choice.
BA
RE: Deflection discussion
RE: Deflection discussion
There is genrally a good rule of thumb check for every typical application, but i would only use these as checks. Cantilevers however alway need a double check.
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"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
RE: Deflection discussion
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?
RE: Deflection discussion
Every point you brought up had been spoken to me when I worked at my first structural consulting engineering firm. He'd mentioned every point you bring up including the humility of knowing you're regurgitating old solutions/wisdom and >90% of the time just standing on the shoulders of giants.
The boss there was well experienced from all directions - worked as a steel erector through high school and college. He made an impressive dance between cost and "looks" as you mention in #3 and both owners and contractors seemed to have great respect and TRUST in the man. The sense of 'teamwork' that inspired between the trades was great. He even somehow found ways to make architects happy who needed thirty pounds of beam in a ten pound sack. I haven't worked with an engineer like him since. I like to think I'd still be working with him if I didn't relocate across the country for the betterment of my personal life.
I like the cut of your jib, KootK ;) Much wisdom in that post.
RE: Deflection discussion
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Deflection discussion
I disagree with the "under any circumstance". However, if that is the stance of the "older guy engineer", he has that right.
Apparently, there is some reasonable flexibility.
I wouldn't. However, circumstances vary and are necessary to consider.
bisandcan, consider all the aforementioned, valuable and well-spoken advice. Perform all necessary and applicable checks and make the decision.
Well said. But, adhering to the rules of thumbs "under every circumstance" was not implied.
This is engineering: time-honored, time-tested, experienced, solid, foundational, confident.
This is typical, contemporary, architecture: (not necessarily negative and often to be appreciated) innovative, whimsical, inexperienced, in need of restraint, not able to stand without engineering.
Architecture and innovation IS to be appreciated and it CAN be beautiful and impressive. BUT, it CANNOT stand without foundational engineering!
RE: Deflection discussion
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Deflection discussion
TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Deflection discussion
My question is about the OPs 11:50 post, which reads in part,
"...simple 3' wide elevated walkways... I used a HSS3x2 tube, and was told that I was crazy and needed to use a 5" tube..."
and can't help but think that there's something more to the conversation than this rule of thumb.
If it's a 3' wide platform, L/24 = 1-1/2" deep member.
Is there a second rule of thumb being applied here?
"Use L/24, but no less than 5in"?
RE: Deflection discussion
He didn't say the span was 3', he said the walkway was 3' wide.
RE: Deflection discussion
TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Deflection discussion
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.
RE: Deflection discussion
RE: Deflection discussion
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.
RE: Deflection discussion
RE: Deflection discussion
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.
RE: Deflection discussion
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.
RE: Deflection discussion
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
www.americanconcrete.com
RE: Deflection discussion
RE: Deflection discussion
RE: Deflection discussion
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Deflection discussion
Room Bm: d = 0.5*span
Floor Bm: d = 0.6*span
Composite Bm: d = 0.55*span
Everyone has different thumbs.
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I think this is as true for engineering as it is for grammar.