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 I understand the rule of thumbs come from somewhere, but don't they eventually get outdated?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?

 

[KootK]

I'm generally in agreement with your colleague. I've had a version of this discussion with most every EIT that's ever worked with me. I enforce the span to depth ratios the vast majority of the time although not 100% of the time. That said, if you design me a beam and fail to have an excellent reason for ignoring the span to depth ratios, you can expect to suffer the wrath of my displeasure. Some great reasons to follow span to depth ratios and other rules of thumb, even when deflection is checked:

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.

I understand the rule of thumbs come from somewhere, but don't they eventually get outdated?

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.

 

[mike20793]

Koot summed it up perfectly. Spot on.

 

[XR250]

Funny, I have never heard of that rule, but historically, I generally exceed that by not trying to design stuff to the serviceability gnat's ass.
Of course there have been some retrofit situations that I had no choice.

 

[hokie66]

KootK,

You have outdone yourself this time. Great reply in the spirit of what Eng-Tips is all about.

 

[MacGruber22]

bisandcan,

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."

 

[SlideRuleEra]

Well said, KootK.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[bookowski]

OP is saying that this is a good rule of thumb and starting point but his colleague is saying 'never, everm under any circumstance'.

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"

 

[BAretired]

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.

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 50f_b/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

 

[Ron]

Nice job.....kootk and BAretired. Both bring back concepts that should be drilled in engineering school, but sadly, are not as likely to occur as we have fewer and fewer academics with any real experience.

 

[rowingengineer]

I have to assume we are talking about steel beams, looked above and found mention of w beams and composite members, so am assuming steel, didnt read all of the replies however. But if this was concrete or frp i would use different depths.

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.

http://www.nceng.com.au/

"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."

 

[bisandcan]

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?

 

[JNieman]

@KootK

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.

 

[TehMightyEngineer]

Bisandcan: What's the cost difference between the two and will this make connections design easier? I suspect that you'll find that it's a negligible cost increase and your connections will be easier to design and fabricate.

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

http://www.americanconcrete.com

 

[BSVBD]

He states that he would never, ever, under any circumstance...

I disagree with the "under any circumstance". However, if that is the stance of the "older guy engineer", he has that right.

What good is engineering if you're glued to some rules that fit on an index card...

If we're talking about 20 beams that are 50% over designed, will I refine them? You bet.

Apparently, there is some reasonable flexibility.

...would you really double your steel weight to control deflections more than L/1347?

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.

I understand the rule of thumbs come from somewhere, but don't they eventually get outdated?

Nope. This is like saying that wisdom eventually goes out of style.

Well said. But, adhering to the rules of thumbs "under every circumstance" was not implied.

95% of it (engineering) is repeating solutions developed by the brilliant engineers that preceded you.

This is engineering: time-honored, time-tested, experienced, solid, foundational, confident.

The little scraps of innovation that we all introduce into our projects are minor and, frankly, often the stuff that lawsuits are made of.

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!

 

[TehMightyEngineer]

bisandcan: Just want to point out that none of this is negative towards you or your opinion. You've asked a very important question that Ron pointed out should be taught better in school or early in an engineers career. Take the advice given and form your own opinion but keep in mind the general consensus here is backed by a lot of good experience. 10 years from now it might be you giving this advice to another young engineer and you'll think back on questions like this that you had and smile.

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

http://www.americanconcrete.com

 

[IRstuff]

don't forget that the best answer isn't necessarily better than the fastest answer, i.e., for something where time is of the essence, are you really going to do the full calculation to optimize each beam when the cost saving is negligible but the analysis time is expensive?

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[Once20036]

I appreciate all of the posts and conversation above - this is great stuff.

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"?

 

[hokie66]

Once,
He didn't say the span was 3', he said the walkway was 3' wide.

 

[IRstuff]

Implication being that the span was at most 10'

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529