Deflection discussion

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 I understand the rule of thumbs come from somewhere, but don’t they eventually get outdated?



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.

To me it’s a guide only, for initial sizing depth.


The opinions expressed in this thread will not deflect my practice.