Engineering Ethics
Engineering Ethics
(OP)
Here's the situation:
NAAMM 510 says to use PROJECTED stair stringer length for calculating deflection. Well, doing it this way gives overall deflection (Perpendicular to beam) that is 44% less than actual and 20% less vertical deflection than actual.
When I say actual I mean using the members actual length and adjusting projected live loads accordingly.
Now the stringers I'm designing are supposed to comply with NAAMM AMP 510 and the stringers a deflection limit of 0.25". If I use NAAMMs estimated deflection calculation, I meet the deflection limit. If I use the right deflection calculation I'm over and need to use a larger size.
Ethically, what's the right answer here? Use bad deflection equations and a smaller size beam because that's what they want, or use correct engineering deflection equations and a higher size?
I prefer the latter but I got attitude from a drafter about how to design stair stringers that made me want to go through the phone...
And how did NAAMM become a standard when it's deflection estimate is so blatantly wrong. I sent them an email asking.
NAAMM 510 says to use PROJECTED stair stringer length for calculating deflection. Well, doing it this way gives overall deflection (Perpendicular to beam) that is 44% less than actual and 20% less vertical deflection than actual.
When I say actual I mean using the members actual length and adjusting projected live loads accordingly.
Now the stringers I'm designing are supposed to comply with NAAMM AMP 510 and the stringers a deflection limit of 0.25". If I use NAAMMs estimated deflection calculation, I meet the deflection limit. If I use the right deflection calculation I'm over and need to use a larger size.
Ethically, what's the right answer here? Use bad deflection equations and a smaller size beam because that's what they want, or use correct engineering deflection equations and a higher size?
I prefer the latter but I got attitude from a drafter about how to design stair stringers that made me want to go through the phone...
And how did NAAMM become a standard when it's deflection estimate is so blatantly wrong. I sent them an email asking.






RE: Engineering Ethics
You may find if you do that the deflection might work as the normal loads only with cause the deflection but the axial loads will cause stress in the stringer.
The NAAMM code (I'm unfamiliar with it so can't comment directly) may be using the projected area as a simplifying method so you don't have to break your loads down into their components.
RE: Engineering Ethics
So we have:
a) L = 8ft with W load and
b) L = 10ft and 0.64W load
8^4 * W *5/384EI = 4096*W *5/384EI
10^4 * 0.64W *5/384EI = 6400*W *5/384EI
Check in RAM if you don't believe me that NAAMM is wrong.
RE: Engineering Ethics
Mike McCann
MMC Engineering
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You're reading things into the spec that aren't there. The spec says not to exceed L/240 or 0.25". Do they define L? Do they say "projected length" or "actual length"? Do they say how the 0.25" is to be measured? Vertically? Or perpendicularly to the stringer? I doubt it elaborates on any of these. So you go to the industry experts, NAAMM, who have a handy design manual to help you. They say use the projected length and compare it to your limit. No need to feel like you've violated your ethical obligations.
Besides, your spec probably tells you to design the stairs per NAAMM. Why would they complain afterward if you did it just like NAAMM says?
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well because what if I had something underneath the stairs or something attached that couldn't exceed a certain limit and I used NAAMM instead of actual length? we'd be blamed if it got damaged.
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What kind of math is this?
Say the load along the l=8' lenght is W=100#.
Than te load perpendicular to the l=10' lenght is W=80#, parallel is W=60#.
From 6^2*8^2=10^2
Where are you getting the 8^2/10^2 factor from?
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
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the first 8/10 takes it from an 8 foot beam to a 10 foot beam
the second 8/10 takes it from a vertical load to the perpendicular component. draw it out.
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Stair stringers should be designed on actual length, but keep in mind that the loading from the treads will be at an angle to the stringer and must be resolved. You will have an axial component and a bending component.
Stairs are not considered structural steel by AISC, so the serviceability issues do not apply. L/240 is too stringent for stair stringers loaded to full code loading....which is awfully difficult to achieve in practicality.
L/180 should be sufficient as there is typically nothing attached to the bottom of the stringers.
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Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
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See table 5.26. At 19' they allow around 3/4" deflection (L/300).
https://www.naamm.org/landing_pages/AMP_510-92.pdf
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Link of calcs to what I'm talking about. I hope it's clear enough, my scanner here didn't seem to pick up my pencil very well...
Here's a graphic from ram. The 200plf for the diagonal beam is a "projected load" not an "actual load" just to make sure there wasn't any confusion. The nodes are at midheight. Visual2 shows the max local beam deflection w/ the global deflections.
https://www.dropbox.com/s/8c1euwds762dvov/Visual.j...
https://www.dropbox.com/s/l8ztjkral2brici/Visual2....
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The other document is NAAMM 510, to which CANEIT gave a link. I cannot find anywhere in NAAMM 510 that limits deflection in any way.
If so, your client may have special (unstated) requirements to limit deflections, therefore 0.25", per the spec. The solution to your problem is now easy. Select structural members that meet the client's "spec", using the NAAMM calculations. The fact that these members EXCEED the NAAMM recommendations is NOT a problem.
www.SlideRuleEra.net
www.VacuumTubeEra.net
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An engineering company hires a manufacturer to do its stairs. The manufacturer hires us to do the stair calcs. The spec comes from the engineering company.
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The NAAMM 510 document is from 1992. On page 5-2, NAAMM 510 mentions that everything in it is calculated using Allowable Stress Design (ASD). The way ASD was practiced in 1992 is different from the ASD of today you are most likely familiar with.
Could this discrepancy be a reason for your inconsistent results? Just a question, not an answer.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Engineering Ethics
RE: Engineering Ethics
The solutions to the example problems certainly need to be coordinated by NAAMM. A general update seems to be justified to modernized the ASD / LRFD references. Am sure there are other issues, too.
One of your original questions concerned why NAAMM is the standard for stairs. A quick answer is because the American National Standards Institute (ASNI) says so. See this link on the NAAMM page:
http://www.naamm.org/ansi/
ANSI is considered to be the final authority on setting technical standards in the USA.
Since the NAAMM 510 manual makes it clear that its contends are "recommendations", suggest that you treat them as such. IMHO, perform your calculations in accordance with good engineering practice and general conformance with NAAMM's intent. Meet your clients "spec" requirements. However, make sure you go over your calcs with your mentor (find one, if you don't have one), or at least someone with more experience on this subject.
www.SlideRuleEra.net
www.VacuumTubeEra.net
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NAAMM may be wrong, but not as wrong as you suggest. You are comparing vertical deflection in the 8' projected span to sloping deflection in the 10' span. The vertical deflection in the 10' span is 0.8*6400*W *5/384EI = 5120*W *5/384EI or approximately 1.25 times the deflection in the 8' span.
BA
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Per the "Design of Wood Structures 3rd ed." by D. E. Breyer page 26 "NOTE: The horizontal plane method is commonly used in practice to calculate design values for inclined beams and rafters. The approach is convenient and gives equivalent design moments and conservative values for shear compared with the sloping beam analysis..."
Thus, you are comparing an equivalent method to an exact method. Of course the answers are difference. If you want them to be the same you need to fix the horizontal plane method before using it.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
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BAretired - Yes you are correct, I noted that in my attached calcs. But shouldn't we be looking at overall deflection not vertical anyways? idk-
The guy I emailed at NAAMM wasn't very nice to me lol. I think he purposefully misspelled my name in his response mail...
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Jerehmy, I'm curious as to what NAAMM's respoinse was..?
I still maintain that this is much ado about nothing.
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The gravity loads act vertically, not on the actual sloped length. If you
vector the load into components one of which is perpendicular to the actual
length and one parallel to the actual length you will find the results are
the same.
so and so"
my response
"so and so,
They are not the same. Deflection changes with a 4th power to length, the gravity loads only change to the 2nd power when you distribute them across the proper length, then change them into their parallel and perpendicular components. Check in RAM Elements or some other analysis software if you don't believe me."
his response
"Gerehmy,
I understand the problem. I will stick with my original recommendation that you should work with the projected span."
^^ was sent from a phone
That was frustrating because he didn't even look at the stuff I sent him as attachments, or analyze it himself. He kind of just blew me off. I was going to email him back that he clearly doesn't understand the problem if he thinks they are the same deflection but whatever, I give up.
And come on, the G and J are two spaces apart lol.
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Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
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Shear varies with a power of 1 to length
moment is 2
deflection is 4 (for distributed)
the loading changes with a power of 2 for a sloped beam, thus moments are the same, calculated deflection is less than actual and calculated shear is more than actual.
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Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
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I won't disagree that the measured, actual numeric deflection in inches, is more for the sloped case than the horizontal projection case. But nobody checks it that way.
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I just find the way NAAMM does it to be unintuitive, misleading, and confusing when it seems like they're trying to simplify things. If I just blindly followed NAAMM it'd be OK, but when I try to understand what they're doing and why they're doing it, it takes more time and over-complicates things. Especially for someone, like myself, who has never used NAAMM before.
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Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
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Is that true here? That would be extremely rare.
Part of the issue here is that 19' is an unusually long span for a stair stringer. That 0.25" requirement is a boiler plate deflection criteria that I've seen from time to time and it is not rooted in reality. However, it isn't a very onerous requirement for typical spans (~12). But for a 19' span, 0.25" deflections corresponds to L/912! That's very stringent. If you can get it approved using the approximate method, you can still sleep easy, even if the real vertical deflection is less than that.