Engineering Ethics 2

[Jerehmy]

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.

 

[woodman88]

Maybe you should take a class in Engineering History. The method use was started in the old days before computers. It work then and now as long as the person using it takes the time to learn it's limitations.

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.

 

[abusementpark]

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.

Is that true here? That would be extremely rare.

I wish it was L/240. They have it set at 0.25" for a 19ft actual length stringer. I have to comply with it.

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.