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.

 

[jayrod12]

When you ran the deflection calculations using the real member length did you break up your applied loads into normal and axial?

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.

 

[Jerehmy]

Yes I broke it up into components. If we have a 6-8-10 triangle, and a projected live load of W, breaking the load into actual length yields 8/10*W, and for perpendicular component gives 8^2/10^2*W = 0.64W.

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.

 

[msquared48]

Even with the normalized forces, with the moment varying depending on the square of the length used, I would expect the properly loaded slope length deflection to be greater here.

Mike McCann
MMC Engineering

 

[WillisV]

Whose to say the 1/4" limit wast calibrated to using the projected length?

 

[Jerehmy]

It's possible, but that's in the spec, not NAAMM. The spec says l/240 or 0.25". it's for every structural component though, not just the stringers.

 

[nutte]

There's a lot to be said for what's standard in the industry. As far as stairs and rails go, NAAMM is that standard. And realistically, you're talking about another 1/8" of deflection? Who's ever going to notice?

 

[Jerehmy]

we're sendin sealed calcs they have to meet their criteria. I use NAAMM and use a c12 I use correct defl. calcs I have to go to c15. I'm just going to make a note when we sen the calcs so they can't come back to us and yell.

 

[nutte]

You're making this into more trouble than it really is. I would hope there is a more senior engineer in your office guiding you through this stuff.

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?

 

[Jerehmy]

talked to my boss we're just gonna put a note.

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.

 

[woodman88]

"If we have a 6-8-10 triangle, and a projected live load of W, breaking the load into actual length yields 8/10*W, and for perpendicular component gives 8^2/10^2*W = 0.64W."

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.

 

[Jerehmy]

woodman

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.

 

[Ron]

A couple of comments...
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.

 

[Jerehmy]

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.

 

[TDIengineer]

C12x20.7

 

[Jerehmy]

aluminum not steel

 

[Ron]

jerehmy...then design accordingly and let them bear the cost of an inefficient design. Not much else you can do.

 

[Jerehmy]

Ron, were do you stand on the whole NAAMM deflection vs actual deflection though?

 

[TDIengineer]

-1

 

[woodman88]

jerehmy - in that case you will have differance I values for the 10' vs the 8' member. Do the calculations.

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.