Skeletons in the closet? 4

[greznik91]

Hi
I have to ask - do you guys have any skeletons in the closet? Have you ever designed something that was built / is going to be build but it wasnt designed as it should be - you have realized there are some errors or incorrect modeling or assumptions in your calculations that you have figured later (while working on another similar projects), what have you done about it? How often do you think about it? Ever nervous of consequences? I know there are safety factors and all that but still... This topic may not be popular but I think its a part of our job and would like to hear some thoughts.

 

[TehMightyEngineer]

Short version of the story:

Bungee cords were being used to hold some equipment together. EIT TME was given the task of replacing the bungee cords, outside my expertise but my mentor was looking over my shoulder. New system did not work as intended; client not pleased. My system was replaced with bungee cords. I kept one of the old bungee cords as a reminder to not work outside my expertise.

Moving to my own employment I decided I didn't need the bungee cord anymore. Now I hold things together with hopes, dreams, and a savings account.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[kingnero]

"When in doubt, make it stout"

While I agree with the principle, I don't agree to live by this. Anybody can "engineer" a building using the "oversizing" method, only a good engineer can make it just stout enough.

 

[EngrPaper]

I've similarly heard from a civil: "Anyone can build a bridge that will stand, but an engineer can build a bridge that will barely stand."

 

[SlideRuleEra]

kingnero, EngrPaper - Problem is not everything structural is a building or bridge (which usually have well defined loading code guidance). In this thread, the "...make it stout" comment refers to to railcar positioner track (on the right of this image) where impact loading is unknown:

The railcar positioner provides 100% of the force needed to pull an entire (coupled together) train with four large (idling) locomotives plus 80 fully loader coal railcars through a rotary car dumper:

Can't precisely design something if some of the loads are unknown, probably not knowable, and can vary greatly depending on condition of the rolling stock.

http://www.SlideRuleEra.net

 

[TLHS]

Part of being an engineer is understanding when conditions are outside of normal and deserve extra contingency and when providing extra material is a better cost option than the associated risk of failure or the risk of seeing unexpected loading conditions. So yeah, I think a good engineer will definitely beef things up when appropriate even if exact figures or analysis don't require it.

It's not over-sizing. It's knowing when conventional sizing methods may not fully define the problem or when there are unusual consequences for failure or serviceability requirements.. You can make the whole thing a lot fancier sounding by calling it a design contingency, allowance for future expansion, supplementary safety factor, dynamic factor or something like that, but at the end of the day it comes down to a judgement call on what's appropriate.