Design by rule of thumb 20

[HvZ]

Hi all,

I am collecting all "rules of thumb" examples in engineering used to either design or give a design a quick sanity check. Could you please e-mail any that you use or know of to me. If you know the origin or explanation (maths) behind it, I would appreaciate it.

I think it is very interesting and have to date not yet found a database of these "quick short cuts", which is the result of years of experince in engineering.

 

[Rjeffery]

Construction super's rule of thumb for how to handle a suggestion by the architect/engineer or anyone concerned with quality:

"I have never done (add your recommendation here) in the (add speeker's age - 10 years, here) I've been doin' (add project title here). thats (add your recommendation here) Just plain (explative deleted) stupid."

 

[StephenEd]

If it draws hard, it builds hard.

 

[nealt]

Here's my contribution:

If it ain't broke, don't fix it.

You can't make sugar out of s_ _ t, no matter what kind of stick you stir it with.

Short form specifications:
1. Use good stuff, do good work.
2. Hammer to fit.
3. Caulk to patch.
4. Paint to match.

Plumbing: Hot on the left, cold on the right, S_ _ t won't flow up hill.

The more I learn, the more I realize just how little I know!

 

[Rjeffery]

plumbing rule of thumb #1:

Sh_t flows down hill.

plumbing rule of thumb #2:

Always clean your fingernails.

plumbing rule of thumb #3:

Don't cut your apple with your pocket knife.

 

[dugal]

As far as structural rules of thumb go....... Learned this a long time ago in the trades. "Never weld across a structural column or beam"

 

[ongjo]

Everything you need to know about civil engineering:

Sum of F = 0
Sum of M = 0

The rest of your education/experience is learning when and how to use the above.

(e.g., F = m x a is a form of Sum F = 0, F - m x a = 0, as is an energy balance--conservation of energy: potential = kinetic energy, but the above is a more practical statement of it)

 

[Inzynier]

Greetings!
Here are my rules of thumb for structural engineers and not only:
1) It is better to find approximate solution to an exact problem than to find an exact solution to approximate problem.
2) Instead of learning tricks of trade - learn the trade!

 

[krd]

Time management for new engineers: "20% of your time will be spent on the design concept and general arrangement; 80% will be spent on details". I find the 20% is the fun part.

 

[Wrightguy]

One of my personal...yet highly idealized quotes...

"If you don't have time to do it right, when are you going to find time to do it over?"

 

[itsdebra]

StevenEd:
Yes, sir, I was. Just found it- right on top. How does that happen?? I get a chuckle every time I read this thread. I feel like I'm sitting in a break room with a bunch of engineers having coffee and "talking shop." My favorites are about esitmating costs then doubling them, expecting the client to change something/anything and get everthing in writing. I'm just learning to run my own practice. Great site. Thanks ladies and gentlemen.
Debra

ok here goes my smiley face practice.... Debra

 

[jbanderson]

Does anyone know where I could get the paper by Ira Hooper that JC Wilson mentioned in a previous post. Any help would be appreciated.

 

[glennd]

I once heard of a gret rule of thumb that actually involves your thumb (one suggestion on where the term comes from)

In soil mechanics you can use your thumb to evaluate the compaction of a material (usually clay) by pressing your thumb into the compacted surface.

If your thumb sinks all the way in ~75% standard
If you reach the first knuckle ~90%
If you can dent it only ~95%

of course this only works on the top layer but I thought you might like a rule that actually uses your thumb!!

 

[BantrelStructural]

Sometimes it seems that if there are two ways to approach a problem, that with some clients it pays to do it the wrong way first, because regardless they'll want to see the other way too, and so you only have to change it once. This applied more to the days of manual draughting where it wasn't easy to work on a copy, so you had to make the changes on the original, and if you started with the correct way first, the velum was getting pretty thin by the time you got around to changing the drawing back to the original concept after exploring the "other concept".

Somewhat cynical, but unfortunately true far too many times when dealing with certain clients.

In fact it is amazing how "cost concious" clients can drive their engineering costs even higher by all the unnecessary rework and wasted effort they cause with their cost saving ideas, that often could be explored early on with a simple cost-benefit analysis.

Another favourite of mine is the delayed critical decision that forces the design team to do the final design (virtually a total redesign) in a compressed schedule (within the already tight and frequently unrealistic original schedule), with the inevitable cost overruns from having to substitute overtime in the crunch for properly planned regular time, often using inefficient and poorly skilled agency personnel (the only ones available on short notice), who often actually make the project go backwards. And then the lead engineer is rated as being a poor lead because he wasn't capable of performing magic or miracles or both, despite the fact that he had warned from the get-go that this critical decision was a show stopper.

I've got to stop now.....I could go on for hours.

 

[whyun]

Rule of thumb for owners (often ignored):
An extra dollar spent on "design" will save ten dollars during construction.

Rule of thumb for everyone (who works):
1. There's always someone out there smarter.
2. There's always someone out there cheaper.
3. All the employees think they are underpaid.
4. All the employers think he pays the employees too much.

Good evening, folks!

 

[itsdebra]

Good evening, Ladies and Gentlemen,
I noticed my (architect) client was wearing very bright socks....hmmm....(sorry) I told him that I was calling out 2x floor joists (even thou I only have details for TJI's drawn so far.) He says he prefers TJI's. Makes me wonder what he would have said if I'd said I was calling out TJI's.Guess the rule would be: give them something (easy) to change?
here goes a cannon??
(cannon)
<cannon> or
{cannon} or
or
well, nevermind

Debra

 

[ghghghgh]

I once designed a two span continuous plate girder that got fabricated in the shop upside down (they swapped flanges in the N.M.R.). The mistake was caught in the field during erection when we got the call. After analyzing it we determed that it would still work. Rather than receive thanks from the client for saving significant $$$ in delays and refab, we got rebuked for over desiging the original.

 

[mmh]

Active pressure behind retaining wall in kN/m2 is:

For dry soil = 5 times depth below surface in m

For saturated soil = 15 times depth below surface in m

 

[rquerin]

Here's a few that I don't think have been mentioned:

1. To get an efficient depth of open web steel joist for a given span:
Span(feet) x 0.65 = Efficient OWSJ depth(inches)

2. For lightly loaded beams, to get a beam depth that won't look ridiculously shallow for a given span:
Span (feet)/ 2.5 = Approximate Beam depth not to look skimpy

3. For reasonably efficient gerber beam cantilever lengths:
Double Cantilever: Span x 0.18 = cantilever length
Single Cantilever: Span x 0.21 = cantilever length

4. Try to avoid specifying hollow structural sections with the same outer dimensions and differing wall thicknesses in the same area of a building. If they get switched, you'll never be able to tell..

RQ

 

[trainguy]

If the connection design calls for 8 fasteners, use 10, and hem and haw as the foreman begs you to reduce the amount. Finally, let him use 9, but he has to appreciate the HUGE favour you are doing him...

tg

 

[PEinc]

How about, if the connection design calls for 8 fasteners, using 8? Then make sure all 8 are installed. I wouldn't want to be paying for either 12.5% or 25% extra, un-needed structure or structural components. The answer isn't always to just add more of everything. Someone has to pay for the extras.