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.

 

[itsdebra]

Are you kidding? A homebuilder will probably spend more on the convection oven than the structural engineering.

Debra

 

[PEinc]

True, but that's not what we're talking about. Engineeres have a responsibility to design properly while also considering the economics. A project can be a failure for several reason - such as bad design, bad construction, bad economics. If engineers want to ignore economics in their designs, they should switch to architecture!

 

[itsdebra]

it seems to me that the question is not regarding safe, sufficient, easy to build structures:that is a given, a minimal requirement. the difficulty begins with customer relations. i believe that TRAINGUY was trying to say that once an engineer determines the minimal requirement, he then determines his "factor of saftey" based personal opinion.

selling his decision to the client is another issue. if you sell them on 10 and they get to build it with nine, they are happy. seems that most people have come to assume an "engineered" project will end up "over- built." why else would builders hate getting an engineer involved at all on a project?

rather, why do we not have the reputation of being called into a job to assist, be of service and help the client satisfy his goals in the project?

Debra

 

[PEinc]

The "minimal requirement" should already have the appropriate safety factor or else it would not be the minimal requirement. Anything extra an engineer adds MAY be unnecessary and is certainly more expensive. An engineer should not try to control field construction or do quality control by pumping up the design. Making sure a job is built right should not be the responsibility of the designer (unless that is specifically in the contract). That's the job for the field inspectors, if any. Giving someone a heavier than needed design does not assure that the structure will be built correctly or will perform as intended. If you design 10 and someone builds 9, someone is stealing. You can't control that by design.

Design it right and build it right. Two separate operation, IMHO.

 

[Tibby]

"BETTER" is the enemy of "GOOD".

 

[trainguy]

My post was in the same lighthearted manner as you can't push a rope, etc.

It's strange how a post discussing girlfriends' average ages (see above)and others making sugar out of s--t don't get nearly the emotional response of (g-d forbid)adding an extra fastener or two to a connection design. An interesting comment on engineers' true passions...

 

[whyun]

To add to the latest topic of discussion...

Like PEinc stated, there are factors of safety on both the strength of materials and forces, thus adding an engineer's own factor of safety in addition to the code minimum appears to be an overkill.

It is prudent to be conservative in the right places. At areas where there aren't too many fudge factors (meaning, forces are relatively accurate and material strengths are well known) and at areas that occur most frequently (like typical details), do not be overly conservative. I would do something similar to providing 10 bolts where 8 is requires in less frequently occuring, special conditions where accurate forces are difficult to determine (such as seismic loads).

Seismic force is minimum level force as required by code. Real earthquakes often generate forces much greater than the design level force but building may sustain minimal damage due to overstrength in the members, dampening effects, redundancies in the connections, etc.

 

[IFRs]

I find rules of thumb useful only when estimating, checking work or starting a new project. It's good to know if you are in the ballpark but after that you have much work to do, particularly reviewing your assumptions and boundary conditions.

 

[MadMango]

When scheduling for the day or week, schedule for only 4 hours in the day. So if you know something will only take you 2 days (16 hrs) to do, you tell who's asking it will take you 4 days.

While there are 8 hours in a normal work day, you have to factor in phone calls, questions for co-workers and the like, not to mention things that pop-up out of the blue.

Ray Reynolds
Senior Designer
Read: faq731-376
"Probable impossibilities are to be preferred to improbable possibilities."

 

[epr]

I agree, "rules of thumb" should be available to assist in quick answers.

If you research older engineering books from the later 1800's and early 1900's, you can find many "rules of thumb". Unfortunately, sincethe advent of attorney's ruling the U.S., every "rule of thumb" is a potential lawsuit and engineers are no longer free to consider "rules of thumb", without an entire list of disclaimers and qualifications.

 

[fmathis]

1. You can design something to be "fool-proof", but you can't design it to be "damn fool-proof"

2. There's a BIG difference between accuracy and precision, and it's more important to be accurate than precise -

 

[SlideRuleEra]

Three rules for designing stairs that all give similar answers and fairly good results:
1. Product of riser and tread must be between 70 and 75
2. Riser plus tread must equal 17 to 17.5
3. Sum of the tread and twice the riser must lie between 24 and 25.5

Another Subject:

For cantilever sheet pile retaining walls, the penetration below the bottom should equal approximately the unsupported height above.

 

[plasgears]

EPR,
Rules of thumb are fine for initial sizing. The final calcs will bridge the gap from prelim to final design, and that is what goes into the design books. It becomes difficult to wrap your arms around a design without some prelim calcs to put some approx dimension to the problem.

 

[FreeRangeMonkey]

There are a couple of books that I use regularly that are filled with rule-of-thumb types of formulas.

The first is called "Standard Handbook of Architectural Engineering" by Robert Brown Butler and is filled with short formulas that the author has compiled for all aspects of architectural engineering applications (Structures, HVAC, Elec., etc) It also contains a lot of concise exerpts from various codes, standard beam sizes, weights of materials, construction details. I find it to be a very handy reference for quick calcs and a one-stop shop for all sorts of engineering data.

The second called "Building Construction Illustrated" by Francis Ching, and it includes a lot of descriptions of various building methods with illustrations (as the name suggests) as well as rules of thumb.

Greg

 

[trainguy]

Hey SlideRuleEra and FreeRangeMonkey:

Congrats on your hilarious "handles". Unless of course those are your real names, in which case I simply salute you.

 

[itsdebra]

I too salute you-all. Am going to attempt to print this thread of valuable information. Looking forward to using it to mentor me through the months and years ahead.



Debra

 

[ttommy]

The rule I have found to be most true is, It is never what you do not know that get's you into trouble, it is what you are sure of!

 

[okeng]

If someone from Marketing begins a sentence with "All you've got to do is.....", then you know some rules of physics are about to be broken.

If you are designing a piece of machinery with tires, design around the biggest ones practical, then build the protos with two sizes smaller. In the first 6 months of sales, someone will demand larger tires and then you can supply them without having to redesign.

I have a book called "The Designer Friendly Handbook of Machinery" by Thomas F. Hanson that is a very good compilation of rules of thumb and handy charts and tables. They are available at 805-259-1861. He also wrote the best (and smallest) book on creative design in the world.

Thanks,

Jess Davis, Davis Precision Design

 

[okeng]

Thought of a couple more:

If someone looks at your design and says "Wow--you must be really smart. I could never figure out anything like that." then you better keep working on it for a while. If they look at it and say "It took you a week to design that? Why?" then you are pretty close to finished.

Antoine de Saint-Exupéry, in "Sand, Wind, and Stars" wrote much more eloquently than I can about a design being complete "not when there is nothing left to add, but when there is nothing left to take away".

 

[ckorski]

Every structural designer's design can be considered alright based on his standards. It's the natural passage of time that will tell how strong it is and obviously the natural or manmade events that will happen within its serviceability. Serious considerations to all factors plus "COMMON SENSE OF RESPONSIBILITY" if you're after for the best result that will benifit the common people.