Design by rule of thumb
Design by rule of thumb
3
HvZ (Civil/Environmental)
(OP)
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.
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.






RE: Design by rule of thumb
RE: Design by rule of thumb
RE: Design by rule of thumb
After age 50 (this assumes you have been doing this type of job for a good portion of your working career) you know about these things & have baked this into the estimate with little thought. If a factor of less than one proves to be needed, the over 50 guy may not be accounting for productivity tools. Just a few years ago we waited a week to get photos from staff in the field. Now we can have photos in a few minutes & request more while the person is still on the site. This turns 1-2 weeks into 1 day effort. Same is true with CAD files vs mailing paper prints.
RE: Design by rule of thumb
RE: Design by rule of thumb
Rod
RE: Design by rule of thumb
-assume 'w' shape member
-start selection within steel manual from w14 shapes.
this was one of the so-called short cuts that was taught to my class by my structural steel design professors. it has helped me out when trying to select an economical member during my exams.
RE: Design by rule of thumb
RE: Design by rule of thumb
You can't push a rope.
Regarding how long something will take to complete:
Just refer to Scotty on Star Trek: If you think you can complete it in 2 days, tell them it will take 2 weeks. When you actually complete it in ONE week, they'll think you are a miracle worker.
RE: Design by rule of thumb
RE: Design by rule of thumb
Take the span in feet, divide by 2 and add 1 for hardwood, or add 2 for softwood. The result is the joist depth in inches.
eg 10' joist needs to be 6" hardwood or 7" softwood. This appears to be based on joists @ 18" centres, domestic loads only.
RE: Design by rule of thumb
i got a few to share...
concrete beam area of steel required for flexure:
Areq = Mu / a d
Mu [k-ft]
a = use 4.0 for 3000 psi normal weight conc
d [in]
steel beam deflection with unusual
load condition (without using computer:
Deflection = M L^2 / 161 Ix
M = max moment in kip-ft
L = beam length in ft
Ix = moment of inertia in in^4
Resulting deflection is in inches.
steel beam supporting concrete filled deck:
(assumes normal situation with standard floor live)
Span in feet divide by two = approx. WF depth in inches
steel girder supporting approx 30 bay beams from each side:
Span in feet = approx WF girder depth in inches
High CMU fence wall footing width:
First trial use height divide two and decrease as needed. Final result depends on soil condition but half height is good guess to start even at bad soil conditions.
RE: Design by rule of thumb
RE: Design by rule of thumb
I wish I was 24 again so i can date someone who is 19.
RE: Design by rule of thumb
We also drive on the correct (left) side of the road and buy (very expensive) petrol in gallons but the price is stated in litres. Its all very strange.
Our range of steel sections are all very funny sizes mainly because they are conversions to millimetres from the imperial (i.e. an 8" x 5" steel I beam is denoted as a 203 x 133). More importantly beer is served in pints and not 568ml - it goes on and on.
(OT - You may not believe this but last year in the UK a grocer was JAILED for selling bananas by the pound, even though his customers were happy. He was termed the 'metric martyr'. Thats what I mean by the Euro thought police - if I had my way I would leave the EU tommorrow)
By the way a standard plywood sheet is 1200mm x 2400mm (4' x 8') and I also wish I was 24 again.
Best regards
RE: Design by rule of thumb
Blacksmith
RE: Design by rule of thumb
Is it correct to refer to it as English Units still?
It was nice chatting with you.
RE: Design by rule of thumb
RE: Design by rule of thumb
1. F=ma, and
2. you can't push a rope.
RE: Design by rule of thumb
Two rules of thumb that help me to get along with fabricators and others is:
There is more than 1 way to skin a cat. (applies to all situations)
The client will always try to aalter your design during construction.
regards
sc
RE: Design by rule of thumb
Note: No pun intended.
Regards,
RE: Design by rule of thumb
A pound at the design stage equals a thousand at the rectification stage (in construction).
RE: Design by rule of thumb
I have a very important rule of thumb to design a foundation for vibrating machines. i.e.
Weight of footing = 3 to 5 times weight of the machine.
RE: Design by rule of thumb
1) Temporary repairs will soon be permanent.
2) If you have any doubts, check it again.
3) When you are sure its right, check it again.
4) You can not get a W18 spanning 5 feet to support a 150 Ton Hydraulic Jack in calcs but the workers lifting 150 tons with the jack can place it on 3/8" steel plate spanning 5 feet for thirty years without problem. (Incredible amounts of deflection, however, the steel looks like a bowl instead of a walking surface
I could go on all day, but the biggest one of all
Whatever you think wont matter in a project - is the only
thing that will matter.
OK one more: Technology does not make us dummer, it makes us more efficient.
RE: Design by rule of thumb
Got a tide of emotion answering this question, ain't you?
Repetition is the foundation of technology
RE: Design by rule of thumb
Yet another: Nothing is written in stone. Make sure to have the client write them in stone for you. This way, you can PAID for any changes made in the future.
RE: Design by rule of thumb
* If something can go wrong, it will!
* Thickness of a slab Perimeter/180 (in cms.)
Hello from the Land of Eternal Spring!
RE: Design by rule of thumb
2]A good Engineer=25% of schooling + 25% practical experience + 50% be able to out-bluff the other person!
From Tanzania
RE: Design by rule of thumb
It contains shortcuts (based mostly on ASD) for several structural engineering problems including deflections, beam columns and even portal drift. It is far more involved and technical than the paper by Ioannides and Ruddy.
RE: Design by rule of thumb
1. Divide the steel beam weight per foot by 3.4 to obtain the cross sectional area
2. When converting cubic feet to cubic inches, multiply by 1728 (12^3)
3. To quickly obtain weld shear capacity, 1/16 weld, one inch long is good for 925 pound (so, 3/16 fillet weld, i inch long is good for 3*925=2775 pounds)
4. To do a preliminary beam depth, I use 0.5 inches per foot of span. I think some else stated same earlier.
RE: Design by rule of thumb
How may we obtain a copy of the Ira Hooper paper?
Regards
Lcubed
RE: Design by rule of thumb
"Measure twice and cut once". That should be self-explanatory.
"It's not wrong until it can't be made right" ie: many mistakes can be fixed before it's too late. Example, changing rebar before placing concrete.
RE: Design by rule of thumb
In the spirit of this thread I would like to say that the rule of thumb which is stated as "you can't push a rope," is only true when L>3xDia.
DON'T OVERSIMPLIFY !!
Curvbridger
RE: Design by rule of thumb
If you're hired to watch a contractor perform a particularly complicated structural erection or a complex mechanical startup operation, stand well away from the action(!!), and NEVER let anyone catch you sitting down or stiffling a yawn.
The moment you relax and joke, the scaffold will collapse, or
the steam valve will rupture, or the elevated tank will fall!
The corollary to that is, no matter how stupid the cause for
collapse or startup failure, NEVER let anyone see you smile!
Robert Marmaduke PE
The Anthae Company, Anthae.com
RE: Design by rule of thumb
That being said, there is a pretty good printed set for pipe lines called "Pipe Line Rules of Thumb Handbook" (edited by E.W. McAllister, published by Gulf Publishing Company, ISBN 0-87201-695-1)
RE: Design by rule of thumb
1. Weight in lbs/ft. of metric designation steel beam, divide the metric weight by 1.5 (i.e. W200x27 weighs about 18 lb/ft. - W8x18). - Note although we are officially metric, many of our industrial clients are U.S. based and insist on their projects being in Imperial units. It is not unusual to see calculations that have both metric and imperial numbers in them. Potential for screw-ups there, but it happens frequently.
2. Wind pressure (or any other load per sq. unit) - kPa * 21 gives approximate psf. (within about 1%) - (i.e. 0.4 kPa is about 8.4 psf).
General Short-cut for beam design (assuming hand calculation)
1. Instead of calculating beam deflection after selecting the beam, invert the deflection equation and calculate the required minimum I value for l/360 (or l/240 or absolute value) deflection limit. Then you can look up both properties and quickly select a beam from the handbook such that Mr and I tabulated are greater than your computed values. For UDL load:
For l/360 deflection limit
Min I reqd = 0.0233 * wl^3
(in^4) (klf & ft.)
Remember though when designing with LRFD (US) or LSD (Canada) to use the specified liveload 'w' for the deflection calculation. A time saver though can be to use the factored total load 'w', and then if the section selected for Mr does not need to be upsized for the computed I reqd you have done a conservative deflection serviceability check which does not impact section selection, and have saved a little time. If the required section has to be upsized based on this quick and dirty number, then you can go and calculate the correct value.
2. Another time saver for deflection checks is to take the moment computed from loading that may be a combination of uniform load, point load, several point loads, or even a triangular load, and divide this by length squared and multiply by 8 (i.e. invert wl^2/8), then compute your deflection for this 'equivalent' uniform load. The inaccuracy of this method is roughly as follows:
(a) Point Load @ centre - 25% overestimation of deflection contribution from this load
(b) Equal Point Loads @ 1/4, 1/2 & 3/4 - 5% overestimation of deflection contribution from these loads
(c) Point loads at 1/3 points only - 3% underestimation of deflection contribution from these loads
(d) Point loads at 1/4 and 3/4 points only - 10% underestimation of deflection contribution from these loads
(e) Triangular Load Peaked at mid beam - 4% overestimation of deflection contribution from this load
(f) Triangular Load Peaked at one end of beam - max 2.5% overestimation of deflection contribution from this load (depends on where you compute: at mid span or at point of maximum deflection for this loading)
For the most part this should give a reasonable, approximation of deflection (or reqd I), which if it doesn't govern means that deflection serviceability has been checked quickly and easily.
Example - take a 20 ft. long beam with 2 klf UDL, Triangular Load W = 15 kips, and three point loads of 8 kips each at 1/4, 1/2 & 3/4 points.
Mmax = 100+50+80 = 230 ft.k
--> w eq = 4.6 klf --> Defl. = 1.656E7/EI
Deflections from individual loads:
7.2E6/EI + 3.456E6/EI + 2.304E6/EI + 3.168E6/EI = 1.6128E7/EI
A 2.6% overestimated by eq. UDL method - but you only had to use one deflection equation. Now if you used total loads or factored total loads you still have the conservatism of using these numbers instead of the specified live load, but since most short to medium span beams are sized based on flexural strength, you have done a quick deflection check. This method obviously is not accurate if you require precise deflection estimates, and if beam size needs to be bumped because of this value, then you should do the more detailed calculation with the exact loads - of course now-a-days you probably have a computer to do all this dirty work for you, but it can still can be handy for a quick check in the field or when you don't want to boot up the PC for a quick reality check.
Tongue in cheek
1. Sewer Design Equation: SI-SO = SSIP (Stuff In minus Stuff Out equals Stuff Stuck In Pipe) - you can substitute appropriate alternate for the first S.....
RE: Design by rule of thumb
(B/H =1)
Now, my smiley practice..
RE: Design by rule of thumb
RE: Design by rule of thumb
When your design is done, stand back and look at it, or have a colleague look at it. If it looks wrong in proportion, shape, balance or even artistically, then you had better check it again.
RE: Design by rule of thumb
he said :cause we know that DEEP INSIDE you are all very good people .
RE: Design by rule of thumb
"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."
RE: Design by rule of thumb
RE: Design by rule of thumb
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!
RE: Design by rule of thumb
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.
RE: Design by rule of thumb
RE: Design by rule of thumb
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)
RE: Design by rule of thumb
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!
RE: Design by rule of thumb
RE: Design by rule of thumb
"If you don't have time to do it right, when are you going to find time to do it over?"
RE: Design by rule of thumb
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
RE: Design by rule of thumb
RE: Design by rule of thumb
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!!
RE: Design by rule of thumb
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.
RE: Design by rule of thumb
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!
RE: Design by rule of thumb
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
well, nevermind
Debra
RE: Design by rule of thumb
RE: Design by rule of thumb
For dry soil = 5 times depth below surface in m
For saturated soil = 15 times depth below surface in m
RE: Design by rule of thumb
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
RE: Design by rule of thumb
tg
RE: Design by rule of thumb
RE: Design by rule of thumb
Debra
RE: Design by rule of thumb
RE: Design by rule of thumb
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
RE: Design by rule of thumb
Design it right and build it right. Two separate operation, IMHO.
RE: Design by rule of thumb
RE: Design by rule of thumb
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...
RE: Design by rule of thumb
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.
RE: Design by rule of thumb
RE: Design by rule of thumb
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."
RE: Design by rule of thumb
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.
RE: Design by rule of thumb
2. There's a BIG difference between accuracy and precision, and it's more important to be accurate than precise -
RE: Design by rule of thumb
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.
RE: Design by rule of thumb
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.
RE: Design by rule of thumb
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
RE: Design by rule of thumb
Congrats on your hilarious "handles". Unless of course those are your real names, in which case I simply salute you.
RE: Design by rule of thumb
Debra
RE: Design by rule of thumb
RE: Design by rule of thumb
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
RE: Design by rule of thumb
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".
RE: Design by rule of thumb
RE: Design by rule of thumb
Don't have a copy close at hand, but they are categorized by subject, may for engioneering and building.
A favourite is for parachuting: Open your chute when cars look as big as ants; if you wait until ants look as big as cars, you've waited too long.
RE: Design by rule of thumb
The shear capacity of wood is 1 kip per foot.
That is:
one 2x12 will carry about 1000 pounds,
one 2x6 will carry about 500 pounds,
etc.
Nigel
RE: Design by rule of thumb
A fool proof design will only fool the designer.
RE: Design by rule of thumb
Is that 1 kip/bf (one kip per board foot)?
RE: Design by rule of thumb
when a PM/Owner/Contractor comes to you and says "can't you just....." he is typically trying to oversimplify a situation that he knows just enough about to be dangerous.