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.

 

[quark]

One rule of thumb (perhaps this is the origin of this expression) is that for right handed threads, if you turn the bolt or nut in the direction of folded fingers, the linear movement of bolt or nut is in the direction of your thumb.

Note: No pun intended.

Regards,

 

[cups]

Ajuk is right. I worked in the timber construction industry in the early 70's when metrication was introduced. 1/2" ply became 12.5mm and 3x2's 75 x 50's. It was very easy!

A pound at the design stage equals a thousand at the rectification stage (in construction).

 

[msajjadh_imported]

Hi Guys,
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.

 

[romo]

A few things for everyone:
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.

 

[quark]

Romo!

Got a tide of emotion answering this question, ain't you?

Repetition is the foundation of technology

 

[whyun]

One rule of thumb: When you think you know everything, there is someone who knows a few more things than you. (Key is to find who that is and learn as much as you can from that person)

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.

 

[rmonterroso]

Two rules:

* If something can go wrong, it will!

* Thickness of a slab Perimeter/180 (in cms.)

Hello from the Land of Eternal Spring!

 

[oijammu]

1]For Quick Guesstimates of Quotations for any work,Rate=Material Cost X 4.Prilimininaries =Materials cost x 1.

2]A good Engineer=25% of schooling + 25% practical experience + 50% be able to out-bluff the other person!

From Tanzania

 

[JCWilson]

If you are still looking for bona fide structural engineering short cuts, I have a copy of a graduate level presentation handout given by Ira Hooper about ten years ago. Some of it has been published, some maybe not.

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.

 

[Lutfi]

I like to throw few more items that I learned over the years. I use them all the time (they are correct and therefore may not qualify as 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.

 

[Lcubed]

JCWilson,
How may we obtain a copy of the Ira Hooper paper?

Regards

Lcubed

 

[Clevegar]

Two rules from carpentry:

"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.

 

[curvbridger]

All,

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

 

[ANTHAE]

I'll add one thumb I've learned from hard personal experience:

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

 

[fmathis]

The cost of reinforce concrete (in place) is usually somewhere between $100/cy and $800/cy. This illustrates the fact that for a "rule of thumb" to be any good, the background for it's development needs to be known. That in turn means that most of the "rules of thumb" are most applicable by the engineer that came up with them; and that everybody else better be careful in using them -

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)

 

[BantrelStructural]

Useful in Canada at least:

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.....

 

[flamby]

Base Width of a Heigh Dam = Height of Dam
(B/H =1)

Now, my smiley practice..




flame

 

[JAE]

The brighter the color of your architect's socks, the higher chance you have of being asked to frame something totally beyond the laws of statics.

 

[RiBeneke]

"If it looks funny, check again".

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.

 

[waseem19]

i once asked a contractor ,why do all the contractors have the reputation of trying to get rid of the engineers by burying them ,,

he said :cause we know that DEEP INSIDE you are all very good people .