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Design by rule of thumb
20

Design by rule of thumb

Design by rule of thumb

3
(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.

RE: Design by rule of thumb

2
I will cost twice what you think it should and take twice as long  as you think it should take.

RE: Design by rule of thumb

Socrates Ioannides and John Ruddy have compiled a fairly comprehensive list of steel design rules of thumb and presented the topic at several conferences.  A summary of their presentation is included in the February 2000 issue of Modern Steel Construction.  

RE: Design by rule of thumb

A rule of thumb given to me for evaluating another person's estimate was as follows. Divide 50 by the person's age. Multiple the results by the estimate. Example. estimate 5 weeks, age 35, 50/35 = 1.43. Plan for an effort of 5 x 1.43 = 7.1 weeks. You learn over time that stuff happens. Sick time, phone calls, a customer is down & needs help now....things than have nothing to do with the initial task but will take time from it.

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

Also, David Fanella wrote a series of articles with reinforced concrete design rules of thumb that appear in the August, October, and November 2001 issues of Structural Engineer.

RE: Design by rule of thumb

At the old Bethlehem it was--- complete your estimate, double it and add 25%---Worked that way for a lot of years.    Maybe that's why they arn't around anymore?

Rod

RE: Design by rule of thumb

-span of girder or beam span = 28 feet -> 28/2 = 14
-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

By doubleing the pipe diameter gives you five times the flow.

RE: Design by rule of thumb

2
Regarding column stability:

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

misc. structural steel (crossframes and diaphragms) ~ 3 psf * deck area

RE: Design by rule of thumb

Timber floor joist design:

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

hello sven.  is 18" spacing common in your area for wood joists?  i am from california and typical spacing here is 12", 16", 24".  reason is plywood is 4 ft x 8 ft and 18" is not a good plywood module.

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

Yes 18" is a common joist spacing, the UK at least. However, IMHO, the 18" is incorrect and should be 19" as this would, as you correctly point out, fit exactly with the ply/chip board module. 16" and 2' are also used (oh dear! I seem to have used proper measurements and now the Euro thought police will be after me )

RE: Design by rule of thumb

Thank you Ajuk.  Does UK use inch for spacing?  I was under the impression that they converted to SI units long ago.  Just curious, what are the standard plywood board dimensions there?  I am from the Republic of California and not in-tune with how things are done elsewhere in the world.

I wish I was 24 again so i can date someone who is 19.

RE: Design by rule of thumb

Officially the UK is metric. However, there are some anomolies, mainly in measurement of distances. All UK road signs are stated in miles and in the construction industry it is very common to discuss the job in imperial on site but issue metric drawings We also discuss a persons weight and height in stones/pounds and feet and inches!

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

Yeah, I wish I was 24 again and knew what I know now!  How can you leave out the 90/10 rule - the first 90% takes 10% of the time, the last 10% takes the other 90% of the time - if you're not already over budget.  Also, if everything is going according to the plan, either you have overlooked something or the plan is defective.  Seriously, I often use what we called in the Military "the dumb Major test" - I will have someone I know to be reasonably competent but completely uninvolved give a quick review of the current project - if they understand it and don't have any show stopping questions - its probably sound.

Blacksmith

RE: Design by rule of thumb

Thank you Ajuk for the interesting info.  England sounds alot like USA in the year 2100 or so.  Many established industries here are resisting a "total" conversion to metric since all their steel mills, lumber yards and factories already have equipment set up for the English Units and it will cost them a fortune to convert to metric.

Is it correct to refer to it as English Units still?

It was nice chatting with you.

RE: Design by rule of thumb

Whyun,  to confuse the matter, in England  "English Units" are called "Imperial units". Ho hum. As an aside, remember the beginning of Pulp Fiction and the discussion of burger weights in Europe....?  Also, what's the situation with aircraft/airlines? Presumably, international units for air speed, fuel weights etc. are governed by Boeings market leadership?

RE: Design by rule of thumb

There are two great rules:

1. F=ma, and

2. you can't push a rope.

RE: Design by rule of thumb

Well most of the above has just helped to prove that some of us engineers are really just a little bit mad!

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

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,

RE: Design by rule of thumb

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).

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

Romo!

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

Repetition is the foundation of technology

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

Two rules:

* 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

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

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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

Regards

Lcubed

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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)

RE: Design by rule of thumb

2
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.....

RE: Design by rule of thumb

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

Now, my smiley practice..




flame

RE: Design by rule of thumb

2
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.

RE: Design by rule of thumb

"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.

RE: Design by rule of thumb

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 .

RE: Design by rule of thumb

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."

RE: Design by rule of thumb

If it draws hard, it builds hard.

RE: Design by rule of thumb

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!

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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"

RE: Design by rule of thumb

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)

RE: Design by rule of thumb

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!




RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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?"

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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!!

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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!

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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

Debra

RE: Design by rule of thumb

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!

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

"BETTER" is the enemy of "GOOD".

RE: Design by rule of thumb

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...


RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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."

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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 -

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

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

RE: Design by rule of thumb

Hey SlideRuleEra and FreeRangeMonkey:

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

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

RE: Design by rule of thumb

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!

RE: Design by rule of thumb

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

RE: Design by rule of thumb

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".

RE: Design by rule of thumb

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.

RE: Design by rule of thumb

For a complete canon of rules-of-thumb (not just engineering, but applicable to almost all areas)get Tom Parker's books: Rules of Thumb, Rules of Thumb 2, and Never Trust a Calm Dog - More Rules 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

Here's a rule of thumb for wood framing:
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

When your design is up for review “Don’t lead with your Lips”

A fool proof design will only fool the designer.

RE: Design by rule of thumb

NigelB,

Is that 1 kip/bf (one kip per board foot)?

RE: Design by rule of thumb

"when in doubt, make it stout"...sorry PEinc if it costs a little more, but IMHO, the structural engineer is NOT the person to penny pinch with! Obviously opinions vary.....

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.

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