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.

 

[boo1]

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

 

[Taro]

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.

 

[jaggmi]

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.

 

[Taro]

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.

 

[evelrod]

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

Rod

 

[CCEV]

-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.

 

[Rowland33]

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

 

[trainguy]

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.

 

[Randy2002]

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

 

[sven]

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.

 

[whyun]

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.

 

[AJUK]

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 )

 

[whyun]

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. LOL

 

[AJUK]

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

 

[TheBlacksmith]

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

 

[whyun]

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.

 

[mobe]

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?

 

[daniel2]

There are two great rules:

1. F=ma, and

2. you can't push a rope.

 

[sc]

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