huangyhg

design by rule of thumb
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 will cost twice what you think it should and take twice as long as you think it should take.
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.
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.
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.
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
-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
by doubleing the pipe diameter gives you five times the flow.
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.
misc. structural steel (crossframes and diaphragms) ~ 3 psf * deck area
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.

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