What are your structural engineering tips and rules of thumb? 5

[dassouki]

Here's one:

the thickness of a steel beam in inches is usually half of its size in feet. For example, a 10' beam would require 5" steel.

What are yours?

 

[dassouki]

I realize this question has been posted many times now, and i'm not sure there is a way to delete this thread

 

[MikeHalloran]

Your first stated rule of thumb should use the word 'depth' instead of 'thickness', which has a very different meaning in beams.

You may delete a thread by Red Flagging the root message and leaving a note requesting that the thread be deleted. Site management will take care of the rest.



Mike Halloran
Pembroke Pines, FL, USA

 

[abusementpark]

As = Mu/(4d) for required area of steel in a concrete beam.

1 gallon of water added to 1 cubic yard of concrete increases the slump by 1 inch.

 

[Ron]

Tip....don't rely on "rules of thumb".

Rule of Thumb....don't rely on "tips".

 

[bradleyelwood]

the section modulus of a wide flange beam is beam weight in pounds per foot times the overall depth divided by 10. (depth is in inches and section modulus is inches cubed.)

the section modulus of a channel or a square tube is beam weight in pounds per foot times depth, (inches) divided by 12.

the section modulus of a round solid section is weight per foot times depth (diameter in inches) divided by 27.3

the section modulus of an equal leg angle is weight per foot times depth divided by 25.5

the section modulus of a rectangle is weight per foot times depth divided by 20.4

all of the above apply when the weight per foot is for steel.

for unusual shapes like jumbo beams or for metric rules of thumb, you have to develope your own formulae.

 

[rb1957]

have two thumbs ... a big one and a small one ... sometimes you want to make this oversize, and sometimes a large thumb might obscure something important

 

[ornerynorsk]

Believe half of what you see, and none of what you're told.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[MiketheEngineer]

Cut off your thumb and check the calc!! You might often be surprised.

 

[ToadJones]

Tip: section moduli for rolled shapes are listed in manuals

I often use span of beam in inches / 20 to get beam depth in industrial settings.
This is for deflection only.

 

[rowingengineer]

you wish to browse the FAQ's for this forum.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[msquared48]

KISS

Mike McCann
MMC Engineering

 

[rapt]

I think I will agree with Ron and give him a star. Not sure why the other stars were awarded, they appear very easy to get these days.

I hope no-one working in SI/metric units will use the RC beam rule! And reinforcement strength type might have some bearing on the result, as well as ductility and a few other things.

Personal Rules of Thumb can be useful in quick checks of results/drawings, but you need to know the limitations assumed in developing the Rule of Thumb.

The ones suggested above are simply dangerous for anyone else to use!

 

[bradleyelwood]

Toad,

You are right that rolled shapes are listed in manuals. In my work, I often have to invent other sections which are not in any manual. The rule of thumb for sections is a sanity check for calculated values of unusual shapes.

Here is another rule of thumb: the buckling capacity of a column is approximately equal to the lateral stiffness (at mid length) TIMES the length divided by five. The column can be any reasonable shape such as a tapered wood piling, a stepped column like a telescopic cylinder (variable section). You have to know something about the column to know that buckling will govern. The formula works for varible end conditions, (pinned, fixed, pinned-fixed etc). The mid column lateral stiffness can be determined by calculation, or it can be obtained by measurement. Interestingly, you do not have to know the moment of inertia exactly, or Youngs modulus if you determine the lateral stiffness experimentally.