On how were span-depth ratios derived. 12

[IJR]

My friends

I need a reference text on this subject: How were the popular span-to-depth ratios for steel and reinforced concrete beams derived. I prefer the derivations, not general discussion.

Thanks in advance.
and for making this forum a great place.

ijr

 

[kipfoot]

BAretired: My link was just for general information; I didn't mean to interrupt the deflection discussion.

Like you observe, I don't think the presentation has anything specific about deflection. I think that it's only a suggestion that the relationship [d=1/2" X L (in feet)] has serviceability baked in. These rules of thumb were also put in a MSC article in 2000: Rules of Thumb

 

[BAretired]

kipfoot: Okay, thanks. Some good ideas in there for preliminary design

BA

 

[kipfoot]

A thought about the Fy/800 ratio is that it's another way to say "L/24" or "1/2" per foot of span" from an era when Fy=36 was most common. Fy/800 for A36 steel is L/22.
This excerpt is from Steel Buildings: Analysis and Design By Stanley W. Crawley, Robert M. Dillon:

 

[BAretired]

Thanks, kipfoot. That clears up the mystery. I did not read the AISC reference carefully enough. My bad!

BA

 

[dhengr]

IJR:
The upshot is that rules of thumb can be more complicated than you think. They actually require you to know a little bit about what you are doing in the structural engineering business, or you may not apply them correctly. If we could only come up with one rule of thumb for everything, then we could do away with the guys who make the big bucks and staff all engineering depts. with a bunch of computer monkeys who applied that rule of index finger to everything..

 

[IJR]

dhengr:

You are right and thanks.

But here is the dilemma. It is the experienced engineers, architects, plant planners, planners of bridge layouts etc, who use ROTs regularly. Not the junior guys who are keyboard happy. Those are usually happy showing off the graphics capability of the FE software of their choice or of the great 3D architect software like Revit that has a macro for almost every need. When I go to meetings, I usually joke the senior guys "did you do this yourself?". He/she does not have to reply, because it is the junior one who does all the presentation. A good design is consistent. And consistency has nothing to do with software.

thanks once more and again.