Concrete Column, Design by Capacity Reduction Factor, R?

[Archie264]

I was perusing a concrete textbook and read up on designing concrete columns using a method described as the "R Method", which made use of a capacity reduction factor, R.

Is this method still allowed/in use? Does it go by a different name now? Why haven't I heard of it before? (Well, you know, besides the obvious...)

 

[KootK]

I don't know the answer but I'd like to play along. What's the name and author of the textbook?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IDS]

Does this method differ from what is usually known as LRFD (in the USA) or Limit State Design (in the rest of the World)? If so, in what way?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[KootK]

Apparently it was the concrete column design method used through ACI 318-63. The moment magnifier method that we've come to know and love is it's replacement. Supposedly it was pretty bad at handling slender columns. While I'm sure that the R method is glorious in its simplicity, I think that you'd have a hard time justifying its use in court should you run into problems.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TLHS]

I have a couple of really old ACI code versions and other old concrete manuals. I have pulled some of the formulas for preliminary design use, because they're significantly more straight forward and you end up in a similar ballpark, but would never suggest that you base your final design on them unless you do the math to verify that the model you're doing is inherently more conservative than the current code provisions.

 

[Archie264]

Thank you, all. I'm pleasantly surprised by the responses...I was expecting either ridicule for my lack of knowledge of it or to be told that it's provisions had been incorporated into the code, contained within the latest revision of the squiggly-line, double-subcript phi beta kappa factor, or something. Anyway...

KootK, the book was one I got from a recommendation by AELLC on this site. It's "Practical Design of Reinforced Concrete," by Russell S. Fling. Fling was chairman of ACI in the 1970's and the book is excellent. Here's what he wrote of the R Method (emboldened emphasis mine),

The R Method is approximate, empirical, relatively easy to use, and should be considered whenever applicable. It is limited to l_u/r < 100 for braced columns and l'_u < 40 for unbraced columns. It should not be used for columns hinged at both ends... The R Method may be overly conservative if the column is relative slender, has large eccentricities, has loads of long duration, has high concrete and steel strengths, has a low steel percentage, and is a component of an unbraced frame.

He also described the method as, "straightforward, unsophisticated, "quick and dirty"...(Fyi, he referred to the Moment Magnifier Method as "...enlightened, refined, longer..." and the Second Order Analysis Method as "...elegant, erudite but painfully slow...")

Moreover, he also wrote that, "In actual practice, many engineers will use the R Method more often than the Moment Magnifier Method because the first method requires fewer calculations with less chance for error, as well as saving the engineer's valuable time. Furthermore, the majority of columns require little if any increase in strength to allow for slenderness effects."

And here's the thing: the book was published in 1987...not that long ago in terms of concrete design "philosophy"...or so I thought. And yet I hadn't heard of it otherwise.

He also had some criticism of the method, but my point is that I was surprised that a man of his stature had so much positive to say so relatively recently about a method I hadn't heard of.