Using R instead of J to determine Angle of Twist in steel beam

[1stTimeBridgeDesign]

I am a new bridge engineer, fresh out of college and 2 weeks on the job. My boss uses R, torsional resistance, instead of J, polar moment of inertia, to determine the angle of twist in an I-beam. The reference he showed me only refers to a section with equal plates for the web and flanges. Is it correct to always use R to determine the angle of twist, and if so is it always more accurate.

I have attached the page sumitted as a reference in all his calculations.

Thank you,

A rookie

 

[jnadeau]

When considering torsion of non-circular cross-sections plane sections do not remain plane and as such the polar moment of inertia is not applicable. The notation that I am familiar with for this general cross sectional property is the Torsional Constant, J (I believe that this is what your boss is calling Torsional Resistance, R) which is in general different from the polar moment of inertia Ip=Ix+Iy (for which your boss uses J). ...Check out Roark's Stress and Strain, but note that they use K for what I'm calling J, and for what I believe your boss is calling R.

When you have a circular cross section then plane sections remain plane and the Torsional Constant is equal to the polar moment of inertia--but this is true only for circular cross sections.

 

[theonlynamenottaken]

J really only actually applies to circular shafts or circular tubes. J doesn't readily apply to non-circular and open sections. R = torsional resistance factor is more appropriate.

See Blodgett, "Design of Welded Structures" or ,"Design of Weldments". Both are books I keep on the bookshelf right next to my desk, alongside Roarks, AISC, etc...

In Blodgett's texts he compares the use of J and R in "angle of twist" calculations with physical testing results. For instance, a channel:

Conventional J: .006 degrees
Torsional Resistance R: 9.7 degrees
Actual Testing Results: 9.5 degrees

After writing all of this I looked at the attachment you provided in your post - that page is from "Design of Welded Structures"...

 

[vincentpa]

Your boss is probably using Blodgett's "Design of Welded Structures." His method is extremely easy and comes withing 5% (usually less) than experimentally tested values. I recommend using this because the calculation is simple, accurate and tested. The read is only like 3 pages. Or you could use the AISC design guide on torsion for wide flange sections. I believe it is like 200 pages. Each calculation takes pages. Blodgett is great. Everyone uses it. AISC is a case of professors gone mad.