Using R instead of J to determine Angle of Twist in steel beam
Using R instead of J to determine Angle of Twist in steel beam
(OP)
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
I have attached the page sumitted as a reference in all his calculations.
Thank you,
A rookie






RE: Using R instead of J to determine Angle of Twist in steel beam
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.
RE: Using R instead of J to determine Angle of Twist in steel beam
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"...
RE: Using R instead of J to determine Angle of Twist in steel beam