×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Torsional Resistance (Blodgett)

Torsional Resistance (Blodgett)

Torsional Resistance (Blodgett)

(OP)
I just recently bought "Design of Weldments" and "Design of Welded Structures" by Omer Blodgett and I noticed in his chapters on Torsional Loading he talks about using "Torsional Resistance" rather than polar moment of inertia to calculate torsional stress and deflection. (Chapter 3.6 in "Weldments" or Chapter 2.10 in "Welded Structures")

This is great information but I'm quite surprised as to how much of a discrepancy there is in beams and channels between polar moment of inertia and torsional resistance. If Mr. Blodgett is right (I would imagine he is), then polar moment of inertia J = Ix + Iy is completely irrelevant for beams and channels.

I'm just somewhat shocked that I've never heard of the "Torsional Resistance" method in Blodgett.

I also find it odd that it's presented as an alternate method, when in actual fact when it comes to open members like beams and channels they are totally night and day.


Can anyone shed some light on this?

RE: Torsional Resistance (Blodgett)

As Roark puts it (in 'Roark's Formulas for Stress and Strain', 6th ed.): "For a circular section K is the polar moment of inertia J....for other sections K is less than J and may be only a very small fraction of J."

So yes, for open sections, the "K" value becomes very important. (The "K" value being equivalent to Blodgett's "R" value.)

I personally haven't used Blodgett's method for torsional member design. I've typically used AISC's Design Guide 9. (Not that there is anything wrong with anything by Blodgett.....I just find DG 9 more straight forward.)

RE: Torsional Resistance (Blodgett)

What source says J = I.x+I.y? That doesn't even come close to sniffing an accurate answer. For an open section, J is best estimated as the sum of J of the components. Polar moment of inertia will never give good results for open sections, because warping stresses dominate the response.

RE: Torsional Resistance (Blodgett)

The "R" factor in Blodgett is equivalent to the torsional constant ("J") in AISC. The polar moment of inertia being equal to Ix + Iy is covered in most Mechanics of Materials textbooks. It can also be seen in AISC 360-16 Eq. (E4-2) where the denominator of the (1/Ix + Iy) term is the polar moment of inertia. AISC does not refer to it as Ip or 'polar moment of inertia' since it is not tabulated in the Manual for rolled shapes.

RE: Torsional Resistance (Blodgett)

My mistake, I was thinking torsional constant (same mistake as OP which is probably what led me down this path). Although I'm not sure what use the polar moment of inertia has with respect to open shapes.

RE: Torsional Resistance (Blodgett)

(OP)
Thanks for the input. I've found the reference to this in Roark (I never knew it was in there). I've also gotten my hands on AISC Design Guide 9 which covers this subject in greater depth than Blodgett.

RE: Torsional Resistance (Blodgett)

"the torsional constant for a thin walled tube, and the same tube with a slit in it."

A core from a roll of paper towels and a pair of scissors has made a convincing experiment/demonstration a few times.

When the ice cubes break loose from the plastic ice cube tray the torsional stiffness plummets a bit.

A few months back a big name equipment manufacturer paid us a couple hundred K-bux in part because the concrete "fill" of the three identical steel bases made of channel rails relied on wishful thinking to make a monolithic structure under the motor and coupled equipment. Their 2 attempts to weld on ribs to make a real base were sad, and futile. I got a cool North Face jacket because I was a whiny bee-atch about the design and the project still made some money.

A former employer made a floor mounted vice stand from ~ 5" I-beam. It was hilariously flexible in torsion. I tried to talk them into stitch welding light gage full length plates to the edges of the flanges to make a pair of closed sections, and make it about 20 X stiffer.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper - A Guide to 3D Printing Materials
When it comes to using an FDM 3D printer effectively and efficiently, choosing the right material at the right time is essential. This 3D Printing Materials Guide will help give you and your team a basic understanding of some FDM 3D printing polymers and composites, their strengths and weaknesses, and when to use them. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close