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


re: bolt bending

re: bolt bending

re: bolt bending

I usually use the basic minor diameter for computing the section modulus of a bolt in bending. It's the smallest diameter, and thus conservative. But is there perhaps a way to be less conservative?

Has anybody else used the effective diameter that can be back-calculated from the stress area of a bolt in tension, and used that for computing bolt section modulus?

RE: re: bolt bending

You can use the shank diameter, if the assembly is designed so that only the shank is subject to bending.

If you're bending the threaded part, I wouldn't use the tensile area, because you don't know where the effective section centroid is relative to the centerline, in general, at any point of interest. I.e., the section modulus sort of wobbles along the length of the thread, as the centroid follows a helix, going both above and below the wrapped centerline.

Can you explore uncommon thread forms, e.g. those with large root radii, or just use a larger bolt?

Mike Halloran
Pembroke Pines, FL, USA

RE: re: bolt bending

I'm thinking I probably need to bring up an issue with a legacy design at my company, b/c somewhere along the line, someone decided that loading 100 lbs on a threaded rod in bending was a perfectly good idea. Mind you, it's also a 300 series stainless steel threaded rod as well.

Thanks for the inputs!

RE: re: bolt bending

I have to ask: Have you gotten away with it so far?

Mike Halloran
Pembroke Pines, FL, USA

RE: re: bolt bending

Hi glulambeam

When I do bolted joints in bending, I assume the bolts are infinitely stiff and calculate the bending stresses as increased tensile forces on the bolt see link below and scroll till you see the heading bolts withstanding bending forces:-


“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: re: bolt bending


Bending the threaded section of a bolt is not good design practice. If I were analyzing this, I would use the minor diameter for my bending and stress calculation because it is conservative, as you note. Also, I would consider that the bottom of the thread acts as a stress concentration, making a conservative analysis value that much more important.


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!


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