×
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

Angle of twist - Torsion

Angle of twist - Torsion

Angle of twist - Torsion

(OP)
I have a Channel section and I am trying to check it for torsion. It is simply supported.
I noticed the spread sheets out there calculate the angle of twist but dont know this number has to be check about what number? Where can I find allowable angle of twist?
If is not needed then why these spread sheets calculate it?

If more infor is need please ask and I will provide

Thanks

RE: Angle of twist - Torsion

How much twist is acceptable is a decision that you, as the designer, get to make based on your assessment of what is important about the situation being considered. It's rare that anyone actually cares about twist itself. However, if something is attached to the twisting member, that twist may well result in transnational movement of the attached member that could cause serviceability issues.

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.

RE: Angle of twist - Torsion

(OP)
KootK (Structural),
May I ask this angle of twist we measure for a channel section is about the ceneter of gravity or it is amount of twist about shear center?

RE: Angle of twist - Torsion

I have used the angle of twist around the centroid of the cross section. Be aware that the number you receive may not be a number that you can translate easily (without experience) into real world effects. You also need to calculate the physical displacements at the extreme fiber, i.e. top and bottom of the cross section. If you don't have an understandable amount of displacement you may not be aware of the real world effect of the twist.

Jim

RE: Angle of twist - Torsion

if the beam is truly simply supported, then you're got a problem 'cause there's nothing to react the torsion ... unless you have applied equal and opposite torques to the beam.

can you close the channel with a over plate (and make a tube out of it) ?

consider deflection ... the allowable deflection is based on how springy the people in the building will perceive it to be. torsional deflection is bounded by the same limits ... what is perceptible ? what messes with the operation of your installation ?

another day in paradise, or is paradise one day closer ?

RE: Angle of twist - Torsion

I agree with rb1957. The beam must be fixed against torsional rotation at not less than one point along the length of the beam; it cannot be simply supported.

Also, it makes no difference about which point you measure the angle of twist. It is the amount the cross section has rotated from its original position and has nothing to do with center of gravity, shear center or any other point on the cross section.

BA

RE: Angle of twist - Torsion

Channels are very un-stiff in torsion. Boxing it would make it significantly better, if needed.

RE: Angle of twist - Torsion

Quote (strP88)


I have a Channel section and I am trying to check it for torsion.

The best way to check a channel for torsion is to redistribute the loads so that the torsion on the channel = 0 < the torsion allowed on the channel.

Quote (strP88)


It is simply supported.

When dealing with torsion, this typically means that the pinned connection can take the twisting motion but is allowing the channel flanges to warp (top flange rotates opposite direction from the bottom flange). Verify that the shear plate, or whatever is providing your connection, can take the torsion reaction.

Quote (strP88)


I noticed the spread sheets out there calculate the angle of twist but dont know this number has to be check about what number? Where can I find allowable angle of twist?

If you can't deal with torsion on a channel the proper way (see above), then you will need to do a stress check on your section for the added normal stresses due to warping and shear stresses due to pure torsion. Sharpen your pencil and get your difEQ book out because the distribution of pure torsion to warping is fairly involved. Of course, you could grab a copy of AISC Steel Design Guide 9 and match up your conditions with one of the lookup charts in the back. I would recommend that you not use an excel spreadsheet unless you develop it yourself.

As for allowable twist, if this a serviceability condition, don't use a channel. I calculate twist on my torsion members as part of the process of getting to my pure torsion stresses.

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!


Resources