×
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

Bending stress ~ Tensile & Comp stress

Bending stress ~ Tensile & Comp stress

Bending stress ~ Tensile & Comp stress

(OP)
Howcome allowable bending stress could be different than tensile or compression stress. Bending stress itself is nothing but either tensile or comp stress depending whether below or above the N.A. Any thoughts on this

RE: Bending stress ~ Tensile & Comp stress

non-isotropic materials

RE: Bending stress ~ Tensile & Comp stress

the key is "allowable".  materials usually have different yield strengths in tension and compression, and some sections will have very low compression strength (due to crippling).

RE: Bending stress ~ Tensile & Comp stress

It is due to the different nature of bending stress versus pure tension or compression. It also is usually only used for ultimate strength calculations and not limit or yield calcs. It takes advantage of the plastic deformation range to re-distribute the bending moment to the inner areas of the structural element being bent, resulting in a nonlinear distribution of the bending moment in the plastic range versus a linear one in the elastic range. It also varies according to the cross sectional shape of the element being bent. It results in the "apparent" allowable bending stress to be higher than you would expect. In pure tension or compression, the entire cross section has about the same level of stress, so there is no place to re-distribute the load to. I beam sections do no benefit much from this effect, and a diamond shape is the best since it has more material in the central area where stress is initially low, and therefore can re-distribute alot of load there. Look up plastic bending in a textbook or manual.

RE: Bending stress ~ Tensile & Comp stress

In allowable stress design, the bending stress is generally controled by the compressive stress of the flange. The flange bending stress is controled by lateral torsional buckling, which is why bending stress can be limited by brace spacing.

RE: Bending stress ~ Tensile & Comp stress

The stress at a point is just one factor in the overall problem.  If the goal is to determine the safe load carrying capacity of a member then the mode of failure as it relates to stress is important.  In a column the compressive stress and other factors determine the buckling load of the column. In a beam the compression flange is subject to lateral-torsional buckling as a limiting condition.  Since tension doesn't cause buckling it has still different limits.  It takes an understanding of these conditions and the appropriate use of a design code (ASD or LRFD for example) to insure a safe design.

Regards,
-Mike

RE: Bending stress ~ Tensile & Comp stress

We also have to look at the theory of bending itself. "Plane section remains plane" is the basic assumption. However, it may be that it does not remain so plane, particularly, near failure limits. You have to take this into account and you can see that bending stresses (allowable) are less than tensile stresses as no assumptions are involved in tensile theory.

Ciao.

RE: Bending stress ~ Tensile & Comp stress

When you look at a stress distribution for bending, assuming linear strain distribution or any other for that matter, you see that the maximum compressive stress is highest only at the extreme fiber.  This is not the case for a pure tension or compression member where the stress is highest across the cross section.  When you look at the compressive stress "triangle", the compression force seen by the part of the beam in compression is less than the compressive force seen by the same part of the section when it is a pure compression or tension member.  Therefore it takes a higher stress to get the same amount of force.  

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