Bending stress ~ Tensile & Comp stress

[recentgrad]

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

 

[boo1]

non-isotropic materials

 

[rb1957]

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).

 

[Cardinal177A]

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.

 

[DRC1]

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.

 

[mrMikee]

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

 

[flamby]

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.

 

[UcfSE]

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.