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The book that I'm referring to for studying Strength of Materials states that:

"The loading case discussed here is an example of simple or direct shear, since the shear is caused by the direct action of the applied load F."

What does the author mean when he says "by the direct action of the applied load"

I essentially want to know what is direct and indirect stress. I think these terms are also used in conjunction with normal stresses apart from shear.

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    $\begingroup$ to my experience the term direct and indirect stresses are not commonly used, and some times they depend very much on the context. E.g. in the following url direct stresses are synonymous to normal stress (which is not common). So it would be helpful if you presented a bit more on your example. $\endgroup$
    – NMech
    Commented Dec 19, 2021 at 18:14
  • $\begingroup$ Agree with you, these terms have ambiguity. I'm a newb so I don't really know what terms engineers use in real life, so at present I just try to understand every word that is written in the book 😅. It's nice though that I'm able to learn that not every word will mean the same to everyone in the world and engineers have to keep that in mind. $\endgroup$
    – Harshit Rajput
    Commented Dec 20, 2021 at 14:18

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We can use some examples.

  • Direct shear stress: Punching a hole into a sheet of metal with a tool. Shear stress caused by a pair of scissors on the paper when cutting it. shear stress on a footing due to a load of column.

  • Indirect shear stress: the shear caused by a change of moment in a beam. the shear created at the base of the column due to column's overturning moment.

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  • $\begingroup$ Thank you kamran, I do get some intuition about the words direct and indirect now. I just want to think of a definition which can define direct and indirect stresses. I came up with this, please let me know what you think about it and if its generalized enough - If the internal resistive forces are all directed along the same direction, then the stress corresponding to those internal resistive forces is called a direct stress. If the internal resistive forces are all not directed along the same direction then the corresponding stress is called an indirect stress. $\endgroup$
    – Harshit Rajput
    Commented Dec 19, 2021 at 15:27
  • $\begingroup$ Hence, if we have an area over which internal resistive forces are developed parallel to the area and all are directed in the same direction then the shear stress is a direct one. if the internal resistive forces are all parallel to the area but are not all directed in the same direction (as in torsion) then the shear stress will be an indirect one. $\endgroup$
    – Harshit Rajput
    Commented Dec 19, 2021 at 15:28
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A direct application of applied load/moment are the reason why we experience anything that happens within the structure (like deformations, strains, stresses etc). If not direct load/moment are applied, then nothing will happen and the body will just remain as it is. So anything that happens (for instance, warping in a rectangular shaft due to torsion, which results in tension and compression stresses as well) are a result of a direct application of load/moment. So nothing can be indirect.

Maybe the author tried to use simple words to not to confuse the audience. When you apply a transverse/lateral load to a cross section, it infers that it is a shear load since it is parallel to the cross section (and by definition, loads parallel to the cross sections result in shear stresses). I guess this is what he meant by direct load. According to this analogy, the tension and compression stresses due to warping, and even bending stresses due to transverse/lateral loads are all due to indirect loads, i.e. arising due to torsion and due to transverse/lateral loads respectively. Since it was initially taught that only an axial force could result in axial stresses (like tension and compression).

Moreover, you might also exprience shear stresses when you fix a beam on one side and apply axial force on the other. This can be a case of indirect shear stress. Think it like this, the extreme end is fixed meaning that this face cannot deform, while just its adjacent cross section is expanding/squeezing due to poisson's ratio effect in an axial loading condition. This relative deformation means that shear stress must exist.

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  • $\begingroup$ Thank You Rameez. I kind of do get the feeling what the author and you are trying to convey. I just want to come up with a definition of direct and indirect stresses. I came up with this, please let me know what you think about it and if its generalized enough - If the internal resistive forces are all directed along the same direction, then the stress corresponding to those internal resistive forces is called a direct stress. If the internal resistive forces are all not directed along the same direction then the corresponding stress is called an indirect stress. $\endgroup$
    – Harshit Rajput
    Commented Dec 19, 2021 at 15:23
  • $\begingroup$ Hence, if we have an area over which internal resistive forces are developed parallel to the area and all are directed in the same direction then the shear stress is a direct one. if the internal resistive forces are all parallel to the area but are not all directed in the same direction (as in torsion) then the shear stress will be an indirect one. $\endgroup$
    – Harshit Rajput
    Commented Dec 19, 2021 at 15:23
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    $\begingroup$ I would say that internal resistive forces if only exist in the direction of the applied load, then its a direct stress, but if the internal resistive forces exist in directions other than the direction of applied load, then the stresses originating due to these internal resistive forces can be classified as indirect stresses. $\endgroup$
    – Rameez Ul Haq
    Commented Dec 19, 2021 at 15:28
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    $\begingroup$ The shear stresses originating due to torsion, according to me, are direct shear stresses. The reason being, firstly, the torsional moment is parallel to the face, and secondly, the shear stresses follow the path of the external torsional moment. $\endgroup$
    – Rameez Ul Haq
    Commented Dec 19, 2021 at 15:29
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Example: You provide confinement reinforcing to a column. That is passive confinement, which will automatically activate and provide confining stress, if there is the need, after a certain amount of strain. But if you applied that confinement force actually yourself, instead of the confining reinforcing, then that would be active confinement. You would be actively applying force in that case.

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The definitions below ar quite clear:

enter image description here

https://www.mechanical.in/what-is-stress-and-types-of-stress/

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