Shear force at a distance of "d", why not shorter than that? 8

[Pretty Girl7]

Following equation is for the calculation of shear r/f for a beam according to Eurocode. The Eurocode expects us to calculate the shear force at a distance of "d" from the supports.

Let's suppose the shear force at a distance of "d" is 150kN, and red arrow shear force (from a point load etc) 400 kN, more close to the supports than a distance "d" shear force.

My question is, if there are any bigger shear force (from the red arrow point load) is present more close to the supports, aren't we missing the critical red shear force?

The critical shear force is not the shear force at the distance at "d" in this case, so why we take the shear force at distance "d"?

 

[ThomasH]

I can assure you that shear behavior of concrete might as well have been an magic black box, students just learnt a prescribed method of how to design to code requirements.

That is unfortunate because knowing the machanics of the failure often helps to prevent it.
You mention a truss as something the students would understand and I don't doubt that. But shear in concrete is based on a truss model, that means that the mechanical concept is a truss. If you look att the diagonal cracks, they are held together by the stirrups and the sliding between the upper and lower part is prevented by the roughness in the crack.

Since this discussion relates to load near a support, in the Eurocode 6.2.2 (6) and 6.2.3 (8) both relate to that. But they are limited to loads on the upper side, in 6.2.1 (9) loads near the bottom is mentioned.

When I started using the Eurocode I went to several lectures regarding the different parts. And this shear model was described in detail by one of my "old" professors from uni. I also think he was involved in developing it but I am not sure about that.

 

[KootK]

I felt the 'strut angle' can be like more than '45 degrees' (for the red concentrated load) and it can directly transfer to the bottom area before the supports. Because, we are designing the beam for the shear load at a distance at "d" at that point the angle is ok, but we have a very large load before distance "d", that would not transfer to the supports as it definitely will not form a strut angle lower than 45 degrees. I might be wrong though.

I feel that's about right. Loads imposed closer than [d] from the support can take the favorable, 45 deg strut and tie path from the point of load application to the support. Ditto for shear loads moving towards the support via the stirrup reinforcing. Shear loads beyond [d] and not travelling through the stirrups must travel to the support via the much weaker, diagonal shear resistance mechanism along the hypothesized 45 deg diagonal tension crack.

The code relaxation is basically an acknowledgement of the relative strengths of these different shear travel paths at the support. As the other folks here have mentioned, additional checking would be required for concentrated loads near the support because:

a) Such loads might overload the strut and tie model, which can be checked.

b) Such loads might produce a shear failure within the strut and tie model at an angle steeper than 45 degrees. This can also be checked.

The concept at play is that, since the strut and tie path is so much stronger than the diagonal shear path, a little extra uniform load is unlikely to exceed the capacity of the strut and tie path.

 

[KootK]

@Pretty Girl7:

On another note, I wonder if you might consider changing your handle to something like "Serious Androgynous Human8"? When I respond to your threads, I fear that this "pretty girl" business creates the impression that I've fallen for some manner of odd catfishing scam. I find it a bit uncomfortable.

 

[Just Some Nerd]

creates the impression that I've fallen for some manner of odd catfishing scam.

Put aside your fears Koot, nobody suspects your mind of having gone soft on us. (Or is it more like going hard?)

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Why yes, I do in fact have no idea what I'm talking about