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When we have a T or I section under a shear force $V_y$ I usually must account for a horizontal shear force across the section's Z axis ($𝛕_{xz}$). I understand why a transverse shearing force would obviously cause a sliding action downwards. Mathematically it is possible to show that $Q$ is non zero in the flange for $𝛕_{xz}$ but is there any physical reasoning for why there is $𝛕_{xz}$ present in the flanges of I and T beams?

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If you look at the shear diagram of I section you see shear in the web starts from the top flange at a non-zero value , calaculated by the $q=\frac{VQ}{I} \ .$ That q on top and bottom are coming from the horizontal tranguluar shear flow in the flanges. The diagram shows how different sections distribute the flow. source

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shear flow

Edit

After OP's comment

If you can intuitivly see how the shear flow works on a C-channel loaded vertically parallel to its web with P, then an I section could be imagined as two C-channels loaded each with P/2 glued to each other.

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two c-channels

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  • $\begingroup$ Is there any particular reason why the shear flows this way in the flanges? $\endgroup$ Mar 11 at 20:04
  • $\begingroup$ Shear flows according to section geometry and the shear demand. for example if you subject a hollow sqare tube to transverse shear, the shear flows drastically diferent than if you subjet it to torsion. how else the top flange could bill in the demand to counter the shear at the web ends? the tips can have zero shear, hence the trangular stress. If you cut a hole in the web, the shear flow goes around the hole mor intense. $\endgroup$
    – kamran
    Mar 11 at 20:21

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