In an I-beam, the shear flow is usually shown drawn as on the left, not as drawn on the right. I do not understand why. I do understand:
- The total horizontal shear force must be zero, consistent with statics.This is satisfied in both images.
- The vertical direction matches the direction of the external load.
My fundamental question is, Why is the image on the left correct and the image on the right incorrect?
It matters because when the direct shear due to a horizontal force (pushing to the right) we will subtract the shears on the upper right hand side of the left I-beam to get the net. If we were to use the right I-beam, we'd have to add the shears to get the net.
1. How can I understand why a vertical force produces horizontal shear and 2. Why the direction is chosen as it is?