Maximum of the First Moment of Area

[a1234]

I am reading about beams under distributed loading and shear stresses and needed to use the equation τ = VQ/It, where Q is the first moment of area.

I understand that Q is zero about the neutral axis, and that this is, in fact, how the neutral axis is defined. The first moment of area above the neutral axis is Q, and that below the neutral axis is -Q.

However, I've also read that the maximum value of Q occurs at the neutral axis. What I don't understand is how this is compatible with the fact that Q is 0 about the neutral axis.

It would be great if somebody could clarify. Thanks in advance.

 

[PhanthomJay]

I am reading about beams under distributed loading and shear stresses and needed to use the equation τ = VQ/It, where Q is the first moment of area.

I understand that Q is zero about the neutral axis, and that this is, in fact, how the neutral axis is defined. The first moment of area above the neutral axis is Q, and that below the neutral axis is -Q.

However, I've also read that the maximum value of Q occurs at the neutral axis. What I don't understand is how this is compatible with the fact that Q is 0 about the neutral axis.

It would be great if somebody could clarify. Thanks in advance.

Q is not zero about the Neutral Axis, in fact, it is typically maximum at the Neutral Axis . In order to calculate shear stress at the Neutral Axis, Q is the area above it times the distance from the centroid of that area to it. It is also the area below it times the distance from the centroid of that area to it, taken as a positive value.
You are confusing this with the method to determine the neutral axis, where you sum moment areas of the various areas of the cross section about the bottom of the cross section, then divide that result by the total area of the cross section to get the distance to the neutral axis from the bottom of that section