shear force diagram help

[slowshow]

hi guys
I'm a new user of the site and I need a little help.
I did a quick analysis of a simply supported beam but the shear force diagram is mirrored i.e. negative shear force on the left support is supposed to be positive and the positive on the right support negative.
Is there any way of correcting this?
see attached image

 

[JoshPlumSE]

Ahh yes, but which end is the "left" end of the member and which is the "right"? I say this in quotes because with a 3D software package it all depends on your perspective. For this reason, it may matter if you draw the member from left to right or right to left.

 

[slowshow]

desertfox, the analysis was done at the ultimate limit state (ULS) which is 1.4*Deal load (BS8110). the shear force value is not the problem just the mirrored diagram.

 

[yakpol]

That's a 3d software. Shear on the left end is positive when Y-axis is up and load direction is -Y, negative when Z-axis is up and load direction -Z (like in your case).

 

[desertfox]

Hi slowshow

Yes I appreciate its just the diagram, however you have all the facts in front of you and we here haven't and I was trying to help you find the reason the diagrams mirrored so I wanted to understand that it wasn't caused by the numbers.
desertfox

 

[BAretired]

If the loads were entered as positive values, it would mean they were acting upward (direction of the Z axis). Then the shear on the left side would be negative.

If the loads are downward, they have to be entered as negative values. Then the shears would be positive on the left portion of the span.

BA

 

[yakpol]

By definition, for horizontal frame member parallel to one of the global axis, positive moment creates tension on the bottom of the member and positive shear force tends to rotate material clockwise.
In the example, Y-axis points into the page. Sagging of the beam corresponds to the counter clockwise rotation about Y, therefore, shear force (Vz) is negative. In classical 2-dimensional examples X points to the right, Y points up and Z points out of page, in that case under the same type of loading at the left support Vy > 0.

Hope it makes sense.

 

[BAretired]

If the Y axis is vertical and the X axis is pointing right, then the Z axis points out of the page because the axes follow the right hand rule.

Downward loads in the classical 2D case should still be entered as negative values when using a 3D frame program to solve moments and shears.

The only reason we got away without doing that with hand calculations is that a sign convention was not required because we could "see" the sense of the shear diagram without a sign convention.





BA

 

[hokie66]

Whether the shear is positive or negative indicates whether the moment is increasing or decreasing, and the rate of change. So I think the correct 2D, x and y, convention should be insisted on.