shear force diagram help
shear force diagram help
(OP)
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
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






RE: shear force diagram help
RE: shear force diagram help
RE: shear force diagram help
Seems to be something wrong with the scale of your shear diagram, however. It appears to be symmetrical which is incorrect for the loads shown.
BA
RE: shear force diagram help
RE: shear force diagram help
"The slang name S A P was selected to remind the user that this program, like all programs, lacks intelligence.
It is the responsibility of the engineer to idealize the structure correctly and assume responsibility for the results.”
Ed Wilson 1970
RE: shear force diagram help
A positive y-direction force acting on a positive x-direction face has a positive magnitude.
Dik
RE: shear force diagram help
RE: shear force diagram help
On the lefthand diagram which is the one showing the loadings, where are the upward reactions which are normally
associated with a simply supported beam?
Also I post a link showing positive and negative convention.
http://www.roymech.co.uk/Useful_Tables/Beams/Shear_Bending.html
RE: shear force diagram help
The reaction at the right is upwards, and if the beam extends in the positive x-axis direction, the reaction could be a positive value pointing in the positive y-axis direction.
The shear force would be acting on the positive 'x-axis face' and would therefore be positive (+ve force on a +face).
For the same reason moment diagrams have the negative moments 'pointing down' although I've seen them reversed to reflect the location of the reinforcing.
It's only a convention... and not a hard and fast rule. If you write engineering analysis software, it is sometimes easier to use the above convention just to keep from 'getting messed up' with sign conventions.
Dik
RE: shear force diagram help
RE: shear force diagram help
Which end of the beam is the initial point and which is the final (is length=0 at the left or right end)? Not sure if this is appropriate for SAP (its been a while) but if the initial point is reversed, maybe the shear diagram will flip?
RE: shear force diagram help
RE: shear force diagram help
Picostruct... I didn't ask the question... just attempted to answer it... It's always good to have info... even if you don't need it.
Dik
RE: shear force diagram help
just came back from work so couldn't reply earlier.
The origin of the beam is on the left hand side, with the y-axis pointing inwards into the screen (front face).
RE: shear force diagram help
just uploaded the reactions.
The absolute values are correct, the problem is just the sign convention.
RE: shear force diagram help
RE: shear force diagram help
RE: shear force diagram help
I'm a little confused in the last picture you have uploaded the vertical reaction at the left hand end is 16.88 upward and in your original post on the shear force diagram its also 16.88 but downward, however if you look at your span loads given in your original post its 6.30 downwards at the left hand side surely that should be the difference between 16.88 and 6.30 or I am I missing something?
BTW I agree about the convention not being a hard and fast rule, you can use whichever you want so long as its consistent
throughout the calculations.
desertfox
RE: shear force diagram help
I'm surprised that you went to the computer for a "quick analysis" of such a simple beam, or was it to be an exact analysis performed quickly?
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: shear force diagram help
RE: shear force diagram help
RE: shear force diagram help
RE: shear force diagram help
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
RE: shear force diagram help
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
RE: shear force diagram help
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.
RE: shear force diagram help
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
RE: shear force diagram help