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shear force diagram help

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

RE: shear force diagram help

I must be missing something because I don't get why that would matter.

RE: shear force diagram help

Just flip it over. Don't you just hate it when these u-beaut black boxes don't do what they are supposed to? I have never used SAP programs, but my daughter uses their accounting programs, and she says the acronym stands for "Stupid Ass Program".

RE: shear force diagram help

My guess is that the loads on the left hand diagram are pointing downward while the Z axis is pointing up. Your program interprets the loads as negative.

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

It does look symmetrical, but the values are different...I think. Another thing about the graphics of these programs is that they don't mind if the layers sit on top of one another.

RE: shear force diagram help

On a side note SAP (Structural Analysis Program) is unrelated to SAP the business software. An appropriate quote I've seen from the original author of the software, Ed Wilson, is below:

"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

By international convention, the shear force sign is determined by the direction of the resisting force acting on the normal direction of the element 'face'...

A positive y-direction force acting on a positive x-direction face has a positive magnitude.

Dik

RE: shear force diagram help

Thanks, dik, but that is gobbledegook to me. Are you saying the OP's shear diagram is right or wrong? I was taught you always start at the left end. Left reaction goes up, loads go down, right reaction goes up. If you started at the right end, the picture would look fine, but that's not the way it's done.

RE: shear force diagram help

Hi slowshow

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

His shear force diagram is correct using international convention assuming the origin is located on the left end of the beam and a conventional RH coord system is used.

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

The origin is on the left, and the beam extends in the positive x-axis direction, but why wouldn't you start with the reaction at the origin? That reaction is upwards as well. Maybe it has to do with the perpendicular axis being "z", rather than "y" as would be normal with 2 dimensional axes. I think the convention as shown in desertfox's link is standard.

RE: shear force diagram help

Slowshow,
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

Must be stupid... but how the sign of shear is important in shear design !

RE: shear force diagram help

Hokie... the origin is at the left side of the beam... and the beam extends along the positive x axis (for my example). The force is positive, but on a 'negative face', therefore, the shear is negative. The normal line to the face the left reaction is acting, is pointing to the negative x direction.

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

(OP)
hi guys, thanks for your contributions
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

I can’t see Slowshow’s shear diagram in the format he posted it in, but I would normally draw my beam shear and moment diagrams the way Desertfox and Hokie are suggesting. A typical Structural Engineer’s shear and moment sign convention..., the way Hokie and I were taught. However, I also agree with Dik and have run into this sign convention mix-up when looking at some international engineering literature, and in particular when doing some free body diagrams and in some Theory of Elasticity works where a consistent axis and sign convention are important.

RE: shear force diagram help

Surely you noticed this in college??

RE: shear force diagram help

Hi slowshow
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 have a question, Slowshow: Did you check read the convention section of the program manual? It helps.

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

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.

RE: shear force diagram help

(OP)
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.

RE: shear force diagram help

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).

RE: shear force diagram help

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

RE: shear force diagram help

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

RE: shear force diagram help

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.

RE: shear force diagram help

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

RE: shear force diagram help

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.

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