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

 

[SteelPE]

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

 

[hokie66]

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".

 

[BAretired]

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

 

[hokie66]

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.

 

[TERIO]

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

 

[dik]

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

 

[hokie66]

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.

 

[desertfox]

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

 

[dik]

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

 

[hokie66]

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.

 

[Teguci]

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?

 

[PicoStruc]

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

 

[dik]

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

 

[slowshow]

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).

 

[slowshow]

desertfox,
just uploaded the reactions.
The absolute values are correct, the problem is just the sign convention.

 

[dhengr]

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.

 

[MiketheEngineer]

Surely you noticed this in college??

 

[desertfox]

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

 

[paddingtongreen]

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.