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vlehner1-Visitor
1-Visitor
October 26, 2016
Solved

Shear Force in the Diagramm is not sucessful displayed

  • October 26, 2016
  • 4 replies
  • 11662 views

Hello,

I'm trying to plot a Shear Force-Diagramm, but it fails at its graphical representation.


Normally it should displayed like this:


...but it is like this (see attachment):

Can somebody help me?

Thanks, Volker

Best answer by Fred_Kohlhepp

The sheet calculates the end reactions Fa and Fb, whether correctly or not I haven't tried to determine.

His solution method is to solve the differential equation for deflection from both sides, which is valid (and impressive.)  He then creates a deflection function by patching these two solutions together and takes the double derivative to get moment; again correctly.  To get shear he takes the derivative of moment, which works everywhere except at the ends of the beam and where the two solutions patch together, where the derivative fluctuates wildly.

Mathcad scales the shear graph to show all of it's reported values, so it shows the wildly incorrect fluctuations.  If we rescale the shear graph to reasonable limits and ignore the wild excursions it should make sense.

4 replies

M
MJG1-Visitor
1-Visitor
October 26, 2016

Can you please post more screenshots or a PDF of your worksheet?  Many of us do not have Prime 3.1 installed, so we cannot open your file.

V
VladimirN24-Ruby III
24-Ruby III
October 27, 2016

Please find a PDF document of this worksheet in attachment.

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-MFra-21-Topaz II
21-Topaz II
October 26, 2016

Something is wrong, even here ...

zooming in.jpg

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
October 26, 2016

V
vlehner1-VisitorAuthor
1-Visitor
October 27, 2016

Here is the PDF-File from my calculation.

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
October 27, 2016

Your plot of shear force (F_Q) is seeing large oscillations because there are discontinuities in moment that aren't easily seen in the graph.  Try limiting your F_Q graph to ±20 kN, see the result.

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
October 28, 2016

Since you have expressions for end reactions, you could solve this problem "upside down."

V
vlehner1-VisitorAuthor
1-Visitor
October 30, 2016

Fred,

your sheet looks very interesting, btw the plotting of the deflection takes a lots of time to be displayed.

I want to share my Original Worksheet with you all, this worksheet is about analytical solution of a beam with two fixed bearings and one Load.

I wanted to solve this problem with the solve-blocks, but there was a problem to calculate Fa,Fb,Ma,Mb because they are unknown.

Have Fun

Volker

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
October 31, 2016

The deflection calculation requires integrating three times:

1) Integrate shear to get moment.

2) Integrate moment to get slope.

3) Integrate slope to get deflection.

The graph has to do that for each point.  A potential work-around is to fit high-order polynomials to each step (or at least to the slope function)  This will shorten the integration time, although at the expense of accuracy.

I was just showing that if you knew the end reactions you could do the problem the way we were taught in school.   (How you got those values is still a mystery to me!.)

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