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# Finding max bending moment

## Finding max bending moment

(OP)
Hi folks.
See file attached.
Its an simply supported beam with an concentrated force.
I cant program Mathcad to find the x value were shear force diagram intesects with x axis.
I know at wich value is this going to happen for this loading scenario but how to tell mathcad to slove this?
I tired using the root function but no luck.
Thank you
Replies continue below

### RE: Finding max bending moment

You've created a discontinuous function, which at no point does it actually equal zero. You could possibly create something that might detect the sign change.

TTFN
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### RE: Finding max bending moment

(OP)
I Know this maybe sound silly but this is my First try in programing something like this, so, Im I doing this the right way or the wrong way?
IRatuff I see that you noticed that Ive created an discontinous function is this the wrong way?
WOuld you do it someother way?
thank you

### RE: Finding max bending moment

This is math issue, not a programming issue. A discontinuous function cannot have a value in the middle of the discontinuity, by definition. Since your function cannot possibly have a value where V(x)=0, there is no way to program to solve for solution that cannot exist. An obvious approach would be to make the function piecewise linear.

TTFN
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### RE: Finding max bending moment

(OP)
Im not shure that I undestand you.
If I define a picewise linear function like in the file attached, I still have an discontinuity for the part were the graph is vertical.
So how to find these two values for x were the graph intesects with x=0?

### RE: Finding max bending moment

(OP)
hmmmm...can you explain what you have done.
My goal would be that Mathcad spits 2 numbers out....in this case -1.5 and +1

### RE: Finding max bending moment

(OP)
IRstuff heres an simple egsample.
YOu can see that V(x) cuts the abscisa at 2 loactions.
I Know how to find one of them, but can Mathcad spit out both values if the egsist?
Thank you.

(OP)
@Occupant

### RE: Finding max bending moment

Again, it's a math problem. When you specify 5 at x=1, and -x for x>1, you've created a discontinuity, for which no value of x results in f(x)=0 in that interval. By defining a piecewise linear transition, as shown in the attached file, a root can be found, and you can make it arbitrarily close to x=1

http://files.engineering.com/getfile.aspx?folder=a...

TTFN
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### RE: Finding max bending moment

(OP)
IRStuff
So basicly you defined that vertical portion with an equation BUT it has an very,very small slope?

### RE: Finding max bending moment

(OP)
One question,
In previous egsamples i have a function V(x)=....
I can get an value of V for any given x that I type in th brackets.
Is it possible vice versa?
I wanta to know at wich x would V= defined value?
So bascily I would need to First type eg. V=20kN and then Mathcad would give value for x at wich V=20kN
Possible?

### RE: Finding max bending moment

You can look up "solve block" or "given-find" it would look something like this:

x:=1 [initial guess for solve block]

given [required for solve block]

V(x) <CTRL>= xx [press the <CTRL> key and then the "=", resulting in a bolded equal sign, unless you've dinked with the fonts]

x := find(x) [or] find(x) = [the former assigns the found value of x to x, so that it can be used in subsequent calculation; the latter simply displays the found value, but x remains at the initial guess value]

TTFN
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### RE: Finding max bending moment

(OP)
IRStuff,
one more question,see file attached.
Is there anyway that I can condition the statement so that it looks for values belowe 4m and second statement for values above 4m?

(OP)
Anyone?

### RE: Finding max bending moment

Two things you can try:

> You need to change the guess value of x to be >4 -- for the second set of Finds
> Change find to minerr -- sometimes this works, and sometimes not - it worked a couple times on your sheet when I tried it, and failed most of the other times. The algorithms are all somewhat dependent on having continuous functions, so the discontinuous functions are very challenging

TTFN
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### RE: Finding max bending moment

(OP)
"Change find to minerr"

Is this spelt corectly?
I get an error every time when I try this.
So the solution woul dbe to convert this discountnous function into what you suggested "very steeep slope"wich would make it continous,

### RE: Finding max bending moment

Getting back to your beam4.xmcd file: The guess value for "x" that you put in has to be in the range of available solutions, therefore, x>4m will not give you a solution with x:=0 but it will with x:=6. Of course, V(x)=-20Kn will not work at all, there is no V(x)=-20Kn with x>4m

### RE: Finding max bending moment

(OP)
@irstuff
" You need to change the guess value of x to be >4 -- for the second set of Finds"
this works fine at my place
thanx!

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