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J
justinskillz1-Visitor
1-Visitor
November 15, 2018
Question

Symbolic Keyword Evaluation

  • November 15, 2018
  • 2 replies
  • 1929 views

I've been trying to get my sheet to solve for B but it keeps telling me "No Solution was Found".  Is it possible that I just have too much going on that the program is unable to solve it?  It's a little sloppy right now I know but I wanted to get it working before I spent the time making it look good.

 

On the second page, I was able to get it to solve.  For these, is there a way for it to only give me the positive solution?  I tried also doing "assume, B>0" but it still just gave me the three answers seen.

2 replies

T
21-Topaz II
terryhendicott21-Topaz II
21-Topaz II
November 15, 2018

Hi,

 

In solving for B, B needs to vary.   This is engineering and it is not possible to find an analytical solution for B so symbolics don't work.

 

Solution is to use functions in terms of the variable B.  Then use a numeric solve/find block.

 

I have shown you how to do this.

For serviceability state it does not find a solution so suspect something is wrong with calculations.  With a 1000 mm wide footing get 540 mm deflection.  It reaches a silly conclusion of B = 3.0 mm

For ultimate state Point 1 it solves to B=770 mm which seems OK.

 

Cheers

Terry

    J
    justinskillz1-VisitorAuthor
    1-Visitor
    November 15, 2018

    Thanks for the help!  I was able to fix it and get it to work.

    Unfortunately, it's not giving me the right answers so I may have messed up on the math a bit but at least now I know how to get the sheet to work.

    W
    Werner_E25-Diamond I
    25-Diamond I
    November 15, 2018

    As Terry already said the way to go are Mathcads numeric methods, either a solve block or the "root" command.

    The problem with Mathcads symbolics is twofold:

    1) The symbolics does not know anything about units and treats them as unknowns (which also may be negative or non-real). Considering this, no solution is found. Sometimes it helps to tell MC that a unit is real and positive by adding "assume,m>0" to the symbolic eval, but in your case it does not help. What works is to tell MC that m=1 (as seen in the screenshot below. Thats bad as you lose Mathcads unit checking feature, you may get wrong results as of unit discrepancy and you would have to add the correct unit yourselft afterwards. And ... you may get a highly undesirable result as seen in the picture, because

    2) when the symbolics is forced to float mode because no exact solution is possible, it will not give you all possible results of your equation but only a single one and usually there is no way to control which one (adding "assume,B>0" or the like has no effect.

     

    So the symbolics is of no use in your case and thankfully isn't necessary at all. You should have set up your sheet to make all variables which depend on B functions of B. I was too lazy to change your sheet that way and so I used the symbolics to make the expression delta.c-delta.a1 a function of B which you want to be zero.

    The sheet shows how to use "root" to find a solution. Guess its the same 3 mm which Terry found using a solve block.

    B.png

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