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08-14-2011
09:04 PM

08-14-2011
09:04 PM

I have just started using Mathcad, and i think it is a fantastic resource. I am trying to determine the moment capacity of a reinforced concrete beam. I have a number of equations that all depend on one another and need to solve for one of the variables. I have done this in excel using the goal seek function but i would prefer to use mathcad as it presents the results much better. I have tried to use a solve block with out any luck.

the basic theory is that some of the beam is in compression and the rest is in tension, I need to move the deviding line (the neutral axis) up or down until the forces balance, ie the sum of forces (compression and tension) are equal to zero.

I have attached the worksheet, if anyone can help i would realy appreciate it. I have highlighted the equations in yellow, they are at the bottom of the sheet.

Joe

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08-15-2011
02:45 AM

17 REPLIES 17

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08-15-2011
02:24 AM

08-15-2011
02:24 AM

Re: Using solver to solve a real problem

So you want to find the value of d.n, which make the resultant forces equal.

Mike

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08-15-2011
02:29 AM

08-15-2011
02:29 AM

Re: Using solver to solve a real problem

Thanks for answering, yes that is exactly what I want.

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08-15-2011
02:45 AM

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08-15-2011
02:57 AM

08-15-2011
02:57 AM

Re: Using solver to solve a real problem

Thanks for helping. I realy appreciate it. If you are ever in Adelaide, I will buy you a beer.

Joe.

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08-15-2011
03:02 AM

08-15-2011
03:02 AM

Re: Using solver to solve a real problem

Thanks for helping. I realy appreciate it. If you are ever in Adelaide, I will buy you a beer.

Joe.

Cheers, I'll hold you to that

Mike

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08-15-2011
03:13 AM

08-15-2011
03:13 AM

Re: Using solver to solve a real problem

Mike,

I have tried to apply your solution to a slightly diferent problem but i got an error

It says that "this variable is undefined" but I have defined it above, to you know what I am doing wrong?

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08-15-2011
03:17 AM

08-15-2011
03:17 AM

Re: Using solver to solve a real problem

Not from the image. Can you post the worksheet?

Mike

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08-15-2011
03:26 AM

08-15-2011
03:26 AM

Re: Using solver to solve a real problem

Have a look at the attached.

If you change the guess value of d.n to 60, Mathcad throws an error. Always try changing the guess value when this happens.

Another trick which I find helpful is to use the Minerr function instead of find. Minerr - Returns the values of var1, var2... that satisfy the equations and inequalities in a Solve Block. **Minerr** differs from **Find** or **Minimize** in that, if the chosen algorithm fails to converge, whatever answer found on the last allowable iteration is returned, even if it doesn't meet the convergence criteria.

Mike

Message was edited by: Mike Armstrong - I forgot to mention - put the guess value of d.n to 50 and it works 🙂

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08-15-2011
03:43 AM

08-15-2011
03:43 AM

Re: Using solver to solve a real problem

Mike,

Here is the worksheet with the error, I have highlighted the section that I am interested in in yellow again. thanks again for your time!

Joe

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