User error or glitch? "variable undefined" Mathcad 15

[RyanFehr]

I've been using Mathcad for a couple years now and I think I have a pretty firm grasp on the in's and out's of the program, but I've always found little things that just seem to happen at random and seem to be a glitch or something. One such problem is when I copy and paste formulas from one sheet to another. The definitions sometimes are no longer recognized. And for the life of me, I cannot get Mathcad to recognize them again. Below is an example of this. This is an assignment I am working on for a class. The first part is a calculation of section properties of a channel. Everything from "Vx:" down is copy&paste'd from a previous assignment for calculating shear flow and shear center in channels. The formulas now no longer recognize the "xbar" definition. Can someone help me out with this problem? Thanks.

[URL=http://imageshack.us/photo/my-images/29/mathcad.png/][IMG]http://img29.imageshack.us/img29/2860/mathcad.png[/IMG][/URL]

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[RyanFehr]

Ah, I figured it out. I guess I need to call xbar as well as any other variable I use that's defined by a formula, regardless of whether the it's in symbolic form or not. I was under the impression that I only needed to call variables that were still in symbolic form. Guess I'm still learning.

[RyanFehr]

nevermind, thats just leaves it in symbolic form and useless to me.

BTW, I tried manually retyping the formula...that doesn't solve the problem.

[RichardJ]

The symbolic processor does not care about variable defintions, but if you pass the result frm the symbilic processor to the numeric processor, it does.

You can post images directly to these forums; no need for an external server. What we really need is the worksheet though, not an image. It's very hard to figure anything out from an image.

[RichardJ]

It looks like the variable tf is undefined. Without the worksheet, I can't say why it's flagging xbar.

[VladimirN]

Hello!

Please, attach your Mathcad worksheet.

[RyanFehr]

My appologies. I ended up starting a fresh sheet, dropping units and getting things to work out without errors. I couldn't tell you why it worked this time. Anyway, I just kept chugging through the assignment because its due tomorrow...err today. I'll attach the finished assignment if that helps explain what I was doing wrong before. Also, is there some sort of setting I can change to make Mathcad provide more accurate calculations? I've been having issues with what appears to be Mathcad rounding errors. As you can see in the shear center calculation of this sheet, my shear force in the top flange doesn't quite equal -0.5lbs as it is supposed to. Thanks for the help. Sorry for the delay.

[RichardJ]

I ended up starting a fresh sheet, dropping units and getting things to work out without errors.

It it works without units but not with units that's usually a bad sign.

Also, is there some sort of setting I can change to make Mathcad provide more accurate calculations? I've been having issues with what appears to be Mathcad rounding errors. As you can see in the shear center calculation of this sheet, my shear force in the top flange doesn't quite equal -0.5lbs as it is supposed to.

The number is accurate, given your input values. So yes, it's a roundoff problem, but the roundoff is in the input numbers, not the calculations.

[wayne]

In addition

Form your picture, it looks like you were using lb (mass) unit instead of lbf (force) units.

Constructive criticism:

If you used the dimesnions as variables at the start, bf, tf, bw,tw, much as you defined them in the Shear Center section, you wold have gotten exact results, and would be a lot easier to check.

The elements are relatively thick, are you using centerline dimensions and checking at centerline's, or should you be using the real sections and checking at the real edges? (I am not entirlely sure you were consistent, for example you used h=11", which is an overall dimension, but the section properites I think are based on centerline dimensions)

[RyanFehr]

i wasnt consistent. i used centerline for everything except the moment of inertia (second moment of area) calcs. I had the same problems on a previous homework though and those values were given to me. oh well, i dont think ill lose too many points for that. thanks for the help.