Is mathcad better than excel for structural calculations ?

That's easily so.

• You see the equations in normal mathematical notation thus even people unfamiliar with Mathcad understand the calculations. Excel on the other hand is just code i.e.; a jumble of parenthesees, double asteriks(**) to raise a power and non explanatory cell references.
• Mathcad automatically converts units so that you can calculate with inches and millimeters in one equation. Remember the Mars probe in which miles and meters were mixed up? Wouldn't have happened.

Raiko

Apart from the benefits mentioned by Raiko and Fred, you can also do programming (if at all necessary). So in my opinion, Mathcad is definitely better than excel for performing design calculations and my favorite reason for choosing Mathcad over excel is that I do not need to take care of units as the unit system is already built inside the Mathcad system and I can get the output in any desired units.

There are many excellent answers.  One more is that you can check and defend your math.  As a licensed Engineer, I am required to submit my calculations as part of the legal documents that make up an Engineering project.  

With Mathcad, someone can follow my logic to check my work or challenge it.  I can also defend my work if challenged.  Try that in a spreadsheet where all of the logic is hidden and cryptic.  When using a complex spreadsheet, created by someone else, how do you know that the answers you are getting are valid?

As a University Lecturer, I require electronic PDF submission of assignments.  I can review math done by hand and scanned to PDF.  Better, the University provides Mathcad so that they can submit it in a very clear format.  But a spreadsheet?  How can I look at that and tell where a mistake was made or how the student arrived at the answer?  Assignments are part of the learning process.  If I can't tell how the student made their mistake and explain that to them, they learn nothing.