Refining Bridge Design

I've had two good questions about refining your bridge designs that may be of general interest.

Q1 - Why does an "X" type arrangement fail when using the Hopkins "Bridge Designer".

Response:

Probably the best answer is that you have created a "redundant" structure with an "X".  You can test that by removing one leg of the X and probably it will work as in the example above.  If you add the member from the lower left to the center of square so you have a complete "X" Bridge Designer won't calculate it because you have an unnecessary or "redundant" member.

Q2 - Additional Analysis

Throughout the project, I have been replicating the Knex bridge on WPBD as an extra load test. But thinking about what you said today after lab, I realize that's not the best way to test our truss. What at-home methods do you suggest would be best to see if the truss is stable?

Response:

Unfortunately you're running up against the limits of the tools you will understand in your first year.  There are indeed many tools for additional analysis and testing, but to use them effectively you need the knowledge that you'll gain in your more advanced courses.  WPBD and Bridge Designer have to make simplifying assumptions that your real bridges violate.  Let's hope this is your incentive to continue learning.

Units for Results

At 10:47pm the day before the assignment was due I was asked the following question.

“Is it ok if we converted pounds to newtons and feet to meters to do the calculations?”

Response

You may work internally in any units you wish, but results should always be shown in the units of the country in which you work unless told otherwise.  If I were working in almost anyplace other than the US then newtons and meters would be the appropriate units.  In our backward (in terms of units) country we still use feet, inches and pound-force and expect to see them in the results.

Calculating Triangle Angles and Trigonometric Values

To analyze a truss there is often a good deal of trigonometry involved to calculate the components of forces.  To make things simpler (I hope), I’ve created a simple excel spreadsheet that will allow you to calculate angles, sines, cosines, and tangents if you know the length of the legs of any triangle.  This should ease the calculation burden somewhat.  You might well wish to use this sheet as a starting point to make your truss calculations even readily modifiable.

Here’s the link to the spreadsheet.

Download the spreadsheet to Excel to see the diagram that explains the notation.