Why does the truss bridge app report forces over 500 for a 200 load?

[mishima]

Hi, my high school students enjoy using the applet found here (http://pages.jh.edu/~virtlab/bridge/truss.htm) to design model (basswood) bridges for our annual regional contest. It seems to require firefox these days.

Recently, some designs have been causing extremely large forces to be reported. For example, a 200 load causing forces of 500+ in multiple members. A screenshot of the design which causes this is below.

So naturally both myself and my students are curious why some designs cause this. I am familiar with truss analysis using the method of joints, but the software appears to use some kind of matrix to do the calculations all at once. I was wondering if anyone had insight into how this software might do its calculations.

My guess is that it can only accurately handle simple trusses.

 

[SteamKing]

Hi, my high school students enjoy using the applet found here (http://pages.jh.edu/~virtlab/bridge/truss.htm) to design model (basswood) bridges for our annual regional contest. It seems to require firefox these days.

Recently, some designs have been causing extremely large forces to be reported. For example, a 200 load causing forces of 500+ in multiple members. A screenshot of the design which causes this is below.

So naturally both myself and my students are curious why some designs cause this. I am familiar with truss analysis using the method of joints, but the software appears to use some kind of matrix to do the calculations all at once. I was wondering if anyone had insight into how this software might do its calculations.

My guess is that it can only accurately handle simple trusses.

Matrix truss analysis is a proven technique. If you have concerns about this particular app., there should be some sample trusses that you can find in a textbook or on the web which has results that you can compare to an analysis of the same structure performed by this app.

After you obtain results, you can still use statics to check them if you question whether they are correct.

 

[mishima]

Well, here is more extreme example showing a 100 load magically amplified into the billions by the same truss. There seems to be some prerequisites to the technique that we are unintentionally breaking.

 

[SteamKing]

Whatever this app is doing, it can't handle asymmetrically applied loads. It also has a peculiar requirement for the number of members a structure must have given a fixed number of nodes.

I recommend you find another app which doesn't have the problems this one has.