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.