Mustang_Crazy:
The problem you encountered is one I found under similar circumstances. The I beam and U channel you used is a PVC product and requires stronger glue to bond it together. The unfortunate part is you will have to strip the paint and re-glue everything again. You can use an Oven Cleaner spray and a toothbrush to strip the paint and begin re-gluing once you have cleaned and dried all the pieces.
Or
You could go back to the hobby shop and buy Styrene I beam and U channel. It is white in colour and comes in the same dimensions and thickness as the PVC type. This is what I had to do after a project I built "imploded".
Live and learn.
Sorry I don't have better news for you.
Cheers;
Gregory