I’m still having trouble with this.
A member in tension tends to elongate. Easy to see in the bottom chord of a flat truss. The bottom chord elongates as the truss deflects.
I don’t see how an arched bottom chord can be in tension. If it elongates, the truss sags UPWARD.
I’m not arguing with you, I’m trying to clarify, because if this is true, it is decidely counter-intuitive.
Prez just gave some remarks after getting a close-up look at the site. He talked one-on-one with a man who was on the bridge and who helped get some of the kids off the bus. They stood, just the two of them, right next to the part of the bridge where the bus is.
If the bottom chord in that arched truss is stretched, the top must contract under compression. It will not bow up, it will bow down.
Look at any king post, real or imaginary, in one of the trangular elements near the center. When the arch is stretched it will staighten out. The king post in the center will pull the center of the top chord down.
Now look at the king post at an end. Stretch the bottom chord and the king post will rise up, moving the vertex of the triangle up.
The thought experiment can be done by compressing the bottom chord. The top chord will bow upwards in that case.
Shapes can always be approximated by decomposing them in to triangles. In the arched truss, they just get smaller, then larger along the long axis of the truss. Finite element analysis often uses triangles, as "finite" elements, instead of points to model complex shapes and situations.
Try this for a simple explanation. Most members exist to transfer force from a support to a load; this force will generally be parallel to the member in question. If the support applies force toward the load, the member will be under compression; if the support applies force away from the load, it will be under tension.
Starting at the place where the truss is supported by the pier, it should be clear that the pier exists to supply upward force. Therefore, any structural members that go up away from the pier will most likely be under compression (at least when the bridge is performing as designed). I'll see if I can draw you a picture.