Incidentally, I can't believe the bottom chord that you marked would be under tension. That would imply its function was to pull downward on the upper end, which would make little sense in this sort of bridge.
It's a simple vector relationship. If it’s an eqilateral triangle, and the king post is taken out, a downward load of 1/2*(the load) is applied to each lateral side. The compressive stress in each side beam is 0.707*load. The sides oppose that force and transfer it to the base by existing under compression. At the base, 0.707*load is applied at each end vertex.
Resolving that into it's components shows the resulting stresses perpendicular and parallel to the base. One component of the force points downward and perpendicular to the base. At each vertex, that component is 1/2*load, for a total downward force of the total load applied. The other component points away from the vertex, parrallel to the base, in a direction away from the other vertex in the base chord. The magnitude of that component is 1/2*load at each vertex. That's a tensile load of magnitude 1/2*load in the base chord.
If you want a "sense" regarding the force in the base do the following experiment in your head. Balance 2 sticks on the table top, so that they look like a triangle. Note the length of an imaginary base stick you'd need to make a complete triangle. Then push down on the top vertex. Now, what happened to the imaginary base stick? Was it crushed, or stretched?