Dave wrote:

We have seen many...the bridge girders over our path.

I am wondering if a thinner material, stamped in appropriate designs
(Triangles) would give the strength needed?

My point is, lightweight I beams.

The reason stamping/punching/trusses are used is because in 99% of structural
applications, most of the material is not stressed that much, and simply
serves to keep the rest of it in place.

For example, take a 4" deep wood beam, and drill 2" holes horizontally
through the center every 12".

The strength is almost unaffected.
If, however, you drill the holes at the bottom of the beam (where it's most
stressed) it's quartered.

In many applications, the reason for using large beams is not to
stop them from breaking, but to keep the bending within acceptable limits
for the structure they are in. (plaster ceilings will crack long before
wooden ceiling beams fail)

In general, if this is the case, for a beam like a ceiling joist, tripling the
height of a beam, will give nine times the stiffness, and you can reduce
the width by 9 times to keep the same as the original stiffness.
However, it will now tend to twist.
The I-beam is a compromise that keeps (almost) the stiffness of a solid
beam of the same dimensions, but at a bare fraction of the weight.

The basis of this thought is (groan) The Roswell Wreckage. Many
pieces were described as having "Heiroglyphs".

I translate this to stamping of a metal for structural strength.

Any thoughts?

Unlikely.
IIRC, the metal was also described as flexible.