Well, I'm sure you mean why are arch doors symbolic and also you would need to be more specific. An arch door in general isn't symbolic while the St.Louis arch is very symbolic. It can be a symbol for strength due to the strength of the design, but it is also smooth and relies on a keystone to hold it together. Well at least in Roman archways.
for vertical structural integrity
Arched!
The Parthenon is not arched. Look again.
1 syllable in arched.
Doorways in the Sand was created in 1976.
Doorways to Space was created in 1951.
Doorways in the Sand has 181 pages.
Doorways to Space has 206 pages.
The Three-Arched Bridge was created in 1978.
The ISBN of Doorways in the Sand is 0-060-14789-X.
the keystone is the most important part of an arched bridge. :]
"Doorways of Italy" in English is Vie d'accesso d'Italiain Italian.