06-05-2010, 06:29 PM
Good comments above regarding drainage and falling stuff. It occurred to me that reading my 1929 Railway Engineering and Maintenance Cyclopedia might help!
Here's a couple of relevant sections:
"The choice between these two types is governed largely by local conditions. The open deck is obviously the cheaper form, but permits the dropping of coal, water and other substances from the bridge and is thus likely to become a source of complaint [on bridges crossing streets]"
"... While treatment undoubtedly prolongs the life of the timber in a material degree, it is to be remembered that generally ties in a bridge deck are favorably situated to offer maximum resistance to the influences of decay..." (a very long way to say that they don't hold water!)
" A solid floor on a steel or concrete bridge permanently and effectually precludes the dropping of material from a bridge to the passageway below, and is for this reason the most desirable form at crossings spanning city streets or heavily traveled thoroughfares. The ballast deck further provides somewhat better riding surface over the bridge than an open wooden deck, since on a ballast deck bridge the track structure itself is practically identaical in character with that on an ordinary roadbed, and the track is maintained in line and surface by the ordinary section gangs."
Goes on to say that concrete floors effectively permanently protects the metal structure of the bridge and maintenance costs on such bridges are much lower than open deck, making them "a source of ultimate maintenance economy".
"...where open tie deck is used, the elevation of the rail is, in a measure, permanently established, The work of section gangs on the track , performing the ordinary maintenance tasks of lining, surfacing, tamping, cleaning, ballast, etc.,invariably tends to raise the track [and the open deck bridge will eventually be in a depression]..."
On the other hand:
"...[on solid deck bridges] cases have been known where a bridge which was originally built with 18 inches of ballast aquired in the course of twenty years over 4 ft. of ballast." (!)
So open - cheaper, drain very easily, possibly leading to longer tie life
Solid - Smoother, cheaper to maintain, longer lived, protects streets below, easier to maintain the roadbed
Good book!
Matt
Here's a couple of relevant sections:
"The choice between these two types is governed largely by local conditions. The open deck is obviously the cheaper form, but permits the dropping of coal, water and other substances from the bridge and is thus likely to become a source of complaint [on bridges crossing streets]"
"... While treatment undoubtedly prolongs the life of the timber in a material degree, it is to be remembered that generally ties in a bridge deck are favorably situated to offer maximum resistance to the influences of decay..." (a very long way to say that they don't hold water!)
" A solid floor on a steel or concrete bridge permanently and effectually precludes the dropping of material from a bridge to the passageway below, and is for this reason the most desirable form at crossings spanning city streets or heavily traveled thoroughfares. The ballast deck further provides somewhat better riding surface over the bridge than an open wooden deck, since on a ballast deck bridge the track structure itself is practically identaical in character with that on an ordinary roadbed, and the track is maintained in line and surface by the ordinary section gangs."
Goes on to say that concrete floors effectively permanently protects the metal structure of the bridge and maintenance costs on such bridges are much lower than open deck, making them "a source of ultimate maintenance economy".
"...where open tie deck is used, the elevation of the rail is, in a measure, permanently established, The work of section gangs on the track , performing the ordinary maintenance tasks of lining, surfacing, tamping, cleaning, ballast, etc.,invariably tends to raise the track [and the open deck bridge will eventually be in a depression]..."
On the other hand:
"...[on solid deck bridges] cases have been known where a bridge which was originally built with 18 inches of ballast aquired in the course of twenty years over 4 ft. of ballast." (!)
So open - cheaper, drain very easily, possibly leading to longer tie life
Solid - Smoother, cheaper to maintain, longer lived, protects streets below, easier to maintain the roadbed
Good book!
Matt
Matt Goodman
Columbus, Ohio
Columbus, Ohio