Design-span for moment calculations (timber-deck on bridge)

[KnarfMan]

I have a multi-girder steel bridge with a timber-plank deck. There are nine steel girders (each a W12x50 with 8" flange width) spaced at 27" c/c (i.e., net out/out width of 18'). The deck is made up of 4" x 12" timber planks (spanning over and perpendicular to the girders). The intended design vehicle is an H10-44 truck.

1) What is the design span for calculating max. bending moment in each timber plank (i.e., 27" c/c girders, or 19" clear span between the flanges, or something else)? Specific AASHTO references would be very helpful (A-4.6.2.1.6 was somewhat vague for this timber-over-steel situation).

2) Should I analyze as simply-supported, continuous, maybe a propped-condition, or something else.

The Inventory Ratings for the steel bridge components are all > 1 , but I'm having trouble with the timber planks (all have IR < 1).

All comments/responses appreciated. TIA

 

[ishvaaag]

Have examined AASHTO 2007 SI and don not see anything about using net or clear spans, so one must surmise the intent is to use distance between axes. Anyway you may consider the clear span I think based on the permissible methods for calculation, that would allow that; in this case you may load the inner tips and then the unloaded parts have longer spans if continuous or well represented the connections, flexure at the flanges, and induced rotation to supporting girders, etc.

Just fine references somewhat related follow...

http://books.google.es/books?id=2gn...=wood bridge deck on stringers AASHTO&f=false

http://www.dot.ca.gov/hq/esc/techpu...idge-design-specifications/page/section13.pdf

http://140.194.76.129/publications/armytm/tm5-600/entire.pdf

http://www.aot.state.vt.us/conadmin/Documents/Supplemental Specifications/Suppspec522LumbandTimb.pdf

http://www.tfhrc.gov/structur/pubs/04098/04098.pdf

http://www.apawood.org/pdfs/managed/T625.pdf

http://www.nicet.org/nicetmanuals/bridge.pdf

 

[ishvaaag]

I also see continuity if available perfectly within the code.

 

[ishvaaag]

You see in the code as per your entry in the Bridge forum and the answers there is in AASHTO requirement to use span either between axes or clear plus one deck thickness. Please look there.

 

[KnarfMan]

Thanks to everyone for the help. However, I got my "Ratings" to work for the timber deck-planks by using Allowable-Stress-Design instead of Load-Factor-Design. Never thought it would make that much of a difference, but such is life ....... well, at least in engineering