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 (Load Factor Method) 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

 

[bridgebuster]

From the Standard Spec's 17th Edition, Article 3.25.1.2:

"For transverse flooring the span shall be taken as the clear distance between stringers plus one-half the width of one stringer, but shall not exceed the clear span plus the floor thickness."

 

[PEinc]

If you are designing for H10-44, I assume that you are using AASHTO 17th Edition criteria. Therefore, check AASHTO Figure 3.7.6A footnotes for possible use of lower axle loads when designing a timber floor (deck). Also, per Section 3.8.1.2 Group B, impact shall not be included for timber structures. Read AASHTO carefully for different design requirements for the timber members.

http://www.PeirceEngineering.com

 

[afrancba]

Hi
I dont know AASHTO and never designed timber flooring but..
I wouldn't put the planks directly on the compressed
flange. I would design a smaller longitudinal "slat" upon
the web in order to centralize reaction forces of planks,
avoiding dangerous further deformations of the girder's top
flange. If so, the span would be the clear distance between
the girders.
I attached a picture about this and highlited the "centralizer slat".
I hope I could help you

 

[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