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Live load distribution for bridge girders

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I must do a preliminary design of some bridge girders and bridge deck for
the purposes of doing a cost estimate so that we can submit a proposal for
the girders and deck.  The girders will likely be glulam beams but if
solid White Oak will work then that could be what is used.  The wood deck
will be 4x8 or 3x8 wood planks (they currently want Locust with treated
Pine as an alternative).

Let's just say that it has been a LONG time since I have done bridge live
loads per the AASHTO code.  I have (or had...can find it at the moment)
1990 AASHTO code around here somewhere.  I do have my class notes from my
bridge course.  Regardless, my information is at a minimum rather dated
and my memory is just a hair "fuzzy".

I need to get from an HS-20 loading (as well alternative associated lane
load) to a live load per girder.  What I need help with is remembering how
to do that...

I believe that I would take the wheel load or lane load moment and
multiple it by the distribution factor which would be in the form of
s/some factor, where s is the girder spacing.  The factor would be
(according to my notes) 3.75, so it would be s/3.75.  I will note that
this was the value that I had for "plank" decking on a two lane bridge
(the birdge driving surface is 22 ft and my notes show that as being
considered a two lane bridge).  In addition, I believe one should
increase the live load to account for impact load, but I remember seeing
somewhere or hearing somewhere that timber bridges need not have impact
factors.  If I must do impact loading, then I have that as
I=50(L+125)<=.3, where L is the span of the girders.

So, the question is I am remembering correctly?  Is there anything that I
am missing?  And has any of the above changed?  Any other wise thoughts or

Keep in mind that my goal is to get a preliminary design so that it is in
the ball park so that our proposal is reasonably accurate.  If we get the
project, then we will be hiring a consultant to do the engineering.


Adrian, MI

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