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RE: AASHTO truck loading

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For longitudinal members, the distribution factor takes into consideration 
that multiple lanes may be loaded and that the load on any one longitudinal 
member is shared by adjacent members, *EXCEPT* at the ends of the 
longitudinal members.  Because the longitudinal members do not have any 
deflection at the ends, loads cannot be shared by adjacent members and you 
have to consider the actual wheel load/lane load locations assuming the slab 
is a simply supported span between longitudinal members.  The AASHTO Standard 
Truck actually represents the 1935 truck-train loading that is shown in the 

For transverse members such as pier caps, you do have to consider multiple 
lanes loaded and this is where bridge design gets interesting if you "do it 
by the book."  You first of all have to figure out how many lanes you have 
and fractional lanes don't count.  Trucks or lane loads have to stay in the 
lanes, but the lanes can be shifted from curb-to-curb and trucks/lane loads 
can be shifted from one side of a lane to the other side to produce the 
maximum effect on the transverse element.  Then you can adjust the results 
due to the unlikelihood of all lanes being loaded at the same time by maximum 
loaded truck.


A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Mete Baykyr wrote:

. > Hi list,

. > For the design of the longitudinal elements of a T-girder bridge, I am
. > trying to figure out the maximum M and V. The point I do not understand is
. > that, if the load is thought to be same on every beam using the proper "S"
. > factor (AASHTO 3.23.2) and impact effects as well, how many trucks should 
. > be considered on a single span (isostatic span) ? Just one for each span 
. > ? Any comments will be of great help. 

. > thanks

. > mete

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