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[SEAOC] RE: [SEAOC] AASHTO highway loads

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Assuming you are referring to the difference between the provisions of 
Sections 3.7 and 3.24 (as opposed to the difference between the truck and 
lane loads in section 3.7), the difference is basically that the truck and 
lane loads are generally used for the analysis of the main load carrying 
members (usually longitudinal beams or girders of some sort) through the 
application of "distribution factors" as defined in section 3.23, while the 
distribution of loads for slab design in section 3.24 is an empirical method 
to arrive at moments in the deck slab without explicitly considering the 
truck or lane loads.

More specifically, the main longitudinal beams or girders are typically 
designed as two-dimensional members (unless there is some compelling reason 
to do a three-dimensional analysis, like sharp curvature, heavy skew, 
variable girder spacing, etc.).  The truck load is applied as a single 
vehicle moving in either direction along the length of the bridge, a 
fraction of which (the distribution factor) is applied to each longitudinal 
girder.  This distribution factor (which is empirical) is calculated based 
on the type of girder, type of deck slab, and the girder spacing as 
described in section 3.23.  The lane load is used in a similar manner except 
that it is a distributed load (with one or more added concentrated loads) 
acting on all or part of each span and does not "move" as the truck does. 
 The lane load is intended to represent a single loaded truck preceded and 
followed by a train of smaller vehicles.

In contrast, (as is frequently the case in bridge design, where the loads 
for one portion of the structure are not carried directly down to lower 
portions of the structure) the deck slab analysis is based on a wheel load 
form the truck and the span of the slab between girders, but is otherwise 
empirical and unrelated to the truck/lane loading provisions of section 3.7.

Really confused now?  Try the AASHTO LRFD Specifications where they have 
combined the truck and lane loads and changed all the distribution factors, 
as well as introducing the Ontario method for deck slab design.

Seriously, I hope that  helps a little.

From: seaoc
To: seaoc
Subject: [SEAOC] AASHTO highway loads
Date: Wednesday, August 14, 1996 12:25PM

The AASHTO manuel requires either a standard truck load or lane load for
design, section 3.7. Section 3.24 provides provisions for slab design. Can
somone please describe the difference between the two.