AASHTO Lane Load

[Waxwing]

There was a post that is closed that was asking about the origin of the AASHTO load. There was not really a decent response to the question...so it is still a mystery to me.

I've never understood how the lane load was arrived at. I remember reading the HS20 lane load was supposed to approximate a truck train of an H20 (or HS20, don't remember) preceded and followed by H15 trucks. I remember trying to calculate equivalent moments and loads trying to get some equivalent shears and moments between the lane load scenario and the truck train scenario, to no avail! I remember researching and not finding a justification of the lane load, and don't ever recall hearing an explanation of how this supposed "equivalence" occurred.

That being said, I guess it doesn't matter now since every one is using the LRFD truck, which is a whole new calibration that somewhat retained the old form of the old loads just to maintain the calculations everyone was used to. But after all these years I'm still am in the dark about the old AASHTO H and HS lane load and how it came about.

 

[OSUCivlEng]

The lane load is still required to be used along with the Design Truck for AASHTO LRFD. It hasn't gone anywhere. Actually the lane load AND the design truck OR design tandem are just simply called HL-93 now.

The way I always understood it was the lane load is a notional load representing the effect of vehicular traffic (2 axle cars), while the design truck(s) (don't forget about tandem, double tandem and double truck) are notional loads representing the effects of heavy truck traffic (dump trucks, garbage trucks, concrete trucks, gravel trucks, tractor trailers, etc). However that doesn't seem to hold true to what the commentary says in the bridge code.

The live load model, consisting of either a truck or
tandem coincident with a uniformly distributed load, was
developed as a notional representation of shear and
moment produced by a group of vehicles routinely
permitted on highways of various states under
“grandfather” exclusions to weight laws. The vehicles
considered to be representative of these exclusions were
based on a study conducted by the Transportation
Research Board (Cohen, 1990). The load model is called
“notional” because it is not intended to represent any
particular truck.

In the initial development of the notional live load
model, no attempt was made to relate to escorted permit
loads, illegal overloads, or short duration special permits.
The moment and shear effects were subsequently
compared to the results of truck weight studies (Csagoly
and Knobel, 1981; Nowak, 1992), selected WIM data, and
the 1991 OHBDC live load model. These subsequent
comparisons showed that the notional load could be scaled
by appropriate load factors to be representative of these
other load spectra.