HS 20-44 Loading

[napavine]

I work in the logging industry and we often buy prefabricated steel bridges for installation on our roads. When ordering bridges we specify they be designed for an HS 20-44 Inventory Rating and also designed to pass several operating rating design vehicles that differ depending on the type of logging equipment expected to cross the bridge. The intent is that the HS 20-44 would closely represent a log truck loaded with a legal highway load.

The bridge manufacturers we typically purchase from have always used an HS 20-44 loading diagram that looks like: 8K..12'..16k..4'..16k..10'to26'..16k..4'..16k.

However if I look at our local DOT manual they specify an HS 20-44 loading diagram as looking like: 8k..14'..32k..14'to30'..32k.

Is the DOT diagram with the larger single axle loads more of a commonly used standard?

The reason I ask is that I am trying to decipher a load rating report that was done for one of our existing bridges by a consulting firm which typically works in a more urban environment. There was a significant difference between the rating factor they came up with for an HS20-44 (RF=1.10) and an AASHTO Type 3S2 (RF=1.51). I would have guessed the rating factors would be more similar for the two vehicles, but maybe not if they used the DOT diagram with the heavier single axles.

Also, the report provided an NBI Inventory RF of 0.79. Am I correct that this factor is based off of the new LRFD design methodology? I know very little about that process but can someone tell me what the underlying design vehicle assumed is? Does that use an HS 20-44 as well? If so what does the loading diagram look like.

With the NBI rating below 1 I am not sure we can safely haul logs over the bridge or not. Any insight for a non-structural engineer is greatly appreciated.

Thanks

 

[bridgebuster]

The HS 20 truck is an imaginary design vehicle meant to encompass the typical trucks on the road. Type 3, 3-3, and 3S2 are actual trucks; that's why the ratings for these three are always higher than a rating for an HS truck.

The NBI rating could be either Standard Specs or LRFD, depending on the state. LRFD uses HL-93 loading, which is HS-20 plus a 640plf lane load spread over 10 feet.

A RF of .79 doesn't mean you can't haul logs safely over the bridge. Your engineer should be able to tell you the maximum load your actual trucks can carry over the bridge. In some localities when a bridge rates below 1 for HS20 the owner will post a sign with actual axle configurations showing maimum tonnage for each truck type.