Does HS20-44 always include Alternate Military Loading?

[jmag03]

I do a lot of load ratings using older plans and often the design loading is "HS20-44" or "HS20-44 with Alternate Military Loading". When I compare versus the bridge inventory it will sometimes include Alternate Military Loading when the plans only specify HS20-44. I know that "44" is the year that AASHTO developed the HS20 loading configuration so maybe military loading was implied during WWII, but my intuition wants to consider the loading as HS20 unless Alternate Military Loading is explicitly stated. Can somebody please clarify this for me?

 

[bridgebuster]

The AML came about during the interstate era. My oldest AASHO dates back to 1953 and has no mention of it. The first mention of it I see is 12th ed. (1977) of AASHTO, section 1.2.5 (G)Interstate Highway Bridge Loadings. It specifically says for Interstate bridges. The 13th ed. of the Standard Specs added "bridges that carry heavy loads.


An afterthought: When I started working the the 70's, load ratings were somewhat new. Perhaps some of the bridges you're looking at were rated several years after original construction and the owner wanted both conditions checked.

 

[BridgeSmith]

If I remember right, the AML was required for any bridges that were on the National Highway System (NHS routes) at the time of design.

 

[Lomarandil]

Another standard that was around the same time is the Bureau of Public Roads PPM 20-4.

This applied alternate tandem loading of 24k/axle at 4' spacing for interstate highway bridges approved on or after July 17, 1956 and to bridge components and floor systems with spans under 40 feet.

 

[BridgeSmith]

The tandem axle truck is one of the standard design trucks we use. I think it's 25 kips per axle for the LRFD HL-93 loading, though. We apply it for all bridges, even though it would only control the design for very short spans.

For our bridges on Interstate 80, we apply a tandem train loading, also - 2 tandem trucks 50 feet apart (+ lane load) for moment, shear, and bearing reactions at interior piers.