LRFD vs. Standard Spec

[jedge]

I am part of a team working on an elevated roadway for an international airport. We have restrictions on the raodway depth to accompdate floor to floor heights appropriate to boarding aircraft and 13'-6" minimum clearance for the lower roadway. We compared the AASHTO LRFD Specification and the Standard Specification and found that the combination of the lane and truck loads in the LRFD spec has some impact on the girders but the big hit was in the bents. Since the lower roadway runs parrallel to and below the upper roadway, the bents have a 62-foot span between columns. Our worst case bent has 105-foot span girders on one side and 30' span girders on the other side. Since the LRFD spec also requires that the trucks be located in trains with 50-foot spacing between trucks, the reactions to the bents are far greater in LRFD than in the Standard Spec. We found that the design moments were increased by a factor of three. Has anyone run across this large of a discrepancy between designing by LRFD and designing by the Standard specs? Did we miss something?

 

[JDBolt]

Some thoughts:

Studies found the effects of the exclusion vehicles under estimated negative moments by by up to 80%. {AASHTO fig.C3.6.1.2.1-1.

The effect of two trucks for negative moments may be reduced by 90%. [AASHTO 3.6.1.3.1].

The 50 feet spacing refered to is a minimum, and a greater spcing may in fact produce a worst case.

The deck reinfocement may be used with the steel girders to resist the moments.

 

[JDBolt]

Another thought:

Suspect 30 105 30 span arrangement is causing the negative moments over the interior supports to approach that of a fix end moment condition. You can study this behavior by increasing the first and last span and notice the moments at the interior support will decrease.

A similiar reduction in the stiffness of the girders in the first and last spans will also reduce the end moments.