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The interior girder moment formula for one lane loaded for the AASHTO LRFD method is:

$$\begin{align} mg^{SI}_{moment}&=\left(1.75+\frac{S}{3.6}\right)\left(\frac{1}{L}\right)^{0.35}\left(\frac{1}{N_c}\right)^{0.45} \\ &= \left(1.75+\frac{13}{3.6}\right)\left(\frac{1}{100}\right)^{0.35}\left(\frac{1}{3}\right)^{0.45} \\ &= 0.65\ \mathrm{lane/web} \end{align} $$

How is this formula derived? I have not been able to find the original research paper.

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The live load distribution formulas in the AASHTO LRFD Bridge Design Specifications cannot be derived. As I understand it, they are based on calibration to extensive finite element modeling.

This is probably the NCHRP report you're looking for.

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  • $\begingroup$ This was also my thought as well. The commentary to the code usually does a fairly decent job of describing these situations. I don't have the code next to me, so I can't point to the specific pages. $\endgroup$
    – hazzey
    Jan 6, 2016 at 1:30

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