A classical hand-calculation method often hardcoded into Excel cell formulas to solve for joint moments.
Box culverts experience permanent and transient loads that vary based on the depth of the earth fill. box culvert design calculations xls link
Q=1n⋅A⋅R2/3⋅S1/2cap Q equals 1 over n end-fraction center dot cap A center dot cap R raised to the 2 / 3 power center dot cap S raised to the 1 / 2 power (Where = Manning's roughness coefficient, = hydraulic radius, = slope). Step 2: Load Identification = hydraulic radius