No. For high fills (>20ft), the structure behaves as a deep buried pipe. You need finite element software (e.g., SAP2000 or PLAXIS). The rigid frame assumption fails due to soil-structure interaction.
: Enter your site-specific data in the yellow-shaded cells (Span, Rise, Fill Depth, Soil Density). box culvert design calculations xls link
For a standard box culvert without bottom slab (footing) or with a slab, the corners are assumed rigid. The moment at the corner ($M_c$) is often approximated for preliminary design as: $$M_c \approx \fracw L^212$$ (Note: Exact analysis requires matrix methods or coefficients depending on wall/slab stiffness ratios.) No. For high fills (>