> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/box-culvert-workflow/loading-bc/live-load-surcharge-bc.md).

# Live Load Surcharge \[BC]

Live Load Surcharge applies the lateral pressure that vehicles standing on the fill exert on the culvert's exterior walls, following AASHTO LRFD 3.11.6.4. The vehicle load is represented as an equivalent height of soil, `h_eq`, taken from Table 3.11.6.4-1 and interpolated for the depth in question; the lateral pressure is then `k · γ_s · h_eq`.

Because `h_eq` decreases with depth, the pressure is evaluated at the culvert top and again at the culvert bottom and applied as a linear diagram between them.

MnDOT LRFD 12.2.3 applies the surcharge to one wall with an equal and opposite reaction rather than symmetrically to both. This is modelled by loading both walls in the same global direction with the same diagram, so the pair reproduces the one-wall demand on the frame. The wall loads are sliced along the barrel so they remain correct on a curved alignment.

## General

**Box Culvert:** The culvert barrel this surcharge is applied to. A selection is required.

**Stage:** The construction stage this load is activated in. Leave it empty for a model that is not staged.

**Fill Data From \[Inputs/Culvert Fill]:** Where the fill depth and unit weight come from — the object's own fields, or the Culvert Fill registered on the selected barrel. See Vertical Earth for the full behaviour; the two objects share it so that the cover used for the surcharge cannot differ from the cover used for the vertical earth load.

**Fill Depth Above Top Slab:** The cover depth over the top slab, which sets the depth at which `h_eq` is read. Shown only when Fill Data From is set to Inputs, where a value is required.

**Fill Soil Unit Weight:** Unit weight of the fill. Shown only when Fill Data From is set to Inputs, where a value is required. Must be positive.

**Surcharged Wall \[Left/Right]:** Which wall the vehicle stands behind. It sets the direction of the applied pair, not which wall receives a load — both walls are loaded in the same direction, which is how the one-wall demand is reproduced.

**Lateral Coefficient for Live Load Surcharge:** The lateral earth pressure coefficient applied to the equivalent soil height. Use the at-rest coefficient for a culvert, since a closed box frame does not deflect enough to mobilise active pressure. Must be positive.


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