> 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/substructure-bc/soil-structure-interaction-bc/culvert-bedding-spring-bc.md).

# Culvert Bedding Spring \[BC]

The Culvert Bedding Spring supports the culvert on the ground beneath it. It attaches a vertical spring to every node of the bottom slab, so the barrel bears on the bedding rather than on point supports, and the bearing pressure redistributes as the frame deflects.

It also carries an optional horizontal friction spring representing the shear the bedding can develop along the base of the culvert.

## General

**Box Culvert:** The culvert barrel these bedding springs support. A selection is required. The springs are generated over the bottom slab of the selected barrel.

**Bedding Modulus of Subgrade Reaction:** Vertical stiffness of the bedding per unit area — the pressure developed per unit of settlement. Take it from the geotechnical report for the prepared bedding rather than the natural ground, since a culvert is normally founded on a compacted granular bed. Must be positive. A very stiff value drives the bearing pressure towards the slab edges and stiffens the frame; a soft value spreads it.

**Spring Behavior \[Compression Only/Linear]:** Whether the bedding can only push up on the culvert or can also hold it down. Compression Only is the physical case and the default — the bedding cannot pull the slab back if part of it lifts. Linear keeps the spring active in both directions and is useful only as a diagnostic, since it can hide an uplift condition by manufacturing a tension reaction that the real bedding could not supply.

**Horizontal Friction Modulus (base shear; 0 = none):** Horizontal stiffness per unit area along the base of the culvert, representing the friction between the bottom slab and the bedding. It restrains the sliding that the lateral earth pressure and surcharge would otherwise produce. Set it to 0 to model a frictionless base, which is the conservative assumption for sliding but pushes more lateral demand into the frame. Must not be negative.


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