> 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/openbrimfea/loads-fea/live-loads-fea/vehicles-fea.md). # Vehicles \[FEA] A Vehicle defines an axle configuration — the axle spacings, wheel spacings and wheel loads that make up a design truck, a tandem, a fatigue vehicle or a permit load. It is a **definition**, not a load: a vehicle applies nothing until a Design Lane references it and a Live Load Case runs. The axle and wheel geometry is held in the vehicle's sub-objects, edited through its own sheet. Vehicles ultimately **generate load**, indirectly, through the influence-surface machinery. They add nothing to stiffness or mass. ## Data **Factor:** A multiplier applied to the vehicle's wheel loads. *Engineering influence.* Scales the whole vehicle linearly, so response scales with it in a linear analysis. It is the natural place to express a load factor that belongs to the vehicle itself rather than to the lane or the combination — a permit vehicle at a proportion of its full weight, or a fatigue truck scaled to a code-specified fraction. Take care not to double-count: impact (dynamic allowance) is applied on the **Design Lane** as an impact factor, and load combination factors are applied in the combination. A factor applied here as well as there multiplies them together. A value of 1 leaves the vehicle at its defined weight. **Gross Vehicle Weight (readonly):** The total weight of all wheel loads, after the factor. *Engineering influence.* This is **output** and it is the most useful verification field on the object. Compare it against the published gross weight for the design vehicle you intended — a mismatch means a wheel load, an axle, or the factor is wrong, and it catches the error before it propagates into every live-load result. {% hint style="warning" %} This column is derived and cannot be edited, but it is not flagged read-only, so typing into it appears to work and the value is discarded on redraw. {% endhint %} ### Axle and wheel definition The axle configuration itself — the number of axles, their longitudinal spacing, the transverse wheel spacing, and the load on each wheel — is defined in the vehicle's sub-objects. *Engineering influence.* **Axle spacing** governs which spans and which influence-surface peaks the vehicle can load simultaneously: a closely-spaced tandem concentrates load and maximizes local deck effects and short-span moments, while a long truck spreads load and can straddle two spans to maximize negative moment over an interior support. Variable-spacing axles exist in design vehicles precisely so the critical spacing can be searched. **Wheel spacing** transversely determines the distribution between girders — a wheel line directly over a girder produces a very different girder moment than one between girders — which is the whole reason for running an influence-surface analysis rather than a simple distribution factor. **Wheel loads** scale the response directly, and their distribution between axles matters as much as the total: two vehicles of the same gross weight but different axle distributions produce different local effects. ## Verification * Check **Gross Vehicle Weight** against the published value for the design vehicle. This single check catches most definition errors. * Display the vehicle and confirm the axle and wheel layout matches the code figure, including spacings. * Confirm the number of axles is right — a missing axle is easy to overlook and shows up in the gross weight. * Run a simple single-span case and compare the maximum moment against a hand calculation for the vehicle at its critical position. * Confirm impact is applied once, on the lane, and not duplicated in the vehicle factor. * Compare results against a published live-load moment table for a standard span where one exists. ## Common mistakes * **Double-counting impact** by applying it both in the vehicle Factor and in the lane's impact factor. * **Gross weight not matching the design vehicle**, from a wrong wheel load or a missing axle. * **Entering axle loads where wheel loads are expected**, doubling the vehicle weight — the gross weight check catches this immediately. * **Wrong wheel spacing**, misrepresenting transverse distribution between girders. * **Fixing a variable axle spacing** at one value and losing the critical-spacing search. * **Assuming the vehicle is applied** — it does nothing until a Design Lane references it and a Live Load Case runs. --- # Agent Instructions This documentation is published with GitBook. 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