> 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/design-lanes-fea.md). # Design Lanes \[FEA] A Design Lane defines a traffic lane for live load analysis: its width, which vehicles travel in it, the accompanying lane load, and the rules governing where vehicles may be placed within it. Given the roadway dimensions, OpenBrIM manages lane placement in the transverse direction for influence-surface live load analysis, exploring the alternatives — shifting lanes across the roadway and evaluating enormous numbers of configurations — to find the arrangement that maximizes each response. Design Lanes **generate load**, indirectly, through the influence-surface search. They add nothing to stiffness or mass. ## Properties **Lane Width:** The width of the design lane. *Engineering influence.* Lane width, together with the roadway width from the loading surface, determines **how many lanes fit** — and the number of loaded lanes, combined with the multiple presence factors on the Live Load Case, is often what governs the design. A lane narrower than the code value allows more lanes onto the deck than the code intends; a wider one allows fewer. The lane width is a code quantity (commonly 12 ft / 3.6 m), not the physical marked lane. **Margin btw. Vehicle & Lane Edge:** The clearance kept between the vehicle's outer wheel and the lane edge. *Engineering influence.* This limits how far a vehicle may shift transversely within its lane, and therefore how close a wheel line can get to an exterior girder or a deck edge. Because exterior girder distribution is often governed by the extreme wheel position, increasing the margin **reduces** the worst-case exterior girder demand, and decreasing it increases it. The value is prescribed by code (commonly 2 ft / 600 mm from the lane edge); choosing it arbitrarily changes the exterior girder result directly. ## Vehicles **Vehicle 1–4:** Up to four vehicles that may occupy this lane. *Engineering influence.* These are **alternatives**, and the analysis takes the governing one for each response — a design truck and a design tandem are both listed so that whichever produces the larger effect at each location is used. Listing only one omits the possibility that the other governs, which in short spans is frequently the tandem and in long spans the truck. **Vehicle 1–4 Impact Factor:** The dynamic load allowance applied to each vehicle. *Engineering influence.* Amplifies the vehicle's static effect to account for dynamic interaction with the deck. Codes typically apply it to the truck or tandem but **not** to the lane load, and often use a reduced value for fatigue and none for some limit states. Applying impact to the lane load, or applying it here as well as in the vehicle's own factor, over-counts it. A factor of 1 means no dynamic allowance. **Vehicle 1–4 Left Wheel Factor:** A multiplier on the left wheel line of each vehicle. *Engineering influence.* Allows an unbalanced axle — heavier on one side — which is used to maximize transverse effects such as exterior girder demand or deck overhang moment. A value of 1 keeps the axle symmetric. **Vehicle 1–4 Direction:** The travel direction of each vehicle. *Engineering influence.* Matters for vehicles whose axle configuration is not symmetric front-to-back: reversing such a vehicle changes which axle group leads and therefore the critical position and the resulting effects. For a symmetric vehicle it makes no difference. ## Lane Load **Lane Load Magnitude:** The uniformly distributed lane load, in force per unit length. *Engineering influence.* The lane load represents the general traffic stream and is applied **in addition to** the vehicle, not instead of it. Its relative importance grows with span: for short spans the vehicle dominates, while for long spans and for negative moment over interior supports the lane load contributes a large share. It is also applied to the influence surface in whatever pattern maximizes each response, which is why it can be discontinuous — loaded in some spans and not others. Impact is **not** normally applied to the lane load. **Lane Load Width:** The transverse width the lane load is spread over. *Engineering influence.* The lane load is distributed across this width rather than applied as a knife edge, which affects the transverse distribution between girders. A width narrower than the code value concentrates the load and increases the demand on the girder beneath it. ## Vehicle Placement Settings **Min. Vehicle Spacing (Back to Front) / Max. Vehicle Spacing:** The range of longitudinal spacing allowed between successive vehicles in the lane. *Engineering influence.* This governs the multiple-truck configurations used for effects that require more than one vehicle — most importantly **negative moment over interior supports**, where codes specify two trucks at a defined minimum headway. Setting the minimum too large prevents the critical two-truck arrangement from being found, under-predicting negative moment; the range should reflect the code requirement. **Adjacent Span Placement:** Whether vehicles may be placed in adjacent spans simultaneously. *Engineering influence.* Loading adjacent spans is what maximizes negative moment and interior support reactions, while loading a single span maximizes positive moment in it. Disabling this prevents the adjacent-span pattern from being considered and **understates** negative moment over the supports — one of the more consequential simplifications available on this object. ## Lane Placement Settings **Apply Lane Constraint:** Whether the lane's transverse position is restricted rather than free to shift across the roadway. **Left Transverse Offset / Right Transverse Offset:** The bounds of the permitted transverse position when the constraint is applied. *Engineering influence.* By default lanes shift freely across the roadway and the analysis finds the governing transverse position. Constraining them fixes lanes at particular positions — appropriate when the actual striping is fixed and will not change, or when checking a specific known configuration. Constraining unnecessarily **prevents the search from finding the critical arrangement**, which typically understates exterior girder demand. Leaving the constraint off is the conservative and usual choice. **Allow Multi Lane Placement:** Whether more than one lane may be loaded simultaneously. *Engineering influence.* Multiple loaded lanes maximize total load and interior girder demand; single-lane loading with the higher multiple presence factor often governs exterior girders. Disabling multi-lane placement removes the multi-lane cases from the search entirely and understates the demand on interior members and substructure. ## Verification * Confirm the number of lanes the analysis actually placed matches the roadway width divided by the lane width, and matches the code's requirement. * Check that the governing transverse lane position for an exterior girder puts a wheel line near the edge, subject to the margin — if lanes are not shifting, a lane constraint is probably active. * Confirm impact is applied to the vehicle and not to the lane load, and only once. * Compare a simple span's maximum live-load moment against a published table or hand calculation. * Check negative moment over an interior support with adjacent-span placement enabled and disabled; the difference confirms the setting is working and shows how much it matters. * Confirm both truck and tandem alternatives are being considered by checking which governs at different locations. ## Common mistakes * **Listing only one vehicle**, missing the alternative that governs — commonly the tandem on short spans. * **Applying impact twice**, in both the vehicle Factor and the lane's impact factor. * **Applying impact to the lane load**, which codes generally do not. * **Disabling adjacent span placement**, understating negative moment over interior supports. * **Constraining lane positions unnecessarily**, preventing the search from finding the critical transverse arrangement and understating exterior girder demand. * **Disabling multi-lane placement**, removing the cases that govern interior girders and substructure. * **A lane width that is not the code lane width**, changing how many lanes fit on the deck. * **A minimum vehicle spacing too large** to allow the code's two-truck negative-moment configuration. --- # Agent Instructions This documentation is published with GitBook. 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