> 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/truss-bridge-workflow/load-rating-tb/girder-load-rating-tb.md).

# Girder Load Rating (LFD) \[TB]

Performs an AASHTO Load Factor Rating (LFR) for an individual girder of the truss bridge, using LFD capacities from the AASHTO Standard Specifications (flexure per Article 10.48, shear per Article 10.48.8) and rating per the Manual for Bridge Evaluation Part 6B. Positive- and negative-moment capacities are evaluated separately for a continuous girder, and the composite deck section is used for the positive-moment section modulus. The rating reads the cross section as the LRFR rating does, from the same sources and in the same order, and refuses the same sections. An I-section's plates are read off its outline where the outline shows an I, else taken from its stored plate dimensions, including a rolled shape's flange width and thickness, and the area, inertia and extreme fibers are computed from the plates. A non-I section, such as a box, tube, channel, tee, angle or round, takes its own properties; a box or channel that cannot (its shape cannot be read from the outline, or its section varies at the rating point and cannot be built there) is rated as an I-section with its webs combined, and the report says so. A section without a full set of plate dimensions, or a non-I section whose shape cannot be read, gives a validation issue, as in the LRFR rating. The lateral-torsional buckling reduction uses the same plates, so it also applies to a rolled I-section. The shear capacity reduces the plastic web shear for web buckling from the web depth and thickness: those of a section that carries its plate dimensions (an I-section, or both webs of a library built-up box or HSS), or both webs of a box or tube read off the section's outline, with the web depth of an HSS reduced for its corners as the LRFR rating does; for any other section the reduction cannot be evaluated, full plastic shear is used and the report says so. The object pulls its load factors and live-load stages from a Girder Load Rating (LFD) template and reports Inventory and Operating ratings for Design (HS-20), Legal, Emergency, and Permit live loads defined on the Load Rating Combination Table (LFD).

## General

**Girder:** Pick the girder to rate. The girder's section properties and length are used for the capacity calculations, and its element name is used to extract the analysis results (moment and shear envelopes) from the staged model. The user must provide a value.

A Frame Member can be picked here too, for example a stringer, a longitudinal girder or a cross girder modelled as a frame member. Its spans run between its ends and the insertion points that carry a bearing (with no bearing, the whole member is one span; at an interior insertion point whose offsets move the member along it, a rating point on the short stretch between its two sides is moved to the nearer side); the rating point is given with Location Input Method = *Relative on Span*, and the forces are read from the member's result line at that point. A frame member whose parts (between consecutive insertion points) are 90° or more apart in direction, or that mixes vertical and non-vertical parts, cannot be rated. The rated section is the member's section at the rating point, as for a girder (see **Override Section**); where two Section Data rows of a different shape cannot be interpolated, the shallower of the two is rated. The member is rated as a bare (non-composite) section. Its own length is the span for the impact factor, and the rated span is the unbraced length; both can be overridden below. For a member continuous over several supports, such as a stringer over the floor beams, enter the rated span with **Override Span Length for Impact Factor**, since the whole member length gives a smaller impact factor.

**Load Rating Template:** Pick the Girder Load Rating (LFD) template that holds the generic inputs — the LFR load factors (A1 dead, A2 inventory/operating) and the Design, Legal, Emergency, and Permit live-load stages. All global rating settings come from this template.

**Location Input Method \[Station/Relative on Span]:** How the rating point is given. *Station* (the default) takes a station along the bridge. *Relative on Span* takes a span and a relative position on it. A Frame Member takes *Relative on Span* only, since its spans can run in any direction; with *Station* it gives a validation issue.

**Station:** The station along the girder where the rating is performed, shown when Location Input Method is *Station*. Positive- and negative-moment demands are read at this station, so set it to the critical location for the check (typically midspan for positive moment or over a support for negative moment).

**Span Number:** The span the rating point is on, shown when Location Input Method is *Relative on Span*: the spans between the girder's supports, or between the Frame Member's ends and bearings, counted from the first one.

**Relative Position along Span (0-1):** Where the rating point is on that span, from 0 (its start) to 1 (its end); 0.5 is midspan. Shown when Location Input Method is *Relative on Span*.

**Resolved Station (readonly):** The station of the rating point, from the Station input or from the span and relative position. (Read-only)

**Check For \[All/Flexure Only/Shear Only]:** Selects which limit states are rated. *All* rates both flexure and shear and reports the smaller (governing) rating factor. *Flexure Only* and *Shear Only* restrict the rating to a single effect and hide the other's cells and report section.

**PDF Name (Export):** Name used when the load-rating report is saved as a PDF. When renaming, avoid characters that interfere with parametric equations (see the [Illegal Characters](https://docs.openbrim.org/faq/general/faq-illegal-chars) page).

## Override

**Composite Action \[Non-composite/Composite]:** Whether the girder acts compositely with its deck. *Composite* (the default) rates the positive-moment capacity on the transformed section of steel and deck (Std Spec 10.50, elastic: F<sub>y</sub> times the composite bottom-fiber section modulus, with the deck's modular ratio from its concrete modulus); the negative-moment capacity stays on the bare steel section, since the deck is cracked there. *Non-composite* rates both on the bare steel section even when the girder carries a deck, for example a stringer whose deck has no shear connectors. A Frame Member has no deck, so it is rated as non-composite whichever is set.

**Override Section \[Yes/No]:** By default the rating uses the girder's own cross section at the rating point: the section-data row at or before the point and, where the section varies between two rows, the section at the point, interpolated between the two as the analysis does on a linear stretch. On a parabolic or circular stretch, or where the two rows cannot be interpolated because they differ in shape or in their number of shapes or points, an I-section's plates are interpolated between the two rows along their variation, as the LRFR rating does, and the report says so; a box or channel there is rated as an I-section with its webs combined, and any other non-I section gives a validation issue. Set this to *Yes* to rate a user-picked section instead.

**Cross Section:** The section used for the capacity calculation when Override Section is *Yes*. The user must provide a value when the override is active.

**Override Span Length for Impact Factor \[No/Yes]:** The impact (dynamic load allowance) factor I = 50/(L+125) ≤ 0.30 depends on the loaded length L. By default L is the girder's own span, computed from the model. Set this to *Yes* to enter L manually.

**Span Length L:** The loaded length used in the impact formula when the impact-span override is *Yes*. Available when Override Span Length for Impact Factor is *Yes*.

**Computed Span Length L (readonly):** The span length automatically determined from the girder geometry, used for the impact factor when the override is *No*. (Read-only)

**Override Unbraced Length for LTB \[No/Yes]:** The lateral-torsional buckling (LTB) check (AASHTO Std Spec 10.48.4.1) needs the unbraced length L\_b of the compression flange (the cross-frame spacing). By default L\_b is computed from the girder's bracing. Set this to *Yes* to enter L\_b manually.

**Unbraced Length L\_b:** The compression-flange unbraced length used for the LTB check when the unbraced-length override is *Yes*. Available when Override Unbraced Length for LTB is *Yes*.

**Computed Unbraced Length L\_b (readonly):** The unbraced length automatically determined from the girder's cross-frame spacing, used for the LTB check when the override is *No*. (Read-only)


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