> 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/truss-load-rating-lfd-tb.md). # Truss Load Rating (LFD) \[TB] Performs an AASHTO Load Factor Rating (LFR) for an individual truss member, using LFD capacities from the AASHTO Standard Specifications and rating per the Manual for Bridge Evaluation Part 6B. A truss member is a pure axial element, so it is rated for both axial tension (gross-section yield per Article 10.18 and net-section fracture) and axial compression (column buckling per Article 10.54.1.1). The object pulls its load factors, effective length factor, and live-load stages from a Truss 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 **Truss Member:** Pick the truss member to rate. The member's section properties and length are used for the capacity calculations (including the effective length for buckling), and its element name is used to extract the axial force envelope from the staged model. The user must provide a value. **Load Rating Template (LFD):** Pick the Truss Load Rating (LFD) template that holds the generic inputs — the LFR load factors (A1 dead, A2 inventory/operating), the effective length factor K, the Rp hole-drilling reduction factor, and the Design, Legal, Emergency, and Permit live-load stages. All global rating settings come from this template. **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 **Override Loaded 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 truss member's own length, computed from the model. Set this to *Yes* to enter L manually. **Loaded Length L:** The loaded length used in the impact formula when the impact-length override is *Yes*. Available when Override Loaded Length for Impact Factor is *Yes*. **Computed Loaded Length L (readonly):** The truss member length automatically determined from the model geometry, used for the impact factor when the override is *No*. (Read-only) ## Deterioration **Section Loss - Percent of Gross Area (%):** Section loss (deterioration) taken from the bridge inspection report, entered as a percent of the gross cross-sectional area. It is section-independent, so it works for any member shape. The deteriorated gross area is Ag,loss = Ag,gross × (1 − % / 100), which then feeds both the tension and compression capacity checks, conservatively reducing them. Leave it at 0 for a pristine section. Values at or above 100% are rejected (no effective section remains). ## Net Section Bolt-hole deductions for the net section of a tension member per AASHTO Standard Specifications Article 10.16.14. Leave all at 0 to take the net area equal to the (deteriorated) gross area. **Bolt Holes at Critical Section (count):** Number of bolt holes removed at the critical section. The total hole area removed is (count) × (hole diameter) × (material thickness). **Bolt Hole Diameter (bolt dia + 1/8 in):** Diameter of each bolt hole used for the net-area deduction — typically the bolt diameter plus 1/8 in. Enter a positive length. **Material Thickness at Holes (0 = no deduction):** Material thickness at the bolt holes, used to compute the removed hole area. Leave it at 0 to skip the net-section deduction. The bolt-hole deduction may not exceed the available gross area (that would leave a zero or negative net section). --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.openbrim.org/templates/truss-bridge-workflow/load-rating-tb/truss-load-rating-lfd-tb.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.