AASHTOWare BrDR 7.6 with AASHTO LRFD Bridge Design Specifications, 10th Edition – A Deep Dive

High-level summary — AASHTOWare BrDR 7.6 + AASHTO LRFD Bridge Design Specifications (10th Ed.)

Purpose

• Walkthrough of major 10th‑edition specification changes and how they were implemented in AASHTOWare Bridge (BrDR) version 7.6.
• Primary focus areas: concrete deflections (instantaneous and time‑dependent), concrete development length (Ld), steel cross‑frame stability / axial rigidity, and lateral‑torsional buckling (LTB) changes.

Concrete: instantaneous and time‑dependent deflections

Specification changes

• The deflection article was split into:
  • Part A — instantaneous deflections
  • Part B — time‑dependent deflections (previously called “long‑term”)
• New inputs / coefficients:
  • Effective moment of inertia (Ieff) is now used for instantaneous deflection checks (previously only Igross).
  • Lambda_d factor (λd) from ACI 318 is used to scale sustained‑load deflections for time‑dependent effects.

Implementation workflow in BrDR

1. Initial FE analysis using Igross to obtain forces.
2. Compute Ieff (Part A). A second FE analysis using Ieff may be run to compute instantaneous deflections.
3. For prestressed (PS) members: if tensile stresses are low, the second Ieff analysis may be skipped and Igross deflections used.
4. Time‑dependent deflection = λd × instantaneous (sustained) deflection.
5. Total deflection = instantaneous + time‑dependent.

Outputs / UI

• Igross deflections appear in standard tabular/graph results.
• Ieff / instantaneous‑via‑Ieff and time‑dependent deflections appear under engine outputs (special model runs labeled “deflection”).
• Spec checks reference relevant tables/articles (e.g., Table 4.6.6) and show ratios versus limits.

Note: BrDR 7.6 implements LRFD 10 formulas for reinforced concrete (RC). Time‑dependent treatment for prestressed (PS) members is not taken from LRFD10 — PCI multipliers remain in the PS Design Tool.

• Many example/calculation outputs were added to the engine log for traceability.

Concrete: development length (Ld)

Major LRFD10 changes

• New unified basic development length formula covering straight bars, hooked bars, and headed bars (use FH for force from hook/head).
• Changes in how modification factors are applied.
• New bar type: dual‑coated zinc+epoxy; applicability extended beyond bar #11 to #14 and #18.

BrDR implementation

• UI additions:
  • Options to mark bars as epoxy or dual‑coated.
  • Headed bar start/end flags.
• Engine log outputs show development length calculations per the new spec; values can differ (examples showed increases ~6 in).

• Headed‑bar inputs appear in different places depending on element type (e.g., bar mark definitions for RC).

• Rules enforced: a given location cannot be both head and hook.

• Examples and calculation traces are included in the engine output for auditing.

Steel: cross‑frame stability checks and axial rigidity coefficient

New spec content

• New articles introduce:
  • Stability checks for cross frames and diaphragms (article 6.7.4.22).
  • Stiffness reduction coefficient (βt) and bracing strength requirements, with a 2023 update allowing reduced required strength/stiffness when conditions are met.

Conditions for reduced stiffness/strength

To qualify for reduced required stiffness/strength, the following must be met:

• Cross frame depth ≥ 0.8 × girder depth.
• Full‑depth connection plate.
• Plate positively attached to both girder flanges (weld details required).

Actual and required stiffness

• Actual stiffness (βt_actual) = sum of:
  • Bracing stiffness
  • Cross‑section distortion stiffness
  • Girder stiffness
• If the three qualifying conditions are satisfied, the cross‑section distortion term is treated as infinite (i.e., it drops out of the sum).
• Required stiffness/strength is computed using girder moments (βt_req) and an imperfection factor Lb/500H0 to obtain required bracing moment. Commentary provides methods for distributing this to member forces.

BrDR requirements / inputs to run checks

• Select LRFD 10th edition spec and run the LRFD Designer view.
• Use full 3D FE analysis (diaphragm checks are 3D only in BrDR).
• Include girder elements (mark girders as existing) so girder moment data are available.
• Provide transverse stiffener / connection plate geometry and weld details.
• Diaphragm definition includes a “tension only diagonal system” checkbox to control distribution of required bracing moments (this does not change the FE model, only how the required moments are distributed).

Outputs / reports

• Stability bracing force report (engine output) summarizes required vs actual stiffness and strength and shows distribution and calculation detail.
• New spec check folder (“brace system”) appears in spec check output for system‑level articles.

Axial rigidity / coefficient

• End‑connection eccentricities for single angles and flange‑connected T members reduce axial rigidity. BrDR implements this by reducing the effective axial area in the FE model (bending stiffness is not altered).
• Default coefficients (if left blank) follow AASHTO guidance:
  • 0.65 AE for non‑composite
  • 0.75 AE for composite
• Users can override these values via a table in the UI.

Warning: If you previously simulated reduced axial stiffness by manually adjusting member properties, review those models. Either restore member properties and let BrDR apply the coefficient or use the new override table — otherwise reductions may be applied twice.

Steel: lateral‑torsional buckling (LTB) changes (Section 6.10)

Summary of major changes

• New elastic LTB load ratio gamma_E (γE) introduced to better handle non‑prismatic sections and provide more accurate LTB resistance estimates.
• Moment gradient modifier replaced with the AISC CB equation using quarter‑point moments (MA, MB, MC).
• LP (compact flange limit) coefficient changed from 1.0 to 1.1.
• Compression flange stress at yield onset FYR coefficient commonly reduced (from 0.7FYC to 0.5FYC) — this can reduce calculated capacity in some cases.
• LTB resistance is now evaluated at a single governing location (where FBU/(RB RH FYC) is maximum) within an unbraced length, instead of searching the minimum over the length.

Model / FE changes

• Quarter‑point nodes are inserted in the FE model for each unbraced length to compute CB; expect more nodes.
• Quarter‑point moments can be zero in some conditions; CB can be large (up to ~5) near contraflexure points.

Gamma_E (γE) calculation methods (Appendix D6.6)

• Method A: based on AISC Design Guide 25 (modification factor approach) — implemented.
• Method B: weighted average cross‑section interpolation — implemented where geometry is compatible; otherwise the program falls back to Method A.
• Method C: not implemented in BrDR.

Outputs / navigation

• LTB parameters RT, FE, and CB are reported under specific spec articles depending on whether the unbraced length is prismatic or non‑prismatic:
  • 6.10.8.2.3B for prismatic unbraced lengths
  • 6.10.8.2.3C for non‑prismatic unbraced lengths
• Appendix D6.6 output shows γE computation (Method A or B) when applicable.

Practical notes

• LTB capacities may increase or decrease compared to LRFD9 depending on geometry and location.
• BrDR will automatically add nodes and calculate new parameters; review the new LTB outputs when upgrading models.
• A non‑prismatic steel girder design guide (with detailed examples for Methods A–C) is expected in the future.

Product / UI / workflow features highlighted

• LRFD 10th edition compliance option and associated spec checks.
• Two‑stage FE deflection workflow for Ieff computations; engine output entries named to distinguish Igross vs Ieff deflections.
• Inputs to:
  • Mark bars as epoxy or dual‑coated
  • Flag headed bar start/end
• Diaphragm definitions include the tension‑only distribution checkbox.
• Weld/detail inputs for transverse stiffeners (to indicate positive attachment).
• New engine output reports:
  • Instantaneous / time‑dependent deflection logs
  • Development length logs
  • Stability bracing force report
  • LTB parameter outputs
• Table to override axial rigidity coefficients (for handling legacy model adjustments).
• Extensive traceability: detailed calculation traces added to engine outputs for inspection and auditing.

Guides / references / tutorials / follow‑ups

• Engine output text reports contain detailed step‑by‑step calculations for new checks (deflection, development length, stability bracing, LTB).
• Technical Note 86 is referenced regarding axial rigidity coefficients.
• Upcoming: a non‑prismatic steel girder design guide (expected next year) with detailed LTB worked examples for Methods A–C.
• Webinar follow‑up: presenters will share slides and recording; Q&A answers will be included in follow‑up communications.

Implementation / upgrade considerations for users

• Review existing models for manual property overrides (especially where users previously reduced axial stiffness); avoid double application of reductions.
• Expect more FE nodes (quarter points) and additional output reports when switching to LRFD 10 checks.
• For prestressed members, LRFD10’s λd handling may not be applied in BrDR’s PS tools — PS Design Tool still uses PCI factors for time‑dependent deflection.
• Provide proper diaphragm / connection plate geometry and weld details to enable cross‑frame stability checks.
• Confirm bar type and headed/hooked inputs in RC models so development length calculations follow the new spec.

Speakers / sources

• Daniel Antonak — presenter (PhD in structural engineering; Master’s in software engineering; >20 years in structural engineering software; member of the AASHTOWare Bridge team).
• Ben (Benjamin) Flanagan — presenter (steel topics).
• ProMiles / AASHTOWare webinar hosts and support staff (moderation, Q&A, distribution of slides/recording).

Next steps (offer)

If you want, I can:

• Extract the exact menu/navigation steps in BrDR for each new input (where to find “deflection” engine runs, diaphragm settings, headed‑bar flags, axial‑rigidity override table).
• Produce a checklist for upgrading existing models to LRFD 10 (what to review and change).