LS-DYNA Modeling of T-Headed Bar Reinforcement

LS-DYNA Finite Element Modeling — T-Headed Bar as Shear and Hook Reinforcement in RC Beams

I am looking for an experienced LS-DYNA modeler to assist me in developing nonlinear 3D finite element (FE) models of reinforced concrete (RC) beams and/or connection specimens incorporating T-headed bars as replacements for conventional hook reinforcement and shear stirrups. T-headed bars (headed reinforcement) are straight reinforcing bars with an enlarged anchor plate welded at one or both ends.

The scope of this thesis is strictly limited to anchorage (hook) strength and shear strength of beams — no column behavior, slab behavior, or seismic cyclic loading performance is required.

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PRIMARY REFERENCE PAPER

The modeling methodology must strictly follow — or be directly comparable to — the approach used in the following published paper, which I will provide in full:

Ousalem, H. & Takatsu, H. (2023)
"FE Analysis of Pulled-Out Eccentrically Spliced Longitudinal Headed Bars for Precast Beam-Footing Connections"
Journal of Asian Architecture and Building Engineering, Vol. 22, No. 5, pp. 2660–2674
DOI: 10.1080/13467581.2022.2160640

This paper uses LS-DYNA R11.1.0 and serves as the primary benchmark for modeling methodology, output requirements, and validation approach. The freelancer must be familiar with this paper or equivalent LS-DYNA nonlinear RC modeling work.

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SCOPE OF WORK — MODELING REQUIREMENTS

Following the methodology of Ousalem & Takatsu (2023), the following FE model specifications are required:

1. SOFTWARE
- LS-DYNA (version R11.1.0 or equivalent recent version)
- Please confirm the version you use

2. MODEL GEOMETRY & SYMMETRY
- 3D quarter-model exploiting symmetry (as done in the reference paper)
- I will provide beam geometry (cross section)
- The model must represent the anchorage zone and/or beam specimen with headed bar as longitudinal and transverse reinforcement
3. ELEMENT TYPES
- Concrete body and head anchors (anchor plates): 8-node brick solid elements (3D solid)
- Mesh density at anchor head locations must be relatively high (fine mesh at anchor head zone)
- Aspect ratio of solid elements: between 1.0 and 2.5 for the investigation zone; between 1.1 and 4.9 for the fixed/support zone — consistent with reference paper
- Longitudinal and transverse steel reinforcing bars: explicitly modeled as beam elements incorporated (embedded) into the concrete mesh
- The circular shape of the anchor head may be simplified to a square of equivalent bearing area for meshing convenience, consistent with the reference paper

4. MATERIAL MODELS
a) Concrete:
- Material model: MAT_CSCM (Continuous Surface Cap Model / MAT_159)
- Input parameters: unconfined compressive strength (f'c) and maximum aggregate size — default parameters to be generated automatically by LS-DYNA
- Young's modulus and tensile strength derived from CEB-FIB Model Code 2010 equations
- The model must capture both compressive crushing and tensile cracking behavior of concrete

b) Steel reinforcing bars and head anchors:
- Material model: MAT_PLASTIC_KINEMATIC (MAT_003)
- Input: Young's modulus (E), yield strength (fy), and tangent modulus
- Material data to be taken from the reference experimental data (I will provide)
- For reference: in Ousalem & Takatsu (2023), material properties were D6 (SD295): fy = 357.2 MPa; D16 (SD685): fy = 738.9 MPa; D16 (SD390): fy = 442.0 MPa; Concrete: f'c = 40.9 MPa, E = 28,000 MPa

5. BOND AND INTERFACE MODELING (CRITICAL)
a) Steel bar — concrete interface (bond along bar length):
- Use CONSTRAINED_BEAM_IN_SOLID (CBIS) keyword
- Bond stress–slip relationship based on CEB-FIB Model Code 2010
- Parameters: pull-out case ("All other bond conditions") for deformed bars

b) Head anchor — concrete interface (bearing mechanism at anchor plate):
- Use TIE-BREAK SURFACE CONTACT elements
- Surfaces initially in contact are tied; tangential motion inhibited until interface failure
- Failure criterion: combined normal and shear stress criterion — (σ/σt)² + (τ/τ₀)² = 1
- Input parameters: friction coefficient μ = 0.5; normal failure stress σt = 0.1 MPa; shear failure stress τ₀ = 0.1 MPa (consistent with reference paper, based on Rabbat & Russell 1985)

6. BOUNDARY CONDITIONS & LOADING
- Concrete stub/support: fully restrained against all vertical and horizontal displacements and all rotations
- Symmetry planes: normal displacements and rotations appropriately constrained
- Loading: displacement-controlled monotonic loading applied at the tips of the beam longitudinal steel bars in the upward direction; all other movements at bar tips restrained
- Loading rate: slow enough to avoid dynamic amplification and ensure quasi-static response

7. COMPARATIVE STUDY REQUIREMENT
The model must include at least two configurations for comparison:
(a) Specimen/beam with conventional hooked bars and stirrups (standard detailing)
(b) Specimen/beam with T-headed bars replacing hooks and/or shear stirrups
Additional configurations with different transverse reinforcement detailing are welcome if feasible (e.g., with and without bent shear reinforcement, with and without alternation of headed bars)

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REQUIRED OUTPUT — MINIMUM STANDARD

The minimum required outputs must match or exceed those reported in Ousalem & Takatsu (2023). Specifically:

OUTPUT 1 — Load vs. Displacement Curves
- Tensile load (kN) vs. displacement (mm) at the concrete upper surface for each model configuration
- Curves must be plotted and compared directly against experimental reference data
- Key points to be identified on curves: first crack load (Pcr), yield load, and maximum strength (Pmax)

OUTPUT 2 — Quantitative Comparison Table
For each model/specimen, the following values must be extracted and tabulated:
- Initial stiffness K (kN/mm) — secant stiffness at first crack
- Strength at initial crack Pcr (kN)
- Maximum strength Pmax (kN)
- Ratios: KTest/KAnalysis, Pcr,Test/Pcr,Analysis, Pmax,Test/Pmax,Analysis
(Consistent with Table 3 of the reference paper)

OUTPUT 3 — Crack Pattern & Maximum Principal Strain Distribution
- Contour plots of maximum principal strain distribution at three key load stages:
(i) At first loading cycle peak (P = F1)
(ii) At second loading cycle peak (P = F2)
(iii) At maximum strength (P = Pmax)
- These contour plots must be compared qualitatively against crack patterns observed in experiments
- Failure mode must be clearly identified (e.g., pull-out cone failure, shear failure, or ductile yielding of longitudinal bars)

OUTPUT 4 — Steel Bar Strain vs. Displacement Relationships
For each model, axial strain evolution of key reinforcement must be plotted against displacement:
- Hoop/transverse reinforcement strain (at the location closest to anchor heads)
- Beam longitudinal bar strain (at a location far from anchor heads)
- Footing/support longitudinal bar strain (at a location far from anchor heads)
(Consistent with Figure 11 of the reference paper)

OUTPUT 5 — Stress Distribution Contours
- Stress contour plots in concrete and steel reinforcement at peak load
- Contour plots showing the compression strut development between anchor heads (if applicable)
- Von Mises stress distribution in anchor plates at peak load

OUTPUT 6 — Failure Mode Identification
Clearly document the numerical failure mode for each model:
- Pull-out concrete cone failure: cone initiation from beam anchor heads toward footing anchor heads
- Shear failure: triggered when transverse reinforcement near anchor heads reaches yield
- Ductile failure: yielding of beam longitudinal bars without transverse reinforcement yielding
(Consistent with Section 3.4 of reference paper)

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DELIVERABLES

- LS-DYNA input files (.k or .key) for all models
- All post-processing output files (d3plot, binout, etc.)
- Load vs. displacement plots for all configurations (compared with experimental data)
- Crack pattern / maximum principal strain contour plots at the three key load stages
- Steel bar strain vs. displacement plots for all key reinforcement locations
- Stress distribution contour plots at peak load
- Quantitative comparison table (K, Pcr, Pmax and their ratios vs. experimental values)
- Brief written report (1–3 pages) explaining: modeling decisions, material parameters used, validation results, and identified failure modes
- Mesh convergence study (at least 3 mesh sizes tested to confirm result independence)