3D FE analysis
The 3D FE model builds the whole connection with finite elements: the plates and member walls as nonlinear shells, and the welds, bolts, anchors, contact and concrete with their own stiffness and strength. The plates are judged by plastic strain; welds, bolts, anchors and concrete by the AISC 360 and ACI 318 checks with the forces the model finds. The analytical checks stay on next to it.
The model#
| Part | How it is modeled |
|---|---|
| Plates and member walls | 4-node shells: membrane with Allman drilling rotation, MITC4 bending, 5 integration layers through the thickness (Simpson; Gauss–Lobatto as an option) |
| Steel | Bilinear von Mises, design yield φ·Fy (φ = 0.9, LRFD), hardening E/1000, exact return mapping with consistent tangent |
| Members | Segments of 2 depths from the joint, loaded at their free end; the far member end is supported |
| Welds | Throat elements joined by constraints, with directional strength; double fillets with one element per face |
| Bolts | Distributed bolt: one strand from each plate node under the washer straight through every ply, each with 1/n of the bolt stiffness and strength (shank shear as a short fixed–fixed beam; tension-only, bilinear to φRnt; shear elastoplastic at φRnv). Bearing comes from the plate mesh and is checked per bolt (J3.10) |
| Holes | Bolt holes meshed with 1–3 rings (16 elements around in plates, 8 in member walls), standard hole size J3.3 |
| Contact | Plate-to-plate, compression only, by penalty |
| Concrete | Compression-only support with elastic subsoil stiffness; anchors as tension-only springs |
| Solution | Incremental Newton–Raphson (modified Newton by default, full Newton as an option) with automatic step cutting |
The model stops before 100% of the load if the plastic strain passes 15% (a plastic mechanism) or the solution cannot converge; then the Nonlinear analysis check fails and the connection does not pass.
Checks from the model#
| Component | Check |
|---|---|
| Plates | Equivalent plastic strain εp ≤ 5% (reported as a percentage of the limit) |
| Welds | Most stressed element of each weld against φRn (J2.4), plastic redistribution allowed up to the limit |
| Bolts | Each bolt in the direction of its own force: shear, tension with prying, J3.7 interaction, bearing and tearout (J3.10) |
| Anchors | ACI 318 Ch. 17 by groups with the model forces |
| Concrete | Bearing (J8) on the effective contact area |
| Buckling | Linear buckling factor αcr against the AISC-based limits |
Rupture of net sections and block shear are not seen by a von Mises shell model; they stay as analytical checks next to the 3D FE ones.
Calculation settings#
File → Calculation settings… (applies to the whole project; only the values you change are stored; export and import as .json):
| Setting | Default |
|---|---|
| Plastic strain limit | 5% |
| Stop the analysis if εp exceeds | 15% |
| Bearing area cutoff (concrete) | 0.1 × peak |
| Steel resistance factor φ | 0.90 |
| Hardening | E/1000 |
| Elements per section depth / on plates | by mesh density |
| Minimum / maximum element size | mm |
| Member segment length | 2 depths |
| Bolt holes | With rings (recommended) or without hole (≈ 30–45% faster) |
| Integration through the thickness | Simpson, 5 layers |
| Initial load increments, tolerance, max. iterations | 10⁻⁵ force tolerance |
| Tangent | Modified Newton (recommended) or full Newton |
Buckling#
Linear buckling (αcr) in the Check toolbar computes 6 modes on the governing case. Below the limit, check that plate's buckling by the code or increase its thickness.
Stiffness and design resistance#
With the 3D FE engine, Stiffness computes the M–φ curve including the column flexibility and classifies the joint (AISC 360 Commentary B3.4b), and Capacity finds the load factor at which the first component reaches its limit.
Equilibrium#
Members and equilibrium lists the model type of each member, the net force at each end and the global sum, which must be zero. It is a quick way to confirm that the loads you entered are in equilibrium the way you expect: what does not balance is taken by the support member.
Verification#
See Verification.