Set structural behaviour on one or more existing members: element type, end releases (hinges), rigid end offsets, cross-section rotation/mirroring and cable parameters. Mutates model.members (and model.member_hinges when a release is authored) and returns the model.

member_type: Normal (full 6-DOF beam), Truss (axial only — carries exactly zero moment), Tension (tie that drops out in compression), Compression (strut that drops out in tension), Rigid (near-rigid link; produces NO entry in results.member_results at all) or Cable (analytic elastic catenary; forces the whole solve onto the nonlinear path and only sags when analysis.options.enable_self_weight is true). Tension/Compression drop-out is governed by analysis.options.axial_slack in FORCE units — a member switches off when its axial force passes 0.5*axial_slack on the wrong side — so this tool lowers axial_slack to 1e-3 when it writes one (set keep_axial_slack to skip). With a coarse axial_slack the member never drops out and quietly behaves like a plain Truss. To build a truss use member_type Truss: two double-hinged Normal members meeting at a free node give a singular stiffness matrix.

END RELEASES ARE A TRISTATE and that is the whole point: for each release DOF, null (or simply not naming it) = RIGID, 0.0 = FULLY RELEASED, and a positive number = a spring of that stiffness (force/length for vx/vy/vz, force*length/rad for mx/my/mz/warp). Release DOFs are in the MEMBER LOCAL frame. Verified on an encastre beam under UDL: mz null gives support moment wL^2/12 and midspan wL^2/24; mz 0.0 gives 0 and wL^2/8, i.e. exactly simply supported. Each release object fully defines that end's hinge — DOFs you do not name are written as rigid — and passing "rigid" clears the release. A rotational release is rejected when it would leave a node with NO rotational stiffness at all, because that is a hard "Stiffness matrix is singular or near-singular": either a dead-end node (no other member frames in and the support does not restrain it) or a node where every member meeting there releases the same DOF. Note the asymmetry the check has to respect — an absent RX/RY/RZ on a support is read as FIXED, but an absent warping_condition really is free, so a plain pinned support does NOT hold a warp release. Releasing the same vx, vy, vz or mx at BOTH ends of one member is refused outright: that one is fatal at the element level whatever the model looks like ("apply_end_releases_to_local_beam_k: interface block is singular"), while my/mz/warp at both ends is just an ordinary pin-pin beam.

A release object may also carry rotation_axes: "Global" or {"User":{"x":[...],"y":[...]}} takes mx/my/mz, their curves and their moment-rotation diagrams about those axes instead of the member's own — a beam at an angle in plan, pinned about the axis of the girder it hangs from, needs {"mx":0.0,"rotation_axes":"Global"}. A user frame's first axis runs along x and its second along y made square to it. Translations stay member-local, member results stay in member axes, and the axes stay fixed in space on the corotational path. A model whose hinges release in other axes is written with schema_version 3 (engine 0.2.68), so an older engine refuses it instead of solving the release about the member's axes.

MemberHinge also carries seven capacity fields (max_tension_*, max_moment_*, max_bimoment_warp). They are accepted by the schema and carried through the model but are NEVER ENFORCED by the solver: nothing limits a hinge force by them and they trigger no nonlinearity. Do not author "yielding" hinges with them — this tool does not write them.

Offsets (start_offset/end_offset) are rigid arms between the node and the member work point in the GLOBAL frame, in model length units. rotation_angle_deg rolls the cross-section about the member axis in DEGREES, not radians: on a vertical column FERS builds the local frame from global Z, so sway along global X is resisted by the WEAK axis i_y unless you rotate 90 degrees (verified 13.8x difference on a cantilever column, with no warning from the solver). pretension is valid ONLY on Cable/Tension/Compression (a hard validation error elsewhere) and pins a tie permanently active; unstretched_length only does anything on a Cable and is silently ignored everywhere else. Sign convention for reading results back: local_start_forces.fx is COMPRESSION-POSITIVE.