FERS — structural FEM

FERS: structural analysis in Python

FERS: structural finite-element analysis (3D frames, trusses, plates, Eurocode 3 checks) for engineers and AI agents. This is its Python SDK and the fers command. The solver is a compiled engine that runs on your machine, with no account and no network call: free for non-commercial use up to 100 members.

pip install FERS

from fers_core import check_beam

# IPE200 in S235, simply supported over 6 m, 10 kN/m. SI units: m, N, Pa.
# check_beam adds an EN 1993-1-1 member check on a ULS combination of 1.35 x the load.
model = check_beam(6.0, "IPE200", udl=10_000.0)
model.settings.analysis_options.include_member_deflected_shape = True
model.run_analysis()

beam = model.resultsbundle.loadcases["Q"].member_results["1"]
x_frac, (_, dy, _) = beam.member_displacement_peak
ec3 = model.unity_check_results()[0]
print(f"M {beam.local_minimums.mz / 1e3:.1f} kNm, sag {dy * 1e3:.1f} mm at x/L {x_frac:.2f}")
print(f"EC3 utilization {ec3['max_utilization']:.2f}")

M -45.0 kNm, sag -41.8 mm at x/L 0.50
EC3 utilization 2.85

The sag is 41.3 mm of bending (5wL⁴/384EI) plus 0.4 mm of shear deformation, which the engine includes. The beam fails: the ULS moment of 60.75 kNm exceeds its bending resistance of 51.9 kNm, and with nothing restraining its compression flange, lateral-torsional buckling takes the utilization to 2.85.

• Agents and scripts: fers solve, fers check and fers validate take a model JSON file, print one line (or --json) and say in their exit code whether the numbers can be used; see From the terminal and from coding agents. fers skill --install . gives a coding agent the FERS skill, and MCP clients can use the FERS MCP server. In Claude Code, both come as one plugin: /plugin install fers --marketplace Jeroen124/FERS_core.
• Extras: plots (pip install "FERS[plot]"), section properties meshed from your own dimensions ("FERS[sections]") and DXF import ("FERS[dxf]"); "FERS[all]" installs everything 0.1.x did. Catalogue sections (IPE200, HEB300, UB 457x191x67, W14X30, ...) need no extra.
• Licence: the SDK is BSD-3-Clause; the engine it installs is free for non-commercial use (Licence).

Solving a model

Building a model by hand, create the FERS object first, then its nodes, members and loads. Creating a FERS restarts the id counters, so a node made before it would share an id with one made after it, and the solver refuses node ids that do not run from 1 without gaps.

Reading results

run_analysis() puts the results on model.resultsbundle. Load cases and load combinations are keyed by name. Within each, nodes and members are keyed by their id as a string, because results round-trip through JSON.

case = model.resultsbundle.loadcases["Load"]    # or .loadcombinations["ULS 1"]

case.displacement_nodes["5"].dy                  # dx dy dz rx ry rz warp
case.reaction_nodes["1"].nodal_forces.fy         # fx fy fz mx my mz bw
beam = case.member_results["4"]
beam.local_maximums.mz, beam.local_minimums.mz   # envelope along the member
beam.local_start_forces.fy                       # end forces, member axes

Since 0.1.98, member_results and displacement_nodes are read-only tables rather than dicts of objects. That is what keeps a large result small in memory. They read like dicts: [], get, in, len, .items(), and iteration in the solver's order. Each lookup builds a fresh object, and nothing in it can be changed:

beam.local_maximums.mz = 0.0                     # AttributeError: read-only
editable = case.member_results.copy()            # a plain dict of editable objects
editable["4"].local_maximums.mz = 0.0            # fine

section_forces, internal_force_series and member_displacements come back as tuples. To test for a table, use isinstance(x, collections.abc.Mapping), not dict. reaction_nodes and plate_results are plain dicts, as before.

Saving and loading

from fers_core import FERS

model.save_to_json("beam.json")                  # the model and its results
model = FERS.from_json("beam.json")

FERS.from_json reads any file the solver or the SDK wrote, one entry at a time. A typical result needs less memory than its file to load, and at most about twice the file's size; before 0.1.98 it needed 15 to 20 times. For a large model, let the solver write the file itself. The result then never passes through Python as one string:

model.run_analysis_to_file("beam_results.json")
solved = FERS.from_json("beam_results.json")

fers_core/examples/809_Reading_Results.py runs all of this end to end.

Saving a loaded model writes back everything the solver reads, with the same ids. That includes what the file defines but nothing uses yet, such as a section an editor keeps in its library. Nodes, members and the rest created after loading get ids the loaded model does not use, so saving never lets a new object replace a loaded one.

Only the results you read (engine 0.2.68)

AnalysisOptions(result_filter=[...]) keeps or drops whole result blocks. result_requests goes further. Each ResultRequest names a block and narrows it to chosen members or nodes, load combinations, components and one member end:

from fers_core import ResultBlock, ResultRequest
from fers_core.builders import check_beam
from fers_core.result_requests import nodes_of
from fers_core.unity_checks import classification

model = check_beam(5.0, "IPE300", udl=10000.0)    # one "Beam", one ULS combination
beams = classification("Beam")
model.settings.analysis_options.result_requests = [
    ResultRequest(ResultBlock.LOCAL_ENVELOPES, members=[beams],
                  limit_state="ULS", components=["fx", "my", "mz"]),
    ResultRequest(ResultBlock.NODE_DISPLACEMENTS, nodes=nodes_of(beams),
                  limit_state="ULS", components=["dy"]),
    ResultRequest(ResultBlock.REACTIONS, limit_state="ULS"),
]
model.run_analysis()

The values load into the same member_results, displacement_nodes and reaction_nodes, so code that reads them keeps working. Anything that was not requested reads as None, not zero. A check that reads a value nobody asked for therefore fails instead of passing on a zero.

• members takes the unity checks' selectors: classification(...), members([...]), member_sets([...]) and all_members(). Several selectors form a union.
• nodes takes nodes([...]), or nodes_of(*selectors) for the end nodes of the selected members.
• Load combinations are matched on their limit_state, as unity checks match them, or by load_combination_ids. To select by limit state, set limit_state on each LoadCombination; the solver does not read situation.
• A request with neither limit_state nor load_combination_ids also covers the load cases.
• end="start" or end="end" keeps one end of the end-force and end-displacement blocks.
• The solver refuses a request it cannot meet, before it starts. That includes section_forces, internal_force_series, a component the block does not have, a limit state no combination carries, and result_filter set as well.

The values themselves are on model.resultsbundle.selections, one entry per request and group of result sets. Each entry's values is a read-only numpy array shaped (result_sets, ids, fields, components), with each axis listed beside it:

sel = model.resultsbundle.selections[0]
sel["group"]                     # "loadcombinations" or "loadcases"
sel["result_sets"], sel["ids"]   # combination names, member or node ids
sel["fields"], sel["components"] # e.g. ["local_maximums", "local_minimums"], ["fx", "my", "mz"]
my = sel["values"][..., sel["components"].index("my")]   # every set, member and field

save_to_json writes the selections back as the solver wrote them, so a saved result reloads unchanged. fers_core/examples/810_Result_Requests.py runs this end to end.

Scissor hinges: a continuous beam that turns on what it crosses (engine 0.2.68)

A rail running over beams and resting on each one is continuous along its length, but free to turn on every beam. A MemberHinge releases one member end, so releasing both rail ends at a beam splits the rail into two spans. Put the rail's members in a member set of their own and give the set a ScissorHinge instead. No link member or extra node is needed:

from fers_core import FERS, DistributedLoad, Material, Member, MemberSet, NodalSupport, Node, ScissorHinge, Section
from fers_core.supports.supportcondition import SupportCondition as C

model = FERS()
steel = Material(name="S355", e_mod=210e9, g_mod=81e9, density=7850.0, yield_stress=355e6)
rail_section = Section(name="rail", material=steel, i_y=1e-6, i_z=1e-6, j=1e-7, area=1e-3)
beam_section = Section(name="beam", material=steel, i_y=1e-4, i_z=1e-4, j=1e-4, area=5e-3)

turns = {"X": C.fixed(), "Y": C.free(), "Z": C.free()}
a, m, b = Node(0.0, 0.0, 0.0), Node(1.0, 0.0, 0.0), Node(2.0, 0.0, 0.0)
a.nodal_support = NodalSupport(rotation_conditions=turns)
b.nodal_support = NodalSupport(
    displacement_conditions={"X": C.free(), "Y": C.fixed(), "Z": C.fixed()}, rotation_conditions=turns
)
beam_start, beam_end = Node(1.0, 0.0, -0.5), Node(1.0, 0.0, 0.5)
beam_start.nodal_support = beam_end.nodal_support = NodalSupport()  # clamped

rail_1 = Member(start_node=a, end_node=m, section=rail_section)
rail_2 = Member(start_node=m, end_node=b, section=rail_section)
model.add_member_set(MemberSet(
    members=[rail_1, rail_2],
    scissor_hinge=ScissorHinge(rotational_release_y=0.0, rotational_release_z=0.0),
))
model.add_member_set(MemberSet(members=[
    Member(start_node=beam_start, end_node=m, section=beam_section),
    Member(start_node=m, end_node=beam_end, section=beam_section),
]))

span = model.create_load_case(name="span")
DistributedLoad(member=rail_1, load_case=span, magnitude=1000.0, end_magnitude=1000.0, direction=(0, -1, 0))
model.run_analysis()
mz = model.resultsbundle.loadcases["span"].member_results[str(rail_1.id)].local_end_forces.mz
print(f"{abs(mz):.1f} Nm")  # 62.4; a knife edge gives qL²/16 = 62.5, a rigid joint 122.6

• Where it applies: at every node where a member of the set meets a member outside the set, or a plate. There the set's members stay joined to each other and move relative to the node along and about the released axes. Nothing is released where the set meets only itself or a support. So the continuous beam's members belong in a set of their own; a set holding every member meets nothing, and the solver refuses it.
• Only some of those nodes: MemberSet(scissor_hinge_nodes=[...]) lists where the hinge applies, for a beam that turns on some of the members it meets and is joined to the rest.
• Each value works as a MemberHinge release does: None holds it, 0.0 frees it, and a positive value is a spring between the set and the node, in force per length for a translation (translational_release_x/_y/_z) and moment per radian for a rotation (rotational_release_x/_y/_z).
• The axes are global unless the hinge names its own: ScissorHinge(..., axes=ReleaseAxes.user(x=..., y=...)) takes the first axis along x, the second along y made perpendicular to it, and the third as their cross product, as a rack turned in plan needs.
• Supports and nodal loads at a released node act on the node, meaning the members outside the set. A node's reported displacement is theirs. The set's own motion there is in its members' local_displacement_start_node and local_displacement_end_node.
• Two hinged sets crossing with nothing else are each released from the node, so every axis either one releases is free between them.
• A set of one member gives a release about the hinge's axes at that member's ends.
• Second order: as with member hinges, a model with scissor hinges takes the corotational correction only when NonlinearMethod.COROTATIONAL is named.

A rail that may also slide over its posts frees that translation. Here it slides over the first post only, so the second takes its share of the pull:

from fers_core import (
    FERS, DistributedLoad, Material, Member, MemberPointLoad, MemberSet, NodalSupport, Node,
    ScissorHinge, Section,
)

model = FERS()
steel = Material(name="S355", e_mod=210e9, g_mod=81e9, density=7850.0, yield_stress=355e6)
frame = Section(name="frame", material=steel, i_y=4e-6, i_z=4e-6, j=1e-7, area=2e-3)
anchor, top_1, top_2 = Node(0.0, 0.0, 0.0), Node(2.0, 0.0, 0.0), Node(4.0, 0.0, 0.0)
foot_1, foot_2 = Node(2.0, -1.5, 0.0), Node(4.0, -1.5, 0.0)
anchor.nodal_support = foot_1.nodal_support = foot_2.nodal_support = NodalSupport()

rail_1 = Member(start_node=anchor, end_node=top_1, section=frame)
rail_2 = Member(start_node=top_1, end_node=top_2, section=frame)
model.add_member_set(MemberSet(
    members=[rail_1, rail_2],
    scissor_hinge=ScissorHinge(translational_release_x=0.0),  # slides along X
    scissor_hinge_nodes=[top_1],                               # over the first post only
))
for foot, top in ((foot_1, top_1), (foot_2, top_2)):
    model.add_member_set(MemberSet(members=[Member(start_node=foot, end_node=top, section=frame)]))

pull = model.create_load_case(name="pull")
DistributedLoad(member=rail_1, load_case=pull, magnitude=2000.0, end_magnitude=2000.0, direction=(0, -1, 0))
MemberPointLoad(member=rail_2, load_case=pull, position=0.5, magnitude=1000.0, direction=(1, 0, 0))
model.run_analysis()
reactions = model.resultsbundle.loadcases["pull"].reaction_nodes
takes = [abs(reactions[str(foot.id)].nodal_forces.fx) for foot in (foot_1, foot_2)]
print(f"post 1 takes {takes[0]:.1f} N, post 2 {takes[1]:.1f} N")  # 0.0 N and 67.2 N

A model that uses scissor hinges is written with schema_version 3, so a solver older than 0.2.68 refuses it instead of solving every connection as rigid. Without them, a model is written exactly as before. fers_core/examples/054_Scissor_Hinge_Rail_On_Beam.py compares the rigid connection, member hinges, the scissor hinge and a spring, and fers_core/examples/055_Scissor_Hinge_Sliding_Rail_And_Release_Axes.py slides the rail, limits the hinge to one post, and turns the rack in plan.

Member hinges in global or user axes (engine 0.2.68)

A MemberHinge releases its rotations about its member's own axes. A beam at an angle in plan, pinned about the girder it hangs from, needs a pin about the girder's axis instead, which in the beam's own axes mixes its bending with its torsion. rotation_axes names the axes the rotational releases, their stiffness curves and their moment–rotation diagrams use:

from fers_core import MemberHinge, ReleaseAxes

pin_about_x = MemberHinge(rotational_release_mx=0.0, rotation_axes=ReleaseAxes.GLOBAL)

ReleaseAxes.LOCAL (the default) keeps the member's axes; ReleaseAxes.user(x, y) names a frame as above. A curve on such a hinge reads the end moment about the same axes; the member's results stay in its local axes. Translational releases stay member-local. A model whose member hinges release in other axes is written with schema_version 3 as well. Release axes are fixed in space, so a corotational solve does not turn them with the structure.

A member's self-weight

The solver works out each member's self-weight, density·area·g, and puts it in the load case that self_weight_load_case_id names. A load combination carries the self-weight only if it includes that case, so give it your permanent-load case: create_load_case("G", self_weight=True) creates G and sets both options.

from fers_core.builders import create_beam

model = create_beam(6.0, "IPE300", udl=5_000.0, load_case_name="Q")   # 5 kN/m imposed load
imposed = model.load_cases[0]
dead = model.create_load_case("G", self_weight=True)                   # G takes the self-weight
model.create_load_combination("ULS", {dead: 1.35, imposed: 1.5}, limit_state="ULS")
model.run_analysis()

uls = model.resultsbundle.loadcombinations["ULS"]
print(round(sum(r.nodal_forces.fy for r in uls.reaction_nodes.values()), 1))   # 48357.4

The ULS reactions are 1.5 × 30 kN plus 1.35 × the beam's 2.49 kN. With enable_self_weight set and no self_weight_load_case_id, the solver puts the self-weight in a load case of its own, "Self-weight", which combinations built from your load cases do not include. run_analysis refuses such a model when it has combinations, rather than solve every one of them without self-weight, and the solver warns with self_weight_not_in_combinations on each combination. A model without combinations solves, and its "Self-weight" case is a result of its own, as below.

The solver also works out each member's mass for modal and seismic analysis from the section. A member that carries more, such as cladding or a services run, can override both:

from fers_core.builders import create_beam

model = create_beam(5.0, "IPE180")                  # no load but its own weight
model.settings.analysis_options.enable_self_weight = True
for member in model.members:
    member.weight_override = 1500.0                 # N/m, gravity included
model.run_analysis()

self_weight = model.resultsbundle.loadcases["Self-weight"]
print(round(sum(r.nodal_forces.fy for r in self_weight.reaction_nodes.values()), 1))   # 7500.0

The solver applies 1500 N/m in place of the section's 184 N/m, and takes 1500 / 9.81 kg/m as the member's mass. weight_override is in the model's force/length units. Member.weight is a different thing: the member's mass as the SDK computes it, density·area·length, which is never sent to the solver.

Seismic analysis

add_seismic_analysis asks the solver for a modal response-spectrum analysis (the default), the lateral-force method, or both. The spectrum helpers build EN 1998-1 design spectra. Accelerations are in m/s² and periods in seconds, whatever the model's units; nodal masses are in kg.

from fers_core import (
    FERS, Member, MemberSet, MaterialLibrary, NodalMass, NodalSupport, Node, Section,
    SeismicAnalysisSettings,
)

model = FERS()                                                    # first: it restarts the ids
steel = MaterialLibrary.S235()
column = Section.from_name("HEB200", steel)
nodes = [Node(0.0, float(height), 0.0) for height in range(7)]    # a 6 m mast
nodes[0].nodal_support = NodalSupport()                           # fixed base
model.add_member_set(MemberSet(members=[Member(a, b, section=column) for a, b in zip(nodes, nodes[1:])]))
model.add_nodal_mass(NodalMass(nodes[-1], 2000.0))                # 2 t of equipment on top

model.add_seismic_analysis(
    SeismicAnalysisSettings.eurocode_spectrum(ag=2.5, ground_type="C", q=1.5),
    directions=["X"],
    num_modes=4,
)
model.run_analysis()

x = model.resultsbundle.seismic["modal_response_spectrum"]["per_direction"][0]
print(f"{x['participating_mass_ratio']:.0%} of the mass, base shear {x['base_shear'] / 1000:.1f} kN")

mass_sources=[(load_case, psi)] turns gravity load cases into seismic mass (ψE = 1.0 for permanent loads). direct_spectrum takes S, T_B, T_C and T_D directly for a national annex, and custom_spectrum a table of periods and accelerations. The results are plain dicts in resultsbundle.seismic, one entry per method. fers_core/examples/151_Seismic_Response_Spectrum.py runs a portal frame through both methods.

Unity-check reports

The solver can embed one HTML report of every unity check in the result:

from fers_core.builders import check_beam

model = check_beam(5.0, "IPE300", udl=10000.0)
model.settings.analysis_options.include_report_html = True
model.run_analysis()
html = model.unity_report_html()                    # a complete HTML page

render_unity_reports = True also fills each checked member's rendered_report from its check's report_template, and adds those narratives to the HTML report. Both are off by default, which keeps the result small.

Premium solves and timeouts

Without an API key the solver runs at Free-tier limits (100 members) and makes no network call at all. The free tier is for non-commercial use; commercial use needs a Pro key at any model size. Passing api_key raises those limits:

model.run_analysis(api_key=KEY)

A key means each solve first verifies the licence with ferscloud.com, so that handshake is bounded. If the licence server does not answer within license_timeout seconds (default 30) the call raises FersTimeoutError before the solve starts — so retrying is always safe:

import fers_calculations

try:
    model.run_analysis(api_key=KEY, license_timeout=60)
except fers_calculations.FersTimeoutError:
    ...   # transient stall — retry or skip this model
except RuntimeError:
    ...   # definitive — bad key, non-premium account, or a rejected model

Keeping those two arms apart is the point: retrying a RuntimeError fails identically, while collapsing both into except Exception turns a retryable blip into a lost result. See fers_core/examples/203_Premium_Batch_Solve.py for the full unattended-batch pattern.

FersTimeoutError subclasses the built-in TimeoutError, so a plain except TimeoutError catches it too.

Set FERS_LICENSE_TIMEOUT (seconds) to change the default without editing any call sites. It must be a real environment variable — .env files are not read by the engine.

license_timeout bounds the licence handshake, not the solve. A solve is native code with no cancellation point, so no in-process timeout can interrupt it. For a hard ceiling on total wall time per model, run each solve in a killable subprocess — that also reclaims its memory.

Requires engine fers_calculations >= 0.2.53.

From the terminal and from coding agents

pip install FERS puts a fers command on the path: a thin layer over the objects above and the engine, for shells, scripts, CI jobs and coding agents. Every large output goes to a file; stdout carries one line per command, or a JSON object with --json.

fers validate model.json                 # the input schema: the failing field, and keys the engine ignores
fers solve model.json --out results.json # the engine streams the results to the file
fers check model.json                    # solve, print each unity check's utilization and a verdict
fers report results.json --out report.html
fers schema                              # the schema's definitions; `fers schema Member` prints one
fers sections get IPE200                 # catalogue properties in SI; `sections list` names the series
fers materials list
fers examples list                       # the numbered examples; `examples show 001`, `examples run 001`
fers skill --install .                   # the skill for coding agents, into .claude/skills
fers skill --zip fers-skill.zip          # the same skill as a zip to upload to Claude.ai
fers doctor                              # versions, and whether the key is accepted

fers solve sends the file to the engine as it is, and reads the results back only for its summary line: load cases, combinations, recorded failures, non-converged result sets and messages. A model that writes hundreds of megabytes costs this process nothing.

The exit code says whether to use the numbers:

A check that meets several of these exits with the first of 4, 6, 5. With --json, solve and check add a verdict and its reasons.

FERS_API_KEY supplies the key for Premium limits when --api-key is not given; without a key the engine runs at Free-tier limits and makes no network call, as above. --solve-token (or FERS_SOLVE_TOKEN) takes a short-lived Premium token from FERS Cloud instead, which the engine checks offline: Premium limits on a machine that cannot reach ferscloud.com. In Python it is run_analysis(solve_token=...).

For an agent, the skill is the working loop on one page: which route to take (MCP tools, a shell or code sandbox, or a file for FERS Studio), validate, solve, check, and the conventions that change answers, with runnable models in reference/api.md. It ships in the package (fers_core/skills/fers/); fers skill --install . copies it to .claude/skills/fers/, where Claude Code looks, fers skill --zip fers-skill.zip packs it for upload to Claude.ai, and fers skill --path names SKILL.md for any other agent. MCP clients can run the same engine as a local MCP server instead (npx -y @ferscloud/fers-mcp, which solves on the user's machine); clients that cannot start a program use the hosted server at https://ferscloud.com/api/mcp, which returns a browser-solve link. Setup for each client: https://ferscloud.com/mcp.

fers_core/examples/811_Command_Line_Workflow.py runs the loop end to end.

Licence

This package, FERS, is open source under the BSD 3-Clause License (LICENSE).

The solver engine it installs, fers_calculations, is licensed separately. From 0.2.70 it is under the PolyForm Strict License 1.0.0: free for any non-commercial purpose, such as study, teaching, research and hobby projects. Commercial use needs a paid FERS Cloud plan, which includes the commercial licence.