Section Optimiser

The Section Optimiser sizes steel beams from lists of sections. It analyses the model, checks every beam with every section of its list to EN 1993-1-1 on the forces the beam carried, as Utilisation does, puts each beam on the first section of its list that keeps it at or below MaxUtilisation, and analyses again: the new sections change the self-weight and, in a statically indeterminate structure, where the forces go. It stops when no section changes, or after Iterations analyses.

🔧 Grasshopper component

Section OptimiserAlpaca4d ▸ 12_Optimisation

Inputs

AlpacaModelModelThe assembled model, from Assemble.

Its beams start from the sections they have.

DesignGroupsDesignGrouplist(empty)Which beams to size from which sections: one Design Group per list.

Left empty, every beam is sized within its family in Steel Section Library.

ElementIdTextlist(empty) = every beamOptimise only some of the beams: an ElementId (MyBeam_3), a wildcard (MyBeam*, Col_?), an element tag (47) or a regular expression (regex:^Col), mixed freely.

The beams it leaves out keep their sections, whatever group they are in, but are still analysed and checked.

MaxUtilisationNumber1The utilisation each beam taking part should end up at or below: 1 is exactly at the limit, 0.8 keeps a fifth in hand.

IterationsInteger10How many analyses at most.

Each one sizes the beams on the forces it finds, for the next to check.

FabricationTextRolledHow the sections are made, which sets the buckling curves: Rolled (rolled I-sections, hot-finished hollow sections) or Welded (welded I-sections, cold-formed hollow sections).

In the Design menu.

γM0Number1.0Partial factor for the resistance of cross-sections.

A National Annex may set another. In the Design menu.

γM1Number1.0Partial factor for the resistance of members to instability.

In the Design menu.

LengthYNumberlisteach beam's lengthBuckling length about local y, the minor axis of an I, in m.

One value for every beam, or one per beam, in the order of the model's beams. It stays with the beam whatever its section. In the Design menu.

LengthZNumberlisteach beam's lengthBuckling length about local z, the major axis of an I, in m.

One value, or one per beam. In the Design menu.

LengthLTNumberlisteach beam's lengthDistance between lateral restraints, for lateral-torsional buckling, in m, as in Utilisation.

Left empty, each beam's own length: not safe for a cantilever, whose tip is not held. In the Design menu.

LoadTextlistShearCentreWhere the load along each beam acts on its section, for lateral-torsional buckling: ShearCentre, Destabilising (or Top) or Stabilising (or Bottom).

One value, or one per beam. In the Design menu.

SwayBooleanlistfalseThe beam belongs to a frame that buckles by swaying: Cm = 0.9 about both axes (Table B.3).

One value, or one per beam. In the Design menu.

SystemTextAutoHow the system of equations is solved, as in Analysis Settings: Mumps on every core when the OpenSees has it, BandSPD when not.

Reach for UmfPack if a model refuses to solve. In the Analysis menu.

AlgorithmTextNewtonHow each analysis is iterated to equilibrium.

Linear does one solve with no iteration: a little faster on an elastic model, and only right on one. In the Analysis menu.

Outputs

AlpacaModelModelThe last design, analysed: for Utilisation, Beam Forces or Model View, or for Natural Vibration or Linear Buckling.

Empty when the first analysis failed.

SectionSectionlistEach beam's section in the last design, in the order of Element.

UtilisationNumberlistEach beam's utilisation in the last design, as Utilisation's Max.

Empty where it could not be checked. Torsional and torsional-flexural buckling (6.3.1.4) are not checked yet.

ElementElementlistThe beams, in the order of the other outputs.

MassNumberlistThe model's mass in each iteration, in kg, the last being the last design's.

ConvergedBooleantrue when the last iteration changed nothing: every beam taking part has the first section of its list that works for it.

GoverningTextlistWhich check gives each beam's utilisation, or why there is none.

In the Design menu.

GroupTextlistEach beam's design group, by name, or its family (HEA family) for a beam in no group; empty for a beam that takes no part.

In the Design menu.

HistoryGenericlistOne item per iteration: mass, largest utilisation, beams above MaxUtilisation, sections changed, and how long the analysis and the checks took.

In the Design menu.

logTextWhat OpenSees printed in the last iteration.

In the Design menu.

A beam's list is its Design Group's, in the order given. A beam in no group is sized within the family of the section it starts with in Steel Section Library, smallest area first: an HEA200 among the HEAs, a CHS among the CHS. A section that is not from the library has no family, and keeps its place.

The Design menu holds how the members are made and held, set once and left alone, and the outputs about each beam's check; the Analysis menu, how each analysis is solved. Each analysis is static, in one step. Press Escape to stop after the iteration under way.

📈 When to use it

Use it when

• You want the lightest sections of a steel frame, truss or beam grid that pass EN 1993-1-1 under its loads, and the forces depend on the sections themselves.
• A member drawn as several beams must end up with one section: give it a Design Group of its own.

Do not use it when

• The members are not steel: the check, and so the sizing, is steel only.
• You want to compare a few designs side by side: Explorer Inputs steps through them and records each.

The Two-Span Beam example sizes a curved beam over two spans from a list of HEAs, element by element and as one section.

Note

A beam can move back up its list as well as down it. When a design comes round again, the iterations after it only let beams move on down their lists, so the run ends.