Alpaca4d

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🏗️ How a model is built ​

Every Alpaca4d definition follows the same chain. Materials go into sections, sections into elements, and the elements, supports, load patterns and constraints go into Assemble, which makes them one model. An analysis solves that model and hands it back with its results, which the result components and Model View read.

Material ──► Section ──► Element ─────┐
                         Support ─────┤
Load ──► Load Pattern ────────────────┼──► Assemble ──► AlpacaModel ──► Analysis ──► AlpacaModel ──► Results
                      Constraint ─────┤                                              with results     Model View
                   MPCO Recorder ─────┘

What each stage hands on ​

Material. A stress–strain law and a density. There are two families, and they are not interchangeable: Uniaxial for beam sections and fibres, nD for shells and solids. Of a uniaxial material only E, Eta and Eneg reach OpenSees. Alpaca4d uses G for the section's torsion and shear terms and Rho for the element's mass, which is why they sit on the material and not on the section.

Section. A geometry paired with a material. A beam section reduces to an area, two second moments, a torsion constant and two shear factors; a shell section is a thickness and an nD material. The section carries its material, and with it the density, on to the element. It does not know which way it faces: that comes from the element.

Element. Rhino geometry turned into finite elements: a curve into a beam, mesh faces into shells, a hexahedral or tetrahedral mesh into a solid. Solids take an nD material directly, with no section, and a Spring Link takes one uniaxial material per direction and reads its E as a stiffness. An element knows its own geometry and nothing else: a beam does not know which other beam it meets. Each element can be given an optional ElementId, a name that loads and result components can pick it out by later.

Supports, loads and constraints. A Support holds a node still. A constraint ties nodes to each other. Loads do not go into Assemble on their own: they are grouped into a Load Pattern, which gives them a time series and a factor. The one exception is Mass Point, which adds mass rather than force and goes straight into Assemble. An MPCO Recorder, if there is one, says which results to record.

Assemble. The hinge of the chain. It collects every beam end, shell vertex and solid corner and merges the ones within its Tolerance into one node. The same search attaches point loads, mass points, supports and the nodes named by constraints. It numbers everything and gives each node 6 degrees of freedom when a beam or a shell touches it, 3 when only solids do. Out comes one AlpacaModel, the object every later component reads.

Analysis. Run Analysis solves the model under the recipe of Analysis Settings: the integrator, the convergence test, the steps, the damping and the solver. Natural Vibration and Linear Buckling need no settings. Each returns the same model with its results attached. There is no separate result object.

Results and Model View. The result components read numbers off the analysed model, one Step or every step at once with History. Model View draws the model before the analysis, to check supports and loads, and any of its results after it.

Why the chain is split this way ​

It is how OpenSees builds a model. An OpenSees model is written as materials, sections, elements, nodes, restraints and load patterns, and an analysis as a handful of objects that each answer one question. Most Alpaca4d components stand for one of those commands, and their reference pages show it under Relation to OpenSees.

Each piece is reused. One material goes into many sections, one section onto many elements. Because loads and patterns are separate, the same load can appear in several load cases at different factors.

Connection is decided once, by geometry. Nothing before Assemble knows about anything else, and two members are joined when, and only when, their geometry meets within the Tolerance. So when a connection carries no force, the tolerance is the first thing to check, and Node IDs in Model View shows how many nodes the model really has.

One object flows downstream. Every analysis takes an AlpacaModel and returns one, so the same result components read the answer of any analysis. An analysed model also keeps the model it was analysed from: that is why Natural Vibration and Linear Buckling accept the output of Run Analysis as well as that of Assemble.

See also ​