> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/openbrimfea/properties-fea/sections-fea.md). # Sections \[FEA] A Section supplies the cross-sectional properties an FE Line needs to become a stiff member: area, moments of inertia, torsion constant and shear areas, together with the material and the geometric information used by design checks. Sections can be imported from the database, drawn as a shape and have their properties computed automatically, or entered numerically. A Section contributes to the **stiffness matrix** and, through its area and the material density, to **mass** and **self-weight**. {% hint style="info" %} **Compute Properties From Shape** decides whether the numbers below are inputs or outputs. When it is on, the properties are derived from the drawn geometry and anything you type is recalculated away; when it is off, the values you enter are used directly. Decide which mode you are in before editing anything on this page. {% endhint %} ## Properties **Material:** The material assigned to the section. Supplies modulus (turning `I` into `EI`), Poisson's ratio, density for self-weight, and the strength values used by design. *Engineering influence.* A section with no material has no stiffness and no weight — a common cause of a model that is far too flexible or that reports zero dead load. It also governs which design code rules apply to members using the section. **Area:** The axial cross-sectional area `Ax`. *Engineering influence.* Sets axial stiffness `EA` and, with the material density, the member's self-weight. It governs axial shortening in columns and cable stretch, and it is the dominant property for truss and cable members, which have no bending stiffness to fall back on. Errors show up as wrong dead load before they show up as wrong deflection. **Shear Area Y / Shear Area Z:** The effective areas resisting shear in each local direction. *Engineering influence.* These control shear deformation, which is negligible for slender members and significant for deep beams and short columns — the classic case being a squat pier where ignoring shear deformation overstates stiffness and attracts too much load to it. They are always **less** than the gross area (roughly the web area for an I-section, about 5/6 of the area for a rectangle). Entering **zero** is the switch that means "ignore shear deformation" — it does not mean zero stiffness. That is a reasonable simplification for slender members and a real error for stubby ones. **Torsion Constant:** The St. Venant torsion constant `J`. *Engineering influence.* Governs torsional stiffness `GJ`. Its magnitude depends enormously on whether the section is open or closed: a closed box has a torsion constant orders of magnitude larger than an open I-section of similar size. Because stiffness attracts load in a redundant structure, an overstated `J` pulls load into the torsional path and away from bending — which is why curved and skewed bridges are sensitive to it. For concrete members the uncracked `J` substantially overstates the torsional stiffness actually available once the member cracks; codes commonly require a large reduction, applied through the torsion-constant factor on the FE Line rather than by editing the section. **Inertia Y / Inertia Z:** The second moments of area about the local y and z axes. *Engineering influence.* These are the flexural stiffnesses, `EIy` and `EIz`. Deflection is inversely proportional to them, and in a redundant structure they set how moment distributes between members. The two are almost always very different for a bridge girder — which is why **Beta Angle** on the FE Line matters so much: assigning the section correctly but orienting it wrongly puts the weak axis where the strong one was intended, producing deflections an order of magnitude too large. **Inertia ZY:** The product of inertia `Izy`. *Engineering influence.* Non-zero for a section whose principal axes are not aligned with its local axes — an unequal angle, a singly-symmetric section rotated relative to the frame. It couples bending about the two axes, so a load applied in one plane produces deflection in the other. For doubly symmetric sections it is zero, and leaving it zero for an unsymmetric section misses biaxial coupling the real member has. **Compute Properties From Shape?:** Whether the properties above are computed from the drawn geometry. *Engineering influence.* This is a **mode switch**, not a physical quantity. With it on, the section is driven by its shape and stays consistent as the shape changes — the right choice for parametric work. With it off, the typed values govern, which is what you need to match a reference model, a handbook section, or a built-up member whose geometry is not drawn. Switching it on after entering values by hand discards them silently. **Has Geometry? (readonly):** Reports whether a shape has been defined. A section with no geometry cannot compute properties and cannot be used for section-level design checks or fibre analysis. **Assigned to FELine? (readonly):** Reports whether any element uses the section. Useful for finding orphaned library entries, and for confirming a newly created section is actually in use. **Is Composite? (readonly):** Reports whether the section combines more than one material. {% hint style="warning" %} These three read-only columns are derived and cannot be edited, but they are not flagged read-only, so typing into them appears to work and the value is discarded on redraw. {% endhint %} ## More Properties **Gross Area:** The gross cross-sectional area, before deductions for holes or transformation. *Engineering influence.* Used by design checks that are written against gross rather than effective properties. It does not replace **Area** in the stiffness calculation. **Radius of Gyration Y / Z (readonly):** `√(I/A)` about each axis. *Engineering influence.* Derived values reported for design use. They drive slenderness ratios (`KL/r`) and therefore buckling and compression capacity checks. A small radius of gyration about one axis identifies the direction a compression member will buckle in. **Centroid Location Y / Z:** The position of the section centroid in local coordinates. *Engineering influence.* The centroid is the reference the element's axis passes through, so it defines the eccentricity of everything measured from it — prestressing tendons, composite deck offsets, and the lever arm of any end offset. An incorrect centroid shifts every eccentricity in the member, which changes prestress moment directly. **Shear Center Y / Z:** The position of the shear centre. *Engineering influence.* Load applied away from the shear centre produces torsion. For a doubly symmetric section the shear centre coincides with the centroid and this does not arise; for a channel or an unsymmetric girder they differ, and the offset is what makes such members twist under load applied at the centroid. It also matters for lateral-torsional buckling checks. **Perimeter:** The section perimeter. Used for surface-area-dependent quantities — painting take-offs, and the notional-size terms in creep and shrinkage calculations, where the volume-to-surface ratio governs the drying rate. **Warping Constant:** The warping torsion constant `Cw`. *Engineering influence.* Open sections resist torsion partly by warping, which St. Venant torsion alone does not capture. `Cw` is significant for open sections such as I-girders — especially in lateral-torsional buckling — and effectively negligible for closed boxes. It is used by design checks rather than by the basic beam element stiffness. **Non-prismatic Variation Direction:** For a member whose section varies along its length, the direction in which the variation is measured. Only meaningful for non-prismatic definitions; leave at the default for a constant section. ## Extent **Pos. of Neg. Edge Y / Pos. of Pos. Edge Y** and **Pos. of Neg. Edge Z / Pos. of Pos. Edge Z:** The extreme fibre positions of the section in each local direction. *Engineering influence.* These give the distances from the centroid to the extreme fibres, which convert a moment into an extreme-fibre stress (`σ = M·c/I`). They therefore drive every stress check and every serviceability tension or compression limit. They are also what the model uses to know the physical envelope of the section, for clash and cover purposes. An asymmetric section has different distances to the two edges, and using the wrong one understates stress at the critical fibre — most consequentially at the tension face of a prestressed member. ## Verification * Check a simple span deflection against `5wL⁴/384EI` to confirm the inertia and material are being applied. * Compare the reported self-weight of a member against area × length × unit weight. * For an imported or drawn section, compare the computed properties against the published values for that designation. * Display the section in the section viewer and confirm the shape, centroid and extreme fibres look right. * Check **Assigned to FELine?** to confirm the section you edited is the one actually in use. * Confirm the strong axis is oriented as intended by checking member deflections under a known load, and by displaying local axes on the FE Lines that use the section. * Compare a torsion-sensitive result computed with open-section and closed-section `J` values to see how much the answer depends on it. ## Common mistakes * **Editing numeric properties with Compute From Shape still on**, so the values are silently recalculated away. * **Wrong axis orientation.** The section is right, but Beta Angle on the FE Line puts the weak axis vertical. Deflections come out an order of magnitude too large. * **Entering the gross area as the shear area**, removing shear deformation from members where it matters — or entering a shear area for slender members where zero was intended. * **Using an uncracked concrete torsion constant** without the code-required reduction, attracting load into a torsional path the real structure cannot deliver. * **No material assigned**, giving a section with no stiffness and no weight. * **Leaving `Izy` at zero for an unsymmetric section**, missing biaxial bending coupling. * **Mixing unit systems** between area and inertia, which is easy to miss because both are large numbers. * **Assuming the shear centre coincides with the centroid** on a channel or unsymmetric girder. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. 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