> 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/materials-fea.md).

# Materials \[FEA]

A Material supplies the constitutive properties every element needs to turn geometry into stiffness: elastic modulus, Poisson's ratio, density and thermal expansion, plus the strength values used by design checks and the time-dependent behaviour used by staged construction.

Materials can be imported from OpenBrIM's database or defined by hand. For importing, see [this example](https://docs.openbrim.org/examples/steel-bridge-examples/example-steel-i-girder-bridge-ex1-sig/properties-ex1-sig/materials-ex1-sig).

A Material contributes to the **stiffness matrix** (through modulus and Poisson's ratio), to **mass** (through unit weight), and **generates load** (self-weight, and thermal load in combination with the thermal coefficient). Its strength values are used for **design only** and do not affect analysis.

## Basic

**Modulus of Elasticity:** Young's modulus.

*Engineering influence.* This scales essentially every stiffness in the model: axial as `EA`, flexural as `EI`, shell membrane and bending both linearly in `E`. Under load control, displacements are inversely proportional to it — halving `E` doubles deflections. In a **statically determinate** structure it does not change internal forces at all, only displacements; in a **redundant** structure it changes the distribution, because stiffness attracts load. That distinction is why an incorrect modulus can be invisible in a simple span and decisive in a continuous one.

Periods scale with `1/√E`, so modulus errors propagate into every dynamic result. For concrete the value is strength-dependent and time-dependent; see the Time Dependent tab.

Direct impact on analysis; indirect impact on design through the analysis results.

**Poisson's Ratio:** The ratio of transverse to axial strain.

*Engineering influence.* It couples the two in-plane directions of a shell and the three directions of a solid, so it matters most for plate and solid models and hardly at all for a beam-only frame. It also ties the three elastic constants together — see Shear Modulus below. Typical values are about 0.2 for concrete and 0.3 for steel. Values approaching 0.5 describe an incompressible material and cause volumetric locking in solid elements, making them artificially stiff.

**Shear Modulus:** The shear modulus `G`.

*Engineering influence.* Governs shear deformation and torsional stiffness. For an isotropic material the three constants satisfy `G = E / (2(1+ν))`, and **entering 0 for any one of the three lets the program compute it from the other two** — which is the recommended way to keep them consistent. Entering all three independently allows a physically impossible combination that will not be flagged.

Shear deformation is negligible for slender members and significant for deep beams, thick plates and short columns; torsional stiffness matters wherever a curved or skewed structure carries load in torsion.

**Unit Weight \[10⁻³]:** Weight per unit volume. **The value is entered in units of 10⁻³**, so a density of 0.15×10⁻³ is entered as **0.15**.

*Engineering influence.* This is the source of self-weight load and of the mass matrix in a dynamic analysis. Increasing it increases both the applied gravity load and the inertia, which lengthens periods; the two effects do not cancel, because self-weight is a load while mass is a property. A value of zero makes the element weightless — no self-weight and no contribution to mass, which silently removes the dominant load case from most bridge models.

For reinforced concrete the value normally already includes the reinforcement, which is why rebar and strand weights are reporting-only quantities elsewhere.

Direct impact on analysis; indirect on design.

**Thermal Coeff. \[10⁻⁶]:** Coefficient of thermal expansion. **Entered in units of 10⁻⁶**, so 6.5×10⁻⁶ is entered as **6.5**.

*Engineering influence.* Converts a temperature change into strain. It only has an effect when a temperature load is present, and only produces **force** when the resulting movement is restrained — an unrestrained member simply changes length. In a restrained or redundant structure it can generate very large forces, which is why thermal effects often govern bearing and joint design. Zero means the model ignores temperature entirely, which is optional only if there are no thermal load cases.

Note that steel and concrete coefficients are similar, which is what makes composite construction practical; a large mismatch entered by mistake produces spurious interface forces.

**Type:** Steel, Concrete, Reinforcement Bar or Prestressing Tendon.

*Engineering influence.* Selects the default stress–strain model and the code rules applied in design and section analysis. It has **no effect on finite element analysis** — a linear analysis uses `E`, `ν` and density regardless — but a direct effect on design, capacity and any nonlinear fibre-based section work. Choosing the wrong type gives a member the wrong constitutive curve at ultimate.

**Stress Strain Model (readonly):** Reports the stress–strain curve implied by the Type — bilinear for steel, Menegotto–Pinto for rebar, Mander for concrete, and a prestressing curve for tendons. Read-only output; use it to confirm the Type selection produced the curve you expected.

## Orthotropic

**Orthotropic Ex / Ey:** Direction-dependent moduli in the material's local x and y directions.

**Orthotropic Nuxy:** In-plane Poisson's ratio for the orthotropic definition.

**Orthotropic Gxy / G13 / G23:** In-plane and transverse shear moduli.

*Engineering influence.* These describe a material that is stiffer in one direction than another — an orthotropic steel deck, a one-way ribbed slab smeared into a plate, timber, or a concrete deck idealized with reduced transverse stiffness. They apply to shell elements and are interpreted in the element's **local** axes, so their meaning depends on shell orientation; a mesh with inconsistent local axes will have its stiff direction pointing different ways on different elements.

Increasing `Ex` relative to `Ey` attracts load into the x direction, which is exactly the intent for a deck that spans one way. Leaving these blank uses the isotropic properties from the Basic tab. The transverse shear moduli `G13` and `G23` only matter where transverse shear deformation is significant — thick plates and Mindlin (Thick) shell formulations.

{% hint style="warning" %}
None of the six Orthotropic parameters are declared in the object type, so they can only be set by hand in this sheet — not through the agent API, `set_parameter`, or ParamML authoring.
{% endhint %}

## Steel

**Steel Yield Stress:** The yield strength `Fy`.

**Steel Ultimate Stress:** The ultimate strength `Fu`.

*Engineering influence, both.* These are **design** quantities. They set the capacity a steel member is checked against and the plateau of its stress–strain curve, so they determine utilisation ratios, plastic hinge capacities in a pushover analysis, and the yield point in any nonlinear material model. They have **no effect on a linear elastic analysis** — a steel member with the wrong `Fy` deflects exactly the same and simply reports a wrong utilisation. Increasing them increases capacity and reduces reported demand-to-capacity ratios, without changing the demand itself.

## Concrete

**Concrete Strength at 28 Days:** The characteristic compressive strength `f'c` at 28 days.

*Engineering influence.* A **design** quantity for capacity, and also the anchor for the time-dependent calculations: the CEB-FIP creep, shrinkage and modulus-development relationships are all expressed relative to the 28-day strength, so it governs long-term behaviour in a staged model. It does not change a linear elastic analysis directly — the analysis uses the modulus from the Basic tab — but if time-dependent modulus is enabled, the computed modulus is derived from this value and the Basic modulus is overridden.

## Time Dependent

These five switches enable the long-term effects used by staged-construction analysis. All are **Include/Ignore** switches.

{% hint style="info" %}
Each of these must be enabled in **two** places to take effect: here on the material, **and** on the construction stage. Enabling it on only one is the usual reason a staged model shows no creep or shrinkage at all. Environmental inputs such as relative humidity are properties of the **stage**, not of the material.
{% endhint %}

**Time Dependent Elastic Modulus:** When included, the modulus is computed as a function of concrete age rather than taken from the Basic tab, following CEB-FIP 1990. The computation uses the 28-day strength, cement hardening type, curing humidity and temperature, and the age at loading.

*Engineering influence.* Young concrete is substantially less stiff than mature concrete, so including this softens early stages and changes how load locks in as the structure is built — generally increasing the long-term deflection and shifting force toward members cast later. OpenBrIM works in secant modulus while CEB-FIP computes a tangent modulus, so the CEB-FIP value is divided by 1.05 before use. Overrides the Basic modulus when enabled.

**Concrete Creep Effect:** Long-term deformation under sustained load.

*Engineering influence.* Creep increases deflections over time and, in a redundant or staged structure, **redistributes** force toward members that creep less — which is why a segmentally-built bridge ends up with a different moment diagram than the same geometry built on falsework. It depends on the age at loading, the sustained stress level, member size, humidity and cement type. Ignoring it understates long-term deflection and misstates the final force distribution.

**Concrete Shrinkage Effect:** Volume reduction as the concrete dries.

*Engineering influence.* Shrinkage is a strain that occurs whether or not load is applied, so in a **restrained** member it produces tension and can cause cracking; in a composite member it produces differential strain between deck and girder. Its magnitude depends on humidity, member size, mix and age at the start of drying. Ignoring it understates cracking and deck tension.

**Steel Relaxation Effect:** Loss of stress in prestressing steel held at constant strain.

*Engineering influence.* Reduces the effective prestress over time, and therefore the balancing load and the compression in the concrete. Ignoring it overstates the prestress remaining at long term, which is unconservative for a serviceability tension check.

**PT Losses from Structure:** Elastic shortening losses — the prestress lost because the member itself shortens as it is compressed, and as subsequent tendons are stressed.

*Engineering influence.* Reduces the prestress delivered by tendons stressed earlier in a sequence. The effect grows with the number of tendons and with how much the member shortens, so it is most significant in heavily prestressed members with many stressing operations.

## Dynamics

**Rayleigh Damping Mass:** The mass-proportional damping coefficient `α` in `C = αM + βK`.

**Rayleigh Damping Stiffness:** The stiffness-proportional coefficient `β`.

*Engineering influence.* Rayleigh damping produces a damping ratio that **varies with frequency**: the `α` term damps low frequencies and the `β` term damps high ones, matching the target ratio at exactly two frequencies and over-damping outside that band. Choose the two frequencies to bracket the modes that carry the response, then compute `α` and `β` from them; entering round numbers without that calculation is the usual cause of an analysis that is inexplicably over- or under-damped.

Both default to 0, giving **undamped** behaviour. Increasing damping reduces peak dynamic response, sharply near resonance and mildly away from it. These coefficients are used by direct-integration time-history analysis; modal and FNA cases use the modal damping ratio on the case instead, and ignore these values.

## Verification

* Check a simple span's deflection against `5wL⁴/384EI` to confirm modulus and density are being applied.
* Confirm total model weight matches a hand take-off; a zero or wrong unit weight shows up immediately.
* Verify the 10⁻³ and 10⁻⁶ scaling on unit weight and thermal coefficient by checking a computed self-weight reaction and a computed thermal expansion against hand values. These two fields are the most common unit-slip on this object.
* Enter 0 for shear modulus and confirm the program computes it from `E` and `ν`.
* For a time-dependent model, run with the effects on and off and compare long-term deflection; no difference means the switches are not enabled on the stage as well.
* Check that the Stress Strain Model shown matches the Type you selected.
* For an orthotropic deck, apply a unit load and confirm it spans in the intended direction.

## Common mistakes

* **Missing the 10⁻³ / 10⁻⁶ scaling** on unit weight or thermal coefficient, producing self-weight or thermal forces wrong by a factor of a thousand.
* **Zero unit weight**, silently removing self-weight and mass.
* **Entering all three of `E`, `ν` and `G`** with an inconsistent combination instead of leaving one at zero to be computed.
* **Enabling time-dependent effects on the material only** and not on the construction stage, so nothing happens.
* **Assuming `Fy` affects analysis.** It does not; a wrong yield strength changes only the reported utilisation.
* **Wrong material Type**, giving a member the wrong stress–strain curve in section analysis and pushover.
* **Leaving Rayleigh coefficients at zero** and running an undamped direct-integration time history.
* **Using orthotropic properties on a mesh with inconsistent local axes**, so the stiff direction varies element by element.


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