RISA’s software suite is a staple in structural engineering offices around the world, prized for its flexibility in modeling complex behaviors. One of its most powerful—and often underused—capabilities is the custom material properties feature. Out of the box, RISA includes libraries for common structural materials such as A36 steel, 4000 psi concrete, and Douglas fir timber. But real-world projects rarely fit neatly into predefined categories. Engineers increasingly work with high-strength alloys, fiber-reinforced polymers (FRP), shape-memory alloys, laminated timber, or concrete with optimized admixtures. To capture the true structural response of these materials, you must define custom properties. This article provides a comprehensive guide to using RISA’s custom material properties, from basic definitions to advanced nonlinear parameters, ensuring your models reflect accurate, production-ready behavior.

Why Custom Materials Matter

Standard material libraries are built around code-minimum values and typical grade definitions. When you push beyond those boundaries—specifying a steel with a yield strength of 65 ksi instead of 50 ksi, or modeling an FRP wrap with orthotropic stiffness—the software needs explicit input. Without custom properties, analysis results can be unconservative (using overly soft materials) or unnecessarily expensive (overdesigning with default moduli). Custom material properties let you input exact values for modulus of elasticity, Poisson’s ratio, shear modulus, density, thermal expansion coefficient, and damping. For nonlinear analysis, you can also define stress-strain curves, strain-hardening slopes, tension stiffening, or damage models. This level of control is essential for optimizing designs, reducing material costs, and meeting advanced performance requirements such as those in seismic retrofit or lightweight aerospace structures.

Workflow: Defining a Custom Material in RISA

RISA offers two related tools for material definition: the standard Material Editor (available in RISA-2D, RISA-3D, and RISAFloor) and the more advanced Custom Material Definition within the section property modules. The following steps apply to the most common approach:

  1. Open the Material Editor. From the main menu, select Properties > Material Editor or click the Materials button on the toolbar. This opens a dialog where you can view all predefined materials.
  2. Create a new material. Click Add or Duplicate a similar existing material (e.g., duplicate A36 steel and rename to “A572 Gr65”). Duplicating carries over the material type (steel, concrete, timber, etc.), which sets default behavior for code checks and design.
  3. Input standard isotropic properties. For most materials, you will specify:
    • Modulus of Elasticity (E) – often in ksi or psi.
    • Poisson’s Ratio (ν).
    • Shear Modulus (G) – auto-calculated from E and ν, but you can override for orthotropic cases.
    • Density (weight per unit volume) – critical for seismic mass and self-weight.
    • Thermal expansion coefficient (α) – required for thermal load combinations.
  4. Enter strength and design parameters. These values govern code-based design checks (AISC, ACI, NDS, etc.):
    • Yield stress (Fy) for steel, or compressive strength (f’c) for concrete.
    • Ultimate tensile strength (Fu) for steel.
    • Allowable stress values for timber or aluminum.
    • Modulus of rupture for concrete.
  5. Access custom / advanced properties. In the same editor, look for tabs or buttons labeled Advanced, Nonlinear, or Custom. Here you can define:
    • Multi-linear stress-strain curves (for plastic analysis).
    • Strain hardening modulus or tangent modulus.
    • Temperature-dependent degradation (creep, relaxation).
    • Orthotropic or anisotropic stiffness values (e.g., for composites).
    • Damping ratios for dynamic analysis.
  6. Save and assign. After entering all data, click OK or Save. The material now appears in your project tree. To use it, select the members or plates and assign the custom material through the Properties tab.

For members that require section-specific material orientation (such as composite laminates with different ply angles), you may need to define a Custom Cross Section or use the Composite Section Builder in RISA-3D, which allows layering of materials with individual properties. The same custom material can be assigned to different orientations within that builder.

Key Differences Between Material Types

RISA categorizes materials by structural behavior: isotropic, orthotropic, and anisotropic. Most steel and concrete are treated as isotropic (properties same in all directions). Timber, FRP, and modern composites are orthotropic (different stiffness along grain/ply directions). When you select “Wood” or “FRP” as the material type in the editor, additional fields for E parallel and perpendicular to grain, G values, and allowable stress ratios appear. For true anisotropic behavior (e.g., monocrystals or some 3D printed fill), you can define a full 6x6 stiffness matrix through the custom advanced tab. Use this only when backed by rigorous testing; most structural applications are adequately covered by orthotropic models.

Advanced Custom Properties for Nonlinear Analysis

Linear elastic analysis is the default in RISA, but for seismic design, pushover analysis, or connection detailing, you need nonlinear material definitions. The custom material editor provides several nonlinear models:

  • Elastic-Perfectly Plastic: Simplest nonlinear model. Define yield stress and keep post-yield stiffness zero. Common for steel beams under bending.
  • Strain Hardening: Define a bilinear or trilinear curve with specified hardening stiffness (often 1–2% of E). Improves accuracy for steel frames undergoing large deformation.
  • Concrete Damage Plasticity or Tension Stiffening: For concrete, input the compressive stress-strain curve (Hognestad, Kent-Park, or user-defined) and a tension stiffening factor to capture post-cracking behavior.
  • Temperature-Dependent Yield: Used for fire analysis. RISA allows defining yield strength reduction factors at discrete temperature points.
  • Hysteretic Damping: For dynamic nonlinear analysis, you can assign a material damping ratio that varies with strain amplitude.

When inputting nonlinear curves, use enough points to capture the shape but avoid abrupt jumps that cause convergence issues. A minimum of 5–7 points for the compressive branch and a softening branch for tension is recommended. Always run a preliminary analysis on a single element to verify the response (e.g., apply a monotonic load and plot stress vs. strain).

Validating Nonlinear Custom Materials

A crucial step that many engineers skip is validating the custom material against a known test or hand calculation. Create a small model in RISA—a single truss element or a short column—and apply a load that will push the material past yield. Compare the force-displacement curve with manual calculations or published data. This check catches errors in input units (e.g., entering ksi when model uses psi) or unintended softening.

RISA’s Results>Tabulated List>Material Strain and Material Stress can be exported to verify the state of each integration point. If using a custom concrete model, ensure the compressive strength reaches the specified f’c at the correct strain (typically 0.002–0.003). If using a steel hardening model, verify that the ultimate stress matches Fu at the desired strain. Documentation of this validation should be kept in your project calculation package.

Practical Applications: Specialized Materials

Custom material properties shine in projects that demand performance beyond typical codes. Here are three common scenarios with step-by-step guidance.

1. Fiber-Reinforced Polymer (FRP) Wraps

FRP is used for column wrap strengthening, slab retrofits, and beam shear reinforcement. FRP is orthotropic: high strength and stiffness along the fiber direction, low across it. In RISA, define the material as “Other” or “FRP” and set:

  • E1 (longitudinal): 20,000–30,000 ksi (carbon fiber).
  • E2 (transverse): 1,500–2,000 ksi.
  • G12: 800–1,000 ksi.
  • Poisson’s ratio ν12: 0.2–0.3.
  • Ultimate tensile stress in fiber direction: 350–550 ksi.
  • Tensile failure strain: 1–2%.

Because FRP is nearly linear elastic to failure, no plasticity is needed. However, for seismic retrofit, you may want to assign a small damping ratio (1–2%). Assign the material to shell elements or to a wrapped section using the Composite Section Builder. Ensure the fiber orientation aligns with the local axis of the element (typically local 1-2 axes).

2. High-Strength Steel with Strain Hardening

Steels such as A572 Grade 65, A913 Grade 65, or ASTM A1085 for HSS sections require custom Fy and Fu. For performance-based design (e.g., FEMA P-695), you will also need the expected yield strength (Ry*Fy) and expected tensile strength (Rt*Fu). In RISA, create a duplicate of A992 steel and change the yield to 65 ksi and ultimate to 80 ksi. In the nonlinear tab, define a bilinear curve with a hardening slope of E/100 (typical for steel). The hardening slope should start at a strain of Fy/E + 0.001 (to account for the yield plateau) or directly at Fy/E if no plateau. This custom material allows the analysis to accurately capture drift and ductility demand in a steel moment frame.

3. Cross-Laminated Timber (CLT) with Orthotropic Properties

CLT is a wood composite with orthogonal layers. In RISA, select material type “Wood” and then change the orientation-dependent properties:

  • E parallel to grain (E0): 1,800,000 psi for typical SPF.
  • E perpendicular to grain (E90): 85,000 psi.
  • Shear modulus G: 120,000 psi.
  • Allowable bending stress (Fb) based on NDS adjustments.

Because CLT is often modeled using shell elements, you must define a layered shell or use the Composite Section Builder where each layer can have its own material orientation. RISA 3D allows you to assign a local material angle per ply by using the “Ply Angle” parameter in the laminated section definition. Custom material properties for CLT should also include the rolling shear modulus, which is roughly one-tenth of the in-plane shear modulus.

Best Practices for Reliable Results

Using custom materials introduces additional responsibility. Follow these guidelines to avoid errors:

  • Source data from trusted references. Use manufacturer data sheets, published test results, or code-recognized values (e.g., AISC 360, ACI 318, NDS). For composite materials, refer to ASCE 41 or ICC-ES evaluation reports.
  • Maintain a material library file. RISA allows you to export the current material library to an .xml file. Save this file as a company standard so that the same custom materials are used across projects. This ensures consistency and reduces re-entry.
  • Use consistent units. RISA works in either U.S. customary or metric units. Double-check that the values you input match the units system of your model. A common mistake is entering E in psi when the model is set to ksi (off by a factor of 1000).
  • Test on a submodel. Before running a full building analysis with 10,000 elements, apply the custom material to a simple two-bay frame or a single column. Apply gravity and lateral loads and inspect deflections and stresses. Compare with hand calculations or code provisions.
  • Document all custom parameters. Keep a separate design note (or a sheet within the RISA model) that lists each custom material with its source and date. This is invaluable during peer review and when revisiting the model months later.
  • Update properties with new data. Material properties can change over time due to batch variations or revised standards. Revisit your custom library at least annually and adjust as needed.

Common Pitfalls and How to Avoid Them

Even experienced engineers can stumble when defining custom materials. Here are frequent mistakes and corrections:

  • Nonlinear curves not matching experimental data. Many users input a trilinear curve with a sudden drop after ultimate, causing numerical instability. Instead, use a smooth descending branch or add a large plastic strain capacity (e.g., 5% for concrete) to improve convergence.
  • Ignoring shear modulus for orthotropic materials. If you define different E values but leave G as the default (computed from isotropic formula), the analysis will underpredict shear deformations. Always input G explicitly for orthotropic materials.
  • Using custom materials in code checks incorrectly. RISA’s design modules (e.g., steel design per AISC) use the assigned material strength values. If you set Fy to 65 ksi but the member is a W-section that is only listed as A992 in the section database, RISA may still use the section’s nominal Fy (50 ksi) for certain limit states. Ensure the section properties are also customized if needed, or use the “Section Definition” to create a new user-defined section with the correct material.
  • Overwriting existing materials. Instead of modifying a built-in material (e.g., “A992 Gr50”), duplicate it and rename. This preserves the original default and avoids accidental changes to other models that might use that material name.

Expanding Your Capabilities with Integrated Workflows

RISA’s custom material properties can be further leveraged when combined with other software tools. For example, you can export material definitions to RISAConnection for connection design with the same steel strength, or import material data from MATLAB or Python via the RISA API. For advanced research-oriented models, you can define custom temperature-dependent properties and run thermal-structural coupling. The API allows you to write scripts that generate a family of custom materials based on parametric studies—ideal for optimization or sensitivity analysis.

External resources for further guidance include:

Conclusion

Custom material properties in RISA are not just a niche feature—they are essential for modern structural engineering where standard libraries are insufficient. By mastering the workflow from basic input to advanced nonlinear definitions, you can model everything from high-strength steel to orthotropic composites with confidence. The key is rigorous validation, consistent documentation, and continuous learning. Implement these practices in your next project, and you will see improvements in both design accuracy and efficiency. RISA’s flexibility, combined with your material knowledge, makes the impossible possible in structural analysis.