Understanding Load Cases in RISA: The Foundation of Structural Analysis

Setting up load cases correctly in RISA is one of the most critical steps in any structural engineering workflow. Whether you are designing a simple steel beam or a complex multi-story building, the accuracy of your results depends directly on how well you define and organize the loads your structure will experience. Load cases in RISA represent distinct scenarios such as dead load, live load, wind, snow, seismic, and thermal effects. Each load case captures a specific type of force, and the software uses these individual cases to perform linear or nonlinear analysis, compute reactions, and generate design checks.

Engineers often underestimate the time investment required to set up load cases properly. Rushing through this phase can lead to incorrect member sizing, missed code checks, or even safety hazards in the final design. A well-structured load case setup, however, saves hours of troubleshooting later and ensures that every possible loading condition is evaluated. This article covers the best practices every RISA user should follow, from naming conventions and unit consistency to advanced techniques for managing load combinations and automating repetitive tasks.

Best Practices for Setting Up Load Cases

The following best practices form a solid framework for building robust load case definitions. They apply to both RISA-3D and RISAFloor, but the principles are universal.

Define Clear and Descriptive Load Case Names

Using names like "DL_Roof", "LL_Office", or "Wind_N_S" might seem trivial, but clear naming pays dividends when you later review results or share models with colleagues. Avoid generic names like "Load 1" or "Case A". Instead, adopt a consistent naming convention that includes the load type, location, and direction. For example, "DL_Roof" distinguishes from "DL_Floor2". In large models with dozens of load cases, searchability and readability become essential. RISA allows up to 80 characters for names, so take advantage of that.

Why it matters: During the design review phase, a well-named load case immediately tells an experienced engineer what is being analyzed. It also helps when you export load case results to spreadsheets or reports.

Use Consistent Units Throughout

RISA supports multiple unit systems, but mixing units within a model is a recipe for disaster. Always set your preferred units (kips and feet, or kN and meters, or lb and inches) before you input any loads. This applies not only to the load values themselves but also to material properties, section dimensions, and member lengths. If you import a DXF or link to another model, double-check that unit conversion is handled correctly.

Pro tip: RISA allows you to change the display units without affecting the underlying data. However, it is safest to work in one consistent unit system and convert only at the final output stage. Inconsistent units have been the cause of many structural failures in practice, so treat this as a non-negotiable rule.

Separate Load Types into Individual Cases

A common mistake is to combine multiple load types into a single load case. For example, putting dead load plus live load into one case called "Total Gravity". This approach prevents you from understanding the contribution of each load type to internal forces and deflections. Worse, it makes it impossible to apply different load factors for different code combinations. Always create separate load cases for each load type and each unique direction or distribution. Typical separate load cases include:

  • Dead load (self-weight + superimposed dead loads)
  • Live load (floor live, roof live, reduced live loads)
  • Wind loads (in each principal direction, sometimes with eccentricity)
  • Seismic loads (based on equivalent lateral force or response spectrum)
  • Snow loads (balanced, unbalanced, drift)
  • Thermal loads (if required)
  • Construction loads or special loads

Separating these load types allows you to use RISA's automatic load combination generator, which applies the correct ASCE 7 or IBC load factors to each case. It also lets you view deflection envelopes under service loads separately from strength-level loads.

Apply Appropriate Load Factors

Load factors are multipliers applied to nominal loads to account for uncertainties and to provide a margin of safety. In RISA, you can define load factors globally or per load combination. However, the individual load cases themselves typically use a factor of 1.0 for nominal values. The factored loads appear only in the load combinations. This separation keeps the model clean and auditable.

Be aware that some loads, such as wind and seismic, may require directionality factors (e.g., 0.75 for wind directionality per ASCE 7). RISA allows you to incorporate these factors either within the load case definition or within the combination. The recommended practice is to apply directionality factors in the combination so that the nominal load case remains pure and can be reused for different scenarios (e.g., service wind vs. strength wind).

Check your code edition: Always verify that the load factors you use align with the building code that governs your project. ASCE 7-16, ASCE 7-22, and IBC 2021 have different requirements for factors such as wind load factor (1.0W for LRFD? no, 1.0W under ASD, 1.6W under LRFD – but be careful). RISA's built-in combination generator can be customized, but it's wise to manually review the generated list for completeness.

Include Comprehensive Load Combinations

Individual load cases are only half the story. Load combinations combine multiple load cases with appropriate factors to produce the worst-case scenario. RISA can automatically generate load combinations from a chosen code, but you should verify that the list covers all required combinations (including serviceability, construction, and seismic overload combinations). Common codes like ASCE 7 include dozens of combinations, and missing one could lead to an under-designed member.

Best practice is to generate combinations automatically, then manually review them for completeness. Pay special attention to:

  • Seismic combinations that include the overstrength factor (Ω₀)
  • Partial combinations (e.g., 0.9D + 1.0E for overturning under ASD)
  • Combinations with drifting snow or unbalanced snow
  • Combinations that consider multiple wind directions simultaneously (especially for wind on open structures)

If you are working on an unusual structure, you may need to create custom combinations. RISA allows manual entry of combinations, but be careful to avoid duplicates and logical errors.

Regularly Save and Document Your Setup

Load case definitions are part of the model intellectual property. As you iterate on a design, you may modify loads. Always save versions of the model with different load stages. Use the model notes area in RISA to document assumptions: where loads came from, what code edition was used, and any special considerations. This documentation is invaluable when another engineer takes over the project or when you revisit the model months later.

Version control: Consider using a file naming convention like "ProjectName_v1.r3d", "ProjectName_v2.r3d". Also, export load case definitions to a spreadsheet or PDF for a quick reference.

Advanced Tips for Efficient Load Case Management

Once you have mastered the basics, you can leverage RISA's advanced features to speed up your workflow and reduce errors.

Use Load Case Templates

RISA allows you to save load case templates that can be imported into future models. If you frequently work with similar building types (e.g., steel office buildings or concrete parking structures), create a master template with pre-defined load cases for dead, live, wind, seismic, snow, and common combinations. This saves hours of setup time and ensures consistency across projects.

To create a template, set up your load cases in a clean model, then save it as a standalone file. When starting a new project, import the load cases from that template. You can then adjust specific load values as needed.

Automate Load Case Generation with Spreadsheets

For large models with zones or multiple floor levels, manual load input is tedious. RISA supports copying and pasting load values from Excel spreadsheets. You can create an external spreadsheet that calculates tributary loads, snow drifts, or wind pressures, then paste the results into the appropriate load cases. This reduces manual entry errors and speeds up iteration. Just ensure the units match.

Use Envelope Load Cases for Preliminary Design

During early design stages, you might not need every load combination. Create envelope load cases that combine several gravity loads with a few wind or seismic cases. This lets you quickly size members before running the full set of combinations. RISA's envelope feature helps you see the maximum and minimum forces across a set of load cases. However, be aware that envelope cases do not replace the detailed code combination check for final design.

Take Advantage of Auto Self-Weight

RISA can automatically calculate the self-weight of members based on the defined material and section properties. In the load case definition, simply enable the "Self-Weight" multiplier. For dead load cases, set this multiplier to 1.0 (or 1.05 to account for connections and small attachments). This eliminates the need to manually calculate and input the weight of every beam and column. Remember that automated self-weight does not include superimposed dead loads like flooring, cladding, or MEP systems – those must be added separately.

Common Mistakes to Avoid

Even experienced engineers fall into these traps. Awareness can help you avoid them.

  • Putting loads directly into combinations without creating separate load cases. This makes it impossible to see raw nominal loads and complicates code-compliant factor application.
  • Forgetting to clear previous loads when modifying a load case. RISA keeps cumulative loads unless you delete or overwrite. Always double-check that the load case contains only the intended loads.
  • Ignoring the effect of load duration on wood structures. In RISA, if you are using wood materials, load duration factors (CD) depend on the load case type. Make sure your load case definitions match the CD required (e.g., 1.15 for snow, 1.6 for wind or seismic).
  • Using the same load case for both strength and serviceability. While you can combine factors in the load combination, it's cleaner to create separate load cases for service-level wind (say, 10-year wind) and ultimate wind if needed, or use factors in the combination.
  • Not running an envelope check before final design. An envelope analysis can catch critical forces that might be missed when looking at individual combinations.

External Resources for Further Reading

To deepen your understanding of load case setup and structural analysis, refer to the following authoritative sources:

These references will help you stay current with evolving code requirements and avoid common pitfalls.

Conclusion

Setting up load cases in RISA is not just an administrative task; it is a core engineering activity that directly impacts the safety, efficiency, and reliability of your design. By following the best practices outlined above—using clear naming, separating load types, applying correct factors, generating accurate combinations, and documenting your work—you create models that are easier to review, less prone to errors, and more aligned with governing codes. Investing time upfront in a well-organized load case structure pays off in reduced analysis time, fewer redesign cycles, and greater confidence in your final results. Whether you are a novice or a seasoned RISA user, continuously refining your load case setup process is one of the highest-leverage improvements you can make to your structural engineering practice.