Introduction: Accelerating Early-Stage Structural Decisions with RISA

In the fast-paced world of structural engineering, the ability to quickly assess the feasibility of a design concept can mean the difference between winning a project and losing to a competitor. RISA (Rapid Interactive Structural Analysis) software, developed by RISA Technologies, stands out as a premier tool for conducting rapid structural feasibility studies. By enabling engineers to model, analyze, and iterate on structural systems within hours rather than days, RISA reduces the upfront investment in detailed calculations while still delivering actionable insights. This article provides a comprehensive guide to using RISA for rapid feasibility studies, covering the essential workflows, advanced features, and best practices that allow engineers to evaluate multiple design alternatives efficiently and communicate findings clearly to stakeholders.

Understanding RISA’s Role in Feasibility Studies

A structural feasibility study is a preliminary evaluation performed to determine whether a proposed structural system can meet performance, safety, and cost requirements. Unlike detailed design, feasibility studies rely on simplified models that capture the critical behavior of the structure without requiring every connection or secondary element. RISA is uniquely suited for this task because it combines powerful analysis engines with an intuitive interface that allows engineers to build models quickly and interpret results at a glance. The software’s real-time feedback on member forces, deflections, and stability enables rapid iteration, making it an ideal platform for exploring “what if” scenarios during the earliest stages of a project.

RISA’s suite of products—including RISA-3D, RISAFloor, RISAFoundation, and RISAConnection—address different aspects of structural design. For feasibility studies, RISA-3D is often the primary tool because it handles general 3D frame and truss analysis, but RISAFloor can streamline the assessment of gravity systems in multi-story buildings, and RISAFoundation can quickly check footing sizes and soil bearing requirements. Knowing which product to use for each phase of the feasibility study helps maintain speed without sacrificing accuracy.

Key RISA Products for Preliminary Design

RISA-3D: The Workhorse for Frame and Truss Analysis

RISA-3D supports linear static analysis, P-delta effects, modal analysis, and response spectrum analysis. For feasibility studies, engineers typically stick to linear static analysis with simple load combinations. The ability to quickly assign member sizes from a library of steel, concrete, timber, or aluminum sections allows the model to reflect realistic stiffness without needing to design every connection. The real-time deflection display and member unity checks provide instant feedback on whether a concept is likely to work.

RISAFloor: Rapid Gravity System Evaluation

When the structure involves multiple floors with consistent bay sizes, RISAFloor allows engineers to define floor layouts, assign beam and girder spacing, and apply uniform loads. The software automatically calculates tributary loads and generates live load reduction, producing preliminary member sizes in minutes. This is especially useful for commercial or residential buildings where gravity loads dominate and lateral systems are secondary.

RISAFoundation: Quick Footing and Mat Checks

Foundations are often the first element to be sized during a feasibility study because they dictate the required site preparation and budget. RISAFoundation lets engineers input column loads from the superstructure model (or manually) and check isolated footings, combined footings, or mats against soil bearing capacity and overturning. The ability to run multiple footing configurations rapidly helps establish the foundation footprint early.

Step-by-Step Workflow for Rapid Feasibility Studies

The following workflow outlines how to use RISA to perform a feasibility study from start to finish, emphasizing speed and iteration over perfection.

Step 1: Define Project Parameters and Constraints

Before opening RISA, gather the essential project parameters: building dimensions, number of stories, approximate bay sizes, intended material (steel, concrete, timber, or masonry), and preliminary loads (dead load, live load, wind, seismic, snow). Determine which load combinations are relevant based on the governing code (e.g., ASCE 7). During feasibility, it is acceptable to use conservative load values; refinement comes later. Also establish constraints such as maximum deflection limits (L/240 for floors, L/120 for roofs) and any architectural restrictions that affect column spacing or floor-to-floor height.

Step 2: Build a Simplified Structural Model

One of the biggest time savers in RISA is the ability to create a simplified model that captures only the primary load-resisting elements. Omit secondary beams, infill walls, and non-structural components. Use the “Model” tab in RISA-3D to define grid lines, story levels, and member placement. For a typical frame, add columns, beams, and braces. Apply uniform loads directly to members or use area loads for floors and roofs. If the structure is regular, use the “Copy Up” feature to replicate floors. Resist the urge to model every detail; remember that the goal is feasibility, not final design.

For example, in a 10-story steel office building, model only the perimeter moment frames or concentrically braced frames, plus interior gravity columns and beams. Assign a typical W-section for beams and a heavier column section for lower floors. Do not model shear tabs or column splices at this stage.

Step 3: Assign Material Properties and Section Sizes

RISA includes extensive databases for steel sections (AISC), concrete sections (ACI), and others. For feasibility, you can use “Auto Select” to allow the software to choose the lightest section that meets strength and deflection criteria. This automation dramatically reduces iteration time. However, be cautious with Auto Select for columns because buckling lengths and slenderness ratios need careful input. Alternatively, manually assign a conservative section (e.g., W14x90 for all columns in a low-rise building) to see if deflections are acceptable. For concrete structures, define a rectangular section with estimated reinforcement ratio (1–2% for beams, 1–3% for columns) and let RISA compute the cracked moment of inertia if needed.

Step 4: Run Analysis and Review Results

With the model complete, run a static analysis. RISA-3D quickly computes reactions, member forces, deflections, and unity checks. Use the “Results” tab to scan the maximum story drift, maximum vertical deflection, and the highest unity check. A unity check above 1.0 indicates the member is overstressed; a value below 0.6 suggests the member is too heavy. Feasibility studies typically target unity checks between 0.7 and 0.9 to allow for future adjustments. If drifts exceed allowable limits (e.g., H/400 for wind), consider adding bracing or increasing member sizes in lateral frames.

Use the graphical display features: color-coded member deflections, moment diagrams, and reaction vectors. These visuals help quickly identify problematic regions. For instance, if a beam shows a deflection of 2 inches when the limit is 1 inch, you can immediately resize that beam or add a mid-span column.

Step 5: Iterate Quickly on Design Alternatives

The power of RISA for feasibility lies in its ability to support rapid iteration. After reviewing results, make one or two changes—such as increasing the beam depth, adding a brace, or changing the column grid spacing—and re-run the analysis. Often, multiple alternatives can be explored in a single session. Document each alternative by saving separate files or using the “Copy Project” feature. Compare key performance metrics: total steel weight, floor-to-floor height, and foundation loads. These comparisons enable objective decision-making when presenting options to architects or clients.

For example, in a feasibility study for a 5-story parking garage, you might compare a two-way flat slab system with a one-way pan joist system. Using RISA-3D with simplified models, you can evaluate the structural depth, deflection control, and column loads for each system in under an hour.

Step 6: Validate Key Results with Simplified Hand Calculations

While RISA is reliable, it is prudent to validate critical results—especially for unusual geometries or loadings. Perform a quick hand check for the maximum moment in the longest beam or the overturning moment at the base. This step catches modeling errors such as missing supports, incorrect release conditions, or mis-applied loads. It also builds confidence in the software output. For feasibility studies, validation does not need to be exhaustive; a single cross-check per structural system is sufficient.

Step 7: Prepare a Feasibility Report

Once the analysis is complete and alternatives are compared, compile a concise report that includes the model assumptions, key results (maximum deflections, unity checks, reactions), and recommendations. RISA’s built-in report generator can export tables and graphs, but for a feasibility study a simple summary with screenshots often suffices. Use the visuals to communicate with non-engineers; a color-coded deflection plot is far more persuasive than a table of numbers. Include a recommendation for the preferred structural system along with an estimated tonnage or cubic yards of concrete, which feeds into cost analysis.

Best Practices for Maximizing Efficiency

Use Templates and Pre-Saved Load Combos

RISA allows users to save templates that include common load combinations, material definitions, and even partial models. Create templates for typical building shapes (rectangular, L-shaped, etc.) and for different materials. Starting from a template can cut modeling time by 50% or more. Similarly, define a standard set of load combinations (1.2D + 1.6L, 0.9D + 1.0W, etc.) and reuse them across feasibility studies.

Leverage Automation Features

RISA’s “Auto Mesh” for floors and walls, “Auto Select” for steel sections, and “Auto Seismic Load” generators are powerful automation tools. In a feasibility study, using these features reduces manual input and accelerates the cycle of model → analyze → adjust. However, verify the default settings (e.g., seismic response factors) match the project parameters.

Integrate with BIM and Other Software

For larger projects, consider integrating RISA with building information modeling (BIM) software such as Revit or Tekla. While this adds setup time, it pays off when feasibility studies need to consider architectural constraints or when the model will be further refined later. RISA supports IFC export and direct links to Revit through the RISA-Revit Link. However, for very rapid feasibility studies, manual modeling in RISA is often faster than importing a heavy BIM model.

Collaborate with the Design Team

Feasibility studies are most effective when conducted in collaboration with architects, mechanical engineers, and cost estimators. Use RISA’s ability to quickly change column spacing or floor depth to answer questions in real time during a meeting. For instance, if an architect proposes a longer spanning beam to eliminate a column, you can adjust the model on the spot and show the resulting depth increase and its impact on floor-to-floor height.

Common Pitfalls to Avoid

Over-Modeling the Structure

The most common mistake in feasibility studies is including too much detail—modeling every lintel, stair stringer, or secondary beam. This defeats the purpose of rapid analysis. A good rule of thumb: if an element does not significantly affect the global stiffness or gravity load path, omit it. Focus on columns, major beams, lateral frames, and main load-bearing walls.

Neglecting Secondary Effects

While detailed dynamic analysis is not required for feasibility, ignoring P-delta effects can lead to unconservative results in tall or slender structures. RISA-3D includes a simple P-delta option that adds minimal computation time. Similarly, for structures in regions with moderate to high seismicity, a quick modal analysis to check the fundamental period can help determine whether the structure is stiffness- or flexibility-governed.

Using Inappropriate Loads

Feasibility studies often use higher loads than final design to provide a safety buffer. However, using unrealistically high loads can lead to oversized members that misrepresent costs. Instead, use code-minimum loads and note that future refinement may increase or decrease them. For example, use a live load of 50 psf for office areas rather than 100 psf, unless the client specifies heavy storage.

Failing to Document Assumptions

Rapid analysis means many decisions are made on the fly. Without documentation, it becomes impossible to justify the results later. Always note which load combinations were used, what material strengths were assumed, and any simplifications (e.g., rigid diaphragms, fixed base supports). This documentation also helps when the feasibility study transitions into detailed design.

Conclusion: Turning Concepts into Confidence

Rapid structural feasibility studies with RISA empower engineers to test design hypotheses quickly, providing clients with data-driven recommendations early in the project lifecycle. By following the systematic workflow outlined here—defining parameters, building simplified models, leveraging automation, and iterating—engineers can evaluate multiple structural systems, identify potential issues, and establish a solid foundation for detailed design. The ability to communicate findings through clear visualizations and concise reports further enhances the value of RISA as a decision-making tool.

For engineers new to RISA, the RISA Support and Training resources provide excellent tutorials and webinars. Advanced users can explore scripting with the RISA API to automate repetitive tasks. Ultimately, the key to successful feasibility studies lies not in the complexity of the model but in the clarity of the questions being asked. RISA supplies the speed; your expertise supplies the answers.