Understanding the Complexity of Skyscraper Structural Design

Designing a skyscraper is one of the most demanding challenges in structural engineering. Every high-rise must safely resist gravity loads, lateral forces from wind and earthquakes, and dynamic vibrations while remaining economically feasible. The engineering team must coordinate dozens of disciplines, from architecture to mechanical systems, and the structural system alone can involve thousands of interconnected steel beams, concrete columns, post-tensioned slabs, and deep foundations. RISA Structural Software has emerged as a trusted platform for modeling, analyzing, and designing these complex structures, enabling engineers to iterate quickly and confidently.

This case study walks through the real-world application of RISA’s suite of tools to design a modern skyscraper. We will examine each phase of the workflow, from initial conceptual modeling through final code-compliant design, highlighting how RISA's features streamline the process and improve accuracy.

Overview of the RISA Structural Software Suite

RISA Technologies offers a family of programs tailored to different stages of structural framing. The core product, RISA-3D, is a general-purpose finite element analysis and design tool capable of handling 3D models with various element types, including beams, columns, cables, and plates. For concrete buildings, RISAFloor provides a macro-level modeling environment for gravity systems, while RISAFoundation handles mat, spread, and pile cap design. Additional modules such as RISASection allow engineers to define arbitrary cross-sections, and RISAConnection automates welded and bolted connection design. The software integrates seamlessly with CAD and BIM platforms, enabling efficient exchange of model geometry and loads.

For skyscrapers, the typical workflow begins in RISA-3D or RISAFloor, depending on the structural typology. Engineers define the building's grid, assign material properties, and apply loads. After analysis, they review deflections, stress ratios, and stability checks before iterating on member sizes and reinforcement.

Learn more about the full product line at the RISA software product page.

Step 1: Initial Modeling and Geometry Definition

The design of a skyscraper always starts with geometry. For the case study skyscraper – a 60-story mixed-use tower with a rectangular footprint and a tapering profile – the architectural model provided the base grid and floor levels. Using RISA-3D, the structural engineer imported the architectural layout via DXF files and manually refined the column grid to a 9-meter by 9-meter module. The software's modeling environment allows for both graphical and spreadsheet input, giving the engineer full control over node coordinates, member orientation, and floor diaphragms.

One key advantage of RISA is its ability to handle complex geometry such as setbacks, transfer floors, and curved facades. For the tapered profile, the engineer defined the top floor's reduced footprint by adjusting column offsets and using sloped beams. The model included 120 floors of columns, 60 floor slabs, a concrete core at the building's center, and outrigger trusses at mechanical levels.

Material properties were assigned based on the project's specifications: Grade 50 steel for beams and columns in the lower zone, Grade 65 steel for high-strength columns, and normal-weight concrete with a compressive strength of 48 MPa for floors and the core. RISA's material database includes hundreds of predefined standards from AISC, ACI, and international codes, allowing quick selection and customization.

Defining Structural Elements with Precision

Once the grid and layout were established, the engineer defined each structural element. In RISA-3D, beams and columns are drawn as linear members with assigned cross-sections. For the skyscraper, the engineer created a library of custom wide-flange sections for the steel frame and rectangular hollow sections for the core columns. The software also supports tapered sections, which were used for the outrigger trusses to optimize material usage.

Slabs were modeled using RISAFloor as two-way concrete panels with post-tensioning tendons. The engineer specified the slab thickness (200 mm typical, 300 mm at podium levels), concrete cover, and tendon layout. RISAFloor automatically calculates the equivalent loads for post-tensioning and accounts for the effects of creep and shrinkage in long-term deflection analysis.

For the foundation, RISAFoundation was used to design a 3-meter-thick mat slab beneath the tower, with piles extending 40 meters into the bedrock. The software considered soil springs (Winkler model) to simulate soil-structure interaction, a critical factor for high-rise settlements and mat rotation. Pile capacities were obtained from geotechnical reports and input as compression and uplift stiffness values.

Step 2: Structural Analysis Methods for High-Rise Buildings

With the model complete, the analysis phase began. Skyscrapers require consideration of both gravity and lateral loads. Gravity loads (dead and live) were applied per floor with load factors based on ASCE 7-16. Live loads were reduced according to the code for storage and occupancy loads. For lateral analysis, the building location was assumed to be in a moderate seismic zone with basic wind speeds of 145 kph (90 mph) exposure C.

RISA-3D performed a linear elastic static analysis for gravity loads, followed by a response spectrum analysis for seismic loads. The engineer defined a natural period range (first three modes) and applied the site-specific design spectrum. The software calculated modal participation factors, base shears, and drift ratios. For wind loading, RISA-3D can generate surface loads based on building geometry and wind direction, but for this skyscraper, wind tunnel test data was imported as time-history load cases to account for dynamic cross-wind effects and vortex shedding.

To capture geometric nonlinearity (P-Delta effects), the engineer enabled the P-Delta analysis option in RISA-3D. This accounts for the additional bending moments induced by vertical gravity loads acting on the laterally displaced frame. For a 60-story building, P-Delta can increase second-order drifts by 15-25%, and ignoring it would lead to an unconservative design. RISA automatically performs iterative P-Delta analysis within each load combination, adjusting member stiffness until convergence.

Furthermore, the engineer conducted a modal response history analysis using the Direct Integration method to verify the building's performance under the maximum considered earthquake (MCE). RISA-3D supports nonlinear time history analysis with material nonlinearity defined via hinges for steel beams and fiber sections for concrete walls.

Analyzing Load Paths and Stress Distribution

Once the analysis ran, RISA generated detailed results including member forces, moments, reaction forces, and displacements. The engineer reviewed load path diagrams to ensure forces from gravity and lateral loads were transferred efficiently down to the foundation. The concrete core acted as the primary lateral-force-resisting system, with outrigger trusses connecting the core to perimeter columns to reduce drift.

RISA's Contour Map feature visualized stress distributions across floors and shear walls, revealing areas of high shear or bending that required additional reinforcement. For the steel framing, the software highlighted stress ratio contours per member, showing which beams and columns were overstressed (ratio > 1.0) or lightly stressed. The design team used this to adjust member sizes and optimize weight.

One common challenge in skyscrapers is controlling inter-story drift to prevent damage to cladding and partitions. The analysis showed that the maximum drift under seismic loading was 0.012 radian (H/500), exceeding the target of H/600. To reduce drift, the engineer increased the stiffness of the core walls by adding coupling beams and thickening the walls in the lower 20 floors. RISA's parametric capabilities allowed quick model modifications and reanalysis without rebuilding from scratch.

Step 3: Design Optimization and Code Compliance

After analysis, the engineer used RISA's design modules to check every structural element against applicable codes. The steel frame was designed to AISC 360-16 (Specification for Structural Steel Buildings), while the concrete core and slabs followed ACI 318-14. RISA-3D automatically selects the governing load combinations from ASCE 7 and performs strength checks for yielding, buckling, and lateral-torsional instability.

The design optimization feature in RISA-3D is particularly powerful for skyscrapers. The engineer can set target demand-to-capacity ratios (e.g., 0.90 for gravity members and 0.80 for lateral members) and let the software automatically resize steel sections to minimize weight. For this project, the optimizer reduced the total steel tonnage by 12% compared to the initial design, saving material cost without compromising safety.

Code compliance for concrete elements in RISAFloor involves checking flexural reinforcement, shear capacity, and crack control. The engineer specified a maximum crack width of 0.3 mm under service loads. RISAFloor calculated required reinforcement areas per strip and generated bar schedules for slabs and walls. For the mat foundation, RISAFoundation checked punching shear, flexural reinforcement, and settlement limits.

External link: The American Institute of Steel Construction (AISC) provides the code specifications used in the steel design.

Iterative Design Refinement and Peer Review

Structural design for a skyscraper is never a one-pass process. After initial optimization, the team presented the results to the architect and the geotechnical consultant. The architect requested a change in the core wall layout to accommodate larger elevator shafts. Using RISA's model edit capabilities, the engineer relocated the core walls by moving nodes and redefining boundary conditions. The software automatically updated all member connections and load paths. Reanalysis took less than 10 minutes on a standard workstation, demonstrating the tool's efficiency for iterative design.

The design also underwent an independent peer review. The reviewer requested a verification of the foundation design using an alternative method. The engineer used RISAFoundation to run a finite element analysis of the mat with different soil stiffness profiles and confirmed that the maximum settlement was within 50 mm (2 inches). RISA's reporting features generated comprehensive output tables and graphs that were easily included in the structural design report.

Case Study Walkthrough: 60-Story Mixed-Use Skyscraper

To illustrate the entire workflow, here is a more detailed walkthrough of a specific project: the fictional "Meridian Tower" – a 60-story building with a total height of 250 meters. The structural system combines a reinforced concrete core with a perimeter steel moment frame and two outrigger trusses at levels 20 and 40.

Step 1: Modeling in RISA-3D

  • Imported architectural grid from Revit via DXF.
  • Defined floor elevations: ground floor at 0 m, typical floors at 4 m spacing, mechanical floors at 5 m.
  • Assigned the concrete core as a group of wall panels with user-defined thickness: 800 mm at ground to 400 mm at top.
  • Modeled outrigger trusses as steel members with custom box sections.
  • Pinned connections for beams to reduce moment transfer and simplify detailing.

Step 2: Load Application

  • Dead load: self-weight of concrete and steel (automatically calculated), plus superimposed dead loads of 1.5 kPa for partitions and mechanical.
  • Live load: 2.0 kPa for office floors, 4.5 kPa for mechanical floors, 1.5 kPa for roof (with provisions for snow).
  • Seismic load: response spectrum from ASCE 7-16 for site class C, Ss = 1.2g, S1 = 0.4g.
  • Wind load: imported from CFD analysis as pressure time histories for three wind directions.

Step 3: Analysis and Results

  • First mode period: 5.8 seconds (translation in X-direction).
  • Base shear from response spectrum: 12,500 kN (0.08W).
  • Maximum roof drift under service wind: 600 mm (H/417) – slightly above target, so outrigger truss stiffness was increased.
  • Critical drift: 0.009 radian under MCE – acceptable for life safety performance.

Step 4: Design and Optimization

  • Steel beams: optimized from initial W24 sections to W21 in upper floors, saving 8% steel weight.
  • Core reinforcement: required 1.2% steel ratio in flexural zones; RISAFloor suggested #11 bars at 200 mm spacing.
  • Mat foundation thickness increased to 3.5 m after punching shear check at column locations.

External link: The American Society of Civil Engineers (ASCE) publishes the minimum design loads standard used in this project.

Integration with BIM and Collaborative Workflows

Modern skyscraper design rarely happens in isolation. RISA supports integration with building information modeling (BIM) platforms such as Revit, Tekla, and AutoCAD. For the Meridian Tower, the structural model was linked to the architectural model using RISA's Revit Interface. Changes in the architectural floor plan automatically updated the structural grid and column locations. This bidirectional link reduced errors and saved hours of manual rework. The structural engineer could also export analytical models directly to ETABS or SAP2000 for cross-validation if required by the client, though RISA's native analysis capabilities were sufficient for most design checks.

Additionally, RISA's integration with Microsoft Excel allowed the design team to run parametric studies on member sizes and material grades, evaluating cost scenarios. The software's open API enables custom scripts for automated model generation and result extraction, which advanced users leverage for repeated design tasks.

Conclusion

Designing a skyscraper demands a combination of advanced analysis, iterative optimization, and rigorous code compliance. RISA Structural Software provides the tools necessary to meet these demands in a unified environment. From initial 3D modeling with custom sections to nonlinear time-history analysis and fully automated code checking, RISA reduces the risk of manual errors and accelerates the design cycle. The case study of the Meridian Tower demonstrates that a 60-story building can be thoroughly modeled, analyzed, and designed within a single software platform, with results that are reliable and production-ready. For structural engineers tackling the next generation of supertall projects, RISA remains a vital component of the digital workflow. For further technical resources and case studies, visit the RISA case studies page.