Understanding the Challenge of Snow Loads in Cold Regions

Structural analysis in cold regions demands rigorous attention to snow loads, which can be the dominant live load on a building envelope. Snow accumulation patterns are not uniform; they depend on geographic location, local climate, building geometry, and even microclimatic effects such as wind drifting. In regions such as the northern United States, Canada, Scandinavia, and mountainous areas, ground snow loads can exceed 100 psf (4.8 kN/m²), making accurate modeling essential for safety and compliance. RISA (Rapid Interactive Structural Analysis) provides a comprehensive platform for engineers to define, apply, and analyze snow loads with precision. This article explores a production‑ready workflow that leverages RISA’s capabilities to address the unique demands of cold‑region structures.

Snow Load Fundamentals and Code Requirements

Before using RISA, engineers must understand the underlying snow load provisions. Most building codes in North America reference ASCE 7 (American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures). Key parameters include:

  • Ground snow load (pg) – derived from local weather records and maps provided in the code or via local jurisdiction.
  • Exposure factor (Ce) – accounts for wind exposure (fully exposed, partially exposed, sheltered).
  • Thermal factor (Ct) – reflects the heat loss from the building that can affect snow melt and ice dams.
  • Importance factor (Is) – based on the building’s risk category (I through IV).
  • Snow load on roof (pf) – calculated as a function of ground load multiplied by the above factors, with additional considerations for roof slope (μ) and snow drift.

ASCE 7 Equation 7.3-1 gives the flat roof snow load as pf = 0.7 Ce Ct Is pg. For sloped roofs, a reduction factor (Cs) is applied. Engineers must also account for unbalanced loads on gable roofs and snow drifts at parapets, roof steps, and obstructions. RISA allows these load conditions to be defined precisely using its load case and load combination editors.

Setting Up a RISA Model for Cold‑Region Analysis

Defining Materials and Member Sections

Start by creating a new model in RISA (2D or 3D). Under the Materials tab, define the structural materials – typically steel, reinforced concrete, or timber. Ensure that material properties (yield strength, modulus of elasticity) are set according to the applicable building code (e.g., AISC 360 for steel, ACI 318 for concrete). For member sections, use the section database or define custom sections. Critical sections in cold regions (e.g., roof joists, rafters, girts, and columns) should be sized conservatively to handle combined snow and wind loads.

Geometry and Support Conditions

Draw the structure using RISA’s graphical interface or import from CAD. Accurate geometry is vital because roof slope directly affects snow load calculation. For a gable roof, define the ridge line and ensure that all roof members are oriented correctly. For flat roofs, note that snow may accumulate more heavily near edges. Use fixed, pinned, or roller supports as appropriate; foundations in cold regions must also account for frost heave, though that is outside RISA’s typical scope.

Loading Snow Loads in RISA

Defining Load Cases

Create a dedicated Load Case for snow. In the Loads menu, select “Add New Load Case” and name it (e.g., “Snow Load – Balanced”). Under Source, choose “Area Load” or “Distributed Load” depending on the element type. For roof surfaces, area loads are applied to plates or to members using load distribution. RISA supports multiple load types: point, line, area, and temperature loads. For snow, area loads on the roof plane are most common.

To model the balanced snow load pf, compute the magnitude from code equations and enter it in the appropriate load case. For example: If pg = 60 psf, Ce = 1.0 (partially exposed), Ct = 1.0 (heated structure), Is = 1.0 (Risk Category II), and pf = 0.7 × 1.0 × 1.0 × 1.0 × 60 = 42 psf. For sloped roofs, multiply by the slope factor Cs (e.g., 1.0 for slopes ≤ 30° with certain surface conditions).

Unbalanced and Drift Loads

Beyond balanced loads, engineers must apply unbalanced snow loads and drift loads. In RISA, add additional load cases for each scenario. For gable roofs, ASCE 7 requires unbalanced loads where one side is loaded with 0.5 pf and the other with pd (drift load). Drift loads arise from wind redistributing snow; they can be significant at parapets, roof steps, and valleys. Use RISA’s Load Combination Tool to combine these with other loads (dead, live, wind, seismic) as per the code.

Using RISA’s Snow Load Generator

RISA (particularly RISA‑3D) includes an automated Snow Load Generator that simplifies this process. After setting the roof geometry and specifying the ground snow load, exposure, thermal, and importance factors, the software calculates balanced and unbalanced loads, including drifts, according to ASCE 7. The load generator can apply these loads directly to the roof members. Review the generated loads carefully to ensure they match local code requirements – some jurisdictions adopt modifications (e.g., increased importance factors for essential facilities).

If the snow load generator is not available, manual entry is straightforward: compute pf and drift heights using the equations in ASCE 7 Chapter 7, then apply as distributed loads along roof surfaces. Use RISA’s Area Load or One‑Way Distribution to transfer loads from roof surfaces to beams. For curved roofs (e.g., barrel shells or domes), use the appropriate drift formula and apply variable loads along the member length.

Running the Structural Analysis

With all load cases defined, proceed to Run Analysis. RISA performs linear static analysis by default, but for snow‑laden structures, second‑order effects (P‑Δ) may be important, especially for slender members. Enable P‑Delta analysis in the solution options. The analysis will compute member forces, reactions, deflections, and stability data.

Monitoring Deflections and Code Checks

Long‑term snow loads can cause excessive deflections that damage cladding, disrupt drainage, or lead to ponding. Set deflection limits in accordance with the code (e.g., L/240 for total load, L/180 for snow alone). RISA’s Deflection Report highlights members that exceed user‑defined limits. Pay special attention to roof beams and purlins – they are most vulnerable.

For steel structures, perform an AISC 360 code check using RISA’s design modules. The software evaluates each member’s stress ratio (demand‑to‑capacity) and warns when ratios exceed 1.0. For concrete, use RISA’s RC design tools to verify flexural and shear capacities. Timber members must comply with the National Design Specification (NDS).

Interpreting Results and Iterative Design

After the initial analysis, review the Member Force Diagrams for snow load combinations. Look for axial, shear, and moment envelopes. Common trouble spots include:

  • Eave members where drift loads pile up.
  • Valley beams collecting snow from multiple roof planes.
  • Columns at roof steps where drift loads create high point loads.
  • Cantilevered overhangs prone to ice buildup and heavy snow.

If stress ratios exceed allowable values, resize members using RISA’s Member Optimization tool. Adjust section sizes or material properties and re‑run the analysis. Use load combination envelopes to ensure all limit states are satisfied. For example, a snow‑only combination may produce large deflections, while a snow‑plus‑dead combination could govern strength.

Considering Snow Melt and Rain‑on‑Snow

In cold regions, thermal effects are significant. Heated buildings melt snow from below, potentially causing ice dams at eaves. RISA does not directly model thermal stress from snow melting, but you can account for the reduced load due to melting by adjusting the thermal factor Ct. For unheated structures (e.g., warehouses), Ct = 1.2 (ASCE 7 Table 7.3-2) because snow accumulates more. For heated structures, Ct = 1.0 or less. Additionally, rain‑on‑snow surcharge must be considered where ground snow loads are low (≤ 20 psf). This extra load can be added as a separate load case or included in the snow load per code.

Advanced Topics: Dynamic Effects and Snow Creep

Snow loads are generally treated as static, but drifting and sliding snow can create impact loads. For structures supporting heavy snow, consider snow creep – particularly on steep roofs or where monitoring is required. RISA can model snow creep by applying an equivalent static load profile that increases near the ridge. While beyond typical design, understanding these phenomena helps avoid serviceability issues.

For structures located in regions with very deep snow (e.g., mountain resorts), consult specialists and reference the International Building Code (IBC) snow load provisions. RISA’s flexibility allows you to input custom load patterns derived from site‑specific snow surveys or wind tunnel tests.

Practical Workflow Summary

  1. Gather data: Obtain ground snow load pg, exposure, thermal, and importance factors from the local building code. For U.S. projects, use ASCE 7‑16 or newer.
  2. Create RISA model: Define materials, sections, geometry, and supports. Ensure roof slope and geometry are accurate.
  3. Generate snow loads: Use the Snow Load Generator for balanced, unbalanced, and drift loads. Manually adjust if needed for unusual roof shapes.
  4. Define load combinations: Follow the code (ASCE 7 or IBC) for strength and serviceability combinations. Include dead, live, wind, and seismic as applicable.
  5. Run analysis: Enable P‑Delta if needed. Check reactions, forces, and deflections.
  6. Review code checks: Review stress ratios and deflections. Envelopes should show maximums.
  7. Optimize: Resize members that fail. Use RISA’s optimization or manual iteration.
  8. Document: Export reports showing load cases, reactions, member lists, and code checks for peer review and permitting.

External Resources for Engineers

To deepen your understanding of snow load analysis and RISA’s tools, consider these authoritative references:

Conclusion

RISA offers a powerful and efficient workflow for analyzing structures in cold regions subjected to snow loads. By mastering the input of code‑compliant snow loads – balanced, unbalanced, and drift – and by leveraging RISA’s analysis and design tools, engineers can ensure safe, economical, and serviceable structures. The iterative process of modeling, analyzing, interpreting results, and refining sections is made more manageable with RISA’s built‑in load generation and automated code checks. Whether designing a ski lodge in the Rockies or a school in the Upper Midwest, a disciplined approach to snow load analysis using RISA will lead to robust buildings that withstand the harshest winters.