Study of Snow Load on Steel Frame Structures in Alaska

Alaska’s extreme climate presents unique challenges for steel frame structures, particularly due to heavy snow loads, drifting, and prolonged snow retention. This study examines key considerations for designing steel buildings in Alaska to withstand snow loads effectively.

1. Alaska’s Snow Load Characteristics

Regional Variations

Alaska’s snow loads vary significantly by region:

Key Factors Influencing Snow Loads

• Snow Density – Wet, coastal snow (Southeast) is heavier than dry Interior snow.

• Wind Drifting – Causes uneven accumulation (leeward sides of roofs, parapets).

• Thermal Effects – Heat loss from buildings can lead to ice dams and increased loads.

• Duration of Snow Cover – Long winters mean snow persists for months, increasing creep effects on structures.

2. Design Codes & Standards for Alaska

Alaska follows ASCE 7 (Minimum Design Loads for Buildings) but has additional state-specific amendments:

• Alaska Building Code (2020) – References ASCE 7 with modifications for local conditions.

• ASCE 7-22 Snow Load Calculations – Uses:

  • Ground Snow Load (Pg) – From Alaska-specific maps.

  • Roof Snow Load (Pf) – Adjusted for exposure, thermal, and slope factors.

Critical Design Equations (ASCE 7)

1. Flat Roof Snow Load (Pf)

   $$P_f=0.7\cdot C_e\cdot C_t\cdot I_s\cdot P_g$$

   • $C_e$ = Exposure factor (0.7–1.2, lower for windy areas)

   • $C_t$ = Thermal factor (1.0 for unheated, 1.1–1.2 for heated)

   • $I_s$ = Importance factor (1.0–1.2 for risk category)

2. Sloped Roof Snow Load (Ps)

   $$P_s=C_s\cdot P_f$$

   • $C_s$ = Slope factor (reduces load for steeper roofs).

3. Drift Loads – Must be calculated for parapets, roof steps, and adjacent structures.

3. Structural Design Considerations

A. Roof Shape & Slope

• Steeper Roofs (≥30°) – Encourage snow sliding, reducing load.

• Curved Roofs – May require uneven load analysis due to drifting.

• Mono-Pitch & Gable Roofs – Common in Alaska; must account for leeward drift accumulation.

B. Steel Frame Reinforcement

• Stronger Purlins & Girts – Thicker gauges for high-load areas.

• Increased Column Sizing – To handle unbalanced snow loads.

• Moment-Resisting Frames – For large-span buildings (aircraft hangars, warehouses).

C. Snow Retention Systems

• Snow Guards – Prevent sudden avalanches (critical for steep metal roofs).

• Heated Roof Edges – Reduce ice dam formation.

D. Anchorage & Uplift Resistance

• Wind + Snow Interaction – Heavy snow can increase wind uplift forces.

• Enhanced Base Plate Connections – To prevent steel columns from buckling under combined loads.

4. Case Study: Steel Warehouse in Anchorage

• Location: Anchorage (Pg = 120 psf)

• Roof Type: Gable, 4:12 slope (18.4°)

• Calculated Roof Snow Load:

  $$P_f=0.7\times 1.0(exposure)\times 1.1(heated)\times 1.0(risk)\times 120=92.4\text{psf}$$

  • Slope Reduction (Cs ≈ 0.94) → Final design load: 87 psf

• Drift Load Addition: Up to 1.5 × Pf (130 psf locally).

Design Outcome:

• Steel Frame: Wider flange beams (W12x65 instead of W10x49).

• Roof Purlins: 16-gauge instead of 18-gauge.

• Bracing: Additional cross-bracing for lateral stability.

5. Future Trends & Innovations

• Real-Time Snow Load Monitoring – IoT sensors to detect excessive accumulation.

• Advanced Coatings – Slippery surfaces to encourage snow shedding.

• Climate-Adaptive Designs – Adjusting for increasing snowfall variability.

6. Conclusion

Designing steel frame structures in Alaska requires:
✅ Site-specific snow load analysis (ASCE 7 + Alaska amendments).
✅ Robust steel framing (stronger purlins, drift-resistant shapes).
✅ Snow retention & thermal management (heated edges, snow guards).
✅ Wind-snow interaction checks (uplift resistance).

For custom calculations, consult an Alaskan structural engineer familiar with local conditions.