Environmental Loads: Comprehensive Overview of Natural Forces, Load Types, Calculation Methods, and Applications in Structural Design

Environmental loads are critical to structural engineering, representing forces caused by natural phenomena and environmental conditions. This comprehensive guide explains what environmental loads are, types of environmental loads, how to determine them, and how to apply them in structural design and analysis.

---

What Are Environmental Loads?

Basic Definition

Environmental loads are temporary forces caused by natural phenomena and environmental conditions, including wind, snow, seismic activity, temperature changes, and moisture effects.

Expression:

• Environmental Load = Force from natural phenomena
• Measured in pounds (lbs) or kilopounds (kips)
• Variable magnitude and location
• Location-dependent
• Code-specified values

Characteristics:

• Natural origin
• Variable
• Location-dependent
• Unpredictable
• Code-regulated

Understanding Environmental Load Concept

Environmental loads indicate:

Natural Forces:

• Wind pressure
• Snow accumulation
• Earthquake motion
• Temperature changes
• Design parameter

Location Dependency:

• Varies by geographic location
• Varies by climate
• Varies by elevation
• Varies by terrain
• Design parameter

Magnitude Variation:

• Seasonal variation
• Annual variation
• Multi-year variation
• Extreme events
• Design parameter

Design Requirement:

• Determines member capacity
• Affects section size
• Affects cost
• Affects feasibility
• Critical parameter

---

Types of Environmental Loads

1. Wind Loads

Definition: Wind loads are forces resulting from wind pressure on building surfaces, varying by location, height, and wind speed.

Characteristics:

• Duration: Minutes to hours
• Magnitude: Variable by location
• Location: All surfaces
• Frequency: Regular
• Dynamic effects

Factors Affecting Wind Load:

Basic Wind Speed:

• Varies by location
• Determined by wind data
• Code-specified by region
• Typical: 85-200 mph
• Design parameter

Exposure Category:

• Category A: Urban areas (dense buildings)
• Category B: Suburban areas (scattered buildings)
• Category C: Open terrain (few obstructions)
• Category D: Coastal areas (water exposure)
• Design parameter

Height Factor:

• Increases with height
• Lower at ground level
• Higher at roof level
• Design parameter
• Affects tall structures significantly

Directionality Factor:

• Accounts for wind direction
• Typical: 0.85
• Design parameter

Gust Factor:

• Accounts for wind gusts
• Typical: 1.0-1.3
• Design parameter

Wind Pressure Calculation:

Formula:

• Wind Pressure = 0.5 × ρ × Cd × V²
• ρ = Air density (0.00238 slugs/cu ft)
• Cd = Drag coefficient (0.5-1.3)
• V = Wind velocity
• Design parameter

Example 1:

• Wind velocity: 100 mph
• Drag coefficient: 1.0
• Wind pressure = 0.5 × 0.00238 × 1.0 × 100²
• Wind pressure ≈ 11.9 psf

Example 2:

• Wind velocity: 120 mph
• Drag coefficient: 1.0
• Wind pressure = 0.5 × 0.00238 × 1.0 × 120²
• Wind pressure ≈ 17.2 psf

Example 3:

• Wind velocity: 150 mph
• Drag coefficient: 1.0
• Wind pressure = 0.5 × 0.00238 × 1.0 × 150²
• Wind pressure ≈ 26.8 psf

Typical Values:

Low Wind Areas:

• Basic wind speed: 85-100 mph
• Wind pressure: 10-15 psf
• Typical: 12 psf
• Examples: Southern California, parts of Texas

Moderate Wind Areas:

• Basic wind speed: 100-120 mph
• Wind pressure: 15-25 psf
• Typical: 20 psf
• Examples: Most of United States

High Wind Areas:

• Basic wind speed: 120-150 mph
• Wind pressure: 25-50 psf
• Typical: 35 psf
• Examples: Coastal areas, mountain passes

Hurricane Zones:

• Basic wind speed: 150-200 mph
• Wind pressure: 50-100+ psf
• Typical: 75 psf
• Examples: Florida, Gulf Coast, Hawaii

Wind Load Distribution:

Pressure on Windward Surface:

• Positive pressure (pushing)
• Typical: 0.8 × Wind pressure
• Pushes structure

Suction on Leeward Surface:

• Negative pressure (pulling)
• Typical: -0.5 × Wind pressure
• Pulls structure

Suction on Roof:

• Negative pressure (pulling)
• Typical: -0.7 to -1.0 × Wind pressure
• Pulls roof upward

Design Approach:

• Determine basic wind speed from code
• Apply exposure and height factors
• Calculate wind pressure
• Apply to all surfaces
• Design for lateral loads
• Verify overturning stability

Example:

• Basic wind speed: 100 mph
• Exposure factor: 1.0
• Height factor: 1.0
• Wind pressure ≈ 12 psf
• Design for 12 psf pressure

2. Snow Loads

Definition: Snow loads are forces resulting from snow accumulation on roof surfaces, varying by location and climate.

Characteristics:

• Duration: Seasonal (weeks to months)
• Magnitude: Variable by location
• Location: Roof surfaces
• Frequency: Annual
• Predictable patterns

Factors Affecting Snow Load:

Ground Snow Load:

• Varies by location
• Determined by climate data
• Code-specified by region
• Typical: 20-150 psf
• Design parameter

Exposure Factor:

• Sheltered (trees, buildings): 0.8
• Normal exposure: 1.0
• Exposed (open areas): 1.2
• Affects roof snow load
• Design parameter

Slope Factor:

• Flat roof (0-5°): 1.0
• Sloped roof (5-30°): 0.8-1.0
• Steep roof (30-70°): 0.5-0.8
• Very steep roof (>70°): 0.0
• Depends on slope
• Design parameter

Thermal Factor:

• Heated building: 1.0
• Unheated building: 1.1
• Affects snow load
• Design parameter

Importance Factor:

• Standard buildings: 1.0
• Important buildings: 1.1
• Critical buildings: 1.2
• Design parameter

Snow Load Calculation:

Formula:

• Roof Snow Load = Ground Snow Load × Exposure Factor × Slope Factor × Thermal Factor × Importance Factor
• Accounts for all factors
• Design parameter

Example 1:

• Ground snow load: 50 psf
• Exposure factor: 1.0
• Slope factor: 0.8
• Thermal factor: 1.0
• Importance factor: 1.0
• Roof snow load = 50 × 1.0 × 0.8 × 1.0 × 1.0 = 40 psf

Example 2:

• Ground snow load: 100 psf
• Exposure factor: 0.8
• Slope factor: 0.7
• Thermal factor: 1.0
• Importance factor: 1.0
• Roof snow load = 100 × 0.8 × 0.7 × 1.0 × 1.0 = 56 psf

Example 3:

• Ground snow load: 150 psf
• Exposure factor: 1.2
• Slope factor: 0.9
• Thermal factor: 1.1
• Importance factor: 1.1
• Roof snow load = 150 × 1.2 × 0.9 × 1.1 × 1.1 = 197 psf

Typical Values:

Light Snow Regions:

• Ground snow load: 20-30 psf
• Roof snow load: 15-25 psf
• Typical: 20 psf
• Examples: Southern states, coastal areas

Moderate Snow Regions:

• Ground snow load: 30-50 psf
• Roof snow load: 25-40 psf
• Typical: 30 psf
• Examples: Mid-Atlantic, Midwest

Heavy Snow Regions:

• Ground snow load: 50-100 psf
• Roof snow load: 40-80 psf
• Typical: 60 psf
• Examples: Northern states, mountains

Very Heavy Snow Regions:

• Ground snow load: 100-150 psf
• Roof snow load: 80-120 psf
• Typical: 100 psf
• Examples: High mountains, extreme climates

Drifting Snow:

Definition:

• Snow accumulation in drifts
• Occurs on sloped roofs
• Occurs at roof discontinuities
• Can exceed uniform load
• Design consideration

Drift Height:

• Depends on roof geometry
• Depends on wind exposure
• Typical: 1-4 feet
• Design parameter

Drift Load:

• Can be 2-3× uniform load
• Critical for design
• Must be considered
• Design parameter

Design Approach:

• Determine ground snow load from code
• Apply exposure and slope factors
• Calculate roof snow load
• Consider drifting
• Use in load combinations
• Design for maximum load

Example:

• Ground snow load: 50 psf
• Exposure factor: 1.0
• Slope factor: 0.8
• Roof snow load = 50 × 1.0 × 0.8 = 40 psf
• Design for 40 psf

3. Seismic Loads

Definition: Seismic loads are forces resulting from earthquake motion, varying by location and magnitude.

Characteristics:

• Duration: Seconds to minutes
• Magnitude: Variable by location
• Location: All directions
• Frequency: Infrequent
• Dynamic effects

Factors Affecting Seismic Load:

Seismic Zone:

• Zone 1: Low seismic activity (0.05g)
• Zone 2: Moderate seismic activity (0.10g)
• Zone 3: High seismic activity (0.20g)
• Zone 4: Very high seismic activity (0.40g)
• Design parameter

Soil Type:

• Type A: Hard rock
• Type B: Rock
• Type C: Dense soil
• Type D: Stiff soil
• Type E: Soft soil
• Type F: Special soil
• Affects ground motion
• Design parameter

Building Importance:

• Standard buildings: 1.0
• Important buildings: 1.25
• Critical buildings: 1.5
• Design parameter

Building Period:

• Depends on height and stiffness
• Affects response
• Design parameter

Seismic Force Calculation:

Formula:

• Seismic Force = Seismic Coefficient × Building Weight
• Seismic coefficient varies by zone
• Typical: 5-30% of weight
• Design parameter

Example 1:

• Building weight: 1,000 kips
• Seismic zone: 2
• Seismic coefficient: 0.10
• Seismic force = 0.10 × 1,000 = 100 kips

Example 2:

• Building weight: 500 kips
• Seismic zone: 3
• Seismic coefficient: 0.20
• Seismic force = 0.20 × 500 = 100 kips

Example 3:

• Building weight: 2,000 kips
• Seismic zone: 4
• Seismic coefficient: 0.30
• Seismic force = 0.30 × 2,000 = 600 kips

Typical Values:

Low Seismic Zones:

• Seismic coefficient: 0.05-0.10
• Seismic force: 5-10% of weight
• Typical: 7% of weight
• Examples: Central United States

Moderate Seismic Zones:

• Seismic coefficient: 0.10-0.15
• Seismic force: 10-15% of weight
• Typical: 12% of weight
• Examples: Eastern United States, parts of Midwest

High Seismic Zones:

• Seismic coefficient: 0.15-0.25
• Seismic force: 15-25% of weight
• Typical: 20% of weight
• Examples: Western United States, Pacific Northwest

Very High Seismic Zones:

• Seismic coefficient: 0.25-0.40
• Seismic force: 25-40% of weight
• Typical: 30% of weight
• Examples: California, Alaska, Hawaii

Seismic Force Distribution:

Base Shear:

• Total horizontal force
• Applied at base
• Design parameter

Story Shear:

• Force at each story
• Decreases with height
• Design parameter

Overturning Moment:

• Moment at base
• Affects foundation
• Design parameter

Design Approach:

• Determine seismic zone from code
• Calculate seismic force
• Distribute force to stories
• Design for lateral loads
• Verify stability
• Design connections

Example:

• Building weight: 500 kips
• Seismic zone: 2
• Seismic coefficient: 0.12
• Seismic force = 0.12 × 500 = 60 kips
• Design for 60 kips lateral force

4. Temperature Loads

Definition: Temperature loads are forces resulting from thermal expansion and contraction of materials due to temperature changes.

Characteristics:

• Duration: Hours to days
• Magnitude: Variable by climate
• Location: All members
• Frequency: Regular
• Predictable patterns

Factors Affecting Temperature Load:

Temperature Change:

• Varies by location
• Varies by season
• Typical: 50-100°F
• Design parameter

Coefficient of Thermal Expansion:

• Steel: 6.5 × 10⁻⁶/°F
• Concrete: 5.5 × 10⁻⁶/°F
• Wood: 3.0 × 10⁻⁶/°F
• Aluminum: 13.0 × 10⁻⁶/°F
• Material-dependent

Member Length:

• Longer members: Larger movement
• Shorter members: Smaller movement
• Design parameter

Thermal Stress Calculation:

Formula:

• Thermal Stress = E × α × ΔT
• E = Elastic modulus
• α = Coefficient of thermal expansion
• ΔT = Temperature change
• Design parameter

Example 1:

• Steel beam: E = 29,000 ksi
• Coefficient: 6.5 × 10⁻⁶/°F
• Temperature change: 100°F
• Thermal stress = 29,000 × 6.5 × 10⁻⁶ × 100 = 18.85 ksi

Example 2:

• Concrete slab: E = 3,600 ksi
• Coefficient: 5.5 × 10⁻⁶/°F
• Temperature change: 80°F
• Thermal stress = 3,600 × 5.5 × 10⁻⁶ × 80 = 1.58 ksi

Thermal Deflection Calculation:

Formula:

• Thermal Deflection = α × ΔT × L
• α = Coefficient of thermal expansion
• ΔT = Temperature change
• L = Member length
• Design parameter

Example:

• Steel beam: α = 6.5 × 10⁻⁶/°F
• Temperature change: 100°F
• Length: 100 feet = 1200 inches
• Thermal deflection = 6.5 × 10⁻⁶ × 100 × 1200 = 0.78 inches

Typical Values:

Cold Climates:

• Temperature change: 80-120°F
• Thermal stress: 10-20 ksi (steel)
• Thermal deflection: 0.5-1.0 inches per 100 feet
• Design consideration

Moderate Climates:

• Temperature change: 50-80°F
• Thermal stress: 6-15 ksi (steel)
• Thermal deflection: 0.3-0.6 inches per 100 feet
• Design consideration

Hot Climates:

• Temperature change: 60-100°F
• Thermal stress: 8-18 ksi (steel)
• Thermal deflection: 0.4-0.8 inches per 100 feet
• Design consideration

Design Approach:

• Determine temperature range
• Calculate thermal stress
• Calculate thermal deflection
• Provide expansion joints
• Design for thermal movement
• Verify connections

Example:

• Steel structure: 100-foot span
• Temperature change: 100°F
• Thermal deflection = 6.5 × 10⁻⁶ × 100 × 1200 = 0.78 inches
• Provide expansion joint for 0.78 inches

5. Moisture Loads

Definition: Moisture loads are forces resulting from swelling and shrinkage of materials due to moisture changes.

Characteristics:

• Duration: Hours to days
• Magnitude: Variable by material
• Location: All members
• Frequency: Regular
• Predictable patterns

Affected Materials:

Wood:

• Significant swelling and shrinkage
• Perpendicular to grain: 5-10%
• Along grain: 0.1-0.3%
• Design consideration

Concrete:

• Drying shrinkage: 0.05-0.1%
• Moisture expansion: 0.02-0.05%
• Design consideration

Masonry:

• Moisture expansion: 0.05-0.1%
• Design consideration

Moisture Movement Calculation:

Formula:

• Moisture Movement = Moisture Coefficient × Moisture Change × Length
• Moisture coefficient: Material-dependent
• Moisture change: Varies by climate
• Length: Member length
• Design parameter

Example (Wood):

• Moisture coefficient: 0.003 per 1% moisture change
• Moisture change: 10%
• Length: 20 feet = 240 inches
• Moisture movement = 0.003 × 10 × 240 = 7.2 inches

Design Approach:

• Identify moisture-sensitive materials
• Calculate moisture movement
• Provide expansion joints
• Design for moisture movement
• Verify connections
• Protect from moisture

Example:

• Wood floor: 40-foot span
• Moisture change: 5%
• Moisture movement = 0.003 × 5 × 480 = 7.2 inches
• Provide expansion joint for 7.2 inches

---

Environmental Load Combinations

Building Code Requirements

International Building Code (IBC):

Load Combinations:

• Dead load: 1.0 × DL
• Dead + Live: 1.2 × DL + 1.6 × LL
• Dead + Snow: 1.2 × DL + 1.6 × SL
• Dead + Wind: 1.2 × DL + 1.0 × WL
• Dead + Seismic: 1.2 × DL + 1.0 × EL
• Code-specified values

American Society of Civil Engineers (ASCE):

ASCE 7: Minimum Design Loads

Load Combinations:

• Multiple combinations
• Different safety factors
• Worst-case scenarios
• Design envelope
• Code-specified values

Typical Combinations

Dead Load Only:

• 1.0 × Dead Load
• Minimum case
• Permanent loads only

Dead + Live Load:

• 1.2 × Dead Load + 1.6 × Live Load
• Common case
• Most critical

Dead + Snow Load:

• 1.2 × Dead Load + 1.6 × Snow Load
• Snow case
• Seasonal loading

Dead + Wind Load:

• 1.2 × Dead Load + 1.0 × Wind Load
• Wind case
• Lateral loading

Dead + Seismic Load:

• 1.2 × Dead Load + 1.0 × Seismic Load
• Seismic case
• Dynamic loading

Dead + Live + Wind:

• 1.2 × DL + 1.0 × LL + 1.0 × WL
• Combined case
• Multiple loads

---

Environmental Load in Different Climates

Cold Climates

Wind Loads:

• Typical: 20-30 psf
• Seasonal variation
• Design parameter

Snow Loads:

• Typical: 60-100 psf
• Heavy accumulation
• Critical design load

Seismic Loads:

• Varies by location
• Design parameter

Temperature Loads:

• Typical: 80-120°F change
• Significant thermal stress
• Design consideration

Design Approach:

• Snow load often governs
• Design for maximum snow
• Consider wind on snow
• Account for thermal stress
• Verify all combinations

Moderate Climates

Wind Loads:

• Typical: 15-25 psf
• Regular occurrence
• Design parameter

Snow Loads:

• Typical: 30-50 psf
• Moderate accumulation
• Design consideration

Seismic Loads:

• Varies by location
• Design parameter

Temperature Loads:

• Typical: 50-80°F change
• Moderate thermal stress
• Design consideration

Design Approach:

• Balance wind and snow
• Design for maximum load
• Consider combinations
• Account for thermal stress
• Verify all combinations

Hot Climates

Wind Loads:

• Typical: 10-20 psf
• Regular occurrence
• Design parameter

Snow Loads:

• Typical: 0-20 psf
• Minimal accumulation
• Design consideration

Seismic Loads:

• Varies by location
• Design parameter

Temperature Loads:

• Typical: 60-100°F change
• Significant thermal stress
• Design consideration

Design Approach:

• Wind load often governs
• Design for maximum wind
• Account for thermal stress
• Consider moisture effects
• Verify all combinations

Coastal Climates

Wind Loads:

• Typical: 30-75 psf
• Hurricane potential
• Critical design load

Snow Loads:

• Typical: 0-30 psf
• Minimal accumulation
• Design consideration

Seismic Loads:

• Varies by location
• Design parameter

Salt Spray:

• Corrosion concern
• Material selection critical
• Design consideration

Design Approach:

• Wind load governs
• Design for maximum wind
• Consider hurricane loads
• Corrosion protection critical
• Verify all combinations

---

Conclusion

Environmental loads are critical to structural engineering, representing forces caused by natural phenomena. Understanding environmental load types, determination methods, and design applications is essential for proper structural design.

Key Takeaways:

• Environmental loads from natural phenomena
• Wind loads vary by location and height
• Snow loads vary by location and climate
• Seismic loads vary by location and zone
• Temperature loads affect all structures
• Moisture loads affect wood and masonry
• Location determines environmental loads
• Multiple load combinations required
• Proper design ensures safety
• Professional expertise required

Need help determining environmental loads for your project? Consult with structural engineers to ensure proper analysis and design for your specific needs.

---

Frequently Asked Questions

What are environmental loads?

Environmental loads are temporary forces caused by natural phenomena, including wind, snow, seismic activity, temperature changes, and moisture effects.

How do I determine wind load for my location?

Consult building code for basic wind speed in your area. Apply exposure and height factors to calculate wind pressure.

How do I determine snow load for my location?

Consult building code for ground snow load in your area. Apply exposure, slope, and thermal factors to calculate roof snow load.

How do I determine seismic load for my location?

Consult building code for seismic zone in your area. Calculate seismic force as percentage of building weight.

What is thermal stress?

Thermal stress is stress created by thermal expansion and contraction of materials due to temperature changes.

How do I account for temperature loads?

Calculate thermal stress using elastic modulus, thermal expansion coefficient, and temperature change. Provide expansion joints.

What is moisture movement?

Moisture movement is dimensional change of materials due to moisture absorption or loss, significant in wood and masonry.

Why are load combinations important?

Load combinations represent worst-case scenarios. Designing for only one load type is inadequate and unsafe.