Wind Load Design for HVAC Equipment

Wind loads impose critical lateral and uplift forces on HVAC equipment, particularly rooftop installations where exposure to atmospheric boundary layer flow creates significant design challenges. Proper wind load analysis ensures equipment remains anchored during extreme wind events and prevents catastrophic failure, building envelope damage, and operational disruption.

Basic Wind Speed Determination

ASCE 7 defines basic wind speed (V) as the 3-second gust wind speed at 33 ft (10 m) above ground in Exposure C terrain, representing various return periods. Design wind speeds vary geographically:

Wind Speed Map Zones (ASCE 7-16, Risk Category II):

The designer must determine the applicable Risk Category (I through IV) based on building occupancy and function. HVAC equipment on essential facilities (hospitals, fire stations) uses Risk Category IV with higher design wind speeds and lower return period exceedance probabilities.

Exposure Categories

Exposure category characterizes surface roughness and terrain features upwind of the building site, directly affecting velocity pressure magnitude:

Exposure B: Urban and suburban areas with numerous closely-spaced obstructions (buildings, trees) of 20-30 ft height. Velocity pressure coefficients are lowest due to wind speed reduction from surface friction.

Exposure C: Open terrain with scattered obstructions less than 30 ft tall. This includes flat grassland, airport surfaces, and areas adjacent to large water bodies. ASCE 7 uses Exposure C as the reference condition.

Exposure D: Flat, unobstructed areas directly exposed to wind flowing over open water for distances exceeding 5,000 ft. This produces maximum velocity pressures. Applicable to oceanfront installations and equipment on structures extending over water.

Exposure Category Comparison (Height = 30 ft)

Exposure B:  ||||||||||||||||||||||||||||||||||||
Kz = 0.70    Normalized Velocity Pressure = 1.00

Exposure C:  ||||||||||||||||||||||||||||||||||||||||||||||||
Kz = 0.98    Normalized Velocity Pressure = 1.40

Exposure D:  ||||||||||||||||||||||||||||||||||||||||||||||||||||||
Kz = 1.03    Normalized Velocity Pressure = 1.47

             └─────────────────────────────────────────────────┘
             Relative Wind Pressure Magnitude

The exposure category must be determined for each wind direction, as terrain features vary azimuthally around a structure.

Velocity Pressure Calculation

Velocity pressure (qz) represents the kinetic energy per unit area of moving air, calculated at height z above ground:

qz = 0.00256 Kz Kzt Kd V² (psf)

Where:

• Kz = Velocity pressure exposure coefficient (function of exposure category and height)
• Kzt = Topographic factor (accounts for speed-up over hills, ridges, escarpments)
• Kd = Wind directionality factor (0.85 for buildings, accounts for reduced probability that maximum pressure occurs from critical direction)
• V = Basic wind speed (mph)

For a rooftop unit at 40 ft elevation in Exposure C terrain with V = 120 mph and flat topography (Kzt = 1.0):

Kz = 2.01(40/33)^(2/9.5) = 2.01(1.212)^0.211 = 1.04

qz = 0.00256 × 1.04 × 1.0 × 0.85 × (120)² = 31.6 psf

Topographic Factor (Kzt)

Wind accelerates over topographic features, producing localized velocity increases. The topographic factor applies when:

• The feature is isolated and protruding above surrounding terrain
• Equipment is located in the upper half of the feature height
• Feature height-to-upwind distance ratio exceeds 0.2

For a two-dimensional ridge:

Kzt = (1 + K₁ K₂ K₃)²

Where K₁ depends on feature shape, K₂ on upwind/downwind position, and K₃ on height above local terrain. Maximum speed-up occurs at the crest. For a 100-ft tall hill with equipment 20 ft from crest and 15 ft above local grade, Kzt can reach 1.3-1.5, increasing design loads by 30-50%.

Equipment Wind Load Calculation

Design wind force on equipment components and cladding (C&C):

F = qh GCp Af (lbf)

Where:

• qh = Velocity pressure at mean roof height
• G = Gust effect factor (0.85 for rigid equipment)
• Cp = External pressure coefficient (varies by surface orientation)
• Af = Projected area normal to wind direction

Pressure Coefficients for Rooftop Equipment:

For a rooftop air handler (10 ft × 6 ft × 4 ft tall) with qh = 32 psf:

Windward force = 32 × 0.85 × 0.8 × (6 × 4) = 522 lbf

Leeward force = 32 × 0.85 × 0.5 × (6 × 4) = 326 lbf (suction)

Net horizontal force = 522 + 326 = 848 lbf

Uplift on roof panel = 32 × 0.85 × 1.8 × (10 × 6) = 2,937 lbf

Anchorage Design

Equipment anchorage must resist combined horizontal shear and vertical tension/compression loads. Design considerations:

Load combinations: Combine wind with equipment operating weight, snow loads, and seismic forces per ASCE 7 load combinations. Wind and seismic need not be combined.

Anchor spacing: Distribute anchors to minimize eccentric loading. Typical spacing does not exceed 4-6 ft for large equipment.

Edge distance: Maintain minimum 7-10 anchor diameters from concrete edges to prevent breakout failure.

Embedment depth: Minimum 4-6 inches for cast-in-place anchors, verified by pullout and shear testing per ACI 318.

Calculate required anchor capacity using overturning analysis. For the previous example with 848 lbf horizontal force applied at 2 ft height (equipment center of mass):

Overturning moment = 848 × 2 = 1,696 ft-lbf

With 10-ft equipment length and 4 anchors:

Tension per anchor = 1,696 / 10 / 2 = 84.8 lbf (additional to dead load)

Design anchors for combined tension (uplift + overturning) and shear loads with appropriate safety factors (typically 4.0 for tension, 4.5 for shear).

Design Procedure Summary

1. Determine basic wind speed from ASCE 7 maps based on location and Risk Category
2. Establish exposure category for all wind directions based on site terrain
3. Evaluate topographic effects and calculate Kzt if applicable
4. Calculate velocity pressure qh at mean roof height
5. Determine pressure coefficients based on equipment geometry
6. Calculate wind forces on all equipment surfaces
7. Perform load combinations per ASCE 7 Section 2.3
8. Design anchorage for combined shear, tension, and compression loads
9. Verify structural adequacy of supporting roof structure
10. Detail attachment connections to prevent stress concentrations

Proper wind load design requires coordination between mechanical and structural engineers to ensure load paths transfer effectively from equipment through anchorage, curbs, structural framing, and building foundations.

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