DASMA TDS 178 - Effective Wind Area for Garage Door Wind Load Calculation

ASCE 7 uses the concept of effective wind area (EWA) to determine which pressure coefficient applies to a cladding element. For small components, the code assumes peak local pressures. For larger areas like a garage door, the pressure averaged over the larger area is lower. DASMA TDS 178 explains how to apply this concept correctly when calculating wind load for a sectional garage door.

What this data sheet says

TDS 178 describes the effective wind area concept from ASCE 7 and how it is applied specifically to sectional garage doors to determine the correct external pressure coefficient for the design wind pressure calculation.

"The effective wind area for a garage door may be taken as the area of the door itself, which for a large sectional door can result in a lower pressure coefficient than would apply to a small cladding element."

Key concepts from TDS 178:

• ASCE 7 Chapter 26 defines effective wind area as the area used to determine the external pressure coefficient (GCp) from the C&C pressure coefficient charts.
• For a single-panel sectional door, the EWA is typically the full door face area (width times height). A standard 16x7 door has an area of 112 square feet.
• From ASCE 7 Figure 30.3-1 (for low-rise buildings), the GCp value is lower for larger effective wind areas than for small areas. This means a large garage door may be designed to a lower pressure coefficient than a small window on the same building.
• The result is a more accurate design pressure. Using the correct EWA instead of a worst-case small-area assumption can reduce the calculated design wind pressure. TDS 178 shows how to read the GCp chart correctly for door-sized elements.
• Sectional panels are interconnected. TDS 178 notes that a sectional door's panels act together as a single element, so the EWA is the full door area, not the area of one panel.

When it applies

TDS 178 is most relevant when a design professional is running a full ASCE 7 calculation for a garage door:

• Commercial projects where the structural engineer specifies wind design pressure for the door schedule.
• Residential projects in high-wind areas where the design engineer wants to use the most accurate calculation method rather than a conservative simplification.
• Cases where the simplified TDS 155 approach gives a higher required DP than expected. TDS 178 may allow a lower required rating based on the actual EWA, which could affect door selection and cost.

For most residential replacements in Denver, TDS 155 is sufficient. TDS 178 becomes relevant when a design professional is involved and wants to use the full ASCE 7 C&C methodology.

What this means for you

Using the correct EWA can lower the required door DP rating. This is not a shortcut around code; it is applying the code correctly. A smaller required DP rating may expand the range of door products that qualify at a lower cost.

The calculation requires knowing the ASCE 7 zone for your building. Pressure coefficients vary by zone (corner, edge, field). The garage door typically falls in Zone 4 or Zone 5 depending on its position relative to the building corners.

Share TDS 178 with your project engineer. If an engineer is specifying the door, this data sheet gives them the DASMA industry position on how to apply EWA to the door design. It resolves a point of ambiguity in the ASCE 7 C&C charts.

G Brothers works with project engineers on commercial installations and can provide TDS 178 as a reference during the design phase.

Full text and source

Download DASMA TDS 178 from the official TDS index at https://www.dasma.com/technical-data-sheets/.

This entry is intended for design professionals applying ASCE 7 Chapter 26-30 to garage door specifications. For residential door replacement without engineering involvement, the TDS 155 simplified wind load guide is the appropriate starting point.