DASMA TDS 177 - Applying the Kd Factor to Garage Door Wind Load Calculations

When an engineer calculates design wind pressure per ASCE 7, several factors reduce the raw velocity pressure to account for real-world conditions. One of these is the directionality factor Kd, which reflects the statistical likelihood that the peak wind speed will hit the structure from exactly the worst direction. DASMA TDS 177 explains how Kd applies specifically to garage door design and whether a door specifier can use it to reduce the required design pressure.

What this data sheet says

TDS 177 addresses the application of ASCE 7's directionality factor (Kd) when determining the design wind pressure for a sectional garage door.

"ASCE 7 includes a directionality factor Kd that is part of the load combination process. TDS 177 clarifies how this factor applies to the selection of wind-rated garage doors."

Key points from TDS 177:

• Kd is already embedded in ASCE 7 load combinations. For components and cladding (C&C) wind loads, ASCE 7 incorporates Kd into the velocity pressure equation through the load factor and the combined coefficient. Designers should not double-count it.
• The typical value of Kd for buildings is 0.85. This means the effective velocity pressure used in design is 85% of the worst-case pressure at the full mapped wind speed.
• Door manufacturers rate their products using the DASMA 108 test which is a static uniform pressure test. The pass/fail pressure from that test is the door's rated design pressure.
• Comparison must be apples-to-apples. When comparing the design pressure calculated per ASCE 7 (which includes Kd) to a door's rated DP (from DASMA 108 testing), the calculation and the test method must be consistent in how directionality and load factors are applied.
• TDS 177 helps resolve a common specifier confusion: whether to specify a door at the raw velocity pressure or at the code-adjusted design pressure. The answer depends on whether the calculation already includes Kd and the applicable load combination factors.

When it applies

TDS 177 is used by engineers, architects, and experienced contractors who are specifying doors for projects subject to IBC or ASCE 7 wind load requirements:

• Commercial construction where the structural engineer of record prepares a wind load calculation and the door specifier needs to match the door's DP rating to the calculated pressure.
• Residential projects in high-wind areas where a design professional is involved and wants to verify that the wind load used to select the door is correctly derived from ASCE 7.
• Permit applications in jurisdictions where the plan reviewer asks for documentation of the wind load basis.

For most standard residential replacements in Denver, the simpler DASMA TDS 155 wind load guide is sufficient. TDS 177 is the reference when a full ASCE 7 calculation is in play.

What this means for you

If you are a homeowner replacing a door, TDS 155 is your starting point, not TDS 177. TDS 177 is for situations where a licensed engineer is running the numbers. Using TDS 177 without understanding ASCE 7 load combinations can lead to errors in either direction.

If your project has a structural engineer of record, give them TDS 177. They will understand the Kd context and can confirm that the door DP rating you select is consistent with their calculated design pressure.

Verify that the door manufacturer's test conditions match your project's load combination. Ask the manufacturer whether their DP rating corresponds to the ASCE 7 C&C design pressure as calculated with all relevant factors.

G Brothers works with project engineers on commercial and high-wind residential installations where an ASCE 7 calculation governs the door specification.

Full text and source

Download DASMA TDS 177 from the official TDS index at https://www.dasma.com/technical-data-sheets/.

This entry is oriented toward design professionals applying ASCE 7 to garage door specifications. Residential homeowners selecting a replacement door can typically use the simplified TDS 155 approach without a full ASCE 7 calculation.

Want to put numbers to this? Use the interactive wind load psf / mph converter below, or open the full wind load psf / mph converter with examples and notes.