May/June 1997, Southern Building Magazine

DASMA Garage Door High Winds Load Guide
What's in it for You?

by Joe Hetzel, P.E.
Technical Director, DASMA

The Door and Access Systems Manufacturers Association organization is acutely interested in accurate wind load calculations as applied to residential and commercial garage doors. As a result, DASMA has created a simple, general guide titled the Garage Door High Winds Load Guide. The Guide as shown here is based on wind load calculations per the 1994 Standard Building Code© which is published by the Southern Building Code Congress International. The SBC basis was chosen due to its enforcement in many hurricane-prone regions of the United States. This article shall examine the background of wind forces due to hurricanes, how to use the Guide, and the Guide’s benefits to garage door manufacturers, dealers, distributors and installers as well as building officials and building owners.

Background
Hurricane-related damage in recent years along the eastern seaboard of the United States has prompted great concern about the ability of structures in such high-risk areas to withstand high winds. Aside from building code enforcement and adequacy of building codes, accurate interpretation and application of wind loads as prescribed in the codes has become a major issue. The issue becomes confusing when it is discovered that numerous variables need to be considered when determining wind loads. These variables include the product classification, design wind speed, exposure conditions, height above finished grade, and horizontal location of doors with respect to corners of a building. The Garage Door High Winds Load Guide employs the above variables as applying to garage doors, but such parameters need to be explained. Since the Guide is based on the 1994 Standard Building Code®, that reference will be used as the basis for explaining wind load variables, and will be known hereinafter as the Code.

Product Classification. Products subjected to wind loads may be classified as either “Components and Cladding” (C&C) or a “Main Wind Force Resisting System” (MWFRS). The primary difference between these two classifications is that C&C’s are directly loaded by wind and then transfer such loads to primary structural supporting members making up a MWFRS. The small effective areas associated with components and cladding necessitate higher coefficients than MWFRS coefficients because pressure fluctuations are more likely to be highly correlated for smaller effective areas. Garage doors fit the C&C definition. Although garage doors do not fit the MWFRS classification, they are a vital part of an exterior building envelope. As the typical largest opening in a structure, when a closed garage door is breached by high winds, the resulting internal pressure change to the adjoining structure can be dramatic with respect to the breaching of windows and entry doors.

Design Wind Speed. The design wind speed as determined from the Code is based on “fastest-mile” speed, standardized at 33 feet above ground. “Fastest-mile” wind speed is defined as the average speed for one mile of air passing an anemometer. The wind speeds in Figure 1606 of the Standard Building Code© are fastest-mile wind speeds for a 50-year mean recurrence internal (annual probability of exceedance of 0.02). Weather stations across the country have compiled this data for many years, and this data is the basis for the wind speed map found in the code. In hurricane-prone regions where the code is being used, the design wind speeds vary from 90 mph to 110 mph.

Exposure Conditions. Exposure conditions can have wide ranging effects on wind pressures. Exposure categories include large city centers, urban and suburban areas, open terrain, and near large bodies of water. The prescriptive methods in the code are based on the open terrain alternative.

Height Above Finished Grade. Wind pressures on building products increase as their location above grade increases. The code has prescriptive methods for determining wind load for buildings with mean roof heights less than 60 feet above grade. As a conservative measure, the Guide’s garage door wind pressures are based on the adjoining structure’s roof height, as opposed to the height of the garage door above grade. The explanation is that if an opening is breached in the upper part of a structure, the resulting pressure at that level could find its way to the garage door.

Horizontal Location. Studies have shown that negative wind pressures on walls are higher in the vicinity of wall corners, and this is accounted for in the code. The zone near the wall corners where negative pressures increase is proportional to the overall wall length. The data in the Guide conservatively assumes a 50' wall length, including 3' of wall between the garage door and the wall corner. The resulting 2' of net increased negative pressure is permitted to be spread out over the entire garage door width. Therefore, the Guide is simplified, but somewhat conservative for garage doors located away from building corners.

How to Use the Guide
The objective in using the Guide is to determine the applied positive and negative wind load pressures onto a garage door. These loads are critical in beginning the process of selecting the proper door assembly. However, just as critical is the identification of specific parameters needed to effectively use the Guide. These parameters are:

DASMA Garage Door High Winds Load Guide

At the bottom of the Guide, there are notes which indicate conditions where using other means becomes necessary. These conditions include:

How the Guide Benefits You
The Guide can prove to be of great assistance to all parties involved in the garage door industry from the manufacturer to the building owner, and everyone in between. Manufacturers know what performance standards they must meet and can prepare standard product alternatives to meet these requirements. They also have testing criteria to use in verifying the performance of their products.

Dealers and distributors can guide people who purchase the product toward the appropriate product to use, gaining credibility from a marketing standpoint. Installers can more readily recognize whether a product is adequate for a specific application and can also become more sensitive to installing products properly.

Building officials have a simple means of knowing the performance standards garage doors must meet, making enforcement clearer. And finally, building owners can have peace of mind knowing that their investment has a better chance of withstanding the fury of high winds by not having the largest opening or openings in their structure breached, helping guard against property losses as well as property damage and threats to the life, health and safety of occupants.

Conclusion
The members of DASMA responsible for the research and preparation of the “High Winds Load Guide” are committed to preventing wind-related damages. The association is making every attempt to place this guide in the hands of those who will find the guide useful. This version of the guide, and others to follow based on other enforced codes and standards throughout the United States, underscore DASMA’s leadership in the garage door industry. For copies of the "High Winds Load Guide", write to Technical Department, DASMA, 1300 Sumner Avenue, Cleveland, Ohio 44115.

Southern Building Magazine, May/June 1997
Copyright 1997, SBCCI, Inc.

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