Ellington

Although it may seem that a simple formula can be used to calculate wind load using wind speed, specifiers must incorporate many additional calculations to ensure their structures can withstand high wind. This is particularly true for specifiers of rolling doors and shutters, especially in areas of the country where high wind speeds dictate special design considerations.

Thus, it is crucial for specifiers to understand the many aspects of designing and specifying rolling doors that are tested and certified to meet static wind load, operational wind load, design wind load and ultimate wind load requirements.

It is also important for specifiers to explore and understand recent updates to the American Society of Civil Engineers Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-16) and other regional codes and requirements such as FEMA P-361 and ICC 500.

In an effort to provide specifiers all the necessary information, this article also addresses the primary factors that play a role in selecting the correct commercial rolling door or shutter for regions where wind design is required, along with best practices for other parts of the country.

What Exactly Comprises Wind Load?

Wind load refers to any pressures or forces that wind exerts on a building or structure. There are actually three primary types of wind forces that are exerted, including uplift, shear and lateral wind load.

Uplift wind load is an upward force of the wind that would affect roof structures or similar horizontal structures in a building, such as canopies or awnings. Shear wind load, on the other hand, is a horizontal pressure or force that can cause walls or vertical structural elements to tilt or crack, causing a building to lean. And lateral wind load is another horizontal wind pressure that can make a structure move off its foundations or overturn.

All three of these forces contribute to calculating structural wind loads, but wind shear has a major effect on the performance of rolling doors. Extreme weather, such as hurricanes, tornadoes and thunderstorms with straight line winds, puts the most extreme forces on a building and can lead to blow outs due to wild swings in positive and negative pressure.

Even “regular” gusts of wind due to surrounding topography and common weather patterns can impact the performance of a rolling door and the building envelope. That’s why these doors should also be tested for operational wind load, which specifies the maximum wind load at which a particular door is able to operate safely.

But additional factors must be considered when specifying rolling doors in areas prone to extreme weather. These areas include states that fall within the Federal Emergency Management Agency (FEMA) 250-mph wind speed zone for tornadoes, areas of Florida that are covered by Miami-Dade wind load requirements and other regional codes and requirements areas.

How Are Rolling Doors Designed for Wind Load and Extreme Weather?

When rolling doors are closed, wind loads are transferred to the overhead door guides and wall angles. In this situation, specifiers must consider two kinds of loads, “dead” and “live.” Dead load is the total rolling door hang weight and is constant. Live load is wind pressure forces and is not constant.

“Think of live forces as the gusts of wind during a hurricane or severe storm,” said Heather Bender, Strategic Marketing Manager, CornellCookson. “Live load forces can therefore greatly exceed the dead load force.”

Since overhead rolling doors are generally supported by a guide assembly attached to the jamb construction with no additional header supports required to keep the coiling door up, some areas are critical for support in dead vs. live load situations. The critical wall fasteners in a dead load are above the rolling door opening. Live load’s most critical wall fasteners are within the opening height.

“Rolling doors that are manufactured to withstand hurricanes, tornadoes and high wind events are designed to ride out the storm in the closed mode,” said Bender. “When a standard rolling door is exposed to high winds, the curtain will deflect and push against the guides, eliminating the function to operate,” she added.

“Considering that high wind speeds are seriously damaging in a hurricane or tornado, specifiers can be tempted to simply specify maximum wind speed to make sure your hurricane resistant doors are up to the task. But that’s not always enough,” she added.

Ten primary factors  play a role in determining design wind load – a much more accurate measure of how a rolling door will be able to withstand a hurricane or extreme weather event. These include wind speed, mean roof height, roof slope, surrounding topography, exposure category, directionality factor, door area, zone, positive or negative pressure and internal building pressure.

These ten wind load door factors give specifiers a much more accurate measure of how a rolling door will perform in windy situations. Changing just one of these factors can radically alter the required design wind load for a rolling door.

Beyond design wind load, ultimate wind load testing calculates the most extreme forces a door or structure can withstand and is heavily dependent on the location of the project. Also known as test wind load, it is defined as the engineered limit of a rolling door’s capacity.

Building codes typically require a proof of performance for an ultimate/test wind load that is 1.5 times the specified design wind load. For example, a 30 psf design wind load implies a 45 psf ultimate design wind load for rolling hurricane doors.

While these recommendations are important to consider, it is always critical that specifiers consult national and regional codes. Some of these were recently updated to reflect changes in average wind speed and weather patterns.

Updated Codes Require Updated Solutions

Revamped in 2019, the American Society of Civil Engineers Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-16) was swiftly adopted into the 2018 International Building Code (IBC). Both the 2018 IBC and ASCE-SEI 7-16 became widely used in 2019.

ASCE 7-16 uses basic wind speed, exposure categories, allowable stress load factor, mean roof height, door area, door location on the building and wind directionality factor to figure wind loads on doors. Wind load for a particular structure is also determined by its Risk Category. These range from structures that represent a low hazard to human life (Category I) to buildings and other structures that are designated as essential facilities (Category IV).

Each risk category carries an importance factor–the higher the risk category, the higher the importance factor and, therefore, higher wind loads. Each of the wind speed maps is based on a risk category with the importance factor already built in.

For example, the wind speed for the central U.S. for Risk Category 2 is 115mph, for Risk Category 3 and 4 it is 120mph and for Risk Category 1,  105mph. A specifier needs to know the risk category for the building before selecting the wind speed map.

Two other important codes, FEMA P-361 Safe Rooms for Tornadoes and Hurricanes: Guidance for Community and Residential Safe Rooms and International Code Council (ICC) 500: Standards for the Design and Construction of Storm Shelters, provide guidance on best practices related to the design, construction, and operation of safe rooms.

Initially released in 2000, FEMA P-361 provides guidance on best practices related to the design, construction and operation of safe rooms. However, FEMA P-361 is only a guideline, not a standard, mandatory only when storm safe room shelter is funded by a FEMA grant.

Using the first edition of FEMA P-361 as guidance, ICC 500 was initially published in the summer of 2008 and updated in 2014. It lays out the minimum requirements to safeguard public health, safety, and general welfare in relation to the design, construction, and installation of storm shelters.

ICC 500 applies to both residential and community storm shelters. Residential shelters have an occupant capacity of 16 people. Maximum occupancy for community storm shelters is based on state and local building codes. Storm shelters are permitted to be either separate, detached buildings or fully or partially enclosed spaces within a host structure.

In 2009, the International Building Code (IBC) incorporated ICC 500 to regulate the design and construction of buildings and safe rooms designated as storm shelters or tornado-safe rooms.

All states that have adopted 2015 IBC or newer require storm shelters within the 250-mph zone,commonly referred to as “tornado alley,”to meet ICC 500. It is important to note there is no official “hurricane alley” where ICC 500 or FEMA P-361 is mandated.

That is why some manufacturers have designed advanced rolling steel doors specifically for safe room protection against life-threatening tornadoes and hurricanes. Newer doors on the market are tested and certified to ICC500-2014 and FEMA P-361 standards.

Recently developed products allow the A&D community to specify just a single door to cover multiple openings, or even banks of windows, to maximize natural light. Hence, typical safe room spaces, such as cafeterias, classroom pods and gymnasiums, can be open and airy instead of dark and claustrophobic.

Some doors are designed to automatically deploy when a storm is approaching, converting an open space to an ICC 500/FEMA-361 rated safe room to protect occupants from harsh winds and deadly projectiles. After the storm, the door coils back into the structure until it is needed again.

Other innovative solutions include operational wind load doors as well as several other rolling door options for specifiers looking to protect building occupants and contents.

“Some new service and insulated rolling doors are designed with special wind locks, which allow the door to seamlessly glide through the guides when exposed to a wind load of 20 psf, which equates to roughly 88 miles per hour,” explained Bender. “These doors are designed for facilities that must be accessed regardless of weather, including fire stations, police station garages, hospitals, military facilities and airports.

Where to Turn Next

Specifying the right rolling door for the right facility is complicated enough without factoring in wind load requirements, evolving building codes and special considerations for areas of the country where tornadoes and hurricanes are increasing in intensity and frequency.

Therefore, it is critical for specifiers to rely on their manufacturer partners for the latest information and help when it comes to designing for wind load requirements. Many companies, such as CornellCookson, offer easy-to-use wind load configurators to help reverse engineer closure solutions based on specific wind load requirements and codes. For more information specifiers can access the CornellCookson Architectural Resource Center.