For architects and builders—especially those in hurricane-prone areas—creating projects that meet or exceed wind load codes is incredibly important. It helps maintain the safety of all building tenants and contributes to the operational functionality of the building over its lifetime. 

But what exactly is wind load, and why is it important? Wind load is essentially the wind pressure or wind force, in pounds per square foot, exerted on a building. There can be uplift wind load (affects roof/horizontal structures), shear wind load (horizontal pressure that can damage walls) and lateral wind load (can cause foundational issues).

We are excited to share our knowledge of the building code, the general requirements of wind loads for buildings and information on how wind loads affect every component of commercial and residential buildings.

Wind Loads and How the Building Code Addresses Them

Wind loads relate to the amount of pressure exerted on any given zone of a building and its components. The parameters used to determine wind loads are:

• Basic wind speed
• Wind flow or directionality
• Exposure category
• Topographic factors
• Gust effect factors
• Enclosure classification
• Internal pressure coefficient

Many people think wind loads are just precautionary measures in hurricane-ridden areas, but the truth is that wind loads are defined in all buildings and in all geographical areas. The first five parameters are completely dependent on the geographic location of the structure, but the last two parameters are specific to the structural design itself. 

So, hurricane-prone regions, like Florida, have additional requirements, but all designers are required to determine the required safety measurements for their walls, roofs and cladding. 

The first, and most important parameter, in determining wind loads of structures is basic wind speed. Basic wind speeds are further broken down into one of four possible risk categories dealing primarily with intended use and capacity. Risk I, the lowest risk category, includes unoccupied buildings like silos or barns. The highest risk category, Risk IV, includes highly populated buildings that would require a full-scale evacuation like hospitals.

The higher the risk category, the higher the basic wind speeds. The higher the basic wind speeds, the higher the designed wind loads need to be for that particular structure. 

The International Building Code and the ASCE 7-10 both publish contour basic wind speed maps by risk category. Below is an example of a basic wind speed map for Categories III and IV. 

With permission from ASCE: Page 248b of ASCE 7-10 Figure 26.5.1B; Title: Basic Wind Speeds for Risk Category III&IV Buildings at Exposure C at 33 ft.

The Basic Wind Speed is defined by the Structural Engineering Institute as the “three-second gust speed at 33 ft. above the ground in Exposure Category C.” The Exposure Category refers to the prevailing wind direction relative to the surface roughness of the terrain or landscape surrounding the structure. 

There are three categories: B, C and D. These categories range from urban areas with house dwellings to flat, unobstructed areas and water spaces.

So, how can you find out the exposure category and basic wind speeds in your area when designing a building? 

1. Determine the risk category based on occupancy for your building. 
2. Find the location of your building on your risk category map. 
3. Follow the contour line with the basic wind speed corresponding to your building’s location. 

This knowledge is important because we need to know whether or not the products installed on our buildings will still be standing when a strong storm comes. 

Manufacturers are legally required to provide this information in their third-party Code Evaluation Reports because they have an obligation to provide a safe, long-lasting design for their customers. You should be able to find it on the manufacturer’s website, or you can request it from them directly.

Help Is On The Way

Looking for design or pricing assistance? Nichiha representatives are on-hand to answer your questions, ensure your detailing is technically sound and that your project meets all product and design requirements.

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Design Pressures in Each Building Zone

How do manufacturers determine wind loads capabilities for their products? To understand how manufacturers gain this data, you first have to understand building zones and design pressures.

At any given time, a building can be under multiple types of forces, as a building is subject to it from both the interior and exterior (e.g. shear load, uplift load, lateral load, or multiple load paths.) The design pressures referenced in this section are those exerted on the building envelope (or exterior surrounding wall). Design pressure is defined as the equivalent static pressure to be used in the determination of wind loads for buildings.

Cladding manufacturers provide performance information for their siding according to design pressures they can meet with their prescribed fastening schedule. These numbers should always be published in third-party code evaluation reports and in the manufacturer’s own technical resource center.

There are positive pressures exerted from the inside of the building to the outer walls. Negative and positive pressures are also exerted on the outer wall from outside, including pressure from wind or other physical factors. The direction of these forces can change depending on weather conditions, which can complicate matters for building designers.

The overall design load on a wall takes the parameters we listed above into account, and the values are derived in a variety of ways. In simple terms, some of these values are derived based on historical data, through fluid dynamics and vibration tables in the SEI handbook ASCE/SE17: Minimum Design Loads for Buildings and Other Structures.

There are three possible enclosure classifications for a building:

1. Open
2. Partially Enclosed 
3. Enclosed

Wind load calculations are simplified for low-rise buildings or buildings that are less than 60 feet tall. Buildings that are higher have more complicated calculations. ASCE7 breaks down a building into zones to combat this complexity. Zones 4 and 5 are located on a structure’s exterior walls. The width of zone 5 is determined by a variety of methods and is dependent on building height and overall wind dimensions.

Once a building designer can factor in the gust effect of a location, they can partner with a manufacturer whose fastening schedules can meet the standard necessary for the high basic wind speeds that place pressure on buildings zones 4 and 5. 

How Standardized Tests Show Building Performance in High Winds

In order for a siding manufacturer to know what their fastening schedules should be in any given building, they put their products through rigorous testing to determine performance to ensure customer safety. When performing these tests, there are a number of standards to choose from.

The most widely accepted standard is ASTME330, or Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air Pressure Difference. Using a sample assembly of the siding product and a vacuum within the test chamber, this test simulates different pressure preset intervals and notes when a failure occurs.

Failure can occur by two means:

1. Fastener Pull-Out: When the wind suction pressure exerted on the fastener exceeds the joint’s maximum pull-out force.
2. Fastener Pull-Through: When the fastener breaks through the siding due to wind suction pressures.

Pull-through failure is dependent mostly on the strength of the siding in the through-the-siding direction and on fastener head diameter. The thicker the siding and the larger the diameter head, the stronger the joint.

The strength of the joint is highly dependent on four factors:

1. Length of the fastener: The longer the siding nail, the stronger the joint.
2. Type of fastener treatment: Smooth shank nails yield a stronger joint.
3. Shank diameter: The larger the diameter, the stronger the joint.
4. Density of the lumber used in the framing members: Wood species with higher specific gravities are stronger.

Because wood framing presents complications in terms of the grade of lumber used and the commonality of fastener failure, many tests are done with steel stud framing instead of wood, even though most residences are built with wood studs.

For this reason, it is important for the code official approving job site permits and inspecting installations to know the exact assembly used in testing and obtain a certain design pressure to ensure end-user safety. The correct stud type must be used in order to replicate the performance at the testing lab.

Code officials rely on performance evaluation reports published by an accredited third-party on behalf of any manufacturers. These reports prove a specific product meets or exceeds the requirements in the latest version of the building code. There are code bodies and agencies whose sole job is to ensure that only safe products are installed in buildings.

Code Bodies and Agencies

There are a long list of agencies, public and private, who are involved in the installation of safe siding.

These agencies include:

• ISO (International Standards Organization): Encompasses everything related to manufacturing, electrical, nuclear, quality and more.
• ANSI (American National Standards Institute): The US representative of ISO.
• ASTM(American Society for Testing and Materials): Hundreds of industry experts develop and publish voluntary consensus standards for a wide range of products, materials, systems and services.
• ASCE-SEI(American Society of Civil Engineers-Structural Engineers Institute): Develops standards pertaining to buildings and structures.
• ICC (International Code Council): Responsible for developing the International Building Code, which is published every 3 years and aims to provide a standard for building construction.
• State Insurance Programs: Government-run agencies whose goal is the safety of their constituents, primarily in the case of natural disasters.

Obviously, there are many valuable players in the game when it comes to testing standards. There are over 300 accredited testing lab agencies throughout the United States and Canada. 

There are also many code evaluation entities, with new ones popping up constantly. While the test standards are pretty detailed, there is still room for interpretation by the lab manager and technicians.

It’s in the manufacturer’s best interest to work closely with the testing agency of their choice to determine the best needs, assemblies, materials and outcome of tests. Why? Well, the manufacturers of these products are also members of some of these voluntary organizations.

This makes sense because manufacturers are experts in the building materials industry, and we have a vested interest in seeing our products get out into the marketplace with ease. 

However, it gets tricky when standards are misinterpreted by the testing agencies, which are independent of ASTM. One lab’s interpretation of the standard could prove to be catastrophic for a consumer if safety isn’t the number one concern.

Manufacturers are required to re-test their products because these standards change over time, as does the building code. To keep things uniform and safe, the general rule of thumb is to re-test products every 10 years or any time a product undergoes a significant composition or manufacturing process change. 

To sum it up, manufacturers can take a very active role in the development of standards and policies that directly affect the installation of the products they sell. At Nichiha, we strive to work closely with other industry experts — and even at times, our competitors — to ensure our products will best serve our customers.

Rely on Nichiha

At Nichiha, we have built relationships with officials in all of the government agencies mentioned above and work hard every day to improve the performance of our products to go beyond even the latest published building codes.

For more information on Nichiha’s wind load ratings and how Nichiha siding will keep your buildings and all their occupants safe, even in the most severe weather, visit our website or contact a rep today.