Under Pressure: Wind Load Series Part 1

 By Carolina Albano | Thu, October 01, 2015 | Meet the Blogger | More Posts by Carolina Albano

 By Carolina Albano | Thu, October 01, 2015 | Meet the Blogger | More Posts by Carolina Albano



Introduction

In an attempt to demystify wind load data contained within the product evaluation reports of building material manufacturers, we are launching a 4-part blog series. We are excited to share our knowledge of the building code, the general requirements of wind loads for buildings, and how wind loads affect everyone related to commercial and residential buildings: the designers, the builders, and the eventual occupants.

Part 1 of this series defines wind loads and how the code addresses them. It also defines geographical exposure categories and how they affect building construction.
Part 2 will define design pressures and how they are translated into wind speeds in Product Evaluation Reports. (Teaser: design pressures are related to building heights!) It will also explain the zones within a building itself as defined by ASCE 7, and the International Building Code.
Part 3 details the standardized tests manufacturers use to show performance in high wind situations.
Part 4 puts all of this information together in a simplified manner.

Why should you care about wind loads?

Wind loads are not just for Florida residents, or people living along the coastline. Wind loads are defined in ALL buildings in all geographical areas.

Hurricane prone regions, like Florida and coastal Carolinas have additional requirements and even their own building codes defining more stringent building guidelines, but wind storms, thunderstorms, and tornadoes can happen in most of the USA.

Understanding how wind loads are determined for walls and siding/cladding is a very important step in the specifying phase of a design project, no matter its intended use, or location.

Part 1 – What comprises wind loads?

Wind loads relate to the amount of pressure wind exerts on any given zone of a building and its components.

The parameters used to determine wind loads include the following:

1. Basic wind speed
2. Wind directionality factor
3. Exposure category
4. Topographic factor
5. Gust effect factor
6. Enclosure classification
7. Internal pressure coefficient

The first five parameters are completely dependent on the geographic location of the structure and its surroundings. The last two parameters are specific to the structure design itself.

The Basic Wind Speed number is the most important parameter in determining design wind loads of structures and the term most often heard. [Tweet This]. Basic wind speeds are further broken down into one of four possible Risk Categories. Risk Categories relate to the structure’s intended use and the number of occupants it will house. Assigning risk categories (formerly occupancy categories) to a structure is very important. The risk category takes into account how many people would be affected by a structural failure or collapse in the building, exit options, whether or not toxic chemicals would be emitted as a result of the failure, etc.

Risk Category I buildings are mostly unoccupied, such as silos or barns.

Risk Category II buildings make up the majority of structures and include residences, most commercial and light industrial buildings. In general this is the default risk category when structures do not meet the parameters of another category.

Risk Category III structures generally house a large gathering of people in one place. This includes movie theaters, places of large assemblies, concert halls. Risk Category III also encompasses structures inhabiting people with limited ability to safely egress in the event of failure, such as correctional facilities, small hospitals, and elementary schools.

Risk Category IV is reserved for structures such as police and fire stations and hospitals. When these facilities fail, there would be a large-scale state of emergency. As an example, think about the hospitals in New Orleans during Hurricane Katrina and its aftermath.

The higher the risk category, the higher the basic wind speeds and the higher the design wind loads for that particular structure.

The International Building Code and the ASCE 7-10 both publish contour basic wind speed maps by risk category. That is, each risk category has its own basic wind speed map. 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.” More on Exposure Categories is below.

Figure 1-3 show the basic wind speeds for risk category I, II, III & IV buildings respectively, for the eastern part of the United States.

FIGURE 1

With permission from ASCE: Page 249b of ASCE7-10—Figure 26.5.1C; Title: Basic Wind Speeds for Risk Category I Buildings at Exposure C at 33 ft.

FIGURE 2

With permission from ASCE: Page 247b of ASCE7-10-0- Figure 26.5.1A; Title: Basic Wind Speeds for Risk Category II Buildings at Exposure C at 33 ft.

FIGURE 3

With permission from ASCE: Page 248b of ASCE7-10—Figure 26.5.1B; Title: Basic Wind Speeds for Risk Category III&IV Buildings at Exposure C at 33 ft.

Now what does Exposure Category C, at 33 ft. mean?

The Exposure Category refers to the prevailing wind direction relative to the surface roughness of the terrain or landscape surrounding the structure. There are only three Exposure Categories: B, C, and D. The fourth category, Exposure A, was deleted in the last revision of the ASCE-7 publication.

The 33 feet is an arbitrary height. Wind speeds, and loads, increase with height. After 60 feet, there are additional factors affecting wind loads. The 33 feet number aims to encompass most structures and serves as an average and easy reference point.

Surface Roughness B: Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.

Surface Roughness C: Open terrain with scattered obstructions having heights generally less than 30 ft. This category includes flat open country and grasslands.

Surface Roughness D: Flat, unobstructed areas and water surfaces. This category includes smooth mud flats, salt flats, and unbroken ice

Exposure Categories:

B: Surface Roughness B prevails upwind for > 2600 ft, or 20 times the height of the building.
(Exception: >1500 ft if building is

< 30 ft)

D: Surface Roughness D prevails upwind for > 5000 ft, or 20 times the height of the building. Applicable inland 600 ft and > 10 times the building height.

C: Applies whenever Exposures B&D do not apply

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, to identify the proper map to use for reference. Example: for a single-family dwelling, use Figure 2, because houses are risk category II structures.
2. Find the location on the map where your structure is located.
3. Follow the contour line with the basic wind speed corresponding to the geographical location of your structure. Example: the house in Miami, Florida where I grew up has a basic wind speed of 180 mph at 33 ft., as it is located in exposure category C.

Why is any of this important?

We should know whether or not the products installed on our homes, places of business and anywhere we frequent will still be around when a strong storm comes.

As a consumer, we must ensure the products we install on our homes will be able to withstand the wind loads in our area. Roofing has wind loads, as does siding. As designers and specifiers, we have an obligation to provide a safe, long-lasting design to our customers.

Manufacturers are required to provide third-party Code Evaluation Reports. These come in a variety of names: ESR, NER, Warnock Hersey numbers, UL number, etc. There are several accredited agencies that work with product manufacturers to ensure their products and installation procedures meet the latest local building code and wind load requirements. You can find these code evaluation reports via the manufacturer’s web site or by requesting it directly from them.

Certain jurisdictions such as the Florida Building Commission and the Texas Department of Insurance typically have additional requirements beyond the latest general building code and require another layer of approval and additional reports. The local code officials in these jurisdictions make the final call on whether the products are safe to install based on the evaluation reports for each material.

So how do manufacturers determine wind loads capabilities for their products? How do they make a product that can be installed in the highest of wind areas and still perform as intended? Stay tuned for our next post in the series where we will delve into Building Zones and Design Pressures.

But before you go, we want to know where you are! Leave us a comment with the basic wind speed in your area!

Read More:
Under Pressure: Wind Load Series Part 2
Under Pressure: Wind Load Series Part 3
Under Pressure: Wind Load Series Part 4

Categories: Commercial, Product Education, Residential, Thought Leadership, Wind Loads

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