Under Pressure: Wind Load Series Part 3
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Under Pressure: Wind Load Series Part 3

In our last blog post we discussed how siding is designed to be installed in different structures and the requirements according to geographical and topographical location. We also discussed where on the building the siding is installed. To know how to safely install a siding product on a building, especially those with known high wind pressures, a manufacturer can turn to a number of test standards.

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. The test consists of a 4' by 8' sample assembly of the siding product that is sealed against one face of a test chamber. This creates a vacuum within the test chamber. Air is then forced behind the siding product, simulating suction of wind. The pressures are increased in preset intervals and the pressure at which failure occurs is recorded. There are displacement gages placed in different parts of the assembly wall to determine how much the product moves during the test. Measurement of any deformation that may have occurred during the test is also required.

There is a layer of 2 mil thick polyethylene film sandwiched between the sheathing (OSB or plywood) and the siding material. The standard calls for the film to be applied loosely with extra folds at each of the siding overlaps and at each of the corners of the assembly. The test specimen is supposed to include expansion joints, or vertical joints and most closely simulate real-world installation of the siding.

Back side of test frame in chamber

Front side of test frame showing siding attached

Any given manufacturer can run into obstacles when determining performance of a product, especially where competition abounds, such as with cladding exteriors.

Depending on the testing laboratory's interpretation of the test standard, performance of a product can vary significantly! That is, the test results can be greatly affected.

The ASTM E330 test is performed until failure. Failure can occur by two means:

1. Fastener Pull-Out
2. Fastener Pull-Through

Fastener Pull-Out Occurs when the force exerted on the fastener (in our case, the wind suction pressure) exceeds the joint's maximum pull-out force.

The joint is defined as the connection point between the structure and the siding. The strength of this joint is highly dependent on the following:

' The length of the fastener. The longer the siding nail, the stronger the joint.
' The type of fastener treatment. For example, a ring shank nail creates a stronger joint because of added friction that must be overcome to remove the nail from the framing member.
' A smooth shank nail that has been hot dipped versus electrogalvanized will also yield a stronger joint. The hot dipping process created added friction on the nail's shank.
' Shank diameter is also a factor. The larger the shank diameter, the stronger the joint.
' The density or specific gravity of the lumber used in the framing members also plays a strong role in the strength of the joint. Species of wood with higher specific gravities, such as Douglas Fir Larch and Southern Yellow Pine are best suited for these types of joints, over lower-density lumber such as Spruce Pine Fir.

Fastener pull-out case (the fastener shown is meant to also go into the sheathing and framing member, connecting the siding board to the structure).

Fastener Pull-Through Occurs when the fastener breaks through the siding due to external forces, such as wind suction pressures.

Fastener pull-through occurs when the loads exerted on the joint (fastener/siding/framing member) exceed the strength of the siding in the thickness direction.

This type of 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, the stronger the joint. A larger head diameter acts like a snow shoe, in that it covers a larger amount of surface area over the siding, thereby increasing the strength of the joint.

As siding boards are lapped, the overlap joint becomes the weakest point in the assembly. When overlaps are less than 1-1/2 inches, the joint is significantly weakened, making siding easier to dislodge.

Fastener pull-through occurs most often in steel-framed structures, as the strength of the joint is much higher into steel.

Fastener pull-through

Case Study: ASTM E330 Variations on Plastic Thickness and Application

In a study by Progressive Engineering Inc., commissioned by Nichiha USA, in late 2013, it was proven that differing mil thicknesses of polyethylene plastic, as well as how the plastic is applied on the specimen (tight or loose application and with folds) can lead to inflated test results.

Table 4 below, from this study, shows a 150% increase in wind resistance when the polyethylene plastic in the ASTME330 test is applied tightly. The products tested in this study are of similar composition, they are all the same width, and have almost identical physical properties.

There are other variations within this test. The framing type is a big determining factor in performance of siding and wind resistance. Even though most residences are built with wood studs, the study above was performed with steel stud framing. Why did they do that?

1. PEI used metal framing to conduct these tests in order to prevent fastener pull-out from being a factor in the test. Fastener pull-out as we explained above is highly dependent on the density of the wood stud, the length of the fastener, as well as the head diameter of the fastener, among other factors.
2. Wood framing presents complications in terms of grade of lumber used, specific gravity, etc.
3. When comparing wind resistance, fastener pull-through provides a level failure playing-field for all lap sidings. Steel is the best material for pull-through because the gage of the steel can be controlled better than lumber density.

In most cases regarding this ASTM testing, it has been found that the mode of failure in blind-nailed conditions for fiber cement siding is fastener pull-through; in face-nailed conditions, it tends to be fastener pull-out. However, either can occur in each condition. Therefore, the assembly is not dependent upon the siding itself, but upon the specific gravity of the lumber into which it is being nailed.

For this reason, it is important for the safety of the end-user, and for the code official approving job-site permits and inspecting installations, to know the exact assembly used in testing to obtain a certain design pressure. 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 the manufacturers. These reports prove a specific product meets or exceeds the requirements in the latest version of the building code. Test results, such as those from ASTM E330, are listed in these code evaluation reports in order to allow installation of a given product.

In the next and final part of this wind load series, we delve into the code bodies and agencies that help keep us safe. Stay tuned!

Read More:

Under Pressure: Wind Load Series Part 1
Under Pressure: Wind Load Series Part 2
Under Pressure: Wind Load Series Part 4

Categories: Performance
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