How can a Home Survive a Tornado, Hurricane or Earthquake? - Did You Know?

Glulam Flexural Test
The test set up over here on our reaction beam is a glulam flexural test. In this test we apply a vertical load at the third points of the beam. This load is increased until the beam actually fails or breaks. While the beam is being loaded, deflection measurements are taken at the center of the beam. After the beam has failed, the ultimate load information can be used to calculate a design load for that specific beam. The load information combined with the deflection information can be used to determine the stiffness of the beam, another important design consideration for the beam. A single test can tell us little about all of the beams constructed this may. In order to determine the properties for a whole class of beams constructed in this way we must conduct multiple beams that are theoretically identical. ASTM standards specify exactly how to convert the test results of a number of beams into design values for that design. We have the capability to test at this location a beam that is 12” thick, 48” deep and 60’ long.

The relationship between the ultimate or failure load and the load the designer uses when incorporating these members into a structure is about a factor of 3. (I say “about” because there are a number of adjustments made to the ultimate load that relate to test duration, number of specimen tested, the “scatter” of the collected data, and etc., that make it impossible to exactly determine what that safety factor is.) We maintain and the building codes require this large safety factor for a number of reasons:
• Uncertainty of the actual loads the member will see over the life of the structure.
• To account for field installation errors.
• To account for the natural variability of the product, and most importantly,
• Because the design community recognizes that in an extreme situation such as an earthquake or hurricane, the building is likely to be over stressed at some point. It is that reserve strength that insures that the building does not collapse, killing or harming the occupants.

Cyclic Shearwall Testing
The ability of a structure to withstand forces acting on that structure from the side – such as high wind, seismic, lateral soil pressures – is known to engineers as its lateral stability. The lateral stability of most wood residential structures is accomplished by the large flat rectangular elements that make up the walls of the structure. Wood structural panel siding or sheathing are the most efficient elements for accomplishing this. The apparatus before you is specifically designed to test the ability of these large flat rectangular elements. Not only can this apparatus conduct single-direction tests –monotonic tests – as are required for high wind and lateral soil pressure, this machine can simulate the cyclic nature of an earthquake. This is what makes this machine unique. There are only 4 or 5 in the US, and this is the only such machine outside of universities or privately owned companies.

Hurricane Impact Resistance Test – the Stud Gun
One of the major causes of damage in high wind events is from flying objects. Fences, garden sheds, lawn furniture, garbage cans, bicycles, barbecues, shingles and debris from less fortunate structures all become potential missiles during a hurricane. These missiles can damage the exterior surface of a structure, penetrate the roof sheathing, break windows and cause water damage to the interior of the house. Aside from the direct damage caused by the impact of these flying objects, there is a more sinister issue at stake. If the penetrations in a structure reach a certain point, then the structure is subject to pressurization. Unless the structure has been designed for pressurization – and few are – then the increased load on the structure can cause it to fail catastrophically. Explode. The difference between the two kinds of damage often determines the fate of the folks huddled inside. The difference is between wet and dead.

The point of this test is to simulate a missile impact by firing a 9-1/2 lb 2 x 4 at the building at 50 fps (34 mph). What does this relate to, aside from a 9-1/2 lb 2 x 4 at 50 fps? There have been attempts to relate this test to reality with little success. While it is highly unlikely that any window will be impacted by exactly that size and weight of 2 x 4 at just that speed, analysis has shown that the amount of energy imparted by this 2 x 4 is equal to or greater than that that could be expected by any of the really probable impact items; roof tiles, garbage cans, and other small debris. It is simply an arbitrary minimum level of protection that has been agreed on by the insurance companies, code bodies, academics, trade associations and home builders.

This test is used to certify/qualify windows, permanent, and temporary shutter systems for use in those areas subject to hurricanes and thus, missile damage.

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