Force of a 2x4 2

[onlineeng]

In Florida, for the large missile impact test, a 2x4 (between 7' and 9' long), weighing between 9 lbs. and 9.5 lbs., is shot from a cannon and strikes the surface of a hurricane shutter. The distance from the end of the cannon to the hurricane shutter is 9' plus the length of the 2x4. The 2x4's speed at impact is 50 feet per second. What is the force with which the 2x4 strikes the hurricane shutter?

Using a 2x4 length of 9' and a weight of 9 lbs., I come up with 19.42 pounds of force, which does not seem correct.

Thanks!

 

[Inlander]

I'll take a stab at it. Let me know if my reasoning is flawed.

If you know how far the shutter deflects, you can figure the time. Assuming constant deceleration(probably wrong), the average velocity is 50 x 12/2 = 300 in/sec. So t = x/300 sec. Your acceleration is 50ft/sec / t. For 1" deflection would take 1/300 sec. Acceleration a = v/t = 50 ft/sec / 1/300 sec = 15000 ft/sec^2. F = weight x 15000/32.2 = 4192 lb. If it deflects 10in, F=about 420 lb. If the shutter breaks, who knows?
I guess if you know the max allowable deflection of the shutter, you can guess the allowable speed or wind speed it can withstand (for a given wt. of projectile). If the shutter behaves like a beam (or a spring), the accelleration would be variable. Your max. acceleration and force would be twice that calculated.

 

[concretemasonry]

olineemg -

Since you are in Florida and mentioned an "air cannon", there must be a concern about debris penetration and not structural since most of the life-safety requirements are based on the testing conducted in Texas (Texas Tech?) where they have been testing wall and some window/door/shutter assemblies for use within the FEMA guidelines that are for life safety and not based on numeric analysis. There is a volume of test that have been performed for by groups and associations that want approval for standards inclusion. The standards are very strict - not penetration for walls after a 12' long 2x4 fired from an air cannon at 145 mph. For a long period only 2 wall assemblies were approved, but finally, a laminated wall of layers of 3/4" plywood on 2x6 studs and steel plate was included.

The basis is practical performance and not assumed structural calculations.

The earlier post about the fun of the testing was very accurate. the fun was not an exaggeration. The people testing verified the construction, assemblies, anchorage and took great pride in separating those that met the criteria from the others, but I imagine all test information is test available. None of the failures were disclosed publicly.

Other testing was done for doors and windows with a lower criteria, but could be valuable.

If I could take a month or two off to participate, it would be a real learning experience.

Much of the testing is the basis for coastal code requirements that are part of the regional/local codes and the heavy insurance discounts in addition the FEMA requirements.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[a2mfk]

Dick:
Shudders or impact resistant glazing is required in Florida for all wind-borne debris regions, see attached map.

I would not disagree with your assertion, but I would think the bigger reason to protect the openings is to maintain the envelope so the structure remains enclosed rather than partially enclosed. Garage doors, windows, and doors are the achiles' heel of structures during wind events, especially residential structures. You could add localized roof failures to the list also, once a couple of jack trusses fail then the roof is in trouble.

If people listen to the authorities, they should not be in the wind-borne debris region when a hurricane hits. That is a big IF.

I think you are referring to a "hardened" structure that meets specific FEMA guidelines for use as a shelter. I assume the OP's shudders are for general use in wind-borne debris regions, and Miami-Dade is usually the litmus test for standard wind-related product approval, though maybe now Florida has adopted S FL's standards state-wide when it comes to product approvals. That is a whole specialty area of engineering I am not involved with...

And I am in the same boat as you, I would love to do this type of testing!

 

[concretemasonry]

The Texas tests were the basis showing up in the hardened FEMA structure standards for zero penetration. They were altered downward slightly for other situations/materials in the coastal code areas, because the conditions in Florida were not as severe.

The insurance discounts (40-60%) offered in selected Florida areas were for structures built beyond the minimum code standards that are slow to change. One interesting thing was a weak point in the codes for the gable end of structures was found in damage assessment. This was almost as critical as the openings. Obviously, they also require some sort of additional anchorage of the garage door, which is major cause of roofs going and opening up everything to further damage.

It is interesting to see the window testing. While attending a door and window installation certification class they were testing windows for penetration in the same lab and I saw the same happy smile was on the face of the technicians there as I saw in Texas. It is great to get behind the scenes to see the basis for the codes and standards.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[onlineeng]

Thanks all for the time and thought.

Ash060, in this case, you are right, in a sense: the client has no plans to pay US for the testing. Our goal is to assist in the design of shutters that will pass muster when the client pays a testing facility to do the test.

Concretemasonry, you are right -- would be fun. Kind of reminds me of that great old Letterman segment, "Dropping Stuff from a Five-Story Building." Maybe we could get Letterman to do it and save everyone a bunch of bucks.