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Component Manufacturing dverti$er
June 2017 #10215 Page #68
Adverti$er
Don’t Forget! You Saw it in the
All things Wood
How to Predict a Bouncy Floor
By Frank Woeste and Dan Dolan
The model International Residential Code (IRC) permits a design live load of 30 psf for “sleeping rooms.” The model codes specify 40 psf for all other rooms. Of the annoying floor vibration complaints we have received, the most common scenario stems from the use of: • 30 psf live load, • L/360 live-load deflection limit, and • joists at 12-inches on-center (OC). The combination of a 30 psf floor design load and code minimum L/360 live load deflection limit permits a long joist span for a closer OC spacing (e.g., 2x10 up to a 21-foot span). While floors designed to minimal code-permitted criteria may provide the least cost alternative for the joist framing materials, some designs have created less than desirable floors and contributed to performance complaints. Bouncy residential floors1 are easily prevented by knowledge of the potential issue and follow-up with the homebuilder in the planning stages of new home construction. After construction and the home is furnished, a repair can be difficult, costly, or practically impossible. For example, in the case of a 2nd floor issue, repairs to the joists can’t be made without removal of the drywall. Moreover, after a repair is attempted, there’s no guarantee the result will resolve the issue.
Floor Vibration Research
Professor Dolan and his graduate students tested 13 fullscale (16 ft. x 16 ft.) floors with 23/32" rated T&G plywood floor sheathing, glued and nailed. The joists were supported on the ends by concrete blocks simulating a rigid support condition (not flexible as from a header or girder). In addition to the laboratory tests, he tested a total of 73 in-situ floors in unoccupied and occupied homes. In a nutshell, using standard methods for testing floor vibration, he found that, for the laboratory floors and unoccupied homes, human subjects did not detect annoying vibrations for floors with a vibration frequency of 15 Hz or higher. However, for joists in occupied homes with furniture, human subjects did not detect annoying vibrations for floors with a vibration frequency of 14 Hz or higher.
Example Calculation of Joist Frequency The fundamental frequency of joists can be calculated by the equation:
where: ƒ is the fundamental frequency of the joist in Hz,
In the 1990s, research led by Professor Dan Dolan at Virginia Tech on full-scale solid-sawn joists, I-joists, and metal-plateconnected (MPC) floor trusses addressed the issue of wood floor vibration control. The laboratory testing program and field validation were limited to wood joist floors spaced at 24-inches 1 This article contains material from the background part of a unit on residential floor joist design that has been included in many Virginia Tech short courses over the last two decades. An example of a typical 15-hour introductory design course may be viewed at: http://www.cpe.vt.edu/sdwnds/index.html. PHONE: 800-289-5627
OC with structural wood sheathing, thus the typical design dead load was 10 psf for solid-sawn and I-joists tests and 15 psf for 4x2 floor trusses.
E is the modulus of elasticity in psi, I is the moment of inertia in inches4, W is the actual total supported permanent (dead) load in lbs, and L is the joist span in inches. This formula applies to the joist installations that bear on a sill plate or framed walls-- not on a flexible support such as a header or girder.
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