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The chance that a residential floor design will result in
annoying vibrations can be greatly reduced by applying some simple rules-of-thumb. No rule-of-thumb or design can guarantee that all homeowners will be satisfied with their floors, but many potential problems
can be eliminated at the design stage. Quite surprising, very small changes in span can make the difference between "success" or "failure" with respect to how residents may judge the
performance of floors in anew home.
Bounce Control Made Easy
One rule that may not yield the most cost efficient solution, but can certainly save you time and worry is to
"increase the depth one size. "For example, if the code requires a 2x8 at 16 inches on-center, then use a 2x10 of the same grade and species. Another example would be to use a 14-inch deep floor truss
when a 12-inch deep truss would meet code requirements. You should remember that spans allowed by code meet life safety minimums. Floor vibration, or bounce, is not a safety issue--it is a
performance issue that may be important to a homeowner.
In general, length of span is the most important variable in predicting how a floor will perform for the residents. A shorter span should improve
floor performance over a longer span for the same size, species, and joist grade. Floor sheathing should be glued to the joists, I-joists, or floor trusses for improved floor performance.
Probably the
least efficient way to improve floor performance is to reduce the on-center spacing. This result may surprise some, but it can be understood by realizing that residents feel "bounce" as a result of
afoot impacting a joist. Simply stated, no impact, no vibration. Joists are not normally close enough for the shock of a foot to be carried by two joists.
In Conclusion
In conclusion,
a simple approach to controlling annoying vibration is to increase the code required depth by one size, but this solution may not be optimum based on joist costs. More efficient design solutions to prevent
annoying floor vibrations are given in the Journal of Light Construction, November 1998.The floor sheathing should be glued to the joists along with nails or screws as specified by the building
designer.
Residential floor design to prevent annoying vibrations is very complicated and the response of residents to actual floor constructions is subjective. The major points to remember about
minimizing floor vibration problems are:
Increase the joist depth over the code minimum for good protection against vibration
complaints.
For solid-sawn joist spans more than 15-feet, reduce joist span over the maximum
allowed by code.
Always glue the sheathing to the joists, or trusses. Screws for holding thesheathing to the joists are preferred over nails. The sheathing should be glued to the joists and all nailing of
each panel should be completed before the glue sets.
Reducing the joist spacing is the least effective way to minimize floor vibration complaints.
Drs. Frank Woeste, P.E. and Dan Dolan, P.E. are
Professor and Associate Professor, respectively, Virginia Tech University, Blacksburg, VA 24061.
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