Floors with repetitive parallel wood joists overlain by wood panel sheathing behave as orthotropic two-way structures since their along-joist stiffness is much higher than across-joist stiffness. Due to this orthotropy, they have a tendency to produce amplified vibration motions when excited by human footfalls or similar impacts on floor surfaces. Such amplified motion results from clustering of modal responses and is strongly influenced by the across-joist construction details and the ratio of floor width to joist span. When such amplifications occur they can cause the serviceability problem of human discomfort, as felt by people causing or being vibrated by impacts. As is known from traditional carpentry practice, installing between-joists bracing is a simple, economic and effective means of improving vibration performance of floors with parallel wood joists. The experimental study reported here elucidates why between-joist bracing is effective and quantifies relationships between floor vibration amplitudes and the stiffness of between-joist bracing.
A test method was developed to quantify the isolated component stiffness properties of common bracing elements (cross-bridging and solid blocking) and an innovative bracing element that permitted broad-range manipulation of the stiffness. Complete rectangular on plan floor systems were constructed to investigate their responses in configurations without and then with various types of between-joist bracing elements added. Floor plan dimensions were span 4.22m and width 3.66m. Joists were 45mm by 240mm sawn lumber spaced at 610mm on center, and floor sheathing was 19mm attached by nailing. Measurements of floor response in each configuration determined low level modal properties (frequencies, shapes and damping) under free vibration; frequency-weighted root-mean-square acceleration under controlled forced vibration; and deflection under a concentrated static load. This reflects that modal properties correlate directly with human perceptions of motions they experience within buildings and other structures. Observations of frequency-weighted root-mean-square acceleration and deflection under concentrated static load were included because some suggested empirical practices (aimed at screening out potentially problematic designs for floors with flexible surfaces – as typifies wood joisted floors) utilize those parameters.
As would be expected, it was found that addition of all types of bracing element decreases static deflection at the centre of floors, with the level of decrease being proportional to the stiffness of between-joist bracing elements (as isolated components). The greatest observed decrease in static deflection was 31% obtained using the innovative type of bracing element. Only slight increases in fundamental natural frequencies resulted from installing between-joists bracing, which is attributed to the mass they add counteracting across-joist stiffness gains for that mode. However, installing between-joists bracing was capable of adding significantly to across-joist stiffness for higher modes. In general it can be presumed that, for floors like those investigated, the separation between adjacent modal frequencies of low level modes that combine to produce annoying amplifications of motion is increased by increasing the stiffness of between-joists bracing (ditto increasing the number of lines of bracing). As a consequence, all types of between-joist bracing elements investigated were effective in reducing frequency-weighted root-mean-square accelerations under controlled impact vibration.
The authors have developed complementary modelling techniques to predict the stiffness of between-joist bracing elements; and to predict relationships between those stiffness properties and various mentioned floor performance parameters employed within direct or empirical assessments of vibration serviceability of floor.