Decrease joist spacing:
- Another option to stiffen the floor system is to decrease the spacing between joists. This strategy is not as effective as increasing the depth of the joists, but it will have considerable impact.
- When comparing a 2″ × 10″ (38 × 235 mm) joists at 16″ (400 mm) on center, to the same 2″ × 10″ (38 × 235 mm) joists at 12″ (300mm) on center, the maximum allowable span controlled by vibration increases by about 5%. This option may have an impact on the design of the house if forced air ducting is installed between rather than below for joists.
- Another advantage of closer joist spacing is the reduction in the amount of flexing of the subfloor. This makes the floor feel stronger and helps prevent sub-floor sagging.
Use stiffer species of wood.
- Choosing a species of wood that is stronger will decrease the vibration. Selecting a species such as Douglas fir rather than spruce can increase the allowable vibration-controlled span by about 5%.
- As machine stress-rated (MSR) lumber becomes easier to obtain, it may be specified for floor systems. Using MSR joists instead of S-P-F will result in an increase of about 9% in the allowable vibration-controlled span.
Glue subfloor and/or use thicker panels.
- The floor joists and the subfloor act as a system. Changing the thickness of the subfloor and using a better fastening system can stiffen the floor.
- An excellent technique that can be used to stiffen the floor is to glue the subfloor as well as nailing or stapling it. This increases the bonding of the subfloor to the joists and will result in an increase of about 11% in the allowable vibration-controlled span.
- For maximum benefit when gluing the subfloor to the joists, ensure that both the joists and the subflooring are clean and free of concrete, dirt, moisture or frost.
- Choosing a thicker subfloor will also stiffen the floor. Increasing the subfloor thickness from 5/8″ (15.5 mm) to 3/4″ (19 mm) will result in an increase of about 5.5% in the allowable vibration-controlled span.
Problem: Flooring Vibration - 2.
Inadequate bridging or blocking
The overall floor system stiffens when blocking and bridging transfer loads from one structural member to another. In order for blocking and bridging to work well, they must be installed properly. Blocking and bridging that are too long or too short or not fastened adequately will not perform.
Increase bridging or blocking beyond building code requirements.
- Most building codes require strapping or bridging/blocking once certain spans are exceeded. This improves the load-sharing between structural members, thereby strengthening the floor system.
- In vibration-sensitive areas, it may be wise to install two rows of bridging or blocking within the span instead of the one that may be required by code. When more than one row of bridging or blocking is placed near the center of the floor span, greater stiffness results as shown in Figure 15.
Figure 15 - Extra Bridging and Blocking To Reduce Vibration
Use strapping in addition to bridging and blocking.
- Strapping on the underside of joists will provide some lateral stability to the joists and prevent twisting, but it does not have much effect on load-sharing. Bridging or blocking is more effective in transferring loads from one structural member to another and thus stiffens the floor, decreasing vibration. The combination of strapping plus either bridging or blocking is even more effective as shown in Table 1 - Figures 16a, 16b, 16c, 16d, 16e and 16f.
Types of Bridging, Blocking & Strapping
Figure 16a - Poor
Figure 16b - Good - Strapping
Figure 16c - Better - Bridging
Figure 16d - Better - Blocking
Figure 16e - Best - Bridging & Strapping
Figure 16f - Best - Blocking & Strapping
- A finished ceiling, consisting of panels fastened directly to the underside of a floor, will have an even more significant effect than a single row of strapping. Panel products fastened to the underside of the floor provide excellent restraint in all horizontal directions.
- Consider adding continuous strapping at 12″ (300 mm), 16″ (400 mm), or 24″ (600 mm) o.c. to anticipate future ceiling finishes. In the interim, the strapping will provide additional restraint and assist in reducing floor vibration.