Recommended: Strongback Bridging
A key benefit to using OPEN JOIST is the ability to enhance floor system performance through the installation of “strongback” bridging in OPEN JOIST floor systems. The product’s open-web configuration allows installation of this continuous perpendicular bridging through OPEN JOIST trusses. The result is the most effective and least expensive way to improve floor system performance with regard to deflection and, most significantly, vibration.
While performance is enhanced, it should be understood that strongback bridging does not affect the structural capabilities of OPEN JOIST trusses. Following is commentary from ANSI/TPI I-2002 about the use of strongback bridging to reduce the effects of differential deflection in floor trusses. Section 18.104.22.168 states:
Strongbacking is recognized for serving two purposes: reducing floor vibrations and limiting differential deflection. Strongbacking does not, however, contribute to or enhance the strength or structural integrity of the system.
The ANSI/TPI statement further discusses the effect strongbacks can have on floor system vibration:
Strongbacks are typically used to control potential vibration problems, as the addition of strongbacks has proven to stiffen the trusses and increase the dampening of transient oscillations. Vibration in a floor joist due to normal human activity (e.g. walking) includes vibration movements from side to side, and while floor sheathing prevents lateral vibration of the top chord, the bottom can still vibrate back and forth. Thus, placing a strongback at the bottom of the floor truss helps control the side-to-side movement at the bottom and improves the overall perceptible performance of the floor. Even when there is a ceiling on the bottom of the trusses, in which case the drywall will reduce lateral movement, the addition of strongbacks can still help to further restrict vibration. It should be recognized that, while it will not affect the structural integrity of the system, cutting removing or failing to provide such strongback bracing can result in degradation of the floor system’s ability to dampen vibration.
OPEN JOIST engineers are in agreement with this ANSI/TPI statement. Even though strongbacks are not required by the International Residential Code (IRC) or the International Building Code (IBC), it is strongly recommended that they be installed on the inside bottom or the inside top of OPEN JOIST floor trusses as indicated in the Strongback Installation Details.
And, after consideration of strongback performance in relation to position in a truss’ span, it is recommended that this bridging be installed only at mid-span of the OPEN JOIST, where it has been shown to achieve best effect.
In some installation situations, it may be necessary to cut through or interrupt the strongback bridging. When this occurs, strongbacks should be installed in an alternate area and situated so that at least 3 joists are connected. Strongbacks should be installed (in vertical orientation) at the same time floor joists are installed.
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