Rod tiedown systems have been used by the light-framed wood construction industry to resist wind uplift forces. Yet codes and standards have not provided detailed guidance for design of these systems. Designers, consequently, have been forced to rely entirely on engineering judgment and/or trust a rod manufacturer's literature or substitution submittals to create this load path.
This lack of guidance sometimes led to rod-restraint spacing based on rod tension and bearing plate capacities alone. This design neglects the wood components of the system and may lead to rods spaced too far apart, compromising the continuous load path, causing building damage and creating life-safety issues.
In June 2010, ICC-ES passed and made effective Acceptance Criteria 391 after multiple public hearings that garnered engineer, manufacturer, building official and other third-party input. AC391 established guidelines for the evaluation of either:
These same guidelines in AC391 can be used by project designers themselves to lay out continuous rod tie-down systems to resist wind uplift.
Following the key design considerations, an effective uplift rod system is designed and detailed to:
|Connection Location||AC391 Requirement||Criteria Section|
CRTS allowable loads shall be evaluated and be limited by
||3.1.1 and 3.2.2|
|Top-plate torsion (rotation) must be prevented due to offsets between the point of load application, such as hurricane ties at the sides of the top plates and load resistance (rods at the center of the top plate for example). This can be accomplished by providing a positive connection from the top plate to stud on the same side of the wall as the roof framing to wall connection.||188.8.131.52|
|Approved top plate splice details must be provided for the CRTS to utilize both top plates in bending, otherwise only the capacity of a single top plate may be used.||184.108.40.206|
|The deflection of the top plates in bending occurring between CRTR is limited to L/240, where L is the length of the top plates between tiedown runs. Additionally, the sum of the rod elongation, top plate crushing under bearing plates, deflection of any take-up devices and the deflection of the top plates between tiedown runs shall not exceed 0.25 inches at the applied (ASD) load.||220.127.116.11|
|The effects of wood shrinkage on the overall deflection of the CRTS shall be analyzed by a registered design professional, and a method of addressing wood shrinkage in the system shall be provided. If shrinkage compensating devices are used, they shall meet AC316 requirements. See the Simpson Strong-Tie® Wood Shrinkage Calculator for more information||3.1.1, 18.104.22.168, and 22.214.171.124|
|Steel bearing plates shall be sized for proper length, width and thickness based on steel cantilever bending action and wood bearing. Deflection from bearing compression (up to 0.04") must be included in overall deflection calculations.||126.96.36.199 and Figure 1|
|Rod elongation is limited to 0.18 inches for total rod length at the applied (ASD) load. See the Rod Elongation Calculator for more information.||188.8.131.52|
||1.4.5 and 184.108.40.206|
|Design of the anchorage is the responsibility of the design professional and must be performed in accordance with the applicable code.||220.127.116.11 and 18.104.22.168|