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AC391: Key Design Considerations for Designing Uplift Restraint Systems for Roofs

A Quick History of Wind Uplift Rod Systems

For the past decade, rod tie-down systems have been used by the wood light-framed 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 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.

ac391-wind-uplift
Figure 1: Excessive spacing of rod restraints to resist high uplift forces could cause failure of the top plate

Steps for Developing an Evaluation Report

  • Step 1

    Refer to Building Code

    ac391-codereport
  • Step 2

    IBC Section 104.11 Alternate Materials, Design, and Methods of Construction and Equipment

  • Step 3

    Acceptance Criteria Developed for Products through Public Process

  • Step 4

    Code Report Issued After Testing/Calculations approved by ICC-ES and IAPMO UES

Industry Guidance

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:

  • The steel components comprising continuous rod tie-down runs (CRTR) only
  • The entire continuous rod tie-down system (CRTS), which includes CRTR and the light-framed wood structure used to resist wind uplift

These same guidelines in AC391 can be used by project designers themselves to lay out continuous rod tie-down systems to resist wind uplift. This technical bulletin shows the important steps required in AC391 to design the system properly. These steps are illustrated and summarized on the following pages of this bulletin. The last page describes the two types of evaluation reports that a manufacturer can obtain from ICC-ES and clarifies the workload of the licensed design professional for the project when specifying a product with a CRTR report vs. a product with a CRTS evaluation report.

aci391-key-design-considerations
Connection Location AC391 Requirement Criteria Section
A
  • Use of continuous rod tiedown runs (CRTR) and continuous rod tiedown systems (CRTS) is limited to resisting roof wind uplift in light-frame wood construction.
  • Specifically excluded from AC391 is the use of CRTR to resist shearwall overturning forces or use in cold-formed steel framing.
1.2.1.1
B CRTS allowable loads shall be evaluated and be limited by
  • Tiedown run steel component capacities per 3.1.1, or
  • Wood deflection limitations per 3.2.2.2, or
  • Flexural (bending) stress per 3.2.2.1, or
  • Shear stress perpendicular to grain per 3.2.2.4, or
  • Combined axial (chord/drag force) and flexural (bending) stresses per 3.2.2.5
3.1.1 and 3.2.2
C 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. 3.2.2.3
D 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. 3.2.2.1
E 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. 3.2.2.2
F 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, 6.2.1.3, and 6.3.1.3
G 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. 3.2.1.2 and Figure 1
H 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. 3.2.1.1
I
  • Proof of the positive connection between threaded rod and threaded rod couplers shall be provided, such as Witness Holes™ or other method.
  • Rod couplers must also be tested to prove they can develop at least 100% of the rod’s tensile strength and 125% of the rod’s yield strength.
1.4.5 and 3.4.1.1
J Design of the anchorage is the responsibility of the design professional and must be performed in accordance with the applicable code. 6.2.4.5 and 6.3.3.5