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ac391

AC391

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<h1>AC391</h1>
<h2>A Quick History of Wind Uplift Rod Systems</h2>
<div class="row">
  <div class="col-md-6 col-sm-6">
    <p>
      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.
    </p>
    <p>
      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.
    </p>
  </div>
  <div class="col-md-6 col-sm-6">
    <div class="image">
      <img
        alt="Wind Uplift"
        src="https://embed.widencdn.net/img/ssttoolbox/eykl91aoqg/500px/SR_URS_InstDtl_Ill_Instld_StrongRod-URS-Wind-Uplift-Resistant-Failure_CY.png"
      />
      <div class="image__caption">
        Figure 1 &mdash; Excessive Spacing of Rod Restraints to Resist Uplift
        Forces Causing Top Plate Failure
      </div>
    </div>
  </div>
</div>
<h2>Industry Guidance</h2>
<p>
  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:
</p>
<ul>
  <li>
    The steel components making up continuous rod tiedown runs (CRTR) only. If a
    manufacturer has a CRTR report, the designer or Engineer of Record must take
    the time to evaluate how the light-framed wood members will transfer forces
    to the CRTR.
  </li>
  <li>
    The entire continuous rod tiedown system (CRTS), which includes CRTR and the
    light-framed wood structure used to resist wind uplift. If a manufacturer
    has a CRTS report, this saves the designer or Engineer of Record time.
  </li>
</ul>
<p>
  These same guidelines in AC391 can be used by project designers themselves to
  lay out continuous rod tie-down systems to resist wind uplift.
</p>
<p>
  Following the key design considerations, an effective uplift rod system is
  designed and detailed to:
</p>
<ul>
  <li>
    Efficiently transfer wind uplift loads from wood components to steel
    components of the rod runs
  </li>
  <li>Keep wood top plate bending within acceptable limits</li>
  <li>Control wood top plate rotation</li>
  <li>Limit steel rod elongation</li>
  <li>Restrict crushing of wood top plate under bearing plates</li>
  <li>Address deflection caused by wood shrinkage</li>
</ul>
<h2>From the Roof to the Foundation</h2>
<h3>Strong-Rod Uplift Restraint System Components</h3>
<div>
  <img
    alt="Strong-Rod Uplift Restraint System Components"
    src="https://embed.widencdn.net/img/ssttoolbox/j18mlacl4i/300px@2x/F-L-SRS18-051b-urs-components.png"
  />
</div>
<table class="table-responsive-sm">
  <tr>
    <th>Connection Location</th>
    <th>Application</th>
    <th>Criteria Section</th>
    <th>AC391 Requirement</th>
  </tr>
  <tr>
    <td>
      <img
        alt="A"
        src="https://embed.widencdn.net/img/ssttoolbox/hopxvqkvc0/exact/A.jpeg"
      />
    </td>
    <td>Roof Framing or Truss-to-Top-Plate Restraint</td>
    <td class="la">1.2.1.1</td>
    <td class="la">
      <ul>
        <li>
          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.
        </li>
        <li>
          Specifically excluded from AC391 is the use of CRTR to resist
          shearwall overturning forces or use in cold-formed steel framing.
        </li>
      </ul>
    </td>
  </tr>
  <tr>
    <td rowspan="3">
      <img
        alt="B"
        src="https://embed.widencdn.net/img/ssttoolbox/oa8mfdvlxg/exact/B.jpeg"
      />
    </td>
    <td rowspan="3">Wood Top Plates</td>
    <td class="la"><p>3.2.2</p></td>
    <td class="la">
      <p>CRTS allowable loads shall be evaluated and be limited by</p>
      <ul>
        <li>Wood deflection limitations per 3.2.2.2, or</li>
        <li>Flexural (bending) stress per 3.2.2.1, or</li>
        <li>Shear stress perpendicular to grain per 3.2.2.4, or</li>
        <li>
          Combined axial (chord/drag force) and flexural (bending) stresses per
          3.2.2.5
        </li>
      </ul>
    </td>
  </tr>
  <tr>
    <td class="la">3.2.2.1</td>
    <td class="la">
      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.
    </td>
  </tr>
  <tr>
    <td class="la">3.2.2.2</td>
    <td class="la">
      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.
    </td>
  </tr>
  <tr>
    <td>
      <img
        alt="C"
        src="https://embed.widencdn.net/img/ssttoolbox/nhauz6hv0h/exact/C.jpeg"
      />
    </td>
    <td>Top Plate-to-Stud Rotation Restraint</td>
    <td class="la">3.2.2.3</td>
    <td class="la">
      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.
    </td>
  </tr>
  <tr>
    <td rowspan="2">
      <img
        alt="D"
        src="https://embed.widencdn.net/img/ssttoolbox/de57wyem26/exact/D.jpeg"
      />
    </td>
    <td rowspan="2">Shrinkage Compensating Device and Bearing Plate</td>
    <td class="la">
      3.1.1,<br />
      6.2.1.3, and<br />
      6.3.1.3
    </td>
    <td class="la">
      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&reg;
      <a href="/webapps/woodshrinkage/">Wood Shrinkage Calculator</a> for more
      information.
    </td>
  </tr>
  <tr>
    <td class="la">
      3.2.1.2 and<br />
      Figure 1
    </td>
    <td class="la">
      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&quot;) must be included in overall
      deflection calculations.
    </td>
  </tr>
  <tr>
    <td rowspan="2">
      <img
        alt="E"
        src="https://embed.widencdn.net/img/ssttoolbox/jukmheeyjk/exact/E.jpeg"
      />
    </td>
    <td rowspan="2">Steel Threaded Rod</td>
    <td class="la"><p>3.1.1</p></td>
    <td class="la">
      <p>CRTS allowable loads shall be evaluated and be limited by</p>
      <ul>
        <li>Tiedown run steel component capacities per 3.1.1</li>
      </ul>
    </td>
  </tr>
  <tr>
    <td class="la">3.2.1.1</td>
    <td class="la">
      Rod elongation is limited to 0.18 inches for total rod length at the
      applied (ASD) load. Visit strongtie.com/software to access our Rod
      Elongation Calculator.
    </td>
  </tr>
  <tr>
    <td>
      <img
        alt="F"
        src="https://embed.widencdn.net/img/ssttoolbox/zg9po5odyv/exact/F.jpeg"
      />
    </td>
    <td>Coupler Nut</td>
    <td class="la">
      1.4.6 and<br />
      3.4.1.1
    </td>
    <td class="la">
      <ul>
        <li>
          Proof of the positive connection between threaded rod and threaded rod
          couplers shall be provided, such as Witness Holes&trade; or other
          method.
        </li>
        <li>
          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.
        </li>
      </ul>
    </td>
  </tr>
  <tr>
    <td>
      <img
        alt="G"
        src="https://embed.widencdn.net/img/ssttoolbox/dgpnh79bl7/exact/G.jpeg"
      />
    </td>
    <td>Anchorage</td>
    <td class="la">
      6.2.4.5 and<br />
      6.3.3.5
    </td>
    <td class="la">
      Design of the anchorage is the responsibility of the design professional
      and must be performed in accordance with the applicable code.
    </td>
  </tr>
</table>

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