Shear Wall Plates

Last updated September 8, 2022
By Ian Story

Typically, wall bottom plates are constructed using 2x nominal lumber, just like the wall studs. This is standard practice and efficient to frame, since the framers can attach the wall studs by nailing through the bottom plate into the base of the studs before standing the wall up.

However, you will occasionally find shear wall schedules calling out 3x nominal lumber for bottom plates at high-capacity shear walls. This is guaranteed to upset the framer and elicit calls for substitutions during construction, for several good reasons:

• The framer can no longer end nail the studs into the baseplate (16d nails aren’t long enough) and must toenail them instead.
• Standard wall heights using precut studs are based on 2x plates. Switching to 3x plates requires trimming 1″ off the end of every single stud used to frame the wall.
• 3x lumber is atypical and likely to be missed during the lumber takeoff. So the required sill plates may not make it to the job site by the time the framer arrives.

So, are 3x sill plates really required? This article addresses that question.

Building Code History

Older versions of the building code (before 2009) did require 3x sill plates in certain situations. Many engineers will have created their standard shear wall schedules during this period, which explains why the 3x sill plate callouts can still be found in engineer’s drawings today.

In the 2006 IBC, shear wall bottom plates were required to be 3x nominal lumber (with an exception allowing 2x lumber if the calculated number of anchor bolts was doubled).

(2006 IBC)
<strong>2305.3.11 Sill plate size and anchorage in Seismic Design Category D, E or F.</strong> Anchor bolts for shear walls shall include steel plate washers, a minimum of 0.229 inch by 3 inches by 3 inches in size, between the sill plate and nut. The hole in the plate washer is permitted to be diagonally slotted with a width of up to 3/16 inch larger than the bolt diameter and a slot length not to exceed 1 3/4 inches, provided a standard cut washer is placed between the plate washer and the nut. <em>Sill plates resisting a design load greater than 490 plf using load and resistance factor design or 350 plf using allowable stress design shall not be less than a 3-inch nominal member.</em> Where a single 3-inch nominal sill plate is used, 2-20d box end nails shall be substituted for 2-16d common end nails found in Line 8 of Table 2304.9.1.
    <strong>Exception:</strong> In shear walls where the design load is greater than 490 plf but less than 840 plf using load and resistance factor design or greater than 350 plf but less than 600 plf using allowable stress design, the sill plate is permitted to be a 2-inch nominal member if the sill plate is anchored by two times the number of bolts required by design and 0.229-inch by 3-inch by 3-inch plate washers are used.Code language: HTML, XML (xml)

In the 2009 IBC, section 2305 was modified to reference the AWC Special Design Provisions for Wind & Seismic (SDPWS), and much of the language for shearwalls and diaphragms was deleted from the IBC. The above code snippet no longer appears In the 2009 IBC and following versions.

The 2015 SDPWS does not require 3x nominal sill plates. The only language requiring 3x nominal framing for shear walls is as follows:

(2015 SDPWS)
<strong>4.3.7.1 Wood Structural Panel Shear Walls:</strong> ...The shear wall shall be constructed as follows:
    <strong>5.</strong> Where any of the following conditions occur, the width of the nailed face of a common framing member or blocking at adjoining panel edges shall be 3" nominal or greater and nailing shall be staggered at all panel edges.
        <strong>a.</strong> Nail spacing of 2" on center at adjoining panel edges is specified, or
        <strong>b.</strong> 10d common nails having penetration into framing members or blocking of more than 1 1/2" are specified at 3" on center or less at adjoining panel edges, or
        <strong>c.</strong> The nominal unit shear capacity on either side of the shear wall exceeds 700 plf in Seismic Design Category D, E, or F.

In lieu of a single common framing member, two framing members that are at least 2" in nominal thickness shall be permitted where designed in accordance with 4.3.6.1.1.

<strong>Table 4.3A Nominal Unit Shear Capacities for Wood-Framed Shear Walls</strong>
<strong>Footnote 6. </strong>Where panels are applied on both faces of a shear wall and nail spacing is less than 6" on center on either side, panel joints shall be offset to fall on different framing members. Alternatively, the width of the nailed face of framing members shall be 3" nominal or greater at adjoining panel edges and nails at all panel edges shall be staggered.Code language: HTML, XML (xml)

This language only applies to framing members at adjoining panel edges (i.e. at the joint between 2 shearwall panels). This condition does not occur at sill plates. So, as of the 2009 IBC, 3x sill plates are no longer required by the code.

The change in language clarifies that the purpose of 3x nominal lumber (or 2-2x lumber stitch nailed together) is to prevent splitting in the face of the member at very tight nail spacings (such as when 2 panels both require 3″ o/c nailing into the same member). Since the sill plate receives the nailing of only a single panel, the sill plate size is not driven by splitting concerns.

Sill Plate Anchorage

While the SDPWS does not specify 3x sill plates, it does require that the sill plate anchorage be designed to resist the applied shears. While this can usually be achieved with a 2x sill plate, using a 3x sill plate is an option to reduce the required number of anchor bolts.

Extensive testing on sill plates anchored to concrete has demonstrated that splitting of the wood sill plate is the primary failure mechanism. In light of this, the 2018 IBC allows you to disregard the concrete breakout calculations when determining the capacity of anchor bolts, provided the detail meets certain requirements (this is important, because concrete breakout calculations based on ACI 314 will show the concrete as the weak link, in contradiction to empirical test results). Thus, the relevant calculations for anchor bolt strength are based on NDS chapter 12 (2018 NDS).

(2018 IBC)
Requirements to disregard concrete breakout strength for anchor bolts:
<strong>1905.1.8 ACI 318, Section 17.2.3.</strong> Modify ACI 318 Sections 17.2.3.4.2, 17.2.3.4.3(d) and 17.2.3.5.2 to read as follows:
<strong>17.2.3.5.2</strong> ...anchors and their attachments shall be designed in accordance with 17.2.3.5.3. The anchor design shear strength for resisting earthquake forces shall be determined in accordance with 17.5.
<strong>    Exceptions:</strong>
<strong>    1. </strong>For the calculation of the in-plane shear strength of anchor bolts attaching wood sill plates of bearing or nonbearing walls of light-frame wood structures to foundations or foundation stem walls, the in-plane shear strength in accordance with 17.5.2 and 17.5.3 need not be computed and 17.2.3.5.3 shall be deemed to be satisfied provided all of the following are met:
<strong>    1.1. </strong>The allowable in-plane shear strength of the anchor is determined in accordance with ANSI/AWC NDS Table 12E for lateral design values parallel to grain.
<strong>    1.2. </strong>The maximum anchor nominal diameter is 5/8 inch (16 mm).
<strong>    1.3. </strong>Anchor bolts are embedded into concrete a minimum of 7 inches (178 mm).
<strong>    1.4. </strong>Anchor bolts are located a minimum of 13/4 inches (45 mm) from the edge of the concrete parallel to the length of the wood sill plate.
<strong>    1.5. </strong>Anchor bolts are located a minimum of 15 anchor diameters from the edge of the concrete perpendicular to the length of the wood sill plate.
<strong>    1.6. </strong>The sill plate is 2-inch (51 mm) or 3-inch (76 mm) nominal thickness.Code language: HTML, XML (xml)

Anchor Bolt Capacities

The following anchor bolt capacities are excerpted from the 2018 NDS Table 12 E for an SPF plate. These are adjusted lateral design values (Z’) and include the load duration factor.

	2x sill plate	3x sill plate
1/2″ anchor bolt	944 lb	1,168 lb
5/8″ anchor bolt	1,088 lb	1,356 lb

The table above uses a geometry factor (c_Δ) of 0.8 for 5/8″ anchor bolts (actual edge spacing = 2″, minimum edge spacing for c_Δ = 1.0 is 4D, or 2.5″)

The following limits are required to avoid a further reduction in the geometry factor (c_Δ):

• Spacing to end of plate: 7D (4.5″ for 5/8″ anchor bolts)
• Spacing between bolts: 4D (2.5″ for 5/8″ anchor bolts)

The table above uses a group action factor (c_g) of 1.0. Here is the logic of this decision: the group action factor is used for a series of bolts to represent the fact that the first bolt in series in a member subjected to axial loads tends to take a disproportionately large amount of the total load. In a sill plate situation, however, the applied load is a unit load per length, so the nature of the loading ensures that loads will be distributed more or less evenly to the individual bolts. See https://www.eng-tips.com/viewthread.cfm?qid=160677 for further discussion.