New Treatment of Shear Wall Aspect Ratios in the 2015 SDPWS

This post was co-written by Simpson Engineer Randy Shackelford and AWC Engineer Phil Line.

The 2015 International Building Code references a newly updated 2015 Edition of the American Wood Council Special Design Provisions for Wind and Seismic standard (SDPWS). The updated SDPWS contains new provisions for design of high aspect ratio shear walls. For wood structural panels shear walls, the term high aspect ratio is considered to apply to walls with an aspect ratio greater than 2:1.

Figure 1. SDPWS 2008 Table 4.3.4 and illustration of shear wall with height, h and length, bs

In the 2015 SDPWS, reduction factors for high aspect ratio shear walls are no longer contained in the footnotes to Table 4.3.4 (See Figure 2). Instead, these factors are included in new provisions accounting for the reduced strength and stiffness of high aspect ratio shear walls.

Figure 2.  SDPWS 2015 Table 4.3.4

Figure 2. SDPWS 2015 Table 4.3.4

Deflection Compatibility – Calculation Method

New Section 4.3.3.4.1 states that “Shear distribution to individual shear walls in a shear wall line shall provide the same calculated deflection, δsw, in each shear wall.” Using this equal deflection calculation method for distribution of shear, the unit shear assigned to each shear wall within a shear wall line varies based on its stiffness relative to that of the other shear walls in the shear wall line. Thus, a shear wall having relatively low stiffness, as is the case of a high aspect ratio shear wall within a shear wall line containing a longer shear wall, is assigned a reduced unit shear (see Figure 3).

Figure 3. Illustration of deflection compatibility

 

In addition, Section 4.3.4.2 contains a new aspect ratio factor, 1.25 – 0.125h/bs, that specifically accounts for the reduced unit shear capacity of high aspect ratio shear walls. The strength reduction varies linearly from 1.00 for a 2:1 aspect ratio shear wall to 0.81 for a 3.5:1 aspect ratio shear wall. Notably, this strength reduction applies for shear walls resisting either seismic forces or wind forces. For both wind and seismic, the controlling unit shear capacity is the smaller of the values from strength criteria of 4.3.4.2 or deflection compatibility criteria or 4.3.3.4.1.

Deflection Compatibility – 2bs/h Adjustment Factor Method

The 2bs/h factor, previously addressed by footnote 1 of Table 4.3.4, is now an alternative to the equal deflection calculation method of 4.3.3.4.1 and applies to shear walls resisting either wind or seismic forces. This adjustment factor method allows the designer to distribute shear in proportion to shear wall strength provided that shear walls with high aspect ratio have strength adjusted by the 2bs/h factor. The strength reduction varies linearly from 1.00 for 2:1 aspect ratio shear walls to 0.57 for 3.5:1 aspect ratio shear walls. This adjustment factor method provides roughly similar designs to the equal deflection calculation method for a shear wall line comprised of a 1:1 aspect ratio wall segment in combination with a high aspect ratio shear wall segment.

In prior editions of SDPWS, a common misunderstanding was that the 2bs/h factor represented an actual reduction in unit shear capacity for high aspect ratio shear walls as opposed to a reduction factor to account for stiffness compatibility. The actual reduction in unit shear capacity of high aspect ratio shear walls is represented by the factor, 1.25 – 0.125h/bs, as noted previously. The 2bs/h factor is the more severe of the two factors and is not applied simultaneously with the 1.25-0.125h/bs factor.

What are the major implications for design?

  • For seismic design, the 2bs/h factor method continues unchanged, but is presented as an alternative to the equal deflection method in 4.3.3.4.1 for providing deflection compatibility. The equal deflection calculation method can produce both more and less efficient designs that may result from the 2bs/h factor method depending on the relative stiffness of shear walls in the wall line. For example, design unit shear for shear wall lines comprised entirely of 3.5:1 aspect ratio shear walls can be as much as 40% greater (i.e. 0.81/0.57=1.42) than prior editions if not limited by seismic drift criteria.
  • For wind design, high aspect ratio shear wall factors apply for the first time. For shear walls with 3.5:1 aspect ratio, unit shear capacity is reduced to not more than 81% of that used in prior editions. The actual reduction will vary by actual method used to account for deflection compatibility.
  • The equal deflection calculation method is sensitive to many factors in the shear wall deflection calculation including hold-down slip, sheathing type and nailing, and framing moisture content. The familiar 2bs/h factor method for deflection compatibility is less sensitive to factors that affect shear wall deflection calculations and in many cases will produce more efficient designs.

As the 2015 International Building Code is adopted in various jurisdictions, designers will need to be aware of these new requirements for design of high aspect ratio shear walls. The 2015 SDPWS also contains other important revisions that designers should pay attention to. The American Wood Council provides a read-only version of the standard on their website that is available free of charge.

Please contribute your thoughts to these new requirements in the comments below.

Wood Shear Wall Design Example

Two weeks ago, I had the chance to present to the Young Members Group of the Structural Engineering Association of Metro Washington on the topic of Multi-Story Light-Frame Shear Wall Design. With all of the large firms in the D.C. area, it wasn’t a big surprise to find out that only about one-third of the group had experience with light-frame shear wall design.

However, while researching civil/structural engineering programs in the Midwest and Northeast last week (for our Structural Engineering/Architecture Student Scholarship program), I was disappointed to find that only about a quarter of the top engineering programs offer a wood design course. So I thought it might be helpful to post a wood shear wall design example this week.

The example is fairly basic but includes an individual full-height and perforated shear wall analysis for the same condition. The design is based on wind loading and SPF framing, both common in the Midwest/Northeast, and is based on the provisions and terms listed in the 2008 Special Design Provisions for Wind and Seismic (SDPWS), available for free download here, along with the recently posted 2015 version.

Multi-Story Shear Wall Example: Wind Loads with SPF Framing

Given:SE blog 1

  • 2012 IBC & 2008 SDPWS
  • 3-Story Wood Framed Shear Wall Line
  • ASD Diaphragm Shear Forces from Wind as Shown
  • Wall and Opening Dimensions as Shown

Solution:

  1. Determine total shear force in each shear wall line.
  2. Determine the Induced Unit Shear Force, v, for use with both shear wall types and the Maximum Induced Unit Shear Force, vmax, for the perforated shear wall collectors, shear transfer, and uniform uplift. Note the following:
    1. vmax requires the determination of the Shear Capacity Adjustment Factor, CO, for the perforated shear wall.
    2. The SDPWS provides two methods for determining CO, a tabulated value or a calculated value. This example uses the more precise calculated value.
    3. The perforated method requires the collectors be designed for vmax and the bottom plate to be anchored for a uniform uplift equal to vmax (as illustrated in the following figure).

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SE blog 43. Determine the Tension, T, and Compression, C, forces in the chords (assume no contribution from dead load for this example). Note the following:

    1. Reverse wind loading will require a mirror image of the T & C forces shown in the following figure.
    2. The tension forces, T, shown in the example reflect the cumulative tension forces as they are transferred down from post-to-post, as is typical with traditional holdowns. For continuous rod systems like ATS, the incremental tension forces (resulting from the unit shear, vor vmax, at that level only) must also be determined as shown in the shear wall specification table at the end of this example.

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SE blog 5 SE blog 64. Determine sheathing material and fastening pattern based on v calculated in Step 2. The table below is based on 7/16″ wood structural panel sheathing values in SDPWS Table 4.3A.

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    1. Individual Full-Height Shear Wall:

i.     v3=227 plf: Use 7/16 OSB with a 6:12 nailing pattern which has an allowable load of 336 plf

ii.     v2=409 plf: Use 7/16 OSB with a 4:12 nailing pattern which has an allowable load of 490 plf

iii.     v1=591 plf: Use 7/16 OSB with a 3:12 nailing pattern which has an allowable load of 630 plf

B. Perforated Shear Wall (apply CO factor to allowable shear capacity):

i.     v3=227 plf: Use 7/16 OSB with a 6:12 nailing pattern which has an allowable load of 255 plf

ii.     v2=409 plf: Use 7/16 OSB with a 3:12 nailing pattern which has an allowable load of 479 plf

iii.     v1=591 plf: Use 7/16 OSB on both sides of the wall with a 4:12 nailing pattern which has an allowable load of 338*2=676 plf

5. Size the posts for compression. Simpson provides some useful tables in the back of the connector catalog with allowable tension and compression loads for a variety of sizes, heights, and species of posts.

6. Select holdowns for the tensions loads and verify post sizes are sufficient. For higher aspect ratio shear walls, the post size and holdown type may significantly reduce the moment arm between center of tension and center of compression, resulting in higher tension and compression forces.

The tables below show the shear wall specification for the walls in the example in a typical format. Note that they do not include some detailing that is required for items such as the uniform uplift force on the bottom plate of all perforated shear walls, or the perforated shear walls with OSB sheathing on both sides.

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There are different ways to address the loads, so let us know if you would do anything differently in your designs.

Use of Holdowns During Shearwall Assembly

When designing a shearwall according to the International Building Code (IBC), a holdown connector is used to resist the overturning moment due to lateral loading.  From a structural statics point of view, a shearwall without dead load or holdowns would have zero lateral-resisting capacity without any restraint to resist the overturning moment. Since the wall assembly still has the sill plate anchorage providing resistance to overturning, testing can measure the capacity of a wall assembly without holdowns.

We have performed multiple tests comparing the performance of a shearwall with and without holdowns.  Diagrams of the test setups are provided in Figure 1 below.

Figure 1: Shearwall test setups

Figure 1: Shearwall test setups

The top of wall was attached to the actuating ram using a steel channel and fastened to the double top plates with 3” SDS screws. The ram pushed and pulled the top of wall according to the CUREE test protocol.

No Holdown Wall:

A wall assembly without holdowns can only rely on the wood sill plate members, sill plate anchorage and sheathing to resist the overturning force.  The two limit states commonly observed in the test: 1) The sheathing fasteners prying the sill plate in cross-grain tension. (see Figure 2)  2) Fastener tearing the sheathing at the sill plate. (see Figure 3) Little damage was observed between the sheathing and end post along the height of the post.  Figure 4 is the load vs. displacement graph showing the peak load, 928 lbs., at relatively small displacement, 1.57”.

Sill plate split

Figure 2: Sill plate split

Figure 3: Fastener tearing through sheathing

Figure 3: Fastener tearing through sheathing

Performance of shearwall without holdowns

Figure 4: Graph showing performance of wall without holdowns

Wall With Holdown:

The change in restraining the end posts increases wall stiffness, capacity and ductility of the assembly.  The peak load was 2,907 lbs. at a displacement of 2.3”.  (see Figure 5)  The use of a holdown to restrain the post and engage additional sheathing fasteners minimized cross-grain tension on the sill plate compared with the test without holdowns. (see Figure 6) The increase in both strength and ductility comes from the additional number of fasteners engaged along the height of the post when the post is restrained. (see Figure 7) The assembly with holdowns was able to achieve approximately three times more strength compared with the same amount of material used without holdowns.  Ductility also increased substantially, which can be observed from illustrating the hysteresis curves of both tested assemblies for comparison. (see Figure 5)

Figure 5: Wall with holdowns compared to performance without holdowns

Figure 5: Wall with holdowns compared to performance without holdowns

Figure 6: Post restrained by holdown minimized cross grain tension on sill plate

Figure 6: Post restrained by holdown minimized cross grain tension on sill plate

Figure 7: Sheathing pull through along height of post

Figure 7: Sheathing pull through along height of post

 

 

 

 

 

 

 

 

 

The comparison between the two walls is based on a 4 foot wide by 8 foot tall configuration.  A wall with a different aspect ratio may change the performance, but walls with holdowns will achieve higher loads, and lower displacements, and more ductile performance.

What are your thoughts about shearwall assembly? Let us know in the comments below.

Do You Design with Wood (Shrinkage) in Mind?

Wood is a unique building material because its characteristics are dependent on the environment and its moisture properties, which can vary over time. Often, sawn lumber is delivered to the jobsite with relatively high moisture content. Over the life of the building, moisture content will decrease until equilibrium has been achieved. As moisture content drops, the wood members shrink. Most wood shrinkage occurs during the first six months of the building life and it’s an important design consideration.

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