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.
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.
Deflection Compatibility – Calculation Method
New Section 188.8.131.52.1 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).
In addition, Section 184.108.40.206 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 220.127.116.11 or deflection compatibility criteria or 18.104.22.168.1.
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 22.214.171.124.1 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 126.96.36.199.1 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.
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
- 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
- Determine total shear force in each shear wall line.
- 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:
- vmax requires the determination of the Shear Capacity Adjustment Factor, CO, for the perforated shear wall.
- The SDPWS provides two methods for determining CO, a tabulated value or a calculated value. This example uses the more precise calculated value.
- 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).
- Reverse wind loading will require a mirror image of the T & C forces shown in the following figure.
- 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.
- 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.
There are different ways to address the loads, so let us know if you would do anything differently in your designs.
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.
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.