Shear Wall Analysis Made Easy

Advancements in Force Transfer Around Openings

This course has been approved by AIA (1 HSW/LU) and ICC (0.10 CEU).
This webinar includes an overview of APA's free design tool, the FTAO Calculator, and explains how it can help engineers and code officials in the design implementation of FTAO shear walls, with a focus on asymmetric piers and multiple openings. Design examples are also discussed throughout the presentation.
Note: A downloadable certificate of completion is available after this webinar is viewed in entirety, upon completing a brief questionnaire.

 

 

 
Approximate length: 1 hour.
 

Webinar Participant Questions and Answers

Can you use the APA FTAO calculator at doors?

Since the calculator is based on the Diekmann method, which assumes that the shear above and below the opening is identical, there is no way for a door to work, because there would be no sheathing below.

Is there a maximum aspect ratio for the areas above and below the opening?

As the opening size gets larger, it results in a higher aspect ratio for the segments of sheathing above or below the opening, and some of the fixity assumptions may break down with the Diekmann method. APA testing for Wall 5 (Reference Joint Research Report: Evaluation of Force Transfer Around Openings—Experimental and Analytical Studies, Form M410) has the largest width-to-height aspect ratio for that segment among all the test configurations at 6.5:1. APA recommends that engineering judgement be used for situations with aspect ratios above that value.

For C-shaped panels, does APA have any guidance on how we can assign a capacity?

Unfortunately, APA does not have any information on how to calculate the capacity of C-shaped panels to transfer corner forces, but the Special Design Provisions for Wind and Seismic (SDPWS) does allow an engineer to determine their own rational analysis.

Does the orientation of the sheathing affect the results? Was the testing done with the panels vertical or horizontal?

No, the direction that the sheathing is oriented will not affect the transfer of forces around the opening. Panels can be installed vertical or horizontal but the calculator assumes that all panel joints are blocked.

How tall of an FTAO wall is allowed?

The wall heights of FTAO are only limited by the maximum aspect ratio of the piers and wall height limitations of the IBC.

Can the spreadsheet be used for more than a one-story wall?

It can be used to design shear walls in a single story, whether that story occurs in a one story or multistory building. If someone wants to look at the shear walls spanning over more than one story, they can create their own spreadsheet based on the methodology outlined in Technical Note: Design for Force Transfer Around Openings (FTAO), Form T555.

Do FTAO shear walls have to stack vertically?

Not necessarily. As long as the uplift forces at the end of each wall are resisted and/or continuous to the foundation, and there is a continuous shear load path between walls, there is no reason the shear walls must vertically stack. It is, however, much easier in design to stack the shear walls vertically.
When ANY shear walls do not vertically stack, the engineer will need to account for the vertical structural irregularity by specifically detailing a continuous load path to the foundation. In certain Seismic Design Categories, this vertical irregularity will require connections and collectors to be designed for an amplified load.

Does it matter what direction the interior blocking is oriented within the wall and does the blocking have to be in an exact line?

The blocking can be installed in either orientation. If it is installed with the wider face against the sheathing, this will make the wall more constructible, because this leaves a wider area to attach the sheathing and strapping.
The blocking does need to be placed in line so that the strapping can be nailed through the wall sheathing into the blocking.

If you put the straps on the inside of the wall, how does this affect the rest of the FTAO detailing?

If the straps are installed on the inside of the wall, the sheathing would have to be nailed to the blocking from the exterior. The nailing pattern from the sheathing into the blocking should be the same nailing pattern as what is specified by the strap manufacturer.

Is there any guidance for unblocked walls?

While there is not currently a code provision preventing design of FTAO for unblocked shear walls, all APA tested walls were fully blocked at all panel edges.

In the calculator, what is the definition of a “corner force”?

The corner force is the load being transferred around the opening. Typically, this tension force is resisted by the strapping at the corners and the opposite compression force is resolved by the blocking.

Do the straps need to meet the collector requirements listed in the SDPWS Section 4.3.5.2?

No, this section refers to full length horizontal elements, such as wall top plates or drag struts, that are acting as continuous collectors or diaphragm boundary elements that collect diaphragm shears and transfer them into the line of lateral resistance. The difference between the shear collectors discussed by SDPWS and the strapping at the corners of the openings of an FTAO shear wall is that the strapping does not resist shear load. Instead, the strap forces are dependent on the calculated internal tension and compression forces of the wall at the corners of the openings.

I am using the FTAO calculator to design a shear wall with two openings. Do the straps need to be installed continuously above and below both openings and the pier in between?

No, straps should be provided at the top and bottom corners, so there could be up to 8 total for a wall with 2 openings. If the length of the full-height wall between the 2 openings is small, the straps may be run continuously across that middle wall pier, but that is not required. The straps need to be long enough to allow for the development of the tension load, similar to-hold downs. Lastly, depending on the calculated corner forces, straps may not be required.

Would you recommend minimum hold-downs at window jambs, even though the hold-down force only occurs at the first and last line and the rest of the vertical lines sum to zero when the shears are summed vertically along each line to check the results?

Some engineers do provide a nominal hold-down at the window jambs. However, one of the main advantages of designing a wall with FTAO is eliminating these hold-downs. Adding additional hold-downs diminishes the benefits of FTAO.

In the APA FTAO Calculator, the unit shear above and below the openings is a function of the "hold-down" force at each end of the wall. What happens when we use dead load to reduce the hold-down forces (potentially to net zero), can we also reduce the unit shears above and below the openings?

FTAO is intended to transfer the lateral force around the opening. While this method does begin with the induced uplift force, adding a counteracting force at the end of the wall does not reduce the magnitude of the lateral force we are attempting to transfer around the opening. Dead load can be used to reduce the required hold-down capacity, as may be allowed by code, but it does not reduce the induced shear load. Because of this, the results should not change.

Is the calculator applicable to Canadian standards?

The calculator is based on U.S. building codes and the SDPWS. Since the loads that are input into the calculator are factored by the user, the calculator may be used with any code. The user should be aware of differences between U.S. codes and Canadian codes regarding factors like minimum pier width, the maximum aspect ratio, the aspect ratio adjustment factor or the deflection calculations. Those differences will need to be taken into consideration.

Why did you test with hold-downs rather than flat straps around openings?

Hold-downs were used so that we could attach test equipment in order to measure the corner forces. We attached calibrated load cells at those locations.

Can the FTAO calculator be used with steel studs?

No, the calculator was based on APA research that validated the Diekmann method. The APA research was conducted on wood studs.