Shear Wall Analysis Made Easy

 

Force Transfer Around Openings Calculator

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: Do not navigate to YouTube if you require a certificate. A downloadable certificate of completion is available only when webinars are viewed on this webpage in entirety, after completing a brief questionnaire.

 

 
Approximate length: 1 hour.
 

Webinar Participant Questions and Answers

What force do you use to design the anchorage along the base of the shear wall?

The required anchorage along the base of an FTAO shear wall is determined by dividing the total applied horizontal shear force by the total length of the shear wall.

Is there a minimum allowable width for the full-height piers?

The 2015 SDPWS currently lists a minimum pier width of 24 inches, but APA has done testing to justify a minimum pier width of 18 inches, provided that the aspect ratio of the pier is 3.5 to 1 or less. This is also noted in Technical Note: Design for Force Transfer Around Openings, Form T555.

Does the spreadsheet tell me somewhere whether or not the wood end posts I specified are adequate, or do I need to check this off-sheet? Does the spreadsheet check whether or not the nails and spacing are sufficient?

The calculator does not check sheathing or end post capacity. The only reason the spreadsheet includes end post information is for deflection calculations. The required sheathing capacity is noted so that you can select the required sheathing thickness and nailing.

What spreadsheet software does the calculator work on?

The spreadsheet is written in Excel version 2010 for Windows, but might work with other platforms.

How should we interpret results for large openings when shear in corner zones becomes negative?

In theory, the corner forces can be negative. When the corner forces are negative, it means that the assumed direction of the shears in the corner was not correct. This may happen when the pier is relatively narrow, the window opening is relatively tall, and hold-down forces are not large.

Is the shear in the piers of a FTAO wall the design shear for nailing, or is the highest shear (perhaps above/below openings) the design shear?

The FTAO calculator checks the shear in all areas of the wall configuration to determine the maximum required sheathing capacity. In the calculator, step 2, step 6, and step 9 check the unit shear in the areas above and below openings, beside openings, and in the corner zones, respectively.

Is OSB similar in strength to plywood?

OSB and plywood have similar strength properties. More information about the design properties and capacities of OSB and plywood can be found in 2015 SDPWS Table 4.3A or in APA's Form L350, Diaphragms and Shear Walls

Has including the tributary building dead load been considered in the FTAO calculations?

The calculator does not take into account the dead load that is tributary to the shear wall. The user can consider the tributary dead load off-sheet to help reduce the amount of hold-down anchorage required to resist overturning.

Is this tool also given for the NDS 2018?

The version of the calculator (v1.2) discussed in the webinar is based on the 2015 National Design Specification for Wood Construction (NDS) and the Special Design Provisions for Wind and Seismic (SDPWS) published by the American Wood Council.

As a contractor, I have had issues fastening through metal strapping, considering sheathing fastenings, siding fastening, window installation and trim around a window. Is there a way to accommodate this? What do you do if the strapping location conflicts with window fin installation?

One item to note is that sheathing edge fasteners can be omitted when they coincide with strap locations, assuming the strap nailing is at a closer nail spacing than the panel edge nailing. In addition, strapping can be installed on the interior face of studs in order to prevent conflict with window and siding installation. If the strapping is installed on the interior side, special detailing is required. The blocking should be the full-depth of the wall studs so that the strap can be installed to the blocking with the required fasteners. Also, the nailing pattern from the sheathing into the blocking should be the same nailing as what is specified by the strap manufacturer.

Can the spreadsheet be used for more than one story?

The calculator can only be used to design force transfer around opening shear walls in a single story. However, the user can create their own spreadsheet based on the methodology outlined in Form T555, Technical Note: Design for Force Transfer Around Openings (FTAO).

How long should the strap be? Does it have to continue the full length of the shear wall?

The required strapping is dependent on the calculated internal tension and compression forces of the wall system and is put in place to aid in the resistance of the tension forces through the thickness of the wall at the corners of the openings. The straps only need to be long enough to allow for the full development of the tension load.

How do I select Gt for plywood?

The calculator determines the applicable Gt value, or the referenced rigidity of the wall sheathing panel through the thickness, based on the user's selections for Sheathing Material, Performance Category, and Grade. An override option is provided so that the user can perform an input override for the Gt value of sheathing materials that are not included in the drop-down menus of the spreadsheet, or if the user would like to input values based on specific plywood construction (i.e., 5-ply,) provided panels are available.

Do we need to check bottom plate for compression?

The bottom or sill plate must be checked for compression separately by the user; the calculator does not check sill plate compression.

Is FTAO design possible with steel studs?

The FTAO calculator is intended for use with wood-framed walls with wood structural panel sheathing.

Has any comparison been made with RISA's analysis for FTAO shear walls?

No comparisons have been made with RISA's analysis for FTAO shear walls.

How do you get access to the calculator?

The FTAO calculator can be accessed at www.apawood.org/FTAO after registering for a free account. The calculator is password protected to protect the integrity of the spreadsheet calculations.

Walls used for testing all have vertical joints in the OSB. What about a wall sheathed with horizontal joints. Does it affect the ability of the overall wall to transfer shear and/or overturning from top of wall to base of wall?

Orientating sheathing panels vertically or horizontally on the wall studs does not affect the transfer of forces around the opening. The horizontal sheathing joints should be blocked.

Is the calculator applicable to Canadian standards?

No, the FTAO calculator is not applicable to Canadian standards. The calculator was based on US Building Codes and SDPWS. CSA O86, the Canadian equivalent to the NDS, bases aspect ratio on a height defined as underside of the bottom shear wall plate to topside of the top shear wall plate. So, one doesn’t get the “advantage” of redefining the pier height for aspect ratio calculations. However, Canadian code has a path for performance based solutions, so the engineer may be able to use FTAO methodology based on principles of mechanics to provide a solution. This would be up to the engineer and the AHJ.

How do you handle walls with more than three openings?

At this time, APA's FTAO calculator can only be used for FTAO shear walls with up to three openings. Designers looking to analyze FTAO shear walls with more than three openings can create their own spreadsheet based on methodology outlined in Form T555, Technical Note: Design for Force Transfer Around Openings (FTAO).

Figure 4E in SDPWS showing a FTAO shear wall appears to show a wall that is not legitimate for FTAO, as the center pier is not wide enough and there is a door opening. Is this correct?

Figure 4E in 2015 SPDWS is simply demonstrating the concept of the redefined height based of the wall, based on the height of the adjacent opening. Although APA Calculator does not provide a solution for door openings, there are other mechanics-based solutions that may be used for FTAO. SPDWS does not mandate which method one uses for FTAO design.

Is there a methodology for a wall with doors and windows?

With APA's methodology, which has been adapted from the Diekmann method, doorways must be omitted from the line of resistance. This is due to the Diekmann method's design assumption that the shear above and below openings is identical, therefore there must be sheathing at both locations. Other documented FTAO methods may be used.

Are there limitations to the aspect ratio of segments above and below openings?

There are no minimums that must be applied for the heights above or below openings. The only limitation is being able to effectively resolve the forces in the narrow section. We have come across these instances before, and what we have found is that certain jurisdictions will require that the area be considered as a pier and you will have to invert the aspect ratio. Ratios will need to be checked for b/h in this case, and the shear values must be adjusted using one of the aspect ratio factors accordingly. This tends to make it difficult to properly resist the loading in these locations.

Why is the trailing pier’s height based on the total height when the other piers are based on the opening height when determining the overall wall deflection?

Testing data was used to analyze the overall deflection of the FTAO wall systems, which verified that the sheathing below the openings aided in resisting the overall deflection of the wall. The wall deflection assumption is that the total deflection of the FTAO shear wall is equivalent to the average of the deflection of each wall pier in both the positive and negative directions. The wall pier heights also vary depending on the deflection direction and amount of sheathing below the openings.

Has the method used in the FTAO calculator been verified by testing?

The wall 12 configuration that was tested during APA's joint research study was the basis for developing the FTAO calculator and Technical Note: Design for Force Transfer Around Openings (FTAO). Wall 12 was constructed with multiple openings and asymmetric pier widths. More information about APA's testing results and the Wall 12 configuration can be found in Form M410, Joint Research Report: Evaluation of Force Transfer Around Openings—Experimental and Analytical Studies.

What if the straps above and below the opening continue across the entire length of the trailing pier? Would this change the full height assumption for deflection calculations?

Regardless of whether or not the straps extend for the full length of the shear wall, the deflection calculations are based on the assumption that the sheathing below the openings aids in resisting the overall deflection of the wall, and the wall pier heights vary depending on the deflection direction and amount of sheathing below the openings.

During the presentation there were a couple of references for the APA calculator using the Diekmann’s method. After reviewing the calculations, it appears that the calculator isn’t truly based on Diekmann’s method, but just a rational analysis based on tested values.

The APA Calculator was adapted from the Diekmann methodology to accommodate multiple openings.

For C-shaped sheathing pieces, does APA have any guidance on how we could assign a capacity to this sheathing? What we're looking to do is set a strap force value for a given sheathing specification, below which a metal strap would not be needed.

APA does not currently have a methodology developed to calculate the acceptable resistance of c-shaped panels. However, the designer could calculate the capability of the panel to resist the strap force through principals of mechanics based on the length of the panel remaining at the sill and the panel’s mechanical properties. This would vary depending on the detailing provided and wood structural panel selected by the designer.