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.
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.
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.
The spreadsheet is written in Excel version 2010 for Windows, but might work with other platforms.
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.
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.
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
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.
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.
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.
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.
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.
The bottom or sill plate must be checked for compression separately by the user; the calculator does not check sill plate compression.
The FTAO calculator is intended for use with wood-framed walls with wood structural panel sheathing.
No comparisons have been made with RISA's analysis for FTAO shear walls.
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.
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.
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.
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.
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.
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.
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.
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.
The APA Calculator was adapted from the Diekmann methodology to accommodate multiple openings.
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.