versatile Shear Wall Analysis method lends greater design flexibility
Wood structural panel sheathed shear walls and diaphragms are the primary lateral-load-resisting elements in wood-frame construction. As wood-frame construction is continuously evolving, designers in many parts of the U.S. are optimizing design solutions that require the understanding of force transfer between elements in the lateral load-resisting system.
The force transfer around openings (FTAO) method of shear wall analysis offers some advantages compared to other methods:
- More versatility, because the FTAO method allows for the use of narrower wall segments while meeting required height-to-width ratios, and
- A high likelihood that fewer hold-downs will be required.
Technical Note: Design for Force Transfer Around Openings
This technical note presents a rational analysis for applying FTAO to walls with asymmetric piers and walls with multiple openings. It is based upon APA modeling and testing and uses methodology that assists the design professional in solving for the required sheathing, nailing, hold-downs, straps, and maximum deflection.
APA Force Transfer Around Openings Calculator
This calculator is an Excel-based tool for professional designers that uses FTAO methodology to calculate maximum hold-down force for uplift resistance, the required horizontal strap force for the tension straps above and below openings, the maximum shear force to determine sheathing attachment, and the maximum deflection of the wall system. The calculator includes worksheets for shear walls with one, two, and three openings and a design example.
Options for Shear Walls With Openings
The U.S. building codes provide three solutions to walls with openings. The first, often referred to as the “segmented shear wall method,” ignores the contribution of the wall segments above and below openings and only considers the full-height segments in resisting lateral forces. The second is the “perforated shear wall method,” which accounts for the effects of openings in the walls using an empirical reduction factor.
The final method is “force transfer around openings” (FTAO). Much engineering consideration has been given to this topic and excellent examples have been developed. But until recently, very little test data had been collected to verify various rational analyses. Typically, walls that are designed for force transfer around openings attempt to reinforce the wall such that it performs as if there was no opening. Generally, increased nailing in the vertical and the horizontal directions, as well as blocking and strapping, are common methods used for the reinforcement around openings.
There are three techniques that are most commonly used to predict force transfer around openings: drag strut, cantilever beam, and the Diekmann technique. The drag strut technique is a relatively simple rational analysis that treats segments above and below the openings as drag struts. This analogy assumes that the shear loads in the full-height segments are collected and concentrated into the sheathed segments above and below the openings.
The cantilever beam technique treats the forces above and below the openings as moment couples, which are sensitive to the height of the sheathed area above and below the openings.
The Diekmann technique, named for the California structural engineer who developed it, assumes that the wall behaves as a monolith and internal forces are resolved by creating a series of free body diagrams. Although the technique can be tedious for walls with multiple openings, many engineering offices have developed spreadsheets or software based on the method. A known limitation of this technique is that when the height above the opening is small, the resolved shear forces become quite large, resulting in the apparent overstressing of the wood structural panel sheathing.
Technical Note: Design for Force Transfer Around Openings, Form T555, presents a rational analysis for applying FTAO to walls with asymmetric piers and walls with multiple openings that is based upon APA modeling and testing to assist the design professional in solving for the required sheathing, nailing, hold-downs, straps, and maximum deflection. As a companion to this technical note, APA has developed an Excel-based Force Transfer Around Openings Calculator, available for free download above.
Advancements in Force Transfer Around Openings for Wood Framed Shear Walls
Advancements in Force Transfer Around Openings for Wood Framed Shear Walls is a recorded webinar given by Engineered Wood Specialist Karyn Beebe. It provides an overview of the force transfer around openings method of shear analysis as well as more detailed information on estimating forces around multiple openings with asymmetric piers and deflections for shear walls. Running time about 50 minutes. Note that this webinar is informational; no continuing education credits are available at this time.
Value Engineering with Force Transfer Around Openings Shear Wall Analysis
How switching to wood framing made a California apartment complex pencil out
APA's Karyn Beebe, PE, talks to developer Barbara Monroy and project engineer Amir Deihimi, who explain how the Santa Barbara Apartments were value-engineered to address excessive building material costs.
Located in Rialto, California, an area with significant seismic loads, construction of the complex had stalled due to prohibitive framing and material expenses. Monroy's search for alternative methods and materials led her to Deihimi of Core Structure, Inc., for more affordable engineering solutions.
A switch to I-joist compatible glulam, double-sided wood structural panel portal frames, and shear walls using force transfer around openings (FTAO) analysis got the project back in the black while addressing the seismic loads.
Adding up the savings realized through replacing steel shear walls with wood and using economical glulam beams, "I think the developer, Barbara Monroy, ended up getting her seventh building for free," says Deihimi.
"Having to redesign this project was a tough pill to swallow when you've already paid for it once. But in analyzing and calculating all the numbers, it was well worth it," said Monroy.
APA Case Study: Santa Barbara Apartments
When framing estimates for construction of an a 120-unit apartment complex in Rialto, California, came in high, developer Barbara Monroy of BM Investments didn't give up. "Rental demand is picking up and values have started to climb," said Monroy. The original project design presented several challenges: the foundation design included expensive grade beams; few of the floor plans stacked, which created a complicated framing plan; and it included costly prefabricated steel shear wall frames surrounding each garage. On the advice of her framing contractor, Monroy took the project to Amir Deihimi, PE, of Core Structure, Inc. Core re-engineered the project, substituting glulam for other engineered wood and converting the steel shear wall frames at the garages to wood structural panel portal frames to get the project back on budget.
Read more in the APA Case Study: Santa Barbara Apartments, Form S125.