When it comes to scenic vistas, California is a place like no other. The dramatic terrain and the endless Pacific merge to produce stunning scenery, and it’s no surprise that Californians often opt for floor-to-ceiling windows to enjoy the breathtaking views.
Unfortunately, few places are less suitable than earthquake-prone California for accommodating wide sections of full-height glass. If you’re an architect whose client wants floor-to-ceiling windows in their home, you’ll have to find a seismically appropriate design solution for your client’s vision.
As you know, the problem with large openings, particularly on the ground floor, is that they create a dangerous soft-story condition. Whereas shear panels effectively brace stud walls against earthquake loads, the panels’ width gives them an unfavorable aesthetic when used with wall-to-wall glazing. Fortunately, the California Building Code (CBC) provides a range of options for bracing openings. The digest below describes the various lateral in-plane reinforcement methods available for conventional wood frame buildings.
Braced Wall Lines (BWL)
Per the CBC, a building’s lateral resistance must be located along the so-called Braced Wall Lines. BWLs are straight, imaginary lines that run through the building in the longitudinal and transverse directions, depicting the location of braced walls that provide lateral resistance. Parallel BWLs cannot be spaced farther than 35 feet in buildings falling under Seismic Design Categories (SDC) A to C, and 25 feet in those under SDCs D and E. Structures assigned to the latter SDCs also require BWLs to intersect perpendicularly.
Braced Wall Panels (BWP)
Braced Wall Panels are full-height wall sections placed along the braced wall line that enable stud assemblies to resist in-plane shear loads through the interaction of framing members, sheathing materials, and anchors. Each sheathing material has its height and length limitations to which the braced wall panels must conform to count toward the total amount of required bracing.
The methods described below use various materials to reinforce studs against lateral forces. Each approach calls for panels that are at least 4 feet long. Panels falling short of this requirement do not count towards the total amount of required bracing, whereas panels that are longer than needed are credited for their entire length. Per the CBC, BWPs must be spaced no more than 25 feet on center, while structures placed under SDCs D and E may need BWPs to comprise a certain percentage of the wall. These bracing mechanisms allow clear spans of up to 21 feet, but may not be the most suitable for bracing wide sections of floor-to-ceiling glazing given the panels’ width.
The code-approved BWP variants include:
- Let-in Bracing (LIB)
- Diagonal Wood Boards (DWB)
- Wood Structural Panels (WSP)
- Structural Fiberboard Sheathing (SFB)
- Portland Cement Plaster (PCP)
- Hardboard Panel Siding (HPS), and
- Gypsum Board (GB)
Let-in Bracing: this lateral reinforcement mechanism consists of 1 x 4 wood or metal strips attached to studs at a maximum of 16 inches on center at a diagonal angle of 45° or 60°, with a gypsum board fastened to one or more sides. This bracing method, although outdated, is still permitted in 1-3 story buildings that fall under SDCs A and B, but is forbidden in SDCs E and D.
Diagonal Wood Boards: also attached at a diagonal angle, this sturdier method uses ¾ inch thick 1×6 boards fastened to studs at 24 inches on center.
Wood Structural Panels: this assembly consists of 3/8 inch-thick plywood attached to studs at 16 inches on center.
Structural Fiberboard Sheathing and Particleboard Sheathing: similar to the WSP, these assemblies comprise ½ inch-thick fiberboard, and 3/8 inch or ½ inch particleboard, respectively, fastened to studs at 16 inch centers.
Portland Cement Plaster: this bracing method employs cement boards nailed to studs at 16 inch centers.
Hardboard Panel Siding: normally a siding material, 7/16“-thick hardboard panels are another approved bracing method when nailed to studs.
Gypsum Board: 1/2” or 5/8” double-sided gypsum sheets may be used as lateral bracing. The sheets must be fastened to studs at a maximum spacing of 24” on center. When used only on one side of the assembly, the minimum length of the panel must be doubled.
Alternative Bracing Methods
The building code also allows two alternative bracing mechanisms to be used in lieu of those described above. The 2 approved alternative methods are Alternate Braced Wall (ABW) and Portal Frame with Hold-down (PFH). Both methods allow shorter panels, thus adding flexibility to a building’s architectural design.
Alternate Braced Wall
This reinforcement mechanism relies on anchor bolts and hold-down devices to brace the wall assembly while utilizing a shorter panel. When replacing standard wall bracing methods, the AWB allows openings to span 22 feet 4 inches. Per the CBC, ABW requirements differ depending on the number of stories the subject building has.
When used in 1 story buildings, ABWs must be at least 2 feet 8 inches long, and not more than 10 feet high. Each panel must be secured with 2 anchor bolts, while the stud at the end of every panel must be fastened to the foundation with a hold-down device with a minimum uplift capacity of 1,800 pounds (8,006 Newtons). The ABW must rest directly on a foundation, or on a framing assembly supported by a foundation, which requires a minimum reinforcement of one No.4 bar at the top and bottom.
For 1st floor applications in 2-story buildings, wood structural sheathing is required on both faces of the ABW. At least 3 anchor bolts must be used, and the hold-down device requires a minimum uplift capacity of 3,000 pounds (13,344 Newtons).
Portal Frame with Hold-downs
Developed by the APA, this method consists of one or two vertical panels joined to a header. These mechanisms are commonly used to reinforce the soft story created by garage openings, but thanks to their relatively narrow vertical panel and a maximum span of 18 feet, they are equally effective at bracing window openings.
In 1-story buildings, a PFH must have panels that are at least 16 inches long and not more than 10 feet high. The header, made up of at least 2 2×12 boards, must extend beyond the inner studs of the panel. Straps and hold-down devices must be used to hold the frame together and fasten it to the foundation. The PFH needs to rest on a continuous foundation reinforced with No.4 bars at the top and bottom.
How Design Everest Can Help
If you’re an architect working on a project in California, our engineers can help you find the optimal bracing solution for your project. Our team offers a full suite of civil, structural, and MEP design services, and our engineers are dedicated to finding the most cost-effective solutions for your client. We also provide architectural drafting and 3D modeling and rendering support to architects. Contact us at (877) 739-2591 or info@designeverest.com to find out more.
