Bracing Foam-Sheathed Walls
If you’re building a foam-sheathed home: metal strap braces and 1″x4″ let-in braces won’t provide racking resistance equal to structural sheathing.
Plywood, oriented strandboard and waferboard are staples of light-frame construction. Wall bracing is not required when these structural sheathings are used. But remove structural skin from the skeleton of the frame and you create a vastly different animal. For example, walls sheathed exclusively with rigid foam panels need lateral bracing to resist wind loads.
Wind delivers a horizontal load to a structure as it flows against, around and over a building’s surface. Friction slows wind speed and reduces loads. Actual loads depend on the shape and height of the building, smoothness of its surface, and lay of the land around the home.
The windward wall of a house feels positive pressure: the wind blows directly against this surface. The leeward, or downwind, side of the house feels negative pressure: the wind stream sucks these roof and wall surfaces as it flows over and away from the house. Windward roofs experience either positive or negative forces depending on steepness of the roof pitch.
Wind forces are additive. A 40 mph wind may deliver a 4.1 psf force, but you don’t simply multiply the surface area of the windward wall times the force. You must combine all forces acting on all surfaces. This calculation is complicated by serious design issues and the magic numbers required to solve the puzzle are best understood by structural designers and engineers.
To get a feel for how wind loading influences a structure, imagine toe-nailing a single stud so it stands upright on a deck. Push gently against the top of the stud and — no surprise — it leans in the direction away from the force. Unbraced wall studs act in a similar way when they are racked by wind loads. The bottom of the wall is anchored to the deck, but the top plate (if unrestrained) will slide in a direction away from a significant wind load. Structural sheathing and diagonal braces installed in walls parallel to the wind flow, transmit lateral wind loads down to the foundation. But how much bracing is enough?
Wall bracing regulations are based, in large part, on the Federal Housing Administration (FHA) Technical Circular number 12, published as an interim standard in 1949 (a permanent standard was never introduced). This document established 5,200 pounds as the acceptable base level of racking resistance for wood-framed walls. The FHA minimum value reflects the wind resistance provided by wood-framed walls, sheathed with horizontal boards and braced by 1″x4″ let-in bracing, a common construction practice in 1949.
The conversion of wind speed to 5,200 pound horizontal wall-loading is not a straight-forward procedure and should be left to design professionals.
A rough calculation for a 24 x 48 ranch with a 5/12 pitch, with the wind blowing directly perpendicular to the front wall and using appropriate shape coefficients etc. shows…it takes a 40mph wind to deliver 5,200 lbs. & interestingly 55mph wind to deliver 10,400 lbs(2x the load!)
This doesn’t begin to address the complex interaction of wind direction, terrain, moment connections, shear walls, etc. This is why I think you should leave the calculations to engineers!
Historically, building codes have prescribed specific materials and applications for builders. Prescriptive codes still allow: 1″x4″ diagonal bracing, diagonal board sheathing and plywood sheathing as wall-bracing options. Recently, more progressive performance-based building codes have been developed as alternative solutions to structural problems. If an engineer can guarantee a certain level of performance for a given wall-bracing scheme, it is allowed. Metal strap-bracing and other bracing systems may qualify for use under these performance-based standards.
The overwhelming success of code-accepted light-frame bracing systems lend credence to existing standards. However, building practices are changing. More non-structural sheathing is being used in place of horizontal boards. Insight can be gained by comparing 4 common wall-bracing systems: 1″x4″ let-in bracing, metal bracing, plywood corners and diagonal framing (stud bracing).
Gary Nichols, assistant manager of research for the Southern Building Code Congress International (SBCCI) is not overly concerned about the performance of foam-sheathed homes braced exclusively with let-in braces. But admits, “Prescriptive building codes are fine as long as you don’t go changing other interactive building elements. When you change those, sometimes the old prescription doesn’t hold up.” SBCCI will consider metal bracing only if it has been tested and proven to be effective as a bracing system.
Thomas Frost, manager of technical services, Building Officials and Code Administrators (BOCA) thinks, “Certainly there is room for an argument that says you should engineer all braces. You can take a prescriptive approach or you can put a design value on the wall system regarding lateral resistance. The fact remains that 1″x4″ braces have provided adequate service over the years.” Frost is more concerned about wall bracing in buildings of “…unusual design.” Here he recommends that building officials demand builders to prove their buildings worthy.
Certainly 1″x4″ let-in braces have worked well in the past. Homes built to code have a good track record. But in the past, these braces were used in conjunction with board sheathing. Will unsheathed or foam-sheathed homes perform as well?
The 1″x4″ material used as let-in braces is not structurally graded. It is graded for appearance. There are no standards controlling the method of installation. And research shows that the structural contribution of let-in braces is negligible when used as isolated members.
In 1977, Roger Tuomi and David Gromala, engineers with the Forest Products Laboratory (FPL) in Madison Wisconsin, studied let-in bracing. The increased use of non-structural insulating sheathing during the energy crises of that decade concerned the researchers. Tuomi and Gromala learned that much of a braced wall’s racking strength is owed to the interaction of board sheathing and let-in bracing. Tuomi and Gromala found that even the clear straight-grained 1″x4″ let-in braces used to support their unsheathed test walls provided less than 2/3 of the 5200-pound value specified by the FHA standard. The 1″x4″ “#2 Common” boards typically found on the jobsite can hardly resist comparable loads.
Garden variety braces made from #2 Common 1″x4″ stock have 2-inch wide red knots. That means almost 60% of their crossectional dimension is nonstructural! Add to that the structurally weak crossgrain region surrounding the knots and you have a recipe for failure at low-level loading.
In 1983, Ronald Wolfe, another FPL research engineer, studied the contribution gypsum wallboard makes to the racking resistance of walls. During the study, Wolfe evaluated the structural contributions of 1″x4″ let-in bracing and metal strap bracing in light-frame wall systems. “Off-the-shelf” No. 2 boards provided only 600 pounds of resistance to horizontal loads (like wind) before failing in Wolfe’s tests. According to this test, it would take NINE 1″x4″ unsheathed let-in braces to provide the FHA minimum 5,200 pounds of lateral resistance in a wall.
It’s worth noting that the braces broke (ultimate load) when subjected to a 600 pound lateral load. Typical design values for structural members carry a 2.5 factor of safety, which means that only 240 pounds per brace could be used as a design value based on these findings.
Brian Dunagan, engineer with Simpson Strong-Tie Company, Inc., a San Leandro, California manufacturer of metal wall bracing, doubts that 1″x4″ let-in bracing provides the necessary racking resistance in unsheathed walls. Tests conducted by Simpson showed that wood braces installed at 45 degrees failed with loads of 1125 pounds. Braces installed at a 60 degree angle provided far less resistance.
Wall braces can buckle, fail in compression or fail in tension depending on the material and the way they are loaded. Computations predict that 1″x4″ let-ins installed at 45 degrees in a 24″ o.c. wall system will buckle under a 890 pound lateral load (assuming “construction grade” material was used). Reduce the stud spacing to 16″o.c. and let-in braces will be supported enough to prevent buckling. But instead, they should fail in compression at 1950 pounds.
Several manufacturers of rigid foam insulation recommended the use of metal bracing when constructing foam-sheathed walls. Product literature depicting this practice may be misleading. Tests conducted by both FPL’s Wolfe and Simpson Strong-Tie indicate that metal bracing does not approach the FHA minimum standard of 5,200 pounds.
Wolfe’s metal strap braces delivered 1,500 pounds of lateral resistance. His straps were wrapped over the top and bottom of the wall frame to minimize nail withdrawal. Flat metal braces should be attached this way for maximum strength. Flat metal braces only resist loads in tension, that’s why an X-configuration is recommended at each corner of a wall. Studs aren’t notched as they are in let-in bracing – a real time saver.
Simpson Strong-Tie’s product manual clearly states that metal wall braces prevent walls from racking during construction and are not designed to replace shearwall load-carrying components. Tests commissioned by Simpson show that their T-type wall bracing (the T-shaped type that is inserted into a saw kerf) resists a maximum load of 710 pounds. Strap, or X-type braces, are assigned lower values through calculated estimates. Failure in metal brace tests generally occur as a result of nail slippage. So size and the number of nails in brace ends limit design values.
Gypsum Wallboard Contribution
Gypsum wallboard is without doubt the most popular interior wall sheathing used in light-frame construction. But rarely is wallboard given the credit it deserves when evaluating the structural integrity of a wall system. Its success depends on orientation. In Wolfe’s study, 1/2-inch wallboard provided 150 pounds of resistance per lineal foot of wall length when applied vertically to an 8′ high wall and 250 pounds per foot when applied horizontally to the studs. Studies sponsored by gypsum manufacturers have yielded values as high as 660 lb/ft. The Uniform Building Code (UBC) recognizes a conservative 100 lbs/ft as a structural contribution. Cut-outs for windows and doors obviously reduce the contribution.
Builders often brace foam-sheathed homes with plywood corners. Half-inch thick sheets are installed vertically at the corners and overlaid with half-inch thick rigid foam. One-inch thick foam is then used to sheath the remainder of the house, leaving the exterior wall surface flush.
Each corner panel will resist an ultimate load of 3,120 pounds when nailed with 8d nails, spaced 6-inches at the edges and 12-inches in the field of the panel. So a wall (2 corners) will resist 6,240 pounds. Add the gypsum contribution to the plywood contribution and this system seems to work nicely. Its fast and easy, too.
Stud bracing: this seldom-used, but effective wall bracing system deserves consideration when building foam-sheathed homes. Here, studs run diagonally at the corners within the wall cavity. The depth of the brace is perpendicular to the face of the wall. For maximum strength, these braces should be installed at a 45 degree angle and extend as one length from top plate to bottom shoe. Vertical studs are cut to fit around the continuous diagonal brace. Stud braces only work in compression and must be blocked at the tops and bottoms so that the the stud braces are not allowed to slide out of position when stressed by racking forces. Additional stud braces can be installed along the length of any exterior wall to improve the racking resistance beyond the level of resistance offered by single corner braces.
Walls framed with 2″x6″ studs spaced 24″o.c. are standard practice in many regions. Calculations show that a “construction grade” 2″x6″ stud brace installed at a 45 degree angle will buckle when subjected to a 11,000 lateral load and fail in compression with a 5,400 pound load — above the FHA minimum.
While the buckling load is computed as an ultimate load, the 5,400 compressive load is computed using design stress values, which include a built-in safety factor. Ultimate loading in compression would most likely occur at a much higher level. Stress graded lumber is readily available for stud bracing. So you can improve brace performance further by purchasing a higher structural grade or stronger species of wood. Be careful that the nailed connection at the bottom of the brace is not the weak link!
Builders may turn to let-in and metal braces as substitutes for structural sheathing when exterior rigid foam is used. Yet neither research or mathematical predictions support the use of let-in or metal bracing in unsheathed walls. Use of these bracing systems probably will not result in catastrophic failure. But their use may provide a steady diet of callbacks related to cracked plaster and windows that bind when opened. A competent engineer should carefully review designs that exclude structural sheathing.
Last updated: January 9, 2008