DESIGN OF WOOD FRAME STRUCTURES FOR WIND RESISTANCE – NEW TECHNOLOGIES FOR STRUCTURAL DESIGN AND MATERIAL EFFICIENCY

Bryan T. Readling, PE and Edward L. Keith, PEAPA

Tacoma, WA/USA

Abstract

This paper highlights innovations in structural design methodology to resist wind forces on wood-frame buildings developed since Hurricane Hugo. Included are new provisions for lateral and vertical load paths in Main Wind Force Resisting Systems as well as for resisting wind pressure on Components and Cladding. Common failure modes observed by APA and others during post-disaster evaluations since Hurricane Hugo are used for illustration.

Code changes since Hurricane Hugo have made a significant impact on wind design. These are discussed along with the rationale used to justify these changes.

In most cases these new developments in design technology and building codes result in simpler more economical systems than those previously available.

Introduction – A Quiet before the Storms

After Hurricane Camille in 1969, it was 20 years until another storm of comparable destructive force, Hurricane Hugo, landed in the United States. In the interim, development had intensified along the nation's coastlines with little corresponding development in building code provisions for high wind design. Hurricane Hugo served as a wake-up that hurricanes will come, bringing with them wind and water, and that our buildings will be exposed to fluctuating wind pressure, flying debris, moving water, wave action, scour, and floating debris.

Since then focused research has resulted in better understanding of wind loads, and improved technologies for resisting these loads. Incidentally, during the same period of time, seismic engineers have become less confident in seismic design forces. This has resulted in further separation between lateral design for wind and seismic events.

Shearwall and Diaphragm Design Evolution

Significant improvements have been made in design for wood shearwalls and diaphragms to resist loads commonly associated with wind design. Many of these improvements were developed through physical testing of full-scale wall systems with various opening configurations, and base connections - to resist overturning and base shear forces. Additional testing has been conducted on full-scale 3-dimensional buildings, both cyclic and monotonic, to better understand how wood-frame buildings actually perform in real events. As evidenced by recent whole-house testing at APA, behavior of wood wall systems intended for lateral resistance are significantly affected by walls oriented perpendicularly. More research into the 3-dimensional “box” behavior of wood-frame buildings sheathed with structural materials will improve design capability in the future.

Figure 1. This OSB-sheathed house under construction in Bay St. Louis, Mississippi, was relocated inland approximately 100 feet from its original foundation by storm surge. Despite failure of the connection to the foundation the wood frame structure remained intact without significant deformation. New research

will result in better understanding of 3-dimensional behavior.

Significant Building Code changes include a 40 percent increase in shearwall and diaphragm values for wind design only per the 2006 IBC 2306.3.1. This change is based on a reduction in the safety factor from 2.8 to 2.0 for wood structural sheathing materials. The resulting 1.4 factor may be applied to tabulated shearwall and diaphragm capacities for resisting wind loads only. Due to more accurate field wind measurements since Hurricane Hugo, the certainty as to the resulting loads has improved. This is partly behind the rationale for reducing the safety factor. The increase also takes into effect the short time duration for wind loads. (ref. 2003 IBC Commentary, p 23-41, 23-47, and 2003 IBC Structural Q&A – Application Guide.)

Another Code Change allows summing shear capacities of dissimilar materials for wind design, such as the combination of wood structural panel exterior sheathing and interior gypsum wallboard per the 2006 IBC section 2305.3.8.

Summing capacities for dissimilar sheathing materials applied to either one side of a wall, or both sides of the same wall, is generally not permitted due to differences in load vs. deflection ratios. Shear values for the same sheathing type and strength are cumulative when used on both faces of the same wall. However, when material capacities are not equal, the allowable shear capacity may be either two-times the smaller shear capacity or the capacity of the stronger side, whichever is greater.

An exception has been made in the case of wind design when wood structural panel sheathing and gypsum wallboard are used on the opposite faces. The capacities may now be added. This applies as well to fiberboard structural sheathing and hardboard panel siding.

This partially reflects the results of testing that confirms the additive behavior of certain combinations of exterior sheathing and gypsum wallboard. Test results for wood structural panels and gypsum wallboard are provided in APA Research Report 157: Wood Structural Panel Shear Walls with Gypsum Wallboard and Window/Door Openings, Form W250.

New design methods include the perforated shearwall approach. Based on physical testing of shearwalls with various openings replicating windows and doors, the effect of these openings was

measured on the performance of shearwall units. This approach was first used in Japan by Sugiyama and others and was followed by research in the U.S., much of this by the American Wood Council and APA.

The perforated design methodology has been fully recognized by U.S. building codes (2006 IBC Section 2305.3.8.2, 2009 IBC via ANSI / AF&PA SDPWS-2008 - Special Design Provisions for Wind and Seismic) and often results in more economy than the traditional “segmented” shearwall design approach. The segmented method assumes that openings in a wall are an interruption in shear flow. This results in independent shearwall segments and ignores the fact that shear forces are transferred around wall openings when portions of the wall above and below the openings contain structural sheathing.

With the perforated method, overturning hold-down devices are generally called for only at the ends of more lengthy shearwalls containing openings. Unlike the traditional approach, this eliminates a significant number of hold-down devices otherwise required at the start and stop of each shearwall segment as illustrated in two design examples below. Reducing the number of hold-downs within a given wall reduces hardware costs and placement costs which are significant since they must be precisely placed and coordinated with the job of concrete placement. Otherwise, adhesive-embedded anchors may be required to attach the walls to the foundation.

Perforated shearwall design is based on determination of a Shear Capacity Adjustment Factor (SCAF.) This factor reduces the unit shear capacity of a given shearwall based on the amount of the wall that is sheathed full-height (meeting code-required height-to-width ratios) as well as the maximum opening height. This allows the designer to locate openings within a shearwall without interruption in the shearwall length. Few engineers would argue that a very small window in the middle of a solid shear wall would have little or no impact on the performance of the shear wall. Similarly, these same engineers would recognize that a large picture window or man-door would have a significant impact on the performance of the same wall. The perforated shear wall design provides a way to evaluate the performance between these extremes.

The SCAF is applied to published design capacities for either wind or seismic shearwall design. A comparison of the traditional segmented versus the perforated shearwall approach is given in Design Examples 1 and 2 to follow.

Example 1: Shear Wall Design (Traditional Segmented) – With Specific Gravity Framing Adjustment

Given:

The ASD (allowable stress design) shear load on the wall from the diaphragm is 3,000 lbf. The controlling load is assumed to be from wind pressures.

Find:

The shear wall design for the wall shown below in figure 2

Figure 2, Building Elevation for Segmented Shear Wall Example.

Solution:

The total length of full height segments is 10 feet. Note per IBC Table 2503.3.4, that 3.5:1 is the minimum shear wall aspect ratio for wind loading. For seismic design, shear wall aspect ratios greater than 2:1 but not exceeding 3.5:1 are permitted provided the factored shear resistance values are multiplied by 2w/h, where w and h are equal to the width and height of the shear wall segment respectively.

1. The unit shear is: v = V/L = 3000/10 = 300 plf

2. Assuming the framing will be spruce-pine-fir (with specific gravity, SG = 0.42), the shear values from the capacity table must be adjusted for species other than Southern Pine and Douglas-fir. The specific gravity adjustment factor (SGAF) in this case is:

SGAF = 1 – (0.5-SG) = 1 – (0.5 – 0.42) = 0.92

According to IBC Section 2306.4.1 of the 2006 IBC, for wind loads the allowable shear capacities are permitted to be increased by 40%.

From Table 1, 7/16-inch wood structural panels with 8d common nails at 6 inches o.c. on supported edges will provide an allowable capacity, vallow. of:

vallow. = 260(0.92)1.4 = 335 plf ≥ 300 plf

Note the increase to 15/32-inch panel design values is taken in accordance with Footnote d of Table 1, assuming studs will be placed 16 inches o.c. and the panel will be oriented with the 8-foot direction vertical. Also, as stated in Example 1, the shear wall capacity of the gypsum sheathing can be added to the wood structural panel shear wall capacity, but is not done in this example.

3. The hold downs must be located at the ends of each full height segment and designed to resist uplift tension, T. The compression, C, in the end studs, due to lateral loads acting on the shear wall is equal to the tension uplift:

T = C = V/L(h) = vh = 300(8) = 2400 lbf

Where v = V/L = unit shear (lb/ft)

Note that no dead load is assumed to counter the hold-down uplift. Dead load would take away tension uplift forces but adds to the compression forces and also adds to the compression perpendicular to grain stress on the bottom plate. Due to the compression chord bearing on the bottom plate of the shear wall, the bottom plate should also be checked to ensure adequate compression-perpendicular-to-grain capacity.

4. The end studs to which the hold down is attached, sometimes called chords, must be capable of resisting the tension and compression forces due to the lateral forces in the wall as shown in Figure 5, in addition to the gravity-load forces. The required combination of lateral and gravity loads is provided in IBC Section 1605.

Example 2: Shear Wall Designed with Openings – Perforated Shear Wall Design Method (2006 IBC Section 2305.3.8.2)

Given:

The same wall section as shown in Example 1 will be redesigned as a perforated shear wall to highlight the differences between the two methods. In this empirical-based method, the entire wall, not just full height segments, is considered the shear wall and the openings are accounted for with a shear-resistance-adjustment factor, Co. Hold downs are only required at the ends of the wall since the entire wall is treated as one shear wall with openings.

The shear load on the wall from the diaphragm, V, is 3,000 lbf from wind pressures. The length of the perforated shear wall is defined by hold-down location, H, as shown in figure 3.

Figure 3. Building Elevation for Perforated Shear-Wall Example

FindThe shear wall design for the wall shown in Figure 3.

Solution:

1. The unit shear in the wall’s full-height segments is 300 plf (see Example 1).2. Adjustment factors: The specific gravity adjustment factor (SGAF) is 0.92 (from Example 1).

The allowable shear capacities can be increased by 40% for wind loads, per 2006 IBC Section 2306.4.1.

The total length of full-height segments is 10 feet. Note that all the full-height segments in Figure 6 meet the minimum aspect ratio requirement for shear walls as specified in IBC Table 2305.3.4 (96"/3.5 = min. 27.4") and therefore may be counted when determining length of full-height segments. Full-height segments less than the minimum cannot be counted when determining length of full-height segment.

Two items are needed for finding the shear-resistance-adjustment factor, Co:• percent full-height sheathed and • maximum opening height.

The percent full-height sheathed is the length of the full-height segments divided by the total length of wall = 10/24 = 0.42 (or 42%).

The maximum opening height is 6' 8".

From IBC Table 2305.3.8.2, the shear-resistance adjustment factor, Co, is 0.53 (conservative using 40% full-height sheathed instead of interpolating).

3. From Table 1, 7/16-inch wood structural panels with 8d common nails spaced at 3 inches at panel edges will provide an adjusted allowable capacity, vallow. of:

vallow. = 450(1.4)(0.92)(0.53) = 307 plf ≥ 300 plf ∴ OK

4. The hold downs must be designed to resist:T=V(h)/(Co(Li)) = 3,000(8)/(0.53(10)) = 4,528 lbf (Equation 23-3 of the IBC)Where Li = the sum of the aspect-ratio-qualifying full-height segments

5. The shear and uplift between hold downs, v and u, from Figure 6 must resist:v = u = V/(Co(Li)) = 3,000/(0.53(10)) = 566 plf (Equation 23-4 of the IBC)

6. Provisions for calculating the total shear-wall deflection of a perforated shear wall (IBC Section 2305.3.8.2.9) state that the total deflection shall be based on the maximum deflection of any full-height segment divided by the shear-resistance adjustment factor, Co. Using the deflection equation from Example 4 with all terms the same but with 3- inch o.c. edge nailing, and a higher capacity hold down (5,480-lbf capacity and 0.045-inch deflection), the deflection of the 2.33-foot shear-wall segment becomes 0.151 inch. The total perforated-shear-wall deflection is calculated as: = 0.151/Co = 0.151/0.53 = 0.285 in.

Load-path Design

Design for Combined Shear and Uplift from Wind using wood structural panel wall sheathing is one of the most significant improvements in design efficiency for wood buildings. This method originally appeared in the prescriptive wind design standard SBCCI SSTD 10-92 and was based on calculations. It assumed that additional nails beyond those required for shear design could be utilized to carry uplift forces simultaneously. This approach has been verified by testing of walls sheathed with wood structural panels (plywood and oriented strand board (OSB)) loaded with a combination of shear and uplift forces. The method is especially efficient when used along with oversized OSB panels now supplied by multiple oriented strand board manufacturers. The oversize panels enable builders to use one row of vertically oriented panels per wall level.

Figure 4. In this wall system using oversize OSB panels, tension forces are transferred at the horizontal panel joint by using OSB Rim Board at the floor perimeter. OSB Rim Board is often supplied with

engineered lumber systems and has an advantage for this application since, unlike lumber; there are published tension design capacities perpendicular to member length. Oversize OSB panels allow for more

efficient use of materials when using the wall sheathing to resist combined shear and uplift forces simultaneously. In this home oversize panels eliminate the need for horizontal blocking at panel joints,

make the vertical load-path easier to verify, and act to improve resistance to air infiltration. An alternative outlined in APA Form SR-101is to use a plywood or OSB splice-plate with lumber rim boards to transfer

tension forces.

The resulting methods use wood structural panel sheathing to transfer uplift forces at horizontal discontinuities in wood wall framing. This is done by adding additional nails above and below discontinuities, to those required for shear design alone. These additional nails are used to transfer the uplift forces from the top-plate to the panel, from panel-to-panel at splice locations (if present) and from panel to sill plate at the foundation. This often eliminates uplift straps at wall framing joints, lowering material costs and increasing speed of construction. It is important to note that uplift straps may still be required around window and door openings in exterior walls to transfer uplift forces from headers to the foundation below.

Figure 5. On this building tension is transferred by the studs at the horizontal panel joint. The use of wood structural panels to simultaneously resist shear and uplift forces eliminates uplift straps at the floor framing, lowers material costs and increases the speed of construction.

Wood structural panel sheathing or siding used to simultaneously resist shear and wind uplift loads must adhere to the following conditions:

1. Panels shall have a minimum thickness of 7/16 inch and shall be installed with the strength axis parallel to the studs,

2. Anchor bolt spacing shall be 16 inches or less on center,3. 3- x 3- x 0.229-inch steel plate washers shall be used at anchor bolt locations,4. Nails in any single row shall not be spaced closer than 3 inches on center, and5. Nails in any double rows shall be spaced 1/2 inch between rows.

These conditions effectively eliminate the cross-grain bending as a failure mode in the bottom plate, as shown by full-scale test results. Assuming the above conditions are met, the following steps may be used to design wood structural panel sheathing or siding to simultaneously resist shear and wind uplift loads.

TABLE 3

UPLIFT CAPACITY (ALLOWABLE STRESS DESIGN) OF WOOD STRUCTURAL PANEL SHEATHING OR SIDING WHEN USED TO SIMULTANEOUSLY RESIST SHEAR AND WIND UPLIFT LOADS1,2

Nail Spacing Required for Shear Wall Design3 – See Table 1

6d@6" & 12" 8d@6" & 12" 8d@4" & 12" 10d@6" & 12"

Alternate Nail Spacing at Top and Bottom Plate Edges (in.), S6

6 4 3 6 4 3 6 4 3 6 4 3

Uplift Capacity (plf)1

NAILS-SINGLE ROW4 0 84 168 0 108 216 NP 0 108 0 131 262

NAILS-DOUBLE ROW5 168 336 504 216 432 648 108 324 540 262 524 786

1. The framing species shall have a published specific gravity of 0.42 (spruce-pine-fir) or greater. For framing with a specific gravity of 0.49 or greater, multiply uplift values listed in above table by 1.08.

2. Anchor bolts shall be installed in accordance with this section. Nail dimensions are: 6d – 0.113" x 2", and 8d – 0.131" x 2-1/2", and 10d – 0.148" x 3".

3. Where nail size is 6d or 8d, the tabulated uplift values are applicable to 7/16" minimum OSB panels or 15/32" minimum plywood with species of plies having a specific gravity of 0.49 or greater. Where nail size is 10d, the tabulated uplift values are applicable to 15/32" minimum OSB or plywood with a species of plies having a specific gravity of 0.49 or greater. For plywood with other species, multiply the tabulated uplift values by 0.90.

4. Wood structural panels shall overlap the top member of the double top plate and bottom plate by 1-1/2 inches and a single row of fasteners shall be placed 3/4 inch from the panel edge.

5. Wood structural panels shall overlap the top member of the double top plate and bottom plate by 1-1/2 inches. Rows of fasteners shall be 1/2 inch apart with a minimum edge distance of 1/2 inch. Each row shall have nails at the specified spacing.

6. Alternate nail spacing (S) is nail spacing that includes both the shear nailing and uplift nailing. See Figures 1, 2A and 2B.

Design Examples

A designer wants to use a conventional isolated shear wall segment for combined shear and uplift in a structure being designed for high wind. The shear on the wall segment is determined to be 420 plf and the uplift along this wall segment is 570 plf. The framing, including bottom plate is southern pine (G = 0.55) with studs at 16 inches on center.

Step 1 – Design the shear wallsFrom Table 1, using sheathing-grade wood structural panels, a 7/16-inch thickness is selected and attached with 8d nails at 4 inches on center at panel edges and 12 inches on center in the field of the panel. This yields a shear capacity of 490 plf. Note that this value may be increased to 530 plf (see Footnote 4 to Table 1). 490 > 420, therefore OK.

Step 2 – Determine the required uplift forceThe uplift force is given as 570 plf.

Step 3 - Determine combined shear and uplift nailingFrom second line in Table 3, based on 8d nails at 4 and 12 inches on center, look for any number that is larger than or equal to 570 plf. There is none. Notice, however, that a double row of nails at 3 inches on center yields a capacity of 540 plf and that Footnote 3 provides an 8 percent increase for framing with a specific gravity of 0.49 or higher 540 plf x 1.08 = 586 plf. 586 > 570, therefore OK.

The designer should specify a double row of 8d nails at 3 inches on center at top and bottom plates (S), which satisfies the combined shear and uplift requirements for this wall segment. This segment is required to use 5/8-inch anchor bolts spaced at 16 inches on center with 3- x 3- x 0.229-inch square steel plate washers.

Note that the designer must still size the hold down for the ends of the isolated shear wall segment based only on the unit shear, as is done in shear walls designed for shear only. Similarly, for the perforated shear wall method, hold downs are required at the ends of the perforated wall and are designed in the same manner as walls without wind

uplift. Uplift forces resulting from wind uplift at headers over windows and doors may still have to be resisted by straps or other tie-down devices as when conventionally framed.

Installation Requirements

The installation of wood structural panel walls for resisting combined shear and wind uplift loads shall be as follows:

a) Multiple rows of nails applied at panel ends and edges shall be installed in accordance with Figure 1. Panel splice occurs across studs or horizontal framing such as rim boards shall be installed in accordance with Figure 2.

b) Panels shall be installed with strength axis parallel to studs.

c) All horizontal joints shall occur over framing and shall be attached per Figure 6.

d) On single-story construction, panels shall be attached to bottom plates and top member of the double top plate. Lowest plate shall be attached to foundation with minimum 5/8-inch bolts with minimum embedment of 7 inches or connectors of sufficient capacity to resist the uplift and shear loads developed in the wood structural panel sheathing or siding walls.

e) On two-story construction, upper panels shall be attached to the top member of the upper double top plate and to band joist at bottom of panel. The panel edges need not fall in the center of the band joist. Upper attachment of lower panel shall be made to band joist and lower attachment made to lowest plate at first-floor framing, which shall be attached to foundation with minimum 5/8-inch bolts with minimum embedment of 7 inches or connectors of sufficient capacity to resist the wind uplift and shear loads developed in the wood structural panel sheathing or siding walls. When a shear and uplift connection is made at a band joist or with an inter-story splice, the band joists and/or splice plates must have the ability to withstand the resulting tensile stresses perpendicular to the grain. Since sawn lumber, glulam and most structural composite clumber (SCL) products do not have a published allowable tensile stress perpendicular to the grain, the shear and uplift connection can be made by a wood structural panel splice plate that is sandwiched between the wall sheathing/siding and the band joist or splice plate. This wood structural panel splice plate must be of the same thickness, grade and orientation as the wall sheathing/siding material. This can be seen in Figures 8 and 9. Note that OSB or plywood band joists are a suitable material for the shear and uplift splice connections shown in Figures 7A, 7B, 8 and 9.

If a wood structural panel splice plate is to be used over a lumber band joist, due to the potential for shrinkage of the lumber as it dries out, the wood structural panel splice plate shall be cut slightly under height (approximately 1/4 inch) to permit room for shrinkage of the band joist.

f) Where windows and doors interrupt wood structural panel sheathing or siding, framing anchors or connectors shall be used to resist the appropriate wind uplift loads, as required.

g) Additional installation information is provided in the APA Guide, Wood Structural Panels for Combined Uplift and Shear Resistance, Form K325 (www.apawood.org/publications).

Figure 6.

Figure 7a.

Figure 7b.

Figure 8 Figure 9.

Designing to Resist Negative Pressures on Roof and Wall Sheathing has been identified as critical to building performance. The requirement to resist negative pressure has been a part of the building code for many years but not until Hurricane Hugo did it become apparent that roof and wall sheathing attachment was critical to mitigating water infiltration and financial loss. In Hurricanes Hugo and Andrew, roof and wall sheathing on recently constructed buildings was too often poorly attached to the framing to resist the positive and negative pressures. Similar observations were made in the case of steel roof deck connections to steel framing.

After Hurricane Andrew, APA conducted a focused survey of roof sheathing performance in South Florida. In almost every case, during observations by APA staff, roof sheathing panels did not meet the minimum fastening recommendations as found in the U.S. codes at that time.

A great deal of publicity was focused on the performance of OSB in Hurricane Andrew. The media rushed in to condemn the product after observing the performance of non-structural board materials especially prevalent in mobile home floor construction. In no cases was widespread OSB failure observed due to deficiencies in water exposure durability or from sub-standard material strength. The South Florida Building Code is still written to exclude OSB roof sheathing based on the perception that the OSB was to blame, when in fact missed nailing schedules and fastener failures were the primary cause. Code acceptance of OSB, even after exposure to rainfall, is widespread because these conditions are typically experienced during construction.

Hurricanes Hugo and Andrew revealed a glaring hole in building code provisions during this time period, however, since fastening schedules for common sheathing materials were not designed to resist negative pressures due to high wind. High-wind fastening schedules were soon added to reflect a greater understanding of actual loads on the roof surfaces and reflect standard factors of safety common to engineering analysis. In addition, increased use of deformed-shank nails and improvements in fastener technology have enabled designers to adequately meet pressure requirements for high wind events.

Figure 10. The APA Disaster Assessment team evaluates a case of roof sheathing loss from Hurricane Katrina. This was a relatively rare occurrence as compared to other large and forceful hurricanes.

Staples were observed to be located more than 12 inches on center at panel ends and many more were missing the framing with one or both legs. Interestingly, this roof was the only one with significant roof damage within the development of approximately 12 nearly identical single-story residential buildings.

Wall sheathing attachment has been revealed as a potential vulnerability in more recent storms such as Hurricane Katrina and even some recent thunderstorms in the Midwest where poorly attached wall coverings were lost at wind speeds significantly lower than the design wind event. Breached wall systems resulted in massive loss due to water infiltration. Non-structural (foam) wall sheathing was observed in several of these damaged wall systems in areas affected by Hurricane Katrina, including one single-family development near Gulfport, Mississippi, (see figure 11) where the majority of homes contained non-structural foam sheathing overlaid with vinyl siding.

Figure 11. Foam sheathing and vinyl were lost on gable-ends in Sunkist (Biloxi), Mississippi, just south of I-10. Massive water infiltration resulted in heavy damage to contents. Non-structural sheathing will

generally not meet the wall component and cladding pressure requirements of the IBC and IRC.

Many sidings actually require a structural “solid” backing capable of resisting wind pressures that affect the wall components and cladding. All vinyl siding code reports require a solid backing (e.g., ICC-ES ESR-1022, ESR-1066, ESR-1083, ESR-1565, ESR-1517, ESR-1728, ESR-1020, and ESR-1133).

Traditionally non-structural foam has not been considered a solid backing capable of resisting wind pressures, and most have no published structural design values. If the siding does not have the capacity to resist these required pressures, the sheathing behind it must. This is not only logical, but also a requirement in the 2006 IBC and IRC (Sections 1609.1 and R301.2.1, respectively). Wood structural panel sheathing, however, provides adequate resistance to design wind loads when used on exterior surfaces.

Table 1 below is from APA Technical Note: Understanding the Importance of Structural Wall Sheathing as a Wall Covering, Form J430, and provides a prescriptive approach to satisfy the wind pressure requirements of Section R301.2.1 of the IRC when using wood structural panel wall sheathing. A similar table with the same data will appear as Table R602.3 (3) in the 2009 IRC. Designers should always consult specific ICC Evaluation Service Reports to determine if wall sheathing or sidings meet wind pressure requirements.

Table 1MAXIMUM WIND SPEED (MPH – 3 SECOND GUST) PERMITTED FOR WOOD STRUCTURAL PANEL WALL SHEATHING USED TO RESIST WIND PRESSURES(a)(b)(c)

Minimum Nail Minimum Wood

Structural Panel Span

Rating

Minimum Nominal Panel

Thickness (inches)

Maximum Wall Stud Spacing(inches)

Panel Nail Spacing Maximum Wind Speed (mph)

SizePenetration

(inches)Edges

(inches o.c.)Field

(inches o.c.)

Wind Exposure Category

B C D

6d Common (0.113" x 2.0") 1.5 24/0 3/8 16 6 12 110  90  85

8d Common (0.131" x 2.5") 1.75 24/16 7/16

16 6 12 130 110 105

24 6 12 110  90  85

(a) Panel strength axis parallel or perpendicular to supports. Three-ply plywood sheathing with studs spaced more than 16 inches on center shall be applied with panel strength axis perpendicular to supports.

(b) Table is based on wind pressures acting toward and away from building surfaces per R301.2. Lateral bracing requirements shall be in accordance with R602.10.

(c) Wood Structural Panels with span ratings of Wall-16 or Wall-24 shall be permitted as an alternate to panels with a 24/0 span rating. Plywood siding rated 16 oc or 24 oc shall be permitted as an alternate to panels with a 24/16 span rating. Wall-16 and plywood siding 16 oc shall be used with studs spaced a maximum of 16 inches on center.

Storm shutters are one of the best options for protecting a home’s windows and doors from windborne debris. They also prevent sudden pressurization of a building envelope when a window is broken. APA has recently updated shutter designs that utilize common wood structural panels (plywood and OSB) to help resist both negative pressure and flying debris. These are intended to satisfy the minimum requirements of IRC Section R301.2.1.2 Protection of Openings, and Section 1609.1.2 of the 2006 International Building Code. Additional shutter designs by APA offer enhanced strength and stiffness over that which is considered code minimum. Refer to APA Hurricane Shutter Designs, Form T450 for more information on hurricane shutter designs for wood-frame and masonry block structures.

Prescriptive lateral design tools for coastal development have improved considerably since Hurricane Hugo. Each hurricane brings with it a unique personality and reveals new deficiencies. New technologies, better design and stricter code enforcement are resulting in a new stock of buildings that are better able to survive wind events with less damage.

A few notable prescriptive design documents are available to assist wood designers in resisting high winds. These include:

Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings – The 2001 edition is an ANSI approved document that provides engineered and prescriptive requirements for wood frame construction based on dead, live, snow, seismic and wind loads derived from the 2000 International Building Code (IBC). Included are tabulated engineered design and prescriptive design provisions applicable for 85-150 mph wind speed (3 second gust) Exposure B and C with adjustment factors for Exposure D. The 2001 Edition is the current code-referenced version of the Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings. The 2009 International Residential Code (IRC) contains an erroneous reference to the 2008 WFCM and the ICC has acknowledged this error.

Wood Frame Construction Manual Guides to Wood Construction in High Wind Areas - Separate documents address wind design requirements in 90, 100, 110, 120, and 130 mph (Exposure B) wind zones. These Guides are based on provisions contained in AF&PA's 2001 WFCM, the reference document for high-wind wood-frame construction in the International Residential Code (IRC). Use of the high wind provisions of these Guides will result in design solutions that prescriptively meet the requirements of the WFCM and the IRC.

ANSI/AF&PA SDPWS-2008 – Special Design Provisions for Wind and Seismic covers materials, design, and construction of wood members, fasteners, and assemblies to resist wind and seismic forces. Engineered design of wood structures to resist wind or seismic forces is either by allowable stress design (ASD); or load and resistance factor design (LRFD). The most notable new provisions of the 2008 Wind and Seismic standard include criteria for wood structural panels designed to resist combined shear and uplift from wind. Criteria for proportioning, design, and detailing of engineered wood systems, members, and connections in lateral force resisting systems is provided. Nominal shear capacities of diaphragms and shear walls are provided for reference assemblies. Provisions of the SDPWS are generally consistent with provisions of the International Building Code (IBC), the NEHRP Recommended Provisions for Seismic Regulations for New Buildings (FEMA 368), as well as recommendations in current design standards or guidelines such as ASCE-7.

ICC 600 - 2008: Standard for Residential Construction in High-Wind Regions - The scope of the standard is to specify prescriptive methods to provide wind resistant designs and construction details for residential buildings of masonry, concrete, wood-framed or cold-formed steel framed construction sited in high wind regions. This standard provides prescriptive requirements and other details of construction for buildings sited in wind climates of 100 to 150 mph in 10 mph increments.

References Rose, J.D., Keith, E.L., 1996, APA Research Report 157: Wood Structural Panel Shear Walls with Gypsum Wallboard and Window/Door Opening, Form W250, APA, Tacoma, WA/USA, pp. 4-26.

Keith, E.L., Rose, J.D., 1992, APA Report T92-21: Hurricane Andrew, Structural Performance of Buildings in Southern Florida (August 24, 1992.), Form SPE-1043, APA, Tacoma, WA/USA, pp. 1-14, Appendix A and Appendix B

Skaggs, T.D., Readling, B.T., 2006, Hurricane Katrina, Structural Performance of Wood-Frame Buildings in the Aftermath, Form SPE-1125, APA, Tacoma, WA/USA, pp. 4-26.

ANSI/AF&PA SDPWS-2008 – Special Design Provisions for Wind and Seismic, 2009, AF&PA American Wood Council, Washington, DC/USA, pp. 24-40