This OSB-sheathed house under construction in Bay St. Louis, Miss., was relocated inland approximately 100 feet from its original foundation by Hurricane Katrina’s 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 this behavior.

Each hurricane has a unique personality and reveals new deficiencies in common practice and design tools. New technologies, better design, and stricter code enforcement are resulting in new buildings that are better able to survive wind events with less damage.

After Hurricane Camille in 1969, another 20 years passed before the next 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 progress in building code provisions for high wind design. Hurricane Hugo served as a reminder that hurricanes, in fact, will come, bringing with them wind and water. Furthermore, buildings will be exposed to fluctuating wind pressure, flying debris, moving water, wave action, scour, and floating debris.

Having just passed the 20th anniversary of Hurricane Hugo, a look back at changes made for wind-resistant wood framing is warranted. Innovations include new design provisions for carrying lateral and vertical loads within main wind force resisting systems, as well as for resisting wind pressure on components and cladding.

Code changes since Hugo have also had a significant impact on wind design. Overall, new developments in design technology and building codes result in safer structures that are simpler and more economical than previously possible.

Focused research has resulted in a better understanding of wind loads and improved technologies for resisting these loads. At the same time, seismic design has become more complicated, resulting in further separation between lateral design for wind and seismic events.

Shearwall and diaphragm design
Significant improvements have been made in the design of wood shearwalls and diaphragms to resist loads commonly associated with wind design. These improvements were developed mostly through physical testing of full-scale wall systems with various opening configurations and base connections to resist overturning and base shear forces.

Significant building code changes — Code changes include a 40-percent increase in shearwall and diaphragm values for wind design only, per the 2006 IBC Sections 2306.4.1 and 2306.3.2, respectively. 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 over the past 20 years, the certainty as to the resulting loads has improved. This is partly the rationale for reducing the safety factor. The increase also takes into effect the short time duration for wind loads. [Reference: 2003 International Building Code (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 side of a wall, or both sides of the same wall, is generally not permitted due to differences in load versus 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 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-The Engineered Wood Association (APA) Research Report 157: Wood Structural Panel Shear Walls with Gypsum Wallboard and Window/Door Openings, Form W250.

New design methods — 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 United States, 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, and 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 traditional segmented method assumes that openings in a wall are an interruption in shear flow. This results in the design of 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. Reducing the number of hold-downs within a given wall reduces hardware costs as well as placement costs, which are significant considering they must be precisely placed and coordinated with the placement of concrete. Otherwise, adhesive-embedded anchors may be utilized to attach the walls to the foundation.

The perforated shearwall design method 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 perforated shearwall design methods is contained in APA publication Diaphragms and Shear Walls, form L350.

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 Southern Building Code Congress International’s Standard for Hurricane Resistant Construction (SBCCI SSTD 10-92) and was based on calculations. It assumed that additional nails beyond those required for shear design could be used to carry uplift forces simultaneous to shear resistance. This approach has been verified by the 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 OSB manufacturers. The oversize panels enable builders to use one row of vertically oriented panels per wall level, eliminating the need for added blocking at the horizontal joints.

Foam sheathing and vinyl were lost on gable-ends near Biloxi, Miss. Massive water infiltration resulted in heavy damage to contents. Non-structural sheathing with vinyl siding will generally not meet the wall component and cladding pressure requirements of the IBC and IRC. On this commercial building, tension is transferred by the studs or the splice plate 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 is used to transfer uplift forces at horizontal discontinuities in the wall framing. This is done by adding additional nails above and below framing discontinuities, supplementing those required for shear design alone. These additional nails are used to transfer the uplift forces from the wall 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.

Resisting negative pressures on roofs and walls is critical to building performance. The requirement to resist negative pressure has been a part of the building code for many years. However, it was not until the devastating effects of Hurricane Hugo that the results of ignoring this detail become apparent. Water infiltration and financial loss in a wind event are closely tied. In areas struck by hurricanes Hugo and Andrew, roof and wall sheathing on recently constructed buildings was often poorly attached to the framing and inadequate to resist fluctuating positive and negative pressures. Similar observations were made in the case of steel roof deck connections to steel framing.

Hurricanes Hugo and Andrew revealed a glaring hole in building code provisions during this time period, since fastening schedules for common sheathing materials were not included 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 standard factors of safety common to engineering analysis. Increased use of deformed-shank nails and other improvements in fastener technology have enabled designers to adequately meet pressure requirements for high-wind events.

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. Poorly attached wall coverings were lost at wind speeds significantly lower than the design wind event. Breached wall systems often result in financial loss due to rainwater infiltration. Non-structural (foam) wall sheathing was observed in several areas affected by Hurricane Katrina. 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 (for example, the International Code Council Evaluation Services Report (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 Sections 1609.1 and IRC Section R301.2.1.

Plywood or OSB wood structural panel sheathing provides adequate resistance to design wind loads when properly fastened on exterior surfaces. APA Technical Note: Understanding the Importance of Structural Wall Sheathing as a Wall Covering, Form J430, provides a prescriptive approach to satisfy the wind pressure requirements of Section R301.2.1 of the IRC. Consult specific International Code Council (ICC) Evaluation Service Reports to determine if wall sheathing or sidings meet wind pressure requirements.

Conclusion
Through testing and post-storm observations, we have demonstrated and experienced past building code short-comings. Much has changed in the past 20 years to improve constructability and structural performance of buildings subject to high winds.

Bryan Readling, P.E., is an engineered wood specialist in the APA Field Services Division. Ed Keith, P.E., is a senior engineer in the Technical Services Division of APA-The Engineered Wood Association. They can be reached at bryan.readling@apawood.org and ed.keith@apawood.org, respectively.