Resilient Wood Construction: Designing for Earthquakes and High Winds

How wood-frame wind and seismic-resisting systems can contribute to resilience in the built environment

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Increased Pressure at Edge Zones

The greatest wind pressures acting on a building are negative pressures (suction), and the locations of the greatest suction pressures occur at wall and roof edge zones, as illustrated in Figure 4. Edge zones at wall corners, roof ridge, and roof perimeter represent locations of potentially high suction forces relative to the central portions of the wall or roof. The extent of increase in suction forces for roofs depends on roof geometry, but increases of two or more times the negative pressures associated with central wall area are possible.

For roof sheathing and roof sheathing attachment, the increase in forces between edge zones and interior zones is larger than that observed for walls. While wood structural panel sheathing of consistent grade and thickness is used over the length of the roof, two fastening schedules are often specified.

One fastening schedule is for roof areas outside of edge zones; fasteners here are spaced farther apart. A second fastening schedule is for higher wind pressure roof areas within the edge zones; fasteners here are more closely spaced. The roof sheathing pressures developed from the external pressure coefficients in ASCE 7-16 are higher than in previous versions of ASCE, and thus require more attention to nailing patterns and to nail types and sizes. In some cases, Roof Sheathing Ring Shank nails, which have higher withdrawal design values than smooth shank nails, are required for attachment of roof sheathing to roof framing for resistance to high wind uplift forces.

The withdrawal design values for Roof Sheathing Ring Shank Nails (RSRN) are generally 1.5 to 2 times greater than those for smooth shank nails. When using Roof Sheathing Ring Shank Nails, care should be taken to ensure that the nails are in fact RSRN nails meeting ASTM F1667, as opposed to another nail with ring shank or other deformations. The 2018 NDS for Wood includes new language for use of Roof Sheathing Ring Shank nails and new provisions for fastener head pull-through. These additions were developed to address the high uplift pressures in ASCE 7-16 and are used in the 2018 Wood Frame Construction Manual.

CONSTRUCTION HIGHLIGHTS: SEISMIC DESIGN OF PODIUM-STYLE WOOD-FRAME BUILDINGS

“Podium construction” refers to multiple stories of wood framing over one or more stories of another construction type. The top of the podium is often a transfer level where loads from the wood superstructure are redistributed to the supporting levels below. Concrete podiums are the most common, though podiums of other materials are also permitted. As podium construction increases in popularity, so, too, does the need to understand the seismic design considerations for these types of framing systems.

There are two specific considerations for these buildings under lateral loading conditions:

The inherent differences in lateral-resistance properties of the superstructure above versus the podium below
A growing tendency to provide fewer lateral-resisting elements at the perimeter of the superstructure, limiting the options for shear walls

To gain additional stories, increase building area, stay within the allowable building heights, and remain within the seismic force-resisting system height limits of light-frame wood shear walls, the IBC and ASCE 7 each have provisions that enable podium designs.⁷ Without these provisions, structures of this height and area would have to be constructed using steel or concrete (Type IV construction). The podium provisions enable the continued use of economical light-frame wood construction where it works best—typically multifamily projects. By building light-frame wood structures over one to two stories of concrete or steel, these projects can achieve additional stories and increased building area while staying under the allowable building height and remaining within the seismic force-resisting system height limits of light-frame wood shear walls.

Two-Stage Seismic Analysis

Structurally, ASCE 7-16 Section 12.2.3.2 provides a two-stage analysis procedure that can be beneficial for seismic design of podium projects. The procedure treats the flexible upper and rigid lower portions of the structure as two distinct structures. Under Section 12.2.3.2, only the weight of the flexible upper portion must be considered in its design, not the entire weight of both portions. The two-stage analysis also allows the seismic base of the upper portion to serve as the top of the lower portion.

Diaphragm Flexibility

Current practice for light-frame construction commonly assumes that wood diaphragms are flexible for the purpose of distributing horizontal forces to shear walls. Even though diaphragms may be idealized as flexible, they may not always be so. Verifying the diaphragm flexibility is becoming increasingly important given trends toward larger openings in exterior shear walls, shorter wall lengths, and a greater number of wood-frame stories over the podium.

Resistance to Uplift

Mechanical connections, sheathing, and the dead load of the building can all be used to resist wind uplift and overturning. Dead load includes the weight of foundations and anchorage; however, only the dead load likely to be in place during a design wind event is permitted to be used in design. This is accommodated using reduced dead loads for resistance to wind-induced forces and is accomplished through load combinations provided in 2021 IBC 1605.2.

Conclusion

Proper design and construction based on research and compliance with building code requirements can minimize the negative effects of wind and seismic events. Inherently strong and ductile, and with many repetitive members and multiple connections, wood construction offers several benefits when it comes to seismic- and wind-resistant design. Buildings that protect occupants and can continue to function or recover quickly after a disaster contribute to the overall resilience of communities.

Codes and Standards

End Notes

¹ “Build Resilient with Wood.” American Wood Council. Web. 13 Sept. 2021

² Merolla, Lisa. “Designing a (Wooden) Earthquake-proof Home.” Popular Mechanics. 16 Jul. 2009. Web

³ Building Seismic Safety Council. Homebuilders’ Guide to Earthquake Resistant Design and Construction. FEMA 232—June 2006. Web. 13 Sept. 2021

⁴ National Design Specification (NDS) for Wood Construction 2018. American Wood Council. Web. 13 Sept. 2021

⁵ Wood Frame Construction Manual for One- and Two-Family Dwellings (WFCM) 2018. American Wood Council. Web. 13 Sept. 2021

⁶ Special Design Provisions for Wind and Seismic (SDPWS) 2021. American Wood Council. Web. 13 Sept. 2021

⁷ “How are podium style wood-frame projects (e.g., 5-over-2) seismically designed to resist lateral forces?” WoodWorks. Web. 13 Sept. 2021

⁸ “Reduce Losses from Wind.” Insurance Institute for Business and Home Safety. Web. 13 Sept. 2021

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Originally published in November 2021

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