Differential Movement

Photo: WoodWorks

Plumbing sleeve connected to structure with foam wrap to allow vertical movement of structure.

Photo: WoodWorks

Plumbing joints with pipes nested in joint to allow for vertical movement of structure.

Allowing for differential movement between wood and other structural elements and building finishes is critical. Steel, concrete and brick continue to expand and contract due to temperature changes, while wood generally maintains its dimensions having reached its EMC. Differential movement occurs when, for example, floor joists are supported by a wood-frame wall at one end and by the masonry block of an elevator shaft at the other end. Areas such as stairwells, shafts and vaulted ceilings require attention for differential movements as do plumbing, electrical and mechanical systems. Using flexible joints such as flexible pipe, conduit, couplings, elbows, and tees for electrical, mechanical and plumbing between floors can prevent potential problems. The design of the joints between building envelope components, such as windows and doors, must also allow for differential shrinkage.

Seismic Requirements

Earthquakes and rigorous seismic requirements are a well-known aspect of building on the west coast, but other parts of the country, especially in the east, are not immune to earthquake activity and the need to comply with seismic codes. Wood structures offer a high strength-to-weight ratio over those built with steel and concrete (concrete walls are about seven times heavier than typical wood-frame walls) resulting in a low inertia force during earthquakes. In wood-frame buildings, the large number of walls and floors often used in a project transfer lateral loads induced by winds and seismic forces.

Photo: John Van De Lindt, Univ. Of Alabama

After being lifted to a shake table and subjected to four progressively intense earthquakes, this six-story light-frame building was found to have only minimal damage to the gypsum wallboard and nail connections. (View a video of the test on YouTube.⁷)

Wood-frame construction also provides numerous load paths through shear walls and diaphragms, which typically have hundreds of structural elements and thousands of nail connections, adding ductility and redundancy to the system. Redundant load paths give additional assurance that loads will be transferred should a connection fail. In contrast, structures supported by heavy non-wood frames have relatively few structural members and connections, meaning fewer load paths. Moreover, the large number of walls reduces the loads shared by each wall. Tests and observations from past earthquakes show that wood buildings have performed well. For example, a six-story light-frame wood building tested on the world's largest shake table in Japan resisted a major 2,500-year earthquake with minimal damage.

Current building code requirements for wood diaphragms, shear walls and holdown devices work effectively in creating earthquake-resistive structures. Horizontal diaphragms in roofs and floors transfer the horizontal forces to the shear walls. Shear walls with holdowns such as a continuous tie-down rod system resist the tension forces in an overturning scenario, while wood studs or columns absorb the compression forces.

In high seismic areas, mid-ply shear walls (i.e., walls with sheathing sandwiched between 2x studs oriented flatwize) have twice the load capacity as traditional shear walls and can be used where increased lateral load capacity is required. This solution would be useful in lower floor lobby areas in multi-story buildings or for stacked corridors and isolated exterior walls.

FIVE STORIES OF WOOD OVER CONCRETE SLAB

Photo: Benson & Bohl Architects

Project: Advanced Individual Training Barracks with Company Operations

Location: Fort Lee, Virginia

Architect/design-build: LS3P ASSOCIATES LTD. and Clark Builders Group

Structural engineering: Michael M. Simpson & Associates, Inc.

Size: 360,000 square feet

Type of construction: Type IIIA wood-frame construction on concrete mat slab

Year of completion: 2011

LS3P ASSOCIATES LTD. and Clark Builders Group designed and constructed the design-build Advanced Individual Training Barracks with Company Operations in Fort Lee, Virginia, as the seed project under a Multiple Award Task Order Contract (MATOC) for the U.S. Army Corps of Engineers. The $68,169,000 LEED Silver project consists of two five-story barracks situated on an 11-acre site. Each 180,000-square-foot building has 600 beds and meets IBC 2006, NFPA 101 egress and life safety provisions, plus Anti-Terrorism/Force Protection Criteria. Set on a reinforced concrete mat slab, the buildings are clad primarily in brick.

“The team provided three structural options: cast-in-place concrete, light-gauge steel framing and panelized wood stud framing,” recalls Chris Ions, AIA, LEED AP, vice president/principal of LS3P ASSOCIATES LTD. “The steel came in $2 million cheaper than the cast-in-place concrete and the wood came in $2 million cheaper than the steel, a significant difference.” The advantage of working as a design-build team was being able to explore a number of framing choices to determine which was the most cost efficient.

To minimize the impact of shrinkage, the walls are balloon- framed with flooring rim beams on the inside, rather than on top of the top plates (platform framing) at each floor of vertical framing. The first floor has the heaviest framing with triple studs of southern yellow pine, selected for its high compression strength.

The first three stories of brick veneer were supported on the foundation; steel relieving angles through-bolted to the wood framing were used to support brick at the fourth and fifth floor levels. “The greatest challenge of wood construction is looking at every assembly, transition and penetration to ensure that fire-rated assemblies are truly continuous,” says Ions.