Timber Bridges

Summary

Timber bridges carry vehicles or pedestrians on stringers, girders, or deck panels of glulam, sawn lumber, or stress-laminated wood, usually in short and medium spans. Design follows AASHTO LRFD, not the building code, with design values adjusted for wet service, so involve a bridge engineer early.

Cols

Image

Stringer bridge: deck carried on parallel longitudinal beams
Glulam superstructure: bridge girders or deck panels of glued-laminated timber
Spike-laminated (nail-laminated) deck: timber deck panels fastened with spikes; US code allows panelized, not continuous longitudinal, nail-laminated decks
Stress-laminated deck: timber laminations held together by prestressing rods
AASHTO LRFD: American Association of State Highway and Transportation Officials load and resistance factor design specifications
Dry-use and wet-use: service conditions that set the design values

Standards

Design basis

Wood bridge design follows the current AASHTO LRFD Bridge Design Specifications with interims; FHWA has required LRFD for new bridges since 2007
The longitudinal deck systems in the codes are spike-laminated, glued-laminated, and stress-laminated
Tabulated AASHTO design values assume dry use and must be modified for wet use
Spans of about 20 to 80 ft are common for stringer bridges
Glulam standard plans from the USDA Forest Products Laboratory and Laminated Concepts are designed to the 2017 AASHTO LRFD (8th edition with 2018 revisions)

Materials

Glued-laminated timber: girders and deck panels
Sawn lumber and spike-laminated panels: decks and short spans
Stress-laminated timber: continuous deck slabs

Detailing

Adjust design values for wet use where the members can get wet
Use connectors and fasteners suited to treated wood and exposed conditions
Coordinate bearings, abutments, and foundations with the structural engineer
Protect end grain and connection points from water

Uses

Specific uses

Pedestrian bridges
Short-span road bridges (stringer bridges)

Pros and cons

Pros

Glulam standard plans are available
Several deck systems for different spans

Cons

Wet-use conditions reduce tabulated design values
Governed by highway rather than building specifications

Manufacturing and cost

Construction

Girders, decks, and rails are prefabricated and treated, set on abutments by crane, and connected with steel hardware

Main manufacturers and suppliers

Trade association: APA, The Engineered Wood Association.

Sources

USDA Forest Products Laboratory: wood bridge design reference, https://research.fs.usda.gov/download/treesearch/40280.pdf
Minnesota Department of Transportation: LRFD bridge design manual, section 8 wood structures, https://www.dot.state.mn.us/bridge/pdf/lrfdmanual/section08.pdf
Wood Products Council: standard plans for glued laminated timber bridge superstructures, https://www.woodcenter.org/standard-plans-for-glued-laminated-timber-bridge-superstructures/
AASHTOWare Bridge: single span timber beam glulam, https://www.aashtowarebridge.com/wp-content/uploads/2024/03/TMBR3-Single-Span-Timber-Beam-Glulam.pdf