Timber Stringer Bridges

No-longer-standing Kirby’s Creek Bridge (Northampton County Bridge 77) north of Conway, built in 1935: timber stringers and substructure and state-standard reinforced concrete railings (source: NCDOT bridge inspection files) No-longer-standing Kirby’s Creek Bridge (Northampton County Bridge 77) north of Conway, built in 1935: timber stringers and substructure and state-standard reinforced concrete railings (source: NCDOT bridge inspection files)

In the 1920s the State Highway Department developed standard plans for timber stringer bridges. The bridge type has been used in North Carolina since the earliest days of settlement, simply being a series of parallel wooden beams or logs carrying a deck. Timber stringer bridges have the advantage of using an abundant natural material, but are limited in span length (about 25 feet maximum) and deteriorate rapidly unless the wood is treated. Untreated wood is generally regarded as having a lifespan of ten years in North Carolina's climate. The most common treatment since about 1900 has been creosote, which extends the life of the wood on average to between 25 and 35 years, although some bridges have been known to last considerably longer.

What was new in the 1920s was that the state's standard plans and specifications ensured uniformity of design and economical use of material by contractors and state maintenance crews working across all regions of the state. The earliest surviving examples of timber stringer highway bridges in North Carolina are of the standard design adopted in June 1928 (Standard 600-C—Standard Creosoted Wood Superstructure with Reinforced Concrete Floor and Rails). This standard introduced a reinforced concrete deck and railings, offering greater permanence than wood plank deck and railings, as well as capacity for two 15-ton trucks passing. Later standard plans for timber stringer bridges (with reinforced concrete deck and railings) were based on this successful standard, with variations for different span lengths, roadway widths, live loads, and the substitution of other standard railings.

According to the Historic Bridge Inventory, more than 570 timber stringer bridges have pre-1961 dates of construction, but most have very little, if any, original fabric, because individual members are replaced in-kind as part of routine maintenance. Today, timber stringer bridges remain one of the most common bridge types built on secondary roads by the Bridge Maintenance Unit. Thus, many timber stringer bridges in North Carolina continue a long-lived technology but are composed of modern replacement material.

[June 2013]