log bridge

{{short description|Bridge that uses logs that fall naturally or are intentionally felled or placed across streams}}

{{BridgeTypePix|

|type_name = Log bridge

|image = Passerelle sommaire sur l'Alzou en crue.jpg

|image_title = Simple log bridge over the Alzou in France

|sibling_names = Clapper bridge

|descendant_names = Beam bridge

|ancestor_names = Step-stone bridge

|carries = Pedestrians, vehicles (on multiple parallel logs)

|span_range = Short

|material = Logs, dry set stonework footings, top may be flattened or boards added, topped with rammed earth for vehicles

|movable = No

|design = Low

|falsework = No

|}}

A log bridge is a timber bridge that uses logs{{cite book | last=National Parks Conference| first= Department of the Interior| title=Proceedings of the National parks conference held at Berkeley, California March 11, 12, and 13, 1915| url=https://archive.org/details/proceedingsnati01confgoog| access-date=March 14, 2010| year=1915| publisher=Government Printing Office| location=Washington| page=[https://archive.org/details/proceedingsnati01confgoog/page/n443 60]| quote=(A log bridge) is a bridge composed of log beams, the logs being in natural condition or hewn, which are thrown across two abutments, and over which traffic may pass.| ref=NPC}} that fall naturally or are intentionally felled or placed across streams. The first man-made bridges with significant span were probably intentionally felled trees.{{cite book |last1=Bennett| first1=David| editor1-first=M. J.| editor1-last=Ryall| editor2-first=G.A.R.| editor2-last=Parke| editor3-first=J.E.| editor3-last=Harding| title=The manual of bridge engineering| url=https://books.google.com/books?id=8PGk81gtCywC| format=Google books| access-date=March 14, 2010| year=2000| publisher=Thomas Telford| location=London| isbn=978-0-7277-2774-9| page=1| chapter=The history and development of bridges| chapter-url=https://books.google.com/books?id=8PGk81gtCywC&pg=PA1}}

The use of emplaced logs is now sometimes used in temporary bridges used for logging roads, where a forest tract is to be harvested and the road then abandoned. Such log bridges have a severely limited lifetimeNational Parks Conference 1915, p. 59. "The chief objection to a log bridge ... is the shortness of its life." due to soil contact and subsequent rot and wood-eating insect infestation.

Longer lasting log bridges may be constructed by using treated logs and/or by providing well drained footingsNational Parks Conference 1915, p. 59. "If we can design the abutment so that no moisture can collect under the logs the life of the bridge is materially increased." of stone or concrete combined with regular maintenance to prevent soil infiltration. This care in construction can be seen in the French bridge illustrated below, which has well locked dry set stone abutments and a footpath leveled with boards.