The Burr Arch Truss Covered Bridge

The Burr Arch Truss Covered Bridge

Theodore Burr obtained the first U.S. patent issued for a specific timber truss configuration in 1806. The Burr arch is, basically, a combination of a typical multiple kingpost truss with a superimposed arch. The arch was added to allow heavier loads on the bridges and to stretch their span capabilities to greater lengths. Surviving examples of Burr arch bridges have spans of up to 67.7 m (222 ft).

Burr's development was immediately popular with bridge builders and has proven durable. More existing North American covered bridges use the Burr arch than any other type. The classic, or conventional, Burr arch supports the ends of the arch components at the abutment, with no connection between the bottom chord and arch as they pass each other (the chord is supported by the abutment directly separated from the arch end). A variation of the Burr arch (sometimes referred to as a modified Burr arch) terminates (and ties) the arch with a connection directly to the bottom chord, which is supported on the abutments.

The actual arches of most Burr arches are in pairs; these sandwich a single multiple kingpost truss between them. The most common connection uses a single bolt to join the arches through each of the vertical members of the truss. This means that the load sharing between the truss and the arch components is largely dependent on the relative stiffnesses of those bolts. The floor beams carry the live loads to the truss bottom chords, and the roof loads bear on their top chords. For these vertical loads to be distributed into the arch, the bolts must resist significant vertical shear forces. The initial, traditional Burr arches used arch components sawn from large, single timbers that were lap-spliced to each other at the verticals. Later, use of continuous but laminated (multiple-layer) timber arches became popular with some builders.

In addition to the critical areas of interest cited above for the multiple kingpost truss that comprises the central portion of the Burr arch structure, special attention should be paid to the ends of the arches and the interconnections of the arch to the truss.

There are about 224 remaining bridges supported by the Burr arches and its multiple variations (about 25 percent of all covered bridges). The Burr arch has individual spans that range from 10.0 to 67.7 m (33 to 222 ft); this longest span is 10 percent longer than the next rival configuration of truss (the Howe). The extant Burr arches were built between the early 1800s and 1988.

Iowa

Delta Covered Bridge, Delta Iowa

Delta Covered Bridge, Delta Iowa

Mary's River Covered Bridge, Chester Illinois

Richland-Plummer Creek Covered Bridge, Bloomfield Indiana

Jackson Covered Bridge, Bloomingdale Indiana

Brouilletts Creek Covered Bridge, Clinton Indiana

Longwood Covered Bridge, Connersville Indiana

Adams Mill Covered Bridge - Carroll County Bridge 39, Cutler Indiana

Possum Bottom Covered Bridge, Dana Indiana

Eugene Covered Bridge, Eugene Indiana

Huffman Mill Covered Bridge, Fulda Indiana

Medora Covered Bridge, Medora Indiana

Forsythe Covered Bridge, Moscow Indiana

Moscow Covered Bridge, Moscow Indiana

Newport Covered Bridge, Newport Indiana

Ferree Covered Bridge, Rushville Indiana

Norris Ford Covered Bridge, Rushville Indiana

Offutt Covered Bridge, Rushville Indiana

Smith Covered Bridge, Rushville Indiana

West Union Covered Bridge, West Union Indiana

Westport Covered Bridge, Westport Indiana

Covered Bridge, Cynthiana Kentucky

Mount Zion Covered Bridge, Mooresville Kentucky

Oldtown Covered Bridge, Oldtown Kentucky

Cabin Creek Covered Bridge, Tollesboro Kentucky

Arthur A. Smith Covered Bridge, Colrain Massachusetts

Jericho Covered Bridge, Kingsville Maryland

Gilpin's Falls Covered Bridge, North East Maryland

Utica Covered Bridge, Utica Maryland

Union Covered Bridge, Paris Missouri

Bath Covered Bridge, Bath New Hampshire

Bedell Covered Bridge, Haverhill New Hampshire

Hyde Hall Covered Bridge, New York

Perrine's Covered Bridge, Rosendale New York

Salisbury Center Covered Bridge, Salisbury Center New York

Roberts Covered Bridge, Eaton Ohio

Shinn Covered Bridge, Palmer Township Ohio

Hizey Covered Bridge, Pickerington Ohio

Pomeroy Academia Covered Bridge, Beale Township Pennsylvania

Meems Bottom Covered Bridge, Mt. Jackson Virginia

Bob White Covered Bridge, Woolwine Virginia

Jack's Creek Covered Bridge, Woolwine Virginia

Gates Farm Covered Bridge, Cambridge Vermont

Grist Mill Covered Bridge, Cambridge Vermont

Poland Covered Bridge, Cambridge Junction Vermont

Quinlan's Covered Bridge, East Charlotte Vermont

Sequin Covered Bridge, East Charlotte Vermont

Pulp Mill Covered Bridge, Middlebury Vermont

Great Eddy Covered Bridge, Waitsfield Vermont

Barrackville Covered Bridge West Virginia

Barrackville Covered Bridge, Barrackville West Virginia