Walnut Room this way

Walnut Room this way

Sunday, July 8, 2012

Clear Fork of the Brazos Suspension Bridge

Spanning the Clear Fork of the Brazos River between Woodson and Fort Griffin, this 1896 suspension bridge is one of seven surviving suspension bridges in Texas that were constructed prior to 1940 (Historic American Engineering Record, 1996).  The catenary-type suspension bridge was built by the Flinn-Moyer Company of Weatherford.  All information is from the Texas Historic Bridges Recording Project, Dr. Mark M. Brown, with J. Philip Gruen, 1996.
The bridge has an "...overall length of 312 feet, a 140 foot clear span, and a clear roadway of 13 feet, 8 3/4 inches."
The east abutment is "roughly coursed stone...
...while the western abutment appears to be earth fill."
The piers are "mortared coursed rusticated masonry..."  Towers, which have been encased in concrete in a 1926 modification by the Austin Bridge Company of Dallas,are 17 feet high.  Castings at the top of each tower secure the ends of the pipes and comprise the cable saddle.  Inclined stay cables extend from  the top of the tower to the deck.
Detail of cable saddle.
"The original towers were constructed of three wrought-iron pipes tied together by tension roads within pipe struts" similar to this early photograph of a suspension bridge.  It is unknown why the Austin Bridge Company modified the towers, whether from structural reinforcement concerns or pure aesthetics.  Austin converted the triangular bases to rectangles, hooded the rods in wire and then poured concrete to create the obelisks.
Individual wire strands (about 1/8 inch in diameter) comprise the cables, which are about seven inches in diameter.
Most of the cable anchorages disappear directly into the ground, with the exception of one corner (not visible) which is attached with wires to pipes that are somehow anchored.
 Rods with turnbuckles suspend the deck from the cables.
Rods with turnbuckles are attached to the cables to provide the support for the deck.
Support for the deck spans consist of 4 1/2 inch diameter wrought-iron pipes, with smaller diameter pipes set at angles to further brace the deck beam.  In the 1926 modification, these pipes were reinforced by the addition of the two I-beam posts and two 8 inch channels placed next to the original deck beam pipe.
Additional deck support is evident in the bracing across the span.  
Splice plate and tension rod.
 "The ..deck system is stiffened by...Howe truss fabricated from wrought-iron pipe diagonals, castings, and tension verticals."  The wooden boards attached at the intersection diagonals (see the photo below for intersectionals) served as a wheel hub guard.
The center-span deck was replaced by the Austin company in 1926, as was a railing made of angles and channels to replace the Howe pipe truss.  It was not as functional in "stiffening" the deck as the Howe truss, so additional steel channels were installed on the outer decks.
Embossed steel plate runners--18 inches wide--were installed at the same time.  Most of the bridges I drove over as a child and young adult in the surrounding county had wooden decks with wooden runners until they were replaced by those uninspiring concrete stretches.  I walked from the west approach here, to the opposite end until stopped by the heavy growth of brush.  I could hear a humming sound that initially I thought was bees under the bridge.  However, I noted it stopped each time I did.  I realized that it was the sound of vibrations of the bridge cables.
"The materials and design features of the bridges that Flinn helped construct permitted swift and economical construction in remote areas."  Pipes and wire were easily transported, and no large equipment was needed as it was for truss bridges.  Timber and rock were readily available at the site.  Gruen asserted that another reason suspension bridges were less costly was due to not needing mid-span supports.  This was particularly important in areas where soil was unstable or the area was prone to flooding, such as west Texas.  It is common to see the river shrink to a mere stream, only to overflow the banks during a thunderstorm.

And as evening was approaching and storm clouds were threatening, it was time to head on back to Young County--another mystery solved:  "Where is the suspension bridge on the Clear Fork at Woodson?"

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