As you might have noticed, my blogs are sometimes responses to bits and pieces of information that are of interest to me, or verbalized ponderings from friends or relatives. This week, my sister said she was wondering about the concepts of compression and tension in bridges.  She is reading up on the subject for a trip she is taking later on in the year.  So I turned, in gratefulness, to my personal architect and engineer, Fred Black, for clarity.  Although compression and tension is a hugely complex subject, with civil engineering libraries devoted to it, I thought I would dive into the shallow end and take a peak at it through the camera lens.  

Basics first.  Most historic buildings and bridges predating steel are under compression. The weight of one stone or adobe brick on top of another makes the unit stronger.  With the exception of elements such as window and door lintels made of wood or stone, the buildings are under compression.  This is true of all the buildings at Chaco Culture National Historic Park in New Mexico, including the wall below with a T-shaped doorway.  

Chaco Canyon window

The lintels are apparent in the Temple of the Dolls Mayan ruin at Dzibilchaltun, Mexico, but the remainder of the building is in compression.

Temple of the Seven Dolls Dzibilchaltun, Yucatan, Mexico

One of the entrances in the Bamberg Cathedral in Germany, shown below, is also a good example of compression.  Very simply put - without rope or steel (or lintels) there is no tension.  Everything is in compression, a building method the Romans began to utilize thousands of years ago.

One of the most famous bridges in the United States is the Golden Gate Bridge in San Francisco, California.  It is a suspension bridge, with the towers under compression, and the cables in tension.  The bridge, although extremely strong, is also designed to be flexible, which is achieved through tension.

Golden Gate Bridge (detail) San Francisco, California

The Astoria-Megler Bridge over the Columbia River in Oregon, is a truss bridge, with different elements of the trusses providing compression or tension, acting in unison to transfer the load of the highway on the bridge to the concrete piers in the river bed.  Both this bridge and the Golden Gate Bridge also have design elements which enable each to sustain earthquake and wind forces.

Columbia River Bridge, Astoria, Oregon

Finally, the photograph below of one of the public areas in the downtown Seattle Public Library, is a wonderful example of a light truss structure, supported by heavy steel beams. The top of the beam is in compression, and the bottom is in tension.  The grid work, although aesthetically interesting and compelling, it is actually a truss with individual members in either compression or tension.  (Fred says that moment and shear come into play here, but I'm not EVEN going there!)

Seattle Public Library My head is spinning!

until next Monday,

DB

a passion for the image@