compression and tension

February 22, 2016  •  4 Comments

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 windowChaco 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, MexicoTemple 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.

stonework above entrance, Bamberg Cathedralstonework above entrance, Bamberg Cathedral

 

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, CaliforniaGolden 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, OregonColumbia 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 LibrarySeattle Public Library My head is spinning!

until next Monday,

DB

a passion for the image@


Comments

Elida Hanson-Finelli(non-registered)
Great post and terrific photos. We watched a documentary once, I believe it was a Ken Burns, perhaps about the Brooklyn Bridge and forever remained fascinated with these structures. All structures ... There is so much to them! Lucky you to,have your own personal architect! Architecture is one of our favorite pastimes. Recently we took a trip to ABQ, while there we visited the Spanish Cultural Center which is a very unique composite of old and new. Have you seen it? Thanks again.... Love your defined yet delicate and nuanced work!
Catherine Sobredo(non-registered)
Very interesting information accompanies by beautiful images. Thanks, Daryl.
Daryl Black(non-registered)
Hi Steve!

It was a bit of a brain drain for me. Add shear and moment to the package, as someone requested of Fred this afternoon, and I have had a real lesson in civil engineering. I am sure Fred would be happy to do a mini-session on structures at some point.
Steve Immel(non-registered)
Wow Nellie! So much to learn so early in the morning. Moment and shear? Puhleeze. But seriously, these are perfect images to illustrate compression and tension. I think. Ask Fred if he gives remedial classes in structures. Seems like I need it. Great post!

That Astoria-Megler bridge shot is a keeper Love the pilings in the foreground.
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