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Posted by David Sherman [24.32.202.83] on Thursday, March 04, 2010 at 15:39:43 :
In Reply to: Look at the strands of re-bar sticking out. posted by Chriscase [76.227.80.68] on Thursday, March 04, 2010 at 15:09:52 :
It used to be that concrete columns just had rebar running vertically, to carry the stress of bending the columns side to side, with only enough tie-wire or thin stuff tacked on to hold it in place. The big change per California seismic codes, based on experience with columns failing in earthquakes, was to encircle the column with coils of rebar run circumferentially.
Here's what happens: With enough bending and shaking, the concrete gets crushed to small pieces. Now the column is basically a bunch of vertical strands of rebar sticking through a cylindrical pile of rubble. The rubble falls out, the rebar can't hold any vertical load, and the whole column is gone. By wrapping rebar around the assembly, fairly close together, before pouring, you now have a cage that will hold the rubble in place if an earthquake breaks it up. That's what's known in civil engineering as a gabion -- a wire cage full of loose rocks that's used for structural purposes where a little bit of distortion is okay, like bridge abutments and retaining walls. The concrete outside of the rebar cage isn't counted for anything in the design calculations; it's just there to protect the rebar. They often retrofit old bridge columns by wrapping them with sheet metal, for the same reason. Even a thin sheet metal pipe filled with sane will carry a pretty good vertical load, unlike either the thin pipe by itself or the sand by itself.
In this photo it's not clear if all that visible rebar was from a column, a beam, or a slab, but clearly the strands are mostly parallel to each other, with nothing wrapped around them like would be required in a modern column design.
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