Why do seismic active regions use more structural concrete

aibarr
Tue, 12 Feb 2008 14:23:16 +0000

No problem; glad to help. =)

Concrete actually is typically continuous because it’s easier to build that way. It’s usually poured in different stages, or lifts, but care in design and construction is taken so that the entire thing acts more or less monolithically (in other words, “as one piece”, for those of you who haven’t seen 2001). The only cases in which we provide separations in concrete structures are in controlled situations to allow for long-term effects like creep (non-immediate, long-term deformation of concrete under constant loading) and shrinking of the slab (concrete has a tendency to shrink for a long time after it’s started to cure). We usually do that at things called “control joints,” which are usually a good distance away from the columns and critical areas, where the forces aren’t so complicated. But yeah, connections typically aren’t that big of a deal in concrete structures, like they are in steel structures.

The only sort of corrolary to that is that when you design your rebar layout at all the column and beam intersections in seismic situations, you have to be pretty careful to do certain things like wrap the rebar snugly against one another so it doesn’t jockey loose in an earthquake when concrete starts spalling off (you want your steel rebar cage to stay more or less intact, even when the concrete starts to break), and to keep as many of the rebar in your beams continuous through where they run into columns so that if you lose a column, you won’t start a progressive collapse… But that’s the only sort of area where you have connection issues, per se, in concrete. The connections are within within the concrete, though, and it’s not like you have to bolt or weld one concrete beam to another, like you would have to in steel structures. But the concrete rebar connection design issue is a really huge topic of discussion among structural engineers, and it typically goes by the name “detailing.” We’re always trying to look for better practices of placing rebar within concrete frames so that it works better in weird (earthquake etc.) situations.

On to the second part of your question…

If cost and time and imperfections weren’t an issue, would concrete have an advantage over steel in minor earthquake situations? Well, it might, in a lot of cases… It really depends on the earthquake, the structure, what the structure is used for, how it’s laid out, stuff like that. Whether to design something out of steel or concrete more has to do with a lot of non-earthquakey factors like vibration control (if you have a hospital with sensitive equipment, steel beams make floors a lot bouncier, which would be bad, say, if someone’s OPERATING on my BRAIN with LASERS omg…) or like if you have long, sweeping curves to parts of your building (easy to bend steel gracefully to make architecturally-tasty things like arches or curved-layout walkways, not so easy to build formwork to pour concrete to do the same thing). So, that’s what typically controls the selection process.

Bottom line, we can pretty much make anything work okay in seismic situations (except for certain types of unreinforced masonry… bricks or concrete block without rebar… but that’s an entirely different discussion) without too much hassle, so long as we know what we’re doing (and in California, you have to go through seven years of on-the-job and grad school experience and 34 hours of testing in three or four rounds in order to get your SE… Structural Engineer… license, so the folks who design these things do know what they’re doing) and we take a little extra time to do things properly.

Good questions! Let me know if you (or anybody else) is interested in hearing more about any of this, and anybody who’s reading who also does this stuff and perhaps is lurking, feel free to add anything you think I’ve forgotten!