You would be surprised at how a very minor error can become a very major problem. We also do not know that it was an engineering mistake. The majority of the structural failures that I have been involved with were the result of installation errors. The engineering was just fine but the people that put it together did so improperly resulting in a failure. In 15 years I have seen only 2 engineering errors that resulted in a failure and neither were catastrophic failures merely something sagging more than it should. There is also the possibility of improper materials being uses. If the EOR specified the foundation was to be made using 6000 psi concrete and 3000 psi concrete was used the fault would lie in installation and not engineering.
Very true. The example I gave above regarding the local building with the sagging steel has turned out to be an Engineering mistake. After investigation, it was determined that the steel was made perfectly, and the concrete deck was as specified. Just a minor under-design. It will likely end up with them jacking the beams up and welding some major plates to them.
The pedestrian walkway that collapsed years ago in that mall was an engineering failure. The collapsing parking garage mentioned by another poster could have been either engineering or contractor failures.
A few years ago some big firms were building the new state library here in Indy. There were several concrete columns in the building (parking garage below) and they had put together the rebar cages and had the columns completely formed up. Then the Contractor brought to the attention of the engineer that the aggregate (rocks) used in the specified concrete mixture were too big to pass through the openings in the VERY dense rebar cage.
Long story short, engineer didn't want to down-size the rebar and told the Contractor to "make it work". Contractor poured, aggregate got stuck in all the cages, and they ended up with hollow columns and "honeycombs" everywhere. It was a disaster, and ended up stopping the entire project for several months. The concrete company went out of business at NO fault of theirs, but since payments for work-in-place were held up by attorneys, it put too much of a strain on the company. Plus, it was a huge job and they had dedicated all of their resources to it.
That has been my though for a long long time.
Steel can be replaced if necessary. You can brace existing structures, removed the defective member, and put in a new one. However a foundation re-work is a little different.
I don't know if it would have been a blatant as subbing 3000 for 6000. I assume they were casting test cylinders during the pour and any 2nd year engineering student should be able to spot something like that during testing.
However, there are other things. Incorrect admixtures, incorrect installation processes, a pocket in the pour, something wrong with the steel that was not noticed before the pour. Maybe even the subsurface compaction was not done to specs. If the water content of the soil is too high or too low you CANNOT compact it to the required density. When it is too wet, there is nothing you can do but scarify it and wait for it to dry (or muck it out, and add in new). Contractor hate delays like that, they tend to threaten site Engineers who won't sign off on that lift. Somtimes they even drop heavy equipment near them :lookaroun You never know what happend during the foundation buld - it can be tough being an inspector at times. A foundation defect would go a long way to explaining the state of the Yeti.
-dave
Dave, you're exactly right. SO many corners are cut at the site, it's not even funny. So often it's because the worker with barely a high school education knows more than the engineers because "he's been doin this for 40 years and that engineer is just a kid."
The time/money factor plays a huge part too. The book says you wait 28 days (minimum) before you apply any load to concrete, which allows the lab time to break the test cylinders to see if you met the required strength.
I can't think of a project in my short career where they actually let foundations set for 28 days before they set steel on them. But when you're talking about a 1-story shopping center or church, the loads on the concrete are negligible. If they didn't wait long enough for the Yeti footing to reach compression strength, that could be disastrous. That or soil compaction, like you said.
So many things could have come into play. And it could be from any combination of construction and/or engineering errors.
