We have all been asking, "Why did the building collapse?" Here is my opinion.
According to early reports, the building has been sinking over the past 40 years at a rate between 1 and 3 millimeters per year. Now, most people have no idea what a millimeter is, so they have no idea if this is a little or a lot? Here are some numbers. A meter is about 39.2 inchers and a millimeter is 1/1000 of that or 0.0392 inchers. If you multiply that by 40 years and 1 to 3 per year, you get a range of 1.568 to 4.704 inches since the building was built. That is a lot and here is why.
The building was made out of steel reinforced concrete. That is good in most cases. As it turns out concrete has very little tensile strength. It is good in compression where it is just used to hold something up, but not if the load is from the side. It is also brittle which means that it can not flex to handle even a small side load. The steel is strong in both tension and compression and is much more elastic than concrete. The steel operates like a spring inside the concrete and does not take any load until it is stretched or compressed a little. Do you follow that? By the time the steel has stretched enough to start carrying some of the load, the concrete has already started to crack because it is so brittle. That is why the concrete girders on highway bridges are made from "pre-stressed" concrete. I am sure that this building was not built with "pre-stressed" concrete members.
In a steel or wood structure if the loads change over time, these materials can accommodate some of that change by flexing, creeping and load sharing. In poured in place reinforced concrete construction, this is not possible. Nothing can move or shift and this is exactly what happened with the building in Seaside.
The bottom line is that the shifting of the foundation of this building over the past 40 years is the reason that it collapsed, pure and simple. Steel rod reinforced buildings like this are fine if they do not shift or are impacted by something like an earth quake. Actually, steel rod reinforced concrete is a little like "cheating". It can get the job done at a lower cost but only under special conditions. It has little reserve when comes to changed conditions and that is what happened here.
PS--When there was an earth quake in the San Francisco Bay area a few years ago, the pre-stressed concrete beans on the elevated Embarcadero were fine, but the poured in place supports failed and the whole thing had to be taken down. The World Trade Center in New York City was made from steel and failed when it got hot from the burning fuel from the airplane and lost both its tensile and compression strength.
PS--Here is another observation about the collapse. I think that most steel re-enforced concrete uses to too large of rods and too few. If you look at the remaining part of the building, you will see many steel rods still attached to the building. That means the rod connection to the concrete in the collapsed building failed. They pulled out of the concrete and thus did not provide the force needed to hold the collapsed part of the building to the part that survived. The steel rods, thus did not do their job.
In ferrocement boats, they use much smaller rods, but many more and these boats last over a hundred years. They also add "fines" to the concrete and we know that most concrete is deficient in "fines" that is needed to assure the concrete is both dense and strong and is "water proof".
