Originally Posted by aaron38
Isn't this a good example why buildings should stick to tried and true engineering, instead of going off on wild cantilevers and such? Sure it looks cool to have half a building sticking out unsupported. But that also means that half the building IS sticking out unsupported.
A building meant to last 100-200 years shouldn't depend on a handful of welds on a transfer floor. Don't try to be cleaver, put columns under it.
Generally, yes. As I stated in my post, "tried and true engineering", as you call it, has been tested by multitudes of buildings built over decades. Their designs have been written down into code books, which dictate to engineers the right materials and dimensions to use to provide adequate factor-of-safety in their designs.
But the Citicorp Building was a special case because it had a church sitting on one corner of the lot. The code books won't tell you how to design a skyscraper in such cases. It's up to architects to work around such conundrums, and structural engineers to design a structural system that will work with the architect's designs.
Let's not forget that engineering concerns are usually a mere side note in the construction of new buildings. Economics and demand are the number one factors that drive the creation and design of skyscrapers.
We all know that the Home Insurance Building in Chicago was considered the first skyscraper to use a steel skeleton structure. But don't think that this building was an absolute game-changer that prompted all skyscraper designs to follow suit, because it most certainly wasn't. First off, most of the building's skeleton was actually wrought and cast iron, not steel. But more important is that during the same time period, there were many ideas in how to design the structures of these new skyscrapers.
The traditional method, the "tried and true", if you will, was the masonry bearing wall system. This structure relied on heavy bearing walls, exterior and interior, to support the weight of the building. The "tried and true" aspect of these buildings was that if their structure was primarily made from masonry, they were essentially fireproof.
So if bearing walls were "tried and true", what prompted the switch to steel skeleton structure in skyscraper designs? You might think that wind-resistance engineering played a role, given that the flexible steel skeletons allowed it to resist wind far better than unreinforced masonry. But you'd be wrong. Back then, nobody had ever built anything tall and slim enough to require wind-resistance engineering. In any case, the sheer mass and weight of bearing wall skyscrapers of that time period allowed them to resist winds quite well (remember that the buildings weren't that tall yet).
The answer? Economics. Elevators at that time weren't the reliable modern convenience of today. They were scary newfangled contraptions mostly used to lift freight up and down. So ironically, the most valuable space in skyscrapers was actually in the lower floors, with ground floor space being the most valuable of all.
But which floors in bearing wall skyscrapers had the least amount of usable interior space? The lower floors, with the valuable ground floor space having the least space of all. And as skyscrapers became taller, this problem became worse.
This conundrum prompted building developers to seek a solution that would allow them to create as much usable interior space in the ground floor as possible. Enter the steel skeleton ... or rather, structures that predated the true steel skeleton.
Skyscrapers of this period that were built using these transitional structures were no longer "tried and true". Just like in the Citicorp Center, the occupants of these buildings were essentially test subjects of how well engineered these buildings truly were. But unlike in the Citicorp Center, there were some unhappy endings with these transitional skyscrapers.
As I mentioned before, the "tried and true" engineering aspect of buildings was mainly about their fire resistance (the main engineering focus of that period, and a very important one today as well). The cast irons, wrought irons, and steels at that time were all advertised by their manufacturers as being "fireproof" like masonry, but it was clearly not true.
We all know of steel's poor performance in intense heat from the WTC towers collapse. But steel's performance in fire in relatively benign compared with cast iron. Back then, cast iron was already known to be weak in tension and brittle, just like masonry. It was mostly used for vertical columns in these transitional structure skyscrapers. Unfortunately, it differs from masonry in that it greatly expands in heat. As a result, when the columns expanded during a fire, their connections to the floor beams broke apart (because cast iron is brittle), and the whole structure would collapse without much warning.
But without these "test subjects", if you will, we would never have transitioned as quickly to using true steel skeletons for skyscrapers. And if economics didn't play a role in the transition, we may have likely seen some wind-related failures of bearing wall skyscrapers that had been built too tall. Therefore, though you might think of "tried and true" as the most logical way to design buildings, don't forget that the same thought process may hinder the engineering and design innovations that can be used to create better structures.