The main point is that the modern construction methods used on the World Trade Center and other modern buildings are not nearly as good as the Empire State Building.
In the ESB all columns and floor beams were made of solid steel and then incased in concrete and masonry. The floor slabs were also made of solid concrete and masonry 8 inches thick. In the WTC only a 3 inch thick concrete slab was used and the structural steel was not encased in masonry, instead spray-on fire retardant was used which only last 5-10 minutes in a fire.
They aren’t comparable because they are completely different construction methods. Older engineering principles used to throw raw materials and over-design at any problem. That is certainly durable in many cases but it isn’t efficient. Both the World Trade Centers and the Empire State building worked perfectly fine for their intended purpose but neither was designed to survive the aftermath of a large aircraft impact. The Empire State building gets bonus points in that regard because it was also hit by a B-25 bomber in a freak accident in 1945 and suffered relatively little damage partially because it is so over-engineered but that wasn’t an intentional part of the design.
Modern skyscrapers can be built taller because they use geometry and fairly lightweight materials rather than just throwing a bunch of steel and concrete at the problem. They also have to accommodate complex shapes, multiple elevator shafts and lots of utility conduits. Earthquake resistance is another concern that is easier to address with modern materials and lighter designs.
The towers at WTC were, actually, designed to withstand hits from aircraft (the Empire State Building being a lesson learned). Planes got bigger, though, and 737s are now considered puddle-jumpers. The 767 is much bigger and carries more fuel. The structural integrity of the buildings didn’t fail - these were heat-induced failures. Throw a ton (actually, more than a ton) of fuel at a building full of paper - both of which burn at a steel-damaging temperature, and see what happens.
The towers were built with an innovative concept (at the time); they were core-based buildings, with the floors and exteriors being largely supported by central “shafts.” That’s how they got so tall, so to speak. Once doused in burning awfulness, though, the sprayed-on fire protection (which was knocked off by the collision of planes that didn’t exist during their design) was inadequate.
And then bad things happened.
I was there to clean it up. Very messy, but where the structures failed (also for the other buildings) was clear. The steel failed because of heat. Several pairs of my boots failed, too.
Is there any truth to the urban legend that this is because they didn’t have advanced mathematical modeling, but had to rely on slide rules? Today, we can do computer-simulated stress tests, and so have a more precise knowledge of structural breaking levels.
Which makes sense when we’re talking about trucks and stoves and log cabins. But for something like a skyscraper or bridge, aren’t they designed to last essentially forever? Maybe not until the sun goes red giant, but for hundreds of years? And in that scenario, what does “over-design” even mean?
It means when in doubt (like, because you were using a slide rule and not a server cluster for modelling) use more materials/over design the strength you think you’ll need as a precaution.
IANAE, but a building designed to last hundreds of years is different than a building designed to survive the impact from a passenger jet (coincidentally, I raised this question once in another thread). In the context here, “over-designed” means that a building is built with redundancy over the forces it is expected to encounter. Sometimes civil engineers get their calculations wrong, and the resulting building may need reinforcements as a result – a famous example is the Citigroup Center.
There was nothing wrong with the structural integrity of the building. What wasn’t considered in the design was the possibility of the riser being cut. 20/20 hindsight would have seen the installation of a second riser so there was water for the sprinkler system.
It’s along the lines of the Fukishima nuclear power plant. they put the backup generators on the ground instead of on top of the power plant. The tsunami wiped them out and there was no power to run backup water supplies.
The slide rule versus computer analysis issue is true. Closed form analysis of structures made from lattices of beams only really exists for 2D forms. It is well understood that if you take 2 lattices and connect them together into a 3D structure, the whole is much stronger than twice one lattice. But in order to really work out just how much stronger you need vastly more compute than mere humans are capable of. So bridges, steel frame skyscrapers, tended to be designed essentially as sets of 2D structures that were then interlinked. But where the additional strength from the interlinking of the elements was sufficiently poorly understood that it was not counted towards the design. So, by default, the structures are a lot stronger than the design requirements, but not necessarily in a consistent or efficient manner.
Look at modern bridges. They provide a wonder visual example of how 3D structural analysis allows for vastly more freedom, and designs with a lot less material.
Adding extra steel and concrete to a structure isn’t really a smart answer. You have to hold up all that extra mass, so the entire thing gets much more costly and heavier, simply so it can hold itself up.
Also, at least with earthquakes mass is actually a vulnerability; it means that when the quake starts shoving the building around, its greater mass puts it under greater stress than a lower mass building would be since all that mass has greater inertia once the quake starts it moving.
It’s not an issue of “well built”. A Porsche is not less well built than a Ford Explorer just because that later can crush the former in a crash.
Modern skyscrapers are essentially built as giant glass boxes around a steel frame or reinforced central concrete “core” that holds the elevator shafts, stairwells, cables and ductwork. The floors can either be cantilevered off the main core or supported by columns (hint, if you don’t see massive load-bearing columns oddly space around the perimeter of your office blocking your view, the floors are cantilevered).
Old Art Deco skyscrapers like the Empire State Building, Chrysler Building, and Woolworth Building are made from steel frames supporting a lot of stonework. They are stronger and more dense than modern buildings, but also more expensive to build and don’t offer the massive panoramic views.
The Great Pyramid of Giza is extremely strong, however the interior spaces lack adequate views and lighting and tend to be cramped as most of the structure is dedicated to holding itself up.
All are well designed. It’s just that the modern ones are lighter, more cost effective and designed to narrower tolerances.
And I feel that I should defend the honor of slide rules, here, too. For most purposes, in trained hands, a slide rule is just as good as a modern scientific calculator, and very nearly as fast. The main benefit of the calculator is just that it’s easier to learn to use it. Of course, neither one is as good as a full computer, which can do the massively-iterated calculations needed for three-dimensional stress analysis.
The Brooklyn bridge is something like 130 years old and seems good for another 130 (with proper maintenance). The Tappan Zee bridge and the Champlain Bridge in Montreal are decrepit after 50 years and being replaced. Overdesign? Your call.
The Tappan Zee bridge was opened in 1955. There was essentially no computer analysis available then. It was built as a traditional (slide rule) design, but its current decreptitude could be reasonably sheeted home to the very limited budget that was available for its construction. The Champlain Bridge was designed not long after, with construction starting in 1957, so no modern design either.
Maintenance is usually the key. You must be absolutely on top of the maintenance. However other things can cause you grief. The Silver Bridge disaster is a good example.
Finite element analysis tools - like Nastran were just starting out in the 60’s. We see the effect of this in later designs.
