Didn't any structural engineers ever foresee the possible problems and risks that these structures presented?
What risk was that? These buildings took a direct hit from airliners weighing 450,000 pounds, going 500 miles per hour. At impact, this exerted a force of 632 million ft-lbs at the 70th floor. Neither building snapped at the bottom or even swayed.
What brought the buildings down was fire, primarily caused by the jet fuel. The melting point of steel is about 1600 degrees C. The flame temperature of kerosene (jet fuel) is 1700 degrees C. No steel structure of any kind could have withstood a fire fueled by 24,000 gallons of jet fuel. That they stood as long as they did, giving time for thousands of people to escape, tells us that these buildings were strong indeed. When they failed, they failed exactly the way they were supposed to... straight down. Can you imagine the death toll if buildings that size had fallen over sideways, crashing into other buildings on the way down?
The architect and the structural engineers deserve a round of applause, not approbation.
Didn't any structural engineers ever foresee the possible problems and risks that these structures presented?
Had the planes crashed into buildings of more 'conventional' construction, large parts of the buildings would have collapsed within seconds. The remainder would have collapsed within minutes.
Although buildings in this country are constructed with reasonable engineering margins, in many structures the loss of even one structural element can cause severe damage. One big reason for this is that the strength of a beam required to bridge a span is proportional to the square of the span's length. If the span doubles in length, the beam must be four times as strong. If you have a row of pillars at 20' spacing, loss of even a single pillar will mean that two 20' spans have been replaced by a 40' span. Unless the beams had a four-fold engineering margin, they will not hold.
Actually, the situation is even worse than that, since the failure of the bottom of a pillar will mean that the remainder of the column, which the beams used not to support at all, will now be supported in the center of the beams, which is their worst-case spot. Not good at all.
As though that weren't bad enough, many buildings use either reinforced-concrete or steel-truss structures which are designed only to be supported by columns, and not to support them. If a column fails, these structures will have almost zero strength to support their own weight, much less that of anything above them.
Really, that the WTC was able to survive the impacts structurally was quite remarkable. While they were not able to withstand 20,000lb of jet fuel, I doubt that any 300+-foot-tall structure built in the last 1000 years could do any better.