In the case of 9/11 there's a lot of unanswered questions in regards to how a modern steel framed building could have collapsed symmetrically and at free fall speed, with all the steel columns supporting the building turning into cheese. — Posty McPostface
WTC 7 was, I assume, a conventional steel frame building. WTC 1 and 2, the two big ones, were not. The big buildings weren't supported by interior steel or concrete columns. The floors were supported by the exterior steel walls of the building. The concrete floors were attached to the exterior, and within the floor structure were 'ties' that connected opposing exterior walls to add rigidity to the building.
The fires on and above the floors struck by the planes weakened the ties. The weight of the building floors in and above the fire, caused the exterior supporting structure to bulge, and begin to break away from the ties and the concrete floor--not instantly, but fairly quickly. When the weight exceed the support capacity of the bulging exterior walls, the x number of floors descended straight down (why would they not?) and overwhelmed the support capacity on the next floor, and the next floor... all the way to the bottom.
The 911 attackers did not need to plan this out--and as far as I know, they didn't expect the towers to collapse. They wanted to severely damage the WTC towers, and they would have totally succeeded if the building hadn't collapsed, and the fires had been put out. If they did plan pancaking, then they were that much more ingenious.
You are suspicious of steel giving way. Steel, as it happens, isn't heat resistant the way concrete is, or wood for that matter. The ties had been coated, but not heavily, and the shock of the crash caused much of the coating to fall off. Bear in mind, the ties were not heave H shaped supporting beams--they were rods, very strong, but not big enough to resist any force.
You are probably correct that a building constructed with the usual forest of steel columns would not have collapsed symmetrically, under the conditions of WTC 1 and 2.
WTC 7 was damaged in two ways: first by heavy debris from the collapsing towers, then by fire. The building did not weaken symmetrically, it was sagging and bulging before it collapsed. I haven't read or heard that much about WTC 7, so I can't say more about it.
911 was the second WTC attack. In 1993, if you remember, a truck bomb loaded with urea-nitrate explosives enhanced with hydrogen gas was detonated in the garage under the North Tower (can't remember if that was WTC 1 or 2--doesn't matter). The intention was to tip the North Tower over into the South Tower and cause both of them to hit the ground, killing a whole lot of people.
The truck bomb didn't happen to be just quite properly located to achieve the intended result. Had it been located in just the right place, the plan would probably have worked.
WTC 7 had a similar vulnerability. Certain support beams were critical in maintaining whole-building stability. Why would this be the case? Because of remodeling, design changes, and the required engineering changes needed to achieve stability. As it happens, the vulnerable support structures were damaged, contributing to the collapse.
From a conspiracy theorist POV, all this is probably unsatisfying because it leaves blame too diffused. Engineering decisions, lack of water pressure, electrical failures (no pumps), not enough insulation on rods, and so on and so forth.
Climate change conspiracy theorists think that global warming is a hoax for, I suspect, similar reasons. Blame is so diffused that there is no guilty party to pin it on. 300 years of coal and oil burning leaves a lot of guilty parties -- like, just about everyone. So, they reason, it must be a hoax.
You might be interested in a New Yorker Magazine story about the City Bank Building in NYC -- the one with the steeply canted roof and asymmetric supports at street level. The architect of the building had been thinking about the building, one fine day, and was thinking about what kind of fasteners were supposed to have been used to attach X-shaped structural members together. They were supposed to have been welded. When he checked it out, he found -- much to his horror, that they had not been welded, the bolts had been screwed into place. Fixing this wouldn't be easy, because the building was finished and occupied. He didn't think the building as it stood would be able to withstand an unlikely--but quite possible--hurricane force storm.
He alerted the building owners immediately, and crews began the process of opening up the walls over the X structures, one by one, floor by floor, and welding the many many bolts--this all being done at night so as not to disrupt bank operations. Then the walls had to be closed and refinished. God, think of the overtime!