This post is going to be long, but I want to add detail so that you can visualize what is going on a bit better.
Thanks. You've answered all my questions very well.
Footings: You're correct. The upward forces from wind won't be a problem in high rise. Downward vertical forces shan't be a problem so long as the footings and piers are engineered properly.
No worries. And yes, it'll take a good engineer to do it properly, because the forces on the building will change as every floor is added - the bearing pressure, the lateral loads, and the live loads (the last one is important when you're talking heavy machinery on a roof. If a crane falls through a concrete floor and kills someone, you have big problems. Plus I probably wouldn't be able to live with myself if I did that to someone)
Shear walls: I'm a little lost here. Are you saying that you would be ale to convert outer walls to shear walls during initial construction or further down the track with your extensions? This makes sense if you're talking about extensions, and I think it's a really neat idea. I imagine you'd need to leave some rebar sticking out of the slabs to allow it in the future though? Or would it not really matter?
Think of all walls constructed out of blocks to be hollow, and weak. Since they are, really - building blocks are made of low grade concrete.
The strength of a wall comes from the steel and concrete that you put inside the hollow blocks.
When your engineer designs a building, they choose a few essential walls and say "right, we are going to reenforce these few walls - walls 1 and 2 are load bearing, so we are going to fill every 3rd hollow core with steel/concrete, and walls 3, 4, 5 are shear walls, so we are going to fill every single hollow core up with steel/concrete".
Traditionally, shear walls are situated somewhere in the middle of the building. This is mainly because it is easier to tie the steel in those walls in with the concrete slab in many directions. Walls on the outside of the building, on the other hand, can only have their steel tied in with the slab in one direction, since they only connect with the slab in one direction (inwards).
In the case of vertical expansion, the engineer will say "we need more shear walls to stop the wind from twisting the building apart" and so right from the beginning will say "fill walls 3,4,5 with steel/concrete like usual, but also fill up walls 7,8,9 as well, because we need them to be strong enough to tie in with walls on floors 10 and up, when the wind loads start picking up".
The reason for the confusion in this thread is that in the case of steel truss buildings, like the world trade center, all of the strength came from the central steel core. There was no such thing as adding more support on the periphery, they way I am proposing.
Oh and yes, there will be steel sticking up out of the slab on the roof. It isn't a problem though - you just bend them inwards so that they can't be seen from the street. When it is time to continue building, you simply bend them straight again, and keep building. They only stick up about 3 feet anyway, so it's not a big deal.
Technically, if you didn't want to do that, you could use different methods - you could drill holes in the slab from the top about 2 feet deep and then glue the steel in with a 2 part industrial epoxy, or you could weld steel plates to the rebar on the top floor that are dead flat/flush with the roof concrete. Then, when you wanted to continue building, you weld rebar to the plates and keep going.
Personally I think bending them is the best, because you are 100% confident that you have a solid piece of steel penetrating all the way into the wall all the way through the slab, connected to more steel all the way to the foundation. I wouldn't want to trust welds when you're talking about many tons of wind rocking the building back and forth 24/7 for years.
Elevator capping: that's the term I was looking for. I was indeed thinking about in-situ slab. I was hoping you'd expand and mention another way to do it because I didn't know of any. Now I do. You're right, that'd work a treat then because all you need to replace each time is supporting steel. Winning.
There are 2 different ways to build elevator shafts (generally). The first is pre cast concrete, where they cast slabs of concrete at ground level, raise them up with a crane, and weld them to each other to form the column.
The second is to use regular cinder blocks reinforced with steel.
Personally I prefer the second method, because you don't need heavy machinery to move things around, and are free to alter it at will to whatever specification you need.
So what youd do is build the elevator well, and then right at the top, put steel rebar connected to metal plates within the hollow cores, and fill the cores up with concrete. Then, you sit the pre cast cap on top, and weld it into place. That way, even if your welds are crap, the cap is sitting on top of concrete filled blocks and so has rock solid mechanical support.
Then to extend, youd chop off the welds, remove the cap, put it aside, attach more steel to the top of the newly opened elevator well via whatever method your engineer prefers (drill/epoxy, or welding, or both) then start laying more blocks.
When the hollow blocks are laid to height, you insert more rebar with steel plates like before, fill with concrete, lift the cap into it's new resting place, weld, and finish installing the elevator on the inside. How you do that depends on the elevator - some use the cap as support, others don't and use side mounted rails instead.
Scaffolding: I think the biggest advantage by far here is the access for window cleaning, etc. This extended eaves sort of thing is evident in a lot of high rise residential buildings. I can't imagine too many commercial clients wanting this all around the building though. Perhaps you can start the trend?
Building owners want whatever makes the most money.
The problem with my method is that it takes up valuable land area. So if you're building in Manhattan, the 4 feet of eave overhang is worth much more than scaffold hire or hiring a guy to clean the windows from a tiny platform.
Also, the shape of the land might make my method a bit too wasteful if the block of land is, say, really skinny, or a horrible sharp triangular shape.
Otherwise, my method is better, since in my mind, you can make up any lost space by adding an extra floor, plus benefit from ongoing development, cheaper ongoing costs etc.
The thing is though, that in the building industry, NO ONE wants to be the first to try something new, because it is risky. Once someone does something that works, everyone piles on, and it becomes the standard method of doing things.
So who knows, you might be right - it could be a trend. One day I might be telling my grand kids that I invented the building method, and no doubt the little bastards will say "grandpa has gone crazy, he thinks he invented how to build buildings, someone lock him up in a nursing home" lol
Cranes: this is exactly what I expected your answer to be. It would be mine also. I was initially going to suggest it, but I saw a flaw so I chose not to... The flaw in my logic was "how the hell would I get 2 smaller cranes up there without a tower crane?" And the inevitable "how do I get them down?" I don't imagine you can get ones with parts small enough to fit in a lift, so how do you think it would be done?
There are different types of cranes.
What you would do is figure out when it was economically beneficial to use the 2 crane method.
For the first few floors, theres no point to having 2 cranes, since hiring mobile truck mounted cranes is cheaper and easier.
Once you hit a height where that becomes too expensive, you hire a mobile truck mounted crane to lift both of the little cranes up to the top of the building.
Some of the truck mounted cranes are huge, but you would have to be careful about site access, and how far exactly on the crane arm you can attach something as heavy as another crane (or the pieces of one).
Personally I'm starting to think that it would be best to design some sort of hoist that bolts into the concrete slab, and use that instead. I'll have to run the numbers and chat to my engineer.
I saw an awesome idea on an auction site the other day - it was a boom arm that bolted into concrete, that swiveled - so youd set it up on the roof, lift your material up, and then once it cleared the roof line, youd swivel the material onto the roof.
It was tiny too - 2 men could lift it, and it'd take an hour or 2 to set up, tops.
The only really big stuff you have to lift are sheets of concrete mesh, but I'm thinking if you just bought it in a big roll that was heavy but didn't take up a lot of space, you could lift it up somehow without all of this double crane stuff...gotta think.
This has been a good way to learn about this stuff further, so I thank you for your insight.
No worries, I'm always curious to see how this stuff is done/received in other countries
Agent-based synchronicity is where it's at.
my apologies.