You mean the Tower Cranes?? They get a bloody huge lump of concrete in the ground to bolt too. Use "Floor based cranes" to erect some of the tower then use a bigger "Floor based crane" to lift the top crane part on. After that they erect themselves. What happens is the top section jacks itself up, then lifts up another frame up on its own. A monorail gets hold of it and you slide it under the top. Then the top bit jacks back down and you bolt it back up. So they climb the building on there own. As you go up most tie into the building for extra support. The really tall ones like in dubai whave there own smoko room, kettle oven etc in the cab so you dont have to come down. If that fails theres always the Heli option