which comes first, the crane or...?

Could someone tell me how construction cranes are constructed? I’ve never actually seen one go up, and barring helicopter support [or a second crane], I’m baffled. It seems to me that prefabricated sections would be the only time-efficient way to do it, but how to haul all that up there without a crane has me stumped. Or are they built strut by strut, the way scaffolding is?

You might get an answer if you re-post this in the General Questions section.

I know the answer to this, but I’m not sure I can explain it without a diagram. Bear with me here.

The mast that the crane sits on is indeed made of prefabricated sections; my poor attempt at ascii drawing follows:

|/| <-section of mast.

The crane sits on a special unit that in turn goes on top of the mast. More bad drawing:

------------------------ <-crane
| | | <- adaptor unit
| | |
| <- mast
The unit at the top has one side open, so that as the crane needs to go up, more sections of mast can be fitted at the top, while the mounting unit “holds on” to the mast further down. Imagine buiding a ladder as you go by adding rungs at hand level while you’re standing on the rungs you’ve added previously further down.

I hope this helps.

Sounds good to me, but I guess I still wonder how the sections are hoisted into place and how the crane part is cantilevered out safely. I suppose I’m bad at visualizing this and what I really want is not a text-oriented forum such as this but a diagram with many small words and lots of fig.s, so to speak. Thanks to everyone who tried to get this through my head!

If you’re talking about a free-standing self-propelled crane, the boom is assembled in sections on the ground (using the one section that remains permanently attached to the power unit to lift them off the truck) and is then attached to and lifted into position by the newly completed crane.

This method of course limits the length and weight of the boom to that which can be safely hoisted from a 180-degree angle pointing away from the power car, and huge counterweights are often added to the back of the crane to balance a heavier boom during this initial, crucial operation. (You’ll note, of course, that at this point the boom exerts the most leverage against the mass of the machine).

Other permanent or semi-permanent cranes are built using free standing cranes. In the case of a multi-storey crane used in tall building construction, other smaller cranes mounted to the structure being built are used to hoist addition boomage (is that a word?) into place.

That’s it! I get it. Thanks, Nickrz. It was the boom placement that seemed so difficult to me when I first posted this query. And thanks to the TMs who’ve been replying.