Lately we've been looking at how construction sites for large buildings start off, at the shoring and excavation stages, for example. Today we're going to jump to the next major piece of equipment installed at a site to assist with forming work from the foundations up — a tower crane. The cranes play an essential role in building construction; their reach and ability to hoist large loads allows a huge variety of building materials to be placed wherever they are needed to build a structure. In today's Explainer, we explore the installation and the anatomy of a typical tower crane.

A deeper pit is dug for a concrete slab to hold the crane base, image by UrbanToronto Forum contributor AHK

Above, at the bottom of an excavation pit in Toronto, rain has turned a deeper area of the pit into a pond, while steel rebar waits off to the side for it to be used to create a lattice of cages that will give great strength to the concrete that is poured around it. Below, three days later with the water drained, the rebar cages have been built; horizontal pieces will hold the whole slab together, while vertical pieces will tie future columns and walls into it. In one corner, the base for a crane has been affixed.

Rebar is placed to create cages around which the concrete will be poured, image by UrbanToronto Forum contributor Red Mars

Below, another four days later, the concrete has been poured and the slab is curing. vertical pieces to tie in the future columns and walls can be better made out, and the crane base sits right atop the slab.

The concrete slab, within a couple days of the concrete being poured, image by UrbanToronto Forum contributor AHK

Another week and a half later, and the slab is still curing, waiting a little longer before the crane's mast will start to be attached. Depending on the thickness of the slab, concrete cures within a day or two to the point where it can be walked on, within a week to the point where you can move heavy equipment over it, and within about four weeks to 99% of its ultimate strength. After that, concrete will continue to slowly cure over the course of its life.

The concrete slab continues to cure, image by UrbanToronto Forum contributor bilked

While the project above, 400 King West, awaits for its crane to be installed, we're jumping way-ahead, photo-wise, to images taken of cranes atop buildings fairly far along in the forming process to look at the parts of a crane.

After the concrete slab to which the base of the crane is affixed is ready, the mast of the crane can be attached. The mast is normally made up of many sections bolted together, and may arrive at the construction on the back of a flatbed truck in more than one section. With a mobile crane at the ready at another Toronto construction site, a mobile crane is seen after having hoisted many parts into place. Crew members wait to fasten together whatever is hoisted their way.

The arm of a mobile crane, between hoisting parts of the crane, image by UrbanToronto Forum contrubutor Rascacielo

Once the crane is set up, its work can being. As time passes and the building begins to rise, the mast can be lengthened — the crane made taller — as needed. 

Now, let's look at the names of the various parts.

The parts of a tower crane, image by Marcus Mitanis

At the top of the mast is the slewing unit, which enables the crane to rotate. A long horizontal section called the jib or boom then extends perpendicular to the mast. The jib carries the weight of whatever is being hoisted. Some cranes are equipped with a luffing jib, which is able to move up and down. A fixed jib contains a trolley below which allows goods suspended from the hook to move horizontally.

The parts of a tower crane, image by Marcus Mitanis

On the opposite end of the horizontal section, the counterjib carries the counterweight. Usually composed of concrete blocks, the counterweight provides balance to the crane by offsetting the load of the jib. The crane operator usually sits in a cab situated at the intersection of the jib and mast. 

The crane's operator cab, image by Marcus Mitanis

Have any other construction and development related terms that you would like to see featured on Explainer? Share your thoughts and questions in the comments section below!

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From 2015 to 2017, UrbanToronto and its sister publication, SkyriseCities, ran an occasional series of articles under the heading Explainer. Each one took a concept from Urban Planning, Architecture, Construction, or other topics that often wind up in our publications, and presented an in depth look at it. It's time to revisit (and update where necessary) those articles for readers who are unfamiliar with them. While you may already know what some of these terms mean, others may be new to you. We are publishing or updating and republishing Explainer on a weekly basis. This article is an update of one that first appeared in 2016.

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Do you have other planning terms that you would like to see featured on Explainer? Share your comments and questions in the comments section below!

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UrbanToronto has a research service, UrbanToronto Pro, that provides comprehensive data on construction projects in the Greater Toronto Area—from proposal through to completion. We also offer Instant Reports, downloadable snapshots based on location, and a daily subscription newsletter, New Development Insider, that tracks projects from initial application.