We’ve all seen them, popping up like tall birds above city skylines, gently lifting steel, cement and glass. These are just some of the materials needed to construct a high rise building. Swaying left and right, their hooks bob up and down into construction zones far below; all skillfully done by an operator and ground crew with the finesse of a fly fisherman casting his fly into the river.
As I watched these tower cranes in motion, I saw each one has very different distinctive visual features. Each one performs a given task better then the other. Together they resemble a well choreographed dance troop sharing must-have common attributes.
- Able to lift a tremendous amount of weigh.
- Place loads at outer site boundaries.
- Each with its own distinctive strengths.
- Takes up a small footprint on the construction site.
The following illustration represents the basic structural assemblies of most tower cranes.
Below ground level, a massive 400,000 pound poured concrete block measuring 30 feet x 30 feet x 4 feet typically provides the crane with a firm stable grounding. From here vertical sections, (masts), are added to give the crane the height it requires.
The turntable, (gears and motor), are at the top of the mast providing the crane with rotational ability. The operator’s cab, the counter balance jib, and the main working jib, are above the turntable. From the cab the operator is able to see the worksite below, positions the main jib and sends the motorized trolley over the payload then raises or lowers the load block, (hook), to move materials into place. The counter balance jib and its counterweights section also include the hoist drum, hoist cable and motors. Electronic controls enable smooth and safe operation of the tower crane.
So what can the average tower crane do?
- Maximum unsupported height of 265 feet.
- Can achieve much greater height if tied into the building.
- Can lift 39,690 pounds when load is closer to the mast.
- Lifts less weight when load is at the end of the main jib.
- Can reach out to 230 feet.
Three types of fixed tower cranes
Flat top tower crane.
- No A-frame structure at the top.
- Jibs have greater size and integral strength.
- Easier to transport and assemble.
- Allows multiple cranes to move over each other at similar heights.
- Preferred near restricted air space, (airports}
Hammerhead tower crane.
- A-frame structure performs comparatively better over a flat top crane.
- Greater lifting capacity.
- Precise handling of materials.
Luffing jib tower crane.
- Ideal on congested sites and near existing high rise buildings.
- Greatest lifting capacity.
- Smaller turning radius.
- No trolley, load block, (hook), is at the end of the main jib.
- Main jib can be set from near vertical to near horizontal.
Have a closer look at this picture. You can see a person inside the vertical section of the crane. He’s climbing a ladder up to his workplace, the cab section. That’s over 220 feet on this particular crane; 220 feet up and 220 feet down. He’s wearing a large backpack. Something tells me he’s trying to minimize his daily up and down trips from his lofty nest.
This picture shows an impressive surprise feature. As casual observers we don’t realize, much less ever see, this happen. Notice the red coloured steel band. That’s where the crane is attached to the building for stability. Now look up a little further to the section of the crane that bulges out more than the other vertical sections. The crane can add more vertical sections to increase its height as the building height increases without aid from another crane. Watch this video.
Industry reports indicate that the greater Toronto area will easily remain the number one tower crane capital in North America in the coming years. With ever increasing building heights these cranes are the way to the top. The 95 story Sky Tower in Toronto will be the tallest high rise in Canada at more than 1000 feet. Comparatively speaking, it’ll slide just under the CN Tower’s terrace level.