Selecting a tower crane

To select the proper tower crane for any given project the following aspects should be considered:

1. Weight and radius of the heaviest lift(s) and the farthest pick(s).

2. The boom/jib length and tail swing of the crane.

a. Consider how jib length and tail swing relate to fixed obstructions on or near the jobsite such as high power lines, buildings, bridges or future obstructions such as cranes or equipment to be used or erected on the project.

b. Check for underground obstructions such as septic tanks, underground power and gas lines and soil conditions before determining the type or size of foundation.

3. The amount of tower or mast required. It may be possible to go above or below obstructions. Normally a luffer requires less tower but hammerheads have greater freestanding heights and greater distances between jumps.

4. Is a hammerhead or luffer best for the project? Generally speaking, hammerheads are less expensive. (Note that the standard hammerhead uses a mast and pendants, however, most manufacturers offer small to medium, or large capacity flat-top hammerheads which can reduce swing clearance between tower cranes used on the project by as much as 6m).

Luffers are used when surrounding buildings or obstructions prevent the use of hammerheads. Also luffers can eliminate problems in areas such as California where airspace violations can be legally enforced.

5. Power and hookline speeds:

a. High gear speeds: used for light loads and quick return to the ground speed.

b. The hoist speed of a common lift, i.e., the weight of a typical load such as the weight of a full concrete bucket, and flying forms specified for a concrete building, or the weights of the standard columns and beams used on a steel building. High line speeds are especially significant as buildings grow to super heights and totally insignificant when used on buildings of five floors or less.

c. Be sure the hoist has adequate power to pick the heaviest load at maximum building height when all the layers are on the hoist drum. On any tall structure calculate and add the weight of the load line to the weight of the load on the ground.

Note that, although labour and time intensive, the flying forms used on a concrete structure can be split and the concrete can be pumped, or the size of the concrete bucket used by the crane can be reduced. And on a steel project, the columns and beams used can be split and welded and/or bolted together on the jobsite. This works in areas where labour costs are very low and critical time lines are not a factor, but some fast track contractors like to 'go big and go fast'.

d. Do the heaviest picks require a four part line? Manufacturers may offer the following load line variations in, for example, the 400tm size hammerhead 1) Two-part, 10t capacity (fast and has greater capacity at the tip); 2) Two-part (10t capacity) - four-part (20t capacity) block and trolley. This option has the advantage of quickly changing from two to four parts as well as the 20t (44,000lb) four part line. Evaluate the spooling capacity of the drum and also note that switching from four parts back to two leaves a lot of loosely wrapped load line on the drum for light picks, fertile ground for damaging the load line. 3) 16t medium capacity two part load line.

6. Hook utilisation versus time and labour. Often it is best to add another crane or a larger crane and better utilise the labour force.

7. Noise and environmental restrictions. Most applications tolerate both diesel and electrically powered machines. In an enclosed environment, however, electric power is best.

8. Cost considerations:

a. 1) Transportation to and from the project. 2) Foundations are generally provided by the contractor. The manufacturer supplies the forces that will be going into the foundation. 3) Erection. Normally erection requires at least one operator, one technician (evaluate using one on the ground and one in the air), and an ironworker rigging crew. 4) Size and availability of the crane required for erection and dismantling. 5) Jumps and tie-ins if applicable. 6) Monthly rental. 7) Maintenance (usually 10% of the monthly rental). Also, what services does the maintenance include? 8) Charges for dismantling.

b. Most tower crane providers require full rental of the total tower needed for the project. (i.e. if 360m is required to complete the project, you are charged for all the tower from the beginning). Costs such as freight to a nearby storage facility, offloading, storage, rehandling and freight to the jobsite must also be considered.

c. All the above with respect to time. Consider the time value of money.

9. The window of operation. Unlike mobile cranes they are not moved in or out quickly or inexpensively and the monthly rate is primarily based on the number of months the machine will be in the air. Generally speaking, the monthly rate includes 200 hours/month; prorate overtime; add 50% for a second shift.

10. Safety regulations. Most areas require an erection/ dismantle plan and strict safety certification. Does the machine meet all applicable standards (such as OSHA, ANSI and DIN)? Consider the manufacturer, the year of manufacture and the date of remanufacture. Keep in mind that it is always easier to prepare the machine and work through any potential problems before it goes in the air.