Cranes continue to move further and higher
A mobile crane has a very important job in the metal construction industry. Unlike a fixed crane, it can be quickly and easily set up, and moved around on a job site under its own power with little or no step up or assembly. A typical mobile crane is composed of a control panel, rubber wheels or tracks, chains, a wire rope and a boom. Mobile cranes can start working immediately with powerful lifting capabilities. They can be cable controlled or hydraulic powered; given power by either electric motors or internal combustion engine. They have a maximum height of about 400 feet and can lift hundreds of thousands of pounds.
The global mobile crane market is one of the fastest growing segments in the material handling equipment market as a result of increase in urban infrastructure and residential construction projects. Analysts at Sandler Research forecast the global mobile crane market to grow at 6.92 percent from 2015 to 2019. This market is driven by many growth factors, one of which is increased investment in development of smart cities.
Rob Riess, Riess Construction, Plymouth, N.H., says a mobile crane’s role on a construction site is to safely place materials in areas that regular equipment such as forklifts cannot reach. Mike Noonan, vice president of marketing at Cascade Manufacturing Co., Cascade, Iowa, says a crane is often the primary device that takes his company’s product (cold-formed steel trusses) from the ground and lifts it to the installation point. Mike Reynolds, president, Systems Contractors Inc., Denver, and current president of Metal Building Contractors & Erectors Association (MBCEA), says cranes are used to set steel and load roof materials; and often for installing insulated panels on the walls.
“With regards to roof truss installation, a crane’s role is to lift the trusses off the ground or directly from delivery truck to the plate line,” says David C. Dunbar, PE, national sales manager at Alpine TrusSteel, Orlando, Fla. “Depending upon numerous variables such as the size/weight of the trusses, job-site conditions, installation methodology or crane capacity, the trusses might be lifted individually, in banded bundles or entire sections of the roof that have been pre-assembled on the ground (rafting).” Building height, along with the size/weight of the trusses generally will dictate if a crane will be utilized.
“Buildings that are one story with corridor walls whereby the trusses are designed as ‘half spans’ that are joined at the ridge over the corridor walls may be hand lifted to the bearings,” Dunbar says.
Reynolds agrees that building size matters, believing that smaller construction jobs don’t always warrant the use of cranes. “[Building materials] can be set using extended boom forklifts,” Reynolds adds. “Even some of the big box projects that are not that tall will be set with a forklift.”
From his experience working in metal construction, Riess also agrees that while cranes are not always the best machine for every situation, but when used in the correct situation, cranes are the only way to go. “[When] unloading trucks it is faster and safer to unload with a forklift,” he says. “But to fly rafters and then fill in purlins, a crane and good operator is hands down faster.”
Multiples and Mistakes
Often construction sites have multiple cranes. Depending on the size of a building, multiple cranes may reduce the time to move and remobilize one crane. Noonan advises weighing that versus the hourly crane charge.
“Most of the time multiple cranes on a site are for doing tandem picks when you maculate the steel,” Reynolds says. “This requires a critical lift plan. The reason you use two cranes is to help stabilize and control the load. The only time is if the project is large enough for a second rig and hanging crew.”
When multiple cranes are on a site, the site must permit safe working space for more than one crane and care must be taken to coordinate the swing radius of multiple cranes. Cranes’ swing radiuses must not cross. “It’s highly recommended that the operators maintain radio communication in addition to visual contact so as to prevent collisions,” says Marty Hortman, general manager, ASI Adventure Services Inc., Oakwood, Ga. Riess says remote cameras mounted on crane booms increase visibility.
Coordinated crane communication requires a rigger with signal training. “Many times the general contractor or municipality will require riggers to be certified,” Reynolds says. “Operators need to be [National Commission for the Certification of Crane Operators
(NCCCO)] or state certified. For the most part, hand signals are used unless there are blind spots or sight restrictions. The best radios have the clip on microphones. Usually your connectors are communicating with the crane and this helps to free up their hands.”
Trying to use a crane under its capacity for the job requirements is another crane error. “Not getting the right-size crane for the job is the most common error,” Reynolds says. “Check the charts and calculate the weights then review the radiuses.”
Noonan sees a common crane error to be the lack of pick points along the length of coldformed steel trusses. “If there are not enough pick points, excessive bending may occur, along with the potential for structural integrity of the truss to be compromised,” he says. Dunbar stresses the importance of understanding the lateral stability of the product being lifted by a crane cautioning not all products perform the same.
Reading job-site handling recommendations can prevent many crane operation errors before they occur. Noonan says there are pictures and text to make it easy to understand. He stresses, “It is important that the instructions for installation are reviewed by all parties in the installation process in order to have a safe and successful installation.” Dunbar agrees, saying, “Up-front communication will first and foremost avoid most errors that tend to occur on the job site; [having a] pre-construction meeting, reviewing the framing layout and developing a plan on the best approach for the building being constructed.”
The Structural Building Components Association publishes industry standard documents to address lift points, spreader bar sizes, etc., when lifting cold-formed steel trusses. Dunbar says this information is extremely important and needs to be in the hands of the erection contractor to ensure proper crane techniques are used.
“The biggest thing with cranes is to do your homework,” advises Reynolds. “Calculate your loads, your rigging, crane set points. This should all be done in your site-specific plan. Following the criterion required for a site-specific plan laid out in AC478 [a MBCEA standard] will be the key to a safe and successful job.”
Sidebar: Cranes and Hand Signals
Hand signals provided by a certified signalman to the crane operator are practical when good visual
contact can be maintained by both the signalman and the crane operator. The signalman must also
maintain good visual contact with the placement location of the load being flown. Many times maintaining this visual contact is not possible due to conditions such as height or shape of the building the loads are being placed on. Then, radio communication between the signalman and operator is best and safest. I prefer radio signaling as the operator; I am not affected by distance from signalman, sunlight or lack thereof. The signalman can maintain better visual contact with the load as he is not required to maintain eye contact with the operator and the load. If the signalman is utilizing a headphone device, both of his hands are free also.
Marty Hortman, general manager, ASI Adventure Services Inc., Oakwood, Ga.
Sidebar: Tower Cranes
Tower cranes lift and position loads to the designated position. These modular systems can easily follow the building up to nearly every height. The load chart and the loads to be lifted have to be taken in consideration to choose the right tower crane. On high-rise buildings the weight of the hoist rope has to be considered and may reduce the capacity of the crane. Also, the hoist winch has to be designed to take the rope for the necessary rope length.
Liebherr tower cranes are equipped with Micromove to enable very fine positioning of the load and
reduce swinging of the load to “0”. Therefore, safe work is granted and damages to the metal parts to be placed are avoided. Tower cranes have a very small footprint, can be placed very close to the building, or even inside the building. On high-rise buildings of more than 80 meters (approximately 263 feet) height, the crane may be tied on to the building, these forces have to be taken by the metal structure.
Thorsten Hesselbein, director, tower crane solutions, Liebherr-Werk Biberach GmbH, Biberach, Germany.