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History of Cranes Used in Construction
Prior to World War II, most cranes were steam-powered, locomotive-type or guy derricks. The operating engineer, besides having to operate the crane, had to handle pressure boilers, replace tubes in the boiler, and, in general, look after the complete hoisting and equipment repair.
After World War II, the steam engine was made obsolete by the introduction of gas and diesel engines, but the cranes were operated in pretty much the same manner, except their mobility was improved considerably. During the mid sixties, the hydraulic crane appeared on the scene. Both hydraulic and conventional cranes were now being manufactured with longer booms and jibs, and, although this was an advantage, lifting and placing heavy loads required more experience and technical skills.
Cranes come in many structural types, such as mobile, truck, tower, derrick, overhead, gantry, hoist, and crawler. This report addresses, primarily, the mobile-type of crane used in construction projects. The report also provides basic information on the different types and configurations of mobile cranes, and discusses safety considerations, operations, inspections, maintenance, and electrical safety of these cranes.
Basic Crane Types and Configurations
Mobile cranes are very versatile rigs. The evolution of the mobile crane has lead to many types and designs that satisfy both the general, as well as the specific, needs of construction. The basic operational characteristics of all mobile cranes are essentially the same. They include:
Unlike other mobile cranes, these cranes are mounted on carriers not designed solely for crane service. They are mounted on a commercial truck chassis that has been specially strengthened to accept the crane; they are, however, a type of mobile crane with respectable capacity and boom length. Included in this basic type of crane are two common configurations:
Knuckle Boom. The boom articulates (folds) under hydraulic pressure and may or may not be equipped with a powered drum and wire rope.
Telescoping Boom. Boom sections are usually telescoped, either manually or hydraulically.
These cranes are primarily intended for operation in industrial locations where working surfaces are significantly better than those found on most construction sites. Their characteristics are basically identical to those of telescopic boom mobiles, either the rotating or fixed type. Basic types include:
Carrier/Crawler Mounted Lattice Boom Cranes. This "truck type" carrier should not be confused with the ordinary commercial truck chassis. It is specially designed for crane service and the weight of heavy loads that these cranes are required to withstand. Carrier-mounted cranes are also commonly referred to as "truck cranes," "conventional cranes," "friction cranes," and "mobile cranes." Crawler-mounted cranes are identical to carrier-mounted types except for their base and method of load ratings.
Carrier/Crawler-Mounted Telescopic Boom Cranes. Carrier-mounted machines are mounted on specially designed carriers. They can be equipped with a variety of jibs and boom extensions which can be stowed on or under the heel section of the main boom. The upper works of the crawler-mounted cranes are identical to the carrier-mounted telescopic boom units. Their bases and the method used for load rating are different.
Rough Terrain Cranes
The rough terrain crane's oversized tires facilitate movement across the rough terrain of construction sites and other areas. Their short wheel base and crab-steering improve maneuverability. In "pick and carry" operations on rough terrain, however, they are still subject to the same operating restrictions that apply to other cranes.
Like carrier-mounted telescopic boom cranes, rough terrain units are available with either full-powered booms or pinned booms and the same types of jibs and boom extensions. There are two basic configurations: fixed-cab type and rotating-cab type. Either type also allows for boom extensions to be stored on or under the heel section of the main boom.
Mobile Tower Cranes
These cranes are modified versions of the carrier-mounted lattice boom crane. Some manufacturers offer optional tower attachments to create a mobile tower crane. The base can be either crawler- or carrier-mounted.
Heavy Lift Mobile Cranes
These cranes combine the best features of derricks and lattice boom mobile cranes. Typically, they use very large extended counterweights, masts, and often roller rings that move the boom's fulcrum and the crane's tipping axis further away from the center of gravity.
Booms. Daily inspection of all boom components is vital. Cracked welds and bent or misaligned members weaken the boom and lead to failure. If booms have been bent or are cracked, the manufacturers should be consulted. Any repairs should be done in accordance with their recommendations or if required, under their supervision. No welding or cutting should be attempted on booms because many are made of special alloys. Most booms are balanced by the manufacturer, and excess welding may do more harm than good. Booms should be painted at regular intervals to prevent rusting and pitting.
Wire rope. The Occupational Safety and Health Administration (OSHA) regulations require a thorough inspection of all ropes at least once a month and a full written, dated, and signed report of rope conditions kept on file where it is readily available. Bright spots on ropes indicate the extent of exterior wear, and breaks will show up in these areas first. Close inspection should be made to determine the number of broken strands per lay, reduction of the diameter, and corrosion, in order to decide on further use or replacement. Operators should avoid overspinning the hoisting drum when lowering loads. A cable that free wheels from the drum will be misaligned and crushed when the load is taken up.
Clutches and brakes. The operator and oiler should make daily inspections of all clutches and brakes for thin linings, loose linkage, and lubrication. Replacing the worn linings will prevent scoring of the brake drum surface. For the same reason, foreign matter on linings and drums should be removed. A spare set of linings will reduce "downtime" when re-lining is required. Experienced operators will detect the need for adjustments from how the crane responds during operations. Over-lubrication should be avoided. Friction components should be free of grease and oil for proper performance. Grease and oil should be removed by washing with an approved solvent. If the over-lubrication is severe, the linings should be replaced. Overhanging and leaking seals or other sources of grease and oil should be immediately repaired or replaced.
Swing rollers. The path and rollers must be free of dirt and mud. Light lubrication is required on the path. Inspection and replacement of worn swing pins, flattened rollers, and cracked or misaligned roller brackets are necessary. The bull wheel and idler should be lubricated daily. All rollers should receive the same service to prevent excessive wear. All shaft bolts should be kept tight to prevent fouling of the treads.
Hooks and attachments. Magnetic particle or other suitable crack-detecting inspection should be required on crane hooks and attachments at least once each year. Defective hooks should be replaced. Any type of repair to hooks is strictly prohibited.
Inspection and Maintenance
Proper inspection and maintenance provide the means for detecting potential hazards and are necessary for the safe, reliable, and uninterrupted operation of cranes. Preventive maintenance begins with the operator. The high-production operator who mishandles the equipment for production's sake will ultimately cost the contractor more in maintenance than what was produced. Overloading, stretching, or unauthorized modifications are hazardous and costly.
The design of cranes will vary widely with different manufacturers. As a result, the inspection and preventive maintenance program should be developed according to manufacturer's recommendations. Additional considerations are the inspection, maintenance, and record keeping requirements of OSHA. Two prime targets of preventive maintenance are (1) ensuring adequate lubrication and (2) aligning all moving parts. All programs should include schedules of inspecting, lubricating, adjusting, and replacing worn parts.
Documentation of Crane Inspection and Maintenance
An adequate record system, showing preventive maintenance and repairs performed, is vital to good management. Determinations of whether or not equipment needs replacement and cost of maintenance and repairs can be made from these records. Also, they can be helpful if legal problems arise.
In addition to regular, periodic inspections, it is the owner's, or his/her agent's, responsibility to inspect and test the equipment to ensure that it is capable of safe and reliable operation whenever the following conditions arise:
- Prior to initial use of all new equipment and just-purchased, rebuilt equipment.
- After every major repair, overhaul, or alteration.
- After major modifications have been made to the original design (with the manufacturer's permission and approval).
- After every assembly operation on any crane that was stripped for transportation.
- Prior to using any crane that has been in storage.
- Prior to using any crane that has been involved in an accident or some other mishap.
- Prior to using any rented crane.
Daily Visual Check and a Daily Preventive Maintenance Check
The following Items should be checked during a Daily Visual Check:
- Glass, for cracks or broken.
- Fuel tank(s); fuel gauges; and hose and connections.
- Drive chains and sprockets for cracked or broken pieces.
- Oil or coolant leaking.
- Roller path, house rollers, and hook rollers for chips or cracks.
- Boom hoist, whip line, and hoist wire rope; pendants; load blocks; and sheaves.
- Damaged or missing guards or gear or chain case covers.
- Limit devices; boom/mast stops; and drum pawls.
- Control valves; levers and linkage; and instrument panel(s).
- Fire extinguisher available and in working order.
The following Items should be checked during a Daily Preventive Maintenance Check:
- Radiator coolant level and add when necessary.
- Hydraulic system(s) level (add oil to the reservoir, as necessary).
- Gear case lube level (add, as necessary).
- Engine oil.
- Transmission and/or chain case or reservoir.
- Rotating bed sump (if applicable) level (add oil, as necessary).
- Converter input and/or output housing(s).
- Air compressor.
Crane Operator's Responsibility
The following are some suggested guidelines for crane inspection and maintenance:
- A daily check should be made of brakes, controls, booms, sheaves, all running ropes, and fire suppression equipment. A permanent log should be kept and dated and signed by the person who did the maintenance check.
- When defects in the equipment are known, the equipment should not be operated and should immediately be taken out-of-service. The operator should immediately notify the supervisor who should contact the master mechanic.
- No equipment should be lubricated while it is running or in motion.
- All engines should be stopped and shut off (keys removed) before the equipment is refueled.
- The equipment should always be blocked during repair or when stopped or parked on an incline.
- Operators should know and understand all of the information on the load limit charts and manufacturer's manual kept in the cab of the crane.
The crane rental business is highly competitive and involves a significant capital investment. It is essential that a contractor deal only with those equipment dealers who have a reputation for integrity and reliability. Reliable dealers protect their investment by following a maintenance program that ensures that all equipment is in first-class operating condition when delivered to the contractor. Less reliable dealers may cut corners on maintenance and may supply equipment which is unsuited or unsafe for the work to be done.
It is the responsibility of the contractor to have and accomplish an inspection and maintenance program to ensure that the rented equipment is in safe operating condition when delivered to a job and that it is safely maintained and operated throughout the course of the rental contract.
The following guidelines as for contractors who rent cranes:
- The original manufacturer's documentation (all booklets, load charts, and maintenance procedures) should be available with the rental unit, preferably printed in English and Spanish.
- The contractor is usually responsible for routine maintenance, while the equipment dealer is responsible for the major repairs. All such arrangements should be spelled out in the rental agreement. The contractor should make certain that the dealer has the repair facilities required for making major repairs and the ability to procure a replacement, if needed.
- The equipment being rented should meet OSHA standards.
- The contractor should never assume that the equipment delivered is in safe condition.
- The equipment should be thoroughly checked by an experienced mechanic when delivered to the job. All conditions affecting safe operation of the equipment should be corrected before it is used on the job. Written records/logs of these inspections should be kept up-to-date.
- Rented cranes should be equipped with the applicable safety devices for the project.
- Cranes may be rented with or without an operator. If an operator is provided with the equipment, the contractor should require the operator to demonstrate his/her ability, to detail previous work experience, and, if required, to prove he/she has the proper licenses and certification.
- If a contractor provides the operator, the contractor should make certain that the individual is experienced, qualified, and capable of safely handling the equipment.
Selection and Training of Crane Operators
Operators have direct responsibility for the actual operation of the crane, but all responsibilities should be shared by workers associated with the use of cranes on the jobsite. Operator training programs should include the many aspects of crane operations that have potential to cause accidents. Suggested qualifications for operators are:
- Operators should be able to provide evidence of their qualifications and experience and, if required, prove that they have the proper licenses and certifications needed to operate a crane dragline. The operator should be required to pass a written or oral examination, and a practical operating examination for the specific equipment to be used.
- The operator's hearing should be adequate for his/her specific operation.
- Operators should have normal depth perception, field of vision, reaction time, manual dexterity and coordination, and sufficient strength and endurance. They should not have any conditions likely to cause dizziness.
- The operator should be able to distinguish colors, regardless of the position of the colors, if color differential is required for the job.
- If an operator is subject to seizures or loss of physical control, the operator should not be permitted to operate the equipment; if allowable, specialized medical tests may be required to determine these conditions.
- If there is evidence of physical defects or emotional instability, which could create a hazard, the operator should not be permitted to operate the equipment; if allowable, specialized clinical opinions or medical tests may be required.
Crane Operations and Safety
Safe crane operations depend on planning, training, maintenance, and supervision, and it is management's obligation to oversee each of these critical functions. One of the basic requirements of any crane safety program involves selecting the machine to best suit the requirements of the job. If the crane's basic characteristics do not match the job's requirements, unsafe conditions are created even before any work is started.
Weights and dimensions of the heaviest and largest loads, maximum lift height, maximum lift radius, number of lifts, and precision placement of loads (whether or not loads have to be walked or carried) are all crane selection considerations that must be predetermined. The required tackle and rigging equipment is based on the determined safe working load, and this "limit" should never be exceeded.
Careful, step-by-step procedures and direct supervision are necessary for proper assembly and accident control. Soft or unstable ground conditions must be considered and proper setup of the machine must be planned based on these conditions. Power line contact is the largest single cause of fatalities associated with cranes. The operator needs to identify all overhead power lines as part of pre-job planning and conduct operations based on analysis of this information.
More than half of all mobile crane-overturn accidents relate to machine setup. Operators should not bear the full burden of responsibility for the proper operation of mobile cranes, unless they have the assistance and full support of management. Management should ensure that a written company policy is created and used by all employees regarding crane specifications and operating procedures. This written policy should provide guidance on the following issues:
- The preplanning of all moves, crane setup (including use of mats under outrigger pads), and lifts.
- The site conditions (where the machine is being used), including: ground and wind conditions; access roads and ramps; available space for erection, operation, and dismantling; and potential obstacles. It should be ensured that stable footing is provided, and outriggers and cribbing used, as required.
- The maximum loads at given radii that can be lifted and placed without being pre-engineered and the pre-engineering of all others. Specific instructions should be given to all project managers and superintendents that changes will not be made in any pre-engineered lifts without consultation and mutual agreement (in writing) of the responsible engineers.
- The lifting limitations of the unit are observed. Older hydraulic type cranes gave feedback; it was "lifting by the seat of your pants." Current fluid-drive-type cranes do not give feedback to the operator before structural failure occurs.
- The weighing of all loads that are near capacity, rather than relying on the indicated weight figures. Invoice weight figures should not be used, nor should assumptions or guesses be made
Hazard Precautions When Operating Cranes
The hazards involved when operating mobile cranes are numerous. Overturning, structural damage via human error, structural damage via machine failure, boom "over the cab," wire and rope failures, and high winds are potential hazards that should be anticipated and controlled, where possible. There are many conditions that can cause crane accidents. The following are some safety suggestions to avoid potential losses:
- Boom stops can be pneumatically or hydraulically operated, or spring-operated, and cranes should be equipped with a boom hoist cut-out switch.
- As previously explained, there is absolutely no safe way to operate a crane when close to electrical power lines. Safe distances must be kept.
- The swing perimeter of the crane should be barricaded, keeping a minimum clearance for a 360° swing of counterweight at a 2-ft (0.6-m) minimum.
- Heavy equipment should never be set up near any excavation area without the proper planning, protection, and notification.
- Booms, pendants, and hoist lines flex under strain. Sudden, released loads can cause the boom to spring in the opposite direction. A high boom could spring over backwards and, over a period of time, could cause fatigue to the structure.
- Wind, especially on long booms of high angles, adds additional load factors and can topple the crane.
- Sudden movements of the rig on unlevel surfaces cause stability problems and potential structural failure.
- Booms that are being raised from or lowered to the horizontal position require preplanning and proper crane setup procedures.
- The design of the boom requires that it be properly aligned at all times. Also, booms are not designed for excessive side loading.
- Adequate steps should be provided for getting onto or off from a crane rig.
- Multiple rig picks are very hazardous and should be avoided if possible.
- Required public protection and site security must be available at all times, day or night.
The operator and all workers should wear the required personal protective clothing and equipment at all times on the jobsite. Unauthorized boarding, leaving, or riding on equipment should be prohibited (no one is permitted to ride the load or hook).
Swinging a boom, turning a machine on the blind side, or lowering a bucket without directions from a signaler, or without due respect to worker safety on the jobsite, should be strictly prohibited.
Outriggers on rubber-tired cranes should be fully extended on both sides, the wheels free of the ground, and the machine leveled. Mats should be used under the outrigger pads when required.
Traveling with a load should be avoided whenever possible. If traveling with a load becomes necessary, the operator should consult the appropriate load limit charts and travel very slowly, and only over firm, level ground.
Prohibit the use of intoxicating beverages and drugs, horseplay, wrestling, practical jokes, or unnecessary conduct or conversation during crane operation. The operator should never leave the engine running when not in the cab or leave the controls with a suspended load on the hook.
Cranes should only be moved under the direction of the designated signaler. Mobile cranes and boom-type excavators should be equipped with an audible signal device. When cranes are operated on soft ground, timber mats should be used as a base for the crane.
Moving or operating a crane near an excavation should be done with extreme care. Any questionable situations should be checked with the responsible engineer. The ground over which a crane is to be moved and placed should be thoroughly checked for inclines or declines, especially for cross levelness, soil compaction, and overhead obstructions. Where there is danger of slides or ground movement, the area should be stabilized with mats or shoring.
All crane booms should be equipped with an operable proximity signal device. Boom stops should be equipped with a hoist cut-off switch.
Fast operations should be avoided. Fast swings cause the load to swing out, increasing the radius. Rapid hoisting or braking increases the stress on the equipment.
Additional wind loads should be considered when lifting during wind speeds of 10 mph (16 k/per m) or greater. In lifting a load from water, the weight of the water contained in the load should be added to the load weight. Buoyancy is lost when the load leaves the water. The load should be lifted slowly from the water to keep the suction effect to a minimum.
The load should be tested by raising it a few inches to determine whether or not the sling is properly set, the load is properly balanced, and the brakes will hold. The speed for lowering a load should not exceed the hoisting speed limits for that same load. Loads should be lifted only when the hoist line is in the vertical position over the load. Before attempting a dual lift, operators must be sure of the following:
- One designated person is in charge for the entire operation.
- The lift has been thoroughly engineered.
- The cranes are in good working condition and in proper position.
- The slings are set to balance the load of each crane.
- The load will not shift since this could result in equipment failure.
- Each operator and the signaler are in direct communication at all times.
- There is direct communication between both operators at all times.
Safe load limits should be posted in the cabs of cranes and draglines, and should indicate the safe load for the maximum positions of the boom, and, for at least two intermediate positions; these guidelines should be strictly followed. Counterweights should not exceed the manufacturer's specifications, and all original counterweights should be properly attached to the machine. The use of temporary or makeshift counterweights should be prohibited.
Taglines or guide ropes should be used on loads that are liable to swing or must be guided through a small area, in order to protect the load and to guard workers or scaffolds from swinging loads. Standard operating signals should be used and only one person should be allowed to give signals to the operator, unless a line of sight problem is involved which may require the use of radio communication. When handling loads, the operator should not be permitted to perform any other work and should not leave the controls until the load has been safely returned to ground level. Loads should not be hoisted over work areas unless the proper warning signals are sounded.
Cranes and Proximity to Electricity
High voltage contact is the largest single cause of fatalities associated with crane operations. Contact with even low voltage lines (less than 600 volts) may result in serious injury or death. At minimum, a 10-ft (3.0-m) clearance between energized power lines and all parts of the crane, load, and load line is required.
The crane boom and load can become electrically charged when they are operated near a television, radio, or microwave transmitting tower. The boom acts like an "antenna" and becomes "hot." The charge is not as electrically dangerous as when it is touching a power line, but it can still cause burns to workers handling the load. When workers get "shocked," usually the effect causes them to "jump," possibly causing a trip or fall. One solution (grounding the crane will not likely have any effect) is to insert a synthetic web sling between the load block and the load, which will isolate the riggers from the crane. The crane operator will not be affected when in the cab, but should wear rubber gloves when getting on and off the crane.
Crane Operating Precautions
The following are some suggested operating precautions that should be observed when working in the proximity of electric power lines:
- "Keep Your Distance" - surrounding every live power line is an area that is called the "absolute limit" of approach. It is forbidden to move any crane, boom, load line, or load into this area unless official notice has been given that the line has been de-energized. This limit varies according to local, state, and federal laws or the crane manufacturer's recommendations. Check your locations requirements.
- Inspect the route to be traveled and the area in which the crane will be operated. Revising the travel route may be necessary to avoid energized lines. Check the area to be used for loading and unloading equipment, and select areas which are clear of overhead lines.
- Place barricades to warn workers of overhead dangers the entire length exposed on the jobsite.
- Tie down booms or swinging parts when transporting equipment. Allow clearance for bouncing due to uneven ground.
- If necessary, consult with the power company about de-energizing, raising, or rerouting power lines. Do not attempt, under any circumstances, to raise, move, or cut power lines. Wherever possible, avoid situating a crane adjacent to power lines at all times.
- Always presume that all power lines are energized and that any contact with them may result in bodily harm, damage to equipment, and/or interruption of power or service.
- Use a qualified signaler whenever the crane is within a boom's length of the lines. The signaler must warn the operator when the machine is approaching the lines because the operator may not be able to accurately judge the distance. The signaler should have no other duties while the machine is working near the power line.
- Review on a daily basis safety procedures and warn the ground crew of potential dangers, especially new workers on the jobsite.
- Except for the operator, keep all workers away from the crane whenever it is working close to power lines. Do not allow anyone standing on the ground to touch the crane, the crane hook and lines, or the load until the signaler indicates that it is safe to do so.
- Avoid using taglines except when it is possible for the load to spin into the power line. (Note: Although all ropes will conduct electricity, dry polypropylene provides better insulating properties than most commercially available rope.)
- Do not rely on ground rods, proximity warning devices, hook insulators, insulating boom guards, swing limit stops, or any other similar devices for safety because each type has serious limitations. They provide little or no protection, and people touching the crane or load can still be injured or killed.
- Do not store any equipment or materials under any lines.
- Exercise caution when traveling the crane on uneven ground (boom can weave or bob into lines) or near lines with long spans (Line swing in the wind can result in contact with the crane).
- Whenever an operator positions a crane where the boom can swing close to a power line, the danger is imminent and the operator must be doubly alert. The operator must never leave the crane if the boom, when lowered, can enter the limit of approach.
- Review working procedures with local utility representatives before starting a job where any power line exposure is recognized. It is suggested that the following warning sign be posted and maintained on the crane equipment in plain view of the operator when he/she is at the controls: "Warning - Do not operate this equipment (at minimum) within 10 feet (3.0 m) of Energized Power Lines: Know your required distances."
- Ensure that whenever cranes must repeatedly travel beneath power lines, a route is plainly marked and "rider poles" (temporary pole structure) are erected on each side of the crossing approach, to ensure that the crane structure is lowered to a safe height.
Crane Contact with Energized Power Lines
The following are suggested procedures in the event contact has been made with an energized power line:
- Call the local power company immediately and request assistance. Tell them what happened and the location where contact was made. After the area is cleared, the power company should conduct a full inspection.
- Post a guard(s) to keep everyone away from the rig, its load lines, load, and exposed wire ends, all of which may be energized. If an overhead wire has fallen, stay away from it. Keep all persons away until the utility company arrives.
- The operator should stay in the cab ("Do not panic") and attempt to swing the boom clear or move the equipment. If the crane cannot be moved away or disengaged from the contact, the operator should remain inside the cab where it is safe until the electrical authorities de-energize the circuit and confirm that conditions are safe.
- If the operator "must" leave the crane, he/she must jump clear. The operator must never step down, allowing part of his/her body to be in contact with the ground while any other part is touching the machine. Because of the voltage differential that may exist in the ground, the operator should jump with his/her feet together, maintain balance, and shuffle or hop slowly across the affected area. The operator should not take large steps because it is possible for one foot to be in a high-voltage area and the other to be in a lower voltage area; the difference between the two can injure or cause death.
- Never touch a person who is in contact with an energized power line. A dry board or a long dry rope or pole may be used as a last resort to pull or push the victim free from the contact. Give first aid and call for medical assistance at once. If the rescued victim has stopped breathing, immediately apply CPR. The chances of reviving a victim depend largely on how quickly the resuscitation is started.
- After the incident and prior to operating the crane, it should be inspected for possible damage caused by the electrical contact. Wire rope should be replaced if it has touched a line since the arc may have been of sufficient power to weld, melt, or badly pit the rope. The damaged section of rope will look like it was burned with a torch.
- After the area has been cleared, the local power company and safety inspectors should inspect the lines and make any needed repairs. This is necessary to ensure that the line is fully back in service and prevents the damaged line(s) from falling at a later date.
For more information on loss control and managing business risks, check out the American Family Insurance Loss Control Resource Center.
1. American National Standards Institute. Mobile and Locomotive Truck Cranes. ANSI B30.5. New York, NY: ANSI, 2007.
2. ---. Hooks. ANSI B30.10. New York, NY: ANSI, 2005.
3. Occupational Safety and Health Administration. Crane, Derrick, and Hoist Safety. Washington, DC: U.S. Department of Labor, 2009.
4. Occupational Safety and Health Administration. Cranes, Derricks, Hoists, Elevators, and Conveyors 1926.550. Washington, DC: U.S. Department of Labor, 2009.
5. Shapiro, Howard, Jay, and Lawrence. Cranes and Derricks. 3rd ed. New York, NY: McGraw-Hill, 1998.
COPYRIGHT ©2009, ISO Services, Inc.
The information contained in this publication was obtained from sources believed to be reliable. ISO Services, Inc., its companies and employees make no guarantee of results and assume no liability in connection with either the information herein contained or the safety suggestions herein made. Moreover, it cannot be assumed that every acceptable safety procedure is contained herein or that abnormal or unusual circumstances may not warrant or require further or additional procedure.