An Overview of ASME BTH-1 (spreader bars and lifting beams)

When planning to design lifting beams (or any other below-the-hook lifting devices), there are many aspects that must be considered beyond finding materials that meets a few basic engineering calculations. Along with stress, buckling is also a critical factor in lifting beams that must be addressed in detail to ensure that the structure can handling the loads imposed on it. ASME BTH-1 clearly defines rules for material selection and determining what kind of load would be permitted by the design in question. Following the procedures outlined in BTH-1, you can successfully design a below-the-hook lifting device that will handle the loads imposed on it.

What is ASME BTH-1

ASME BTH-1 “Design of Below-the-Hook Lifting Devices” is an ASME design standard and is to be used in conjunction with ASME B30.20, the ASME safety standard for Below-the-Hook Lifting Devices.

ASME B30.20 defines the safety requirements for below-the-hook lifting devices including marking, inspection, construction and operation, whereas ASME BTH-1 defines the design requirements for developing these lifting devices. BTH-1 addresses design requirements including:

  • classification of the lifting device based on frequency and capacity of lifts,
  • structural design requirements related to member design, connection design (pins, bolts, welds), and fatigue,
  • mechanical design requirements for the mechanical components (sheaves, wire rope, gears, bearings, shafts, fasteners),
  • electrical component requirements for electrical components used to operate below-the-hook lifting devices,
  • lifting magnet design requirements.

While BTH-1 is broad in its coverage of design requirements for below-the-hook lifting devices, only portions of the design standard are applicable to spreader bars and lifting beams, in particular beam classification and the structural design requirements. These items are discussed in more detail below.

Classification of Spreader Bars and Lifting Beams

Spreader bars, lifting beams, and other below-the-hook lifting devices are classified based on the frequency and capacity of lifts that are required of the device. In the latest revision of BTH-1 (2017), there are three design categories, categories A, B and C, as well as five different services classes, service class 0 through 5. Together the design category and the service class define the design requirements of the beam related to material strength and fatigue.

Design Category The design category establishes the different stress factors to be used in beam design:
  • Category A lifters are designated when the magnitude and variation of load is predictable, and require a design factor of 2 on yield/buckling
  • Category B lifters are designed for unpredictable lift scenarios where the magnitude of lift can vary significantly and require a design factor of 3 on yield/buckling. This is the most common design category and is what Basepoint Engineering lifting beams and spreader bars are typically designed for.
  • Category C lifters are designated for special-application lifting devices where a specified design factor is required and require a design factor of 6 on yield/buckling.

Service Class

The service class designates the number of load cycles, or fatigue life for which a beam is designed. Basepoint Engineering lifting beams and spreader bars are typically designed as Service Class 0, which is for 0-20,000 load cycles, although beams with different service classes can be designed on request. Service Classes 1 through 5 cover load cycles from 20,001 through 2 million load cycles.

Structural Design

ASME BTH-1 specifies design calculations for different types of loading of a lifting device including tension, compression, flexure, shear and combined loading of beams. Depending on the style of lifting device, only certain structural design considerations apply to a specific device. For example, a lifting beam with a centre pick point and load suspended on the two ends of the beam are loaded in flexure, therefore compression and tension calculations aren’t specifically relevant to this device. Conversely, for a telescopic spreader bar with a centre pick point and two slings out to the ends of the spreader bar the loading on the beam would be primarily compressive, although if the ends of the spreader bar are set up such that bending is applied to the ends of the spreader bar, then a combination of load calculations must be utilized.

The structural design section of the standard also includes requirements for connection design including bolted connections (bolt quantity, allowable loading, required tightening and hole requirements), pinned connections (pin hole strength, pin clearance), and welded connections (weld size and properties).

Fatigue design is not required for bars classified as service class 0 due to the low number of total load cycles, but as the number of load cycles is increased, the potential for fatigue failure increases. BTH-1 includes a structural design section covering fatigue requirements for lifting devices classified as service classes 1 through 5.

Mechanical Design, Electrical Components and Lifting Magnets

In addition to all the information previously discussed, ASME BTH-1 also addresses design requirements for mechanical design, electrical components and lifting magnets. These sections of the standard apply to components that are loaded in below-the-hook lifting devices, and in some devices or scenarios they would be relevant. They are not discussed in this article because they do not generally apply to typical spreader bars and lifting beams.

Conclusion

This article is a brief overview of what is covered by ASME BTH-1, particularly with respect to the design of spreader bars and lifting beams. Emphasis was made to focus on the areas of BTH-1 that apply to these devices. If you are looking for more information on BTH-1, feel free to contact us, or you can wait for future articles that will discuss applying BTH-1 to design of lifting beams and spreader bars in greater detail. If you are looking to manufacture spreader bars that have been designed in accordance with requirements of ASME BTH-1, we can provide you with engineered shop drawings of designs that are ready to manufacture. They can be found at our store. If you have a unique design challenge where you could use some engineering assistance, please contact us to see how we can help you.