Letter: Crane Stats Were Wrong
The Letters section on page four of ENR's June 4, 2012, edition, in which William Shuzman of the Allied Building Metal Industries commented on an earlier staff editorial regarding crane safety in New York City, gives some numbers that are not correct. The misinformation is why construction safety is not improving.
First, one could infer from the 75% and 85% levels that were stated in the letter for a crane's reduced load chart that these numbers are safety factors when they are not.
In the notes for lifting capacities, a well-known crane manufacturer's manual states, "All rated loads have been tested to and meet minimum requirements of SAEJ1063OCT80-Cantilevered Boom Crane Structures-Method of Test, and do not exceed 85% of the tipping load on outriggers fully and 50% extended, and 75% of the tipping load on outriggers 0% extended (fully retracted) as determined by SAEJ765OCT80 Crane Stability Test Code."
The percentages indicated in the notes for lifting capacities is based on how much weight it takes for a crane to tip, not the structural chart capacity.
With regard to a crane being able to pick 150% of its rated capacity, this is not accurate. When a crane is being tested under the supervision of a qualified person (mainly, someone who is licensed by the Dept. of Labor or another accredited body to survey cranes), the proof load testing during an annual or quadrennial crane certification is not to exceed 110% of the crane's load chart capacity based on the way the crane is set up, the amount of boom on the crane and if the crane is on outriggers or on rubber—that is, static, without the outriggers being down.
Moreover, if a crane is lifting personnel in an approved platform, in addition to other safety requirements, the crane operator has to de-rate the crane's load chart capacity by 50% and then lift the platform with a 125% proof load before lifting personnel in the platform.
In addition, load moment indicators (LMI) are an operator aid. They are not safety devices. Like any other electronic device, an LMI can be inaccurate. For example, an LMI may not have been calibrated properly or recently. If operating near sources of radio frequencies (RFs), the RF waves can interfere with the accuracy of the LMI.