Crane Structural Life AssessmentBusiness & Products
Port cranes are used over long periods of time. As they engage in cargo handling on a daily basis as the crucial part in logistics operations they are not only subjected to damage and general wear and tear to their parts, but also accumulate fatigue in their steel structures, which can lead to the possibility of fatigue damage. Fatigue damage is caused by repetitive loading and unloading tasks (as in handling cargo), resulting in the formation and progressive worsening of fatigue cracks in crane structures, structural materials and joints. Fatigue in steel structures is difficult to see with one’s eyes or feel with one’s hands, and carries the risk of developing into visible cracks, etc., which lead to destructive damage or structural collapse. Therefore, many cargo handlers (i.e. crane users) concerned about fatigue in the cranes they are using. The Mitsui E&S Group conducts structural life assessments, in which we examine and assess the structures of currently active cranes to determine for how many years they can continue to be used. We then offer repair and reinforcement advice based on the findings of those assessments.
- What is the physical life of a crane structure?
- The limit at which the performance of a crane’s steel structure is degraded and becomes unable to withstand use, or to resist subjection to external forces, due to damage, wear, fatigue and accidental damage, etc., after commencing use (Source: Japanese Society of Steel Construction (JSSC) survey concerning the lifespan of steel structures, August 1991)
Japanese Society of Steel Construction (JSSC) Fatigue Design Recommendations for Steel Structures
- Overhead Cranes / Bridge Cranes
- Quayside Cranes and Rubber Tired Gantry Cranes
- Loaders and Unloaders
Objectives of Crane Structural Life Assessments
To enable customers to understand crane structures
Crane structures have a remaining structural lifespan. Quantitative calculation of the crane’s remaining structural lifespan by conducting a structural life assessment helps to deepen the level of users’ concern and interest in structural maintenance.
To alleviate concern over crane structures
When using a crane over the course of many years, rust begins to become obviously visible on the crane structure, and operators involved in daily cargo handling and crane maintenance duties begin to harbor concerns, wondering whether the crane is still OK to use, and for how many years it can continue to be used. Such concerns can be alleviated by conducting a structural life assessment.
To assist in future maintenance of crane structures
The findings of structural life assessment, and the reccommendation for repair and improvement based on those findings act as indicators in determining the timing for efficient maintenance and inspections.
To enable judgments regarding crane replacement
When considering crane replacement, structural life assessments act as a reference for determining whether to continue using an existing crane or replace it with a new one.
To prolong crane structural life
Together with the replacement of mechanical and electrical components, carrying out repairs and reinforcement modifications to structural components based on the results of a structural life assessment can help to prolong the useful life of crane structures.
Outline Explanation of How Crane Structural Life Assessments are Conducted
|Step1: Data Gathering|
Data required for calculations is gathered.
|Step2: Examination & Measurements of Current State of Crane Structure|
Specialied Engineers conduct inspections of the crane, especially focusing on the parts which influence calculations.
Where it is found as a result of actual examinations that parts will influence calculations, these findings are incorporated into calculations.
|Step3: Preparation of Structural Data|
|3D skeletal frame data is prepared for the beam elements of the crane structure.
Structural data is prepared using Structural Analysis Design System Ver.2, which enables calculations to be made for each individual structural component.
Structural Analysis Design System is an original Mitsui E&S program designed for use on crane structures, and which has a real-world track record of over 20 years.
|Step4: Variable Load Selection|
Variable loads during cargo handling are considered and incorporated into calculations.
|Step5: Variable Stress Calculation|
|Structural Analysis Design System Ver.2 is used to calculate variable stresses. (Improvements have been made to the program in order to increase the precision of structural life assessments.)|
|Step6: Joint Classification Selection|
|Joint types and basic permissible stresses are selected in accordance with the standard.
Under the applied standard, joints are classified into 9 different types, and it is possible to select the most suitable joints within the crane structure.
|Step7: Calculation of Degree of Cumulative Damage|
|Structural Analysis Design System Ver.2 is used to calculate the degree of cumulative damage.
The range and frequency of stresses are analyzed to create a stress range frequency distribution for variable amplitude stresses. This is used to find the frequency of specific stress range levels.
|Step8: Calculation of Remaining Structural Life (1)|
|The calculation results of Step 7 are compared with the permissible cumulative damage values for the relevant joints in order to calculate the remaining structural lifespan.
Degree of cumulative damage = number of load (i.e. stress) repetitions / permissible cumulative damage (fatigue life) Where the degree of cumulative damage is less than or equal to 1 / the safety coefficient, it is calculated that the structure has a remaining lifespan.
|Step9: Inspection of Current State of Structure|
|A structural inspection is carried out based on the results of the calculation.
Parts of the structure which are found as a result of the calculations to not have much remaining life, or to have already exceeded their structural lifespan are inspected and compared with the calculated results.
|Step10: Repair & Modification Planning|
|Structural repairs, reinforcements and modifications are recommended based on the remaining structural lifespan.
Repairs, reinforcements and modifications are also recommended to prolong the remaining life of the structure.
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