Ductile link tie-down systems for quay cranes

Introduction

Scientists are observing that the intensity of tropical cyclones (specifically hurricanes and typhoons) is increasing due to global warming. More hurricanes are reaching land, and quay cranes in hurricane-prone regions face an increasing risk. Recently, a number of cranes collapsed when subjected to hurricane forces.

These catastrophic collapses typically stem from failures in the crane tie-down system or wharf hardware, or both. In hurricane regions, two or more independent tie-down links are frequently used at each corner of a crane. The turnbuckles cannot be tightened in a way that creates equal tension between both tiedowns.

Because cranes rotate and shift laterally during a hurricane, the uplift forces in the links are neither equal nor vertical. Often the entire uplift force of two tie-downs at a corner is transferred to one tie-down, causing the more highly loaded tie-down to fail at a fraction of the design corner load. As a result, the remaining tie-down at the same corner fails before the intended load is reached, and the crane collapses.

Liftech invented the ‘ductile link’ as part of the tie-down system to equalise the uplift forces between two or more tie-downs at one corner. The ductile link system is economical, practical, and suitable for new and existing quay cranes.
 

Tie-down failures

Tie-down failures result in serious consequences; cranes tip over and collapse. If the cranes’ stowage pins disengage, cranes may run away and destroy adjacent cranes. Such ‘chain reaction’ failures occurred during Typhoon Maemi after the tie-downs failed on one crane, causing the collapse of five other cranes sequentially (see Figure 1).

Why do failures occur in the tie-down system? Poor workmanship and faulty design are two primary causes. This article focuses on the faulty design aspects.

In moderate wind regions where uplift forces are small, there is usually one tie-down per corner for a total of four tie-downs per crane. In hurricane regions, however, there are usually two to four tie-downs at one corner.

Most wharves with multiple tie-downs at one corner are designed with the assumption that the tie-downs are perfectly vertical and the uplift forces are equally distributed among the tie-downs. These assumptions are incorrect however, since cranes rotate, shift, and deflect during hurricanes. The crane wheels may slide perpendicular to the rails (see Figure 2). The stow pin socket is larger than the pin, and the pin may move inside the socket as a result.

It is impossible to tighten the multiple turnbuckles such that the tension is perfectly equal. A difference of a few millimetres may significantly change the distribution of load between the tiedowns. Our analyses indicate that one tie-down may take up to 100 per cent of the uplift load.

Consider the following scenario. Two tie-downs are at one corner, and the wharf is designed with the assumption that each tie-down carries half the load in that corner. The tiedown pretension is unequal and the crane shifts, causing one tie-down to carry more than its share of the load. The tiedown fails. Consequently, the second tie-down must now carry the full load in that corner. Since the second tie-down is also designed to carry only 50 per cent of the load in that corner, it too fails. Both tie-downs fail before the intended load is reached, even though the total tie-down load, if equally distr ibuted, would not cause failure. Liftech’s ductile link system is designed to equalise the loads, thereby reducing the likelihood of such a scenario.
 

Ductile link system – how it works

A ductile link placed in a tie-down is capable of deforming plastically without losing strength until the other tie-downs share the load, developing the full potential of the tie-down system. Plastic elements, acting as fuses, have been used for decades to protect structures from severe damage during earthquakes. The ductile link is simply a fuse that protects the wharf hardware and tie-downs from premature failure (see Figures 3 and 4). Initially the loads in multiple tie-downs are not equal. The Figure 5 example shows an initial distribution of 2:1. The desired distribution is 1:1.

With the ductile link, the heavily loaded tie-down yields at the design tie-down load, allowing more load to transfer to the lightly loaded tie-down. As the uplift force increases, the ductile link stretches plastically. Eventually, the load is shared equally by both tie-downs, and the full strength of the multiple tiedown anchor is utilised, as intended by the wharf designer. It is important to note that once a ductile link deforms plastically, it will require replacement.

Kenton Lee, Liftech Consultants Inc., Oakland, California, USA
Edition: Edition 31

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