Your wharf may be stronger than you think

Authorship

Erik Soderberg, vice president, Yoshi Oritatsu, structural designer and Michael Jordan, CEO, Liftech, Oakland, California, United States

Publication

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Over time, vessel and crane sizes have increased dramatically. This continues as the maritime industry is experiencing another wave of larger vessels and cranes. The Panama Canal expansion and Triple-E class vessels are already impacting container terminals. Many terminals now face the challenge of accommodating larger vessels and cranes.

Often wharf owners find that they have insufficient crane girder rated capacity and only consider strengthening options. Before proceeding with expensive strengthening, owners should study the capacity of their existing crane girders. For a variety of reasons, crane girders are often stronger than their rated capacity due to early design methods and tools, cautious designers, or both. Using modern methods, engineers can often justify increased girder rated capacities without expensive strengthening upgrades.

Looking at methods of analysis

Various modern engineering methods are available to evaluate girder capacities. The most appropriate method depends on the nature of the structure, what controls the strength of the structure, and what capacity must be justified. Three of the more significant methods are presented here.

Three dimensional finite element analysis

Finite element analysis (FEA) is a relatively modern analysis tool not commonly used in design more than 30 years ago. Compared to earlier analysis methods, FEA provides a more accurate calculation of forces in the wharf structure, in particular when the structure supports the crane loading in multiple ways, eg. if the crane girders are integrated into a deck structure or have transverse beams. In these situations, a three dimensional FEA can often justify additional capacity.

Strut-and-tie model analysis

The strut-and-tie analysis method has only recently been integrated into US design codes by AASHTO in 1989 and ACI in 2002. This method involves considering the girder structure as a truss comprised of compression struts (concrete) and tension ties (steel reinforcing).

This method is most suitable when shear capacity controls the girder strength and when the controlling load is infrequent or extreme, where temporary or limited cracking can be tolerated. This method typically justifies much larger shear capacities than traditional methods.

Pile load testing

If the capacity of the wharf is limited by the estimated soil strength, there are several techniques that can be used to justify additional capacity. The easiest and initial method is to evaluate the pile driving data using modern methods. If driving data is not available or if more capacity must be justified, tests can be performed.

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