Madsen Giersing, an Australian consulting engineering firm that specializes in marine engineering and construction solutions, was commissioned to design a new bulk liquids berth at Dampier, Australia, one of the country's largest ports. The berth or dock consists of four independent berthing and mooring dolphins (or groups of piles) and a 490-meter-long access trestle or bridge that leads out to a 37.4 x 34.3-meter loading platform, from which up to 65,000 Dead Weight Ton (DWT) tankers are to be loaded.
The company had to analyze the trestle structure, which would be subject to marine conditions such as wind, waves, and earthquakes, as well as the heavy-duty loads of the giant tankers. Working with a team of engineers, the company began by determining an initial concept for the geometry of the trestle. Given the basic design concept, the engineers were responsible for the detailed design of the access trestle, the largest part of the berth. It provides support for the product pipelines and allows vehicular access from the shore to the loading platform.
The engineers used the Superdraw 2- and 3-D sketching, modeling, and meshing tools within ALGOR FEMPRO to draw the various parts of the structure. The trestle consisted of headstocks, girders, walls, pipes, piles, and pile braces, all consisting of beam and plate elements with material properties of reinforced concrete for the headstocks, prestressed girders for the roadway, and structural steel pipes for the piles. The piles, aligned in a series of 'pile bents,' or rows of capped piles, support the roadway plates on which the vehicles travel.
Australian standards were used to estimate the dead, live, wind, and earthquake loads as well as traffic-induced loads. In order to cut analysis time, four copies of the model were made and load cases were applied separately. Because the structure was designed to only handle the imposed loads within the linear material range, all of the loads were static.
After the initial model and analysis, engineers noticed that the Trestle 8 and 9 pile bents (T8 and T9), the last bents of the trestle, were highly stressed. To reduce these stresses, they added an extra pile to each pile bent and optimized the locations and rakes (slant) of the six piles. They then reconsidered the various load combinations. Subsequently, pile bent T5 had attracted increased loads. To compensate, they increased the rake of the T5 piles, thus reducing the stiffness of the bent to absorb the stresses.
As further detail was added to the model, the engineers accounted for the fact that no piles could be driven at bents T1 and T2 due to the rock formations at these locations. Therefore, they replaced the raked piles with cross-braced vertical columns and mounted these directly onto the rock. They then defined the rock as a fixed boundary condition. At bent T5, due to deeper rock formations, the piles were unable to be driven into the soil at the length required to obtain a fixed support. Therefore, the structure had to be modeled based on more superficially driven piles. The team did so by using spring elements to simulate the stiffness of the soil over the length of penetration of the piles. As such, they were able to determine and account for the new stresses in the structure due to the lack of pile penetration.
Based on continued design iterations and analyses, engineers were able to determine the size and thickness of the steel piles and the amount of reinforcement needed in the concrete headstocks. They used the results capabilities of ALGOR to determine the area of the maximum deflection or displacement. This was important, as there was a strict criterion on the allowable deflections due to the ability of the product pipes to handle the movement.
Engineers were able to model the global structure of the access trestle and obtain accurate stress responses for the structural members. They determined the thickness of the piles and headstocks, and the allowable distance between piles within each pile bent.
This work was done by Lasse Madsen, structural engineer at Madsen Giersing, Australia, using software from ALGOR, Inc. For more information, visit http://info.ims.ca/5655-121 .