Model Tests on Current Forces on a Large Bridge Pier Near an Existing Pier

[+] Author and Article Information
Subrata K. Chakrabarti

 Offshore Structure Analysis, Inc., 13613 Capista Dr., Plainfield, Illinois,

Mark McBride

 HR Wallingford, Howbery Park, Oxon OX10 8BA UK

J. Offshore Mech. Arct. Eng 127(3), 212-219 (Feb 02, 2005) (8 pages) doi:10.1115/1.1951772 History: Received June 16, 2004; Revised January 27, 2005; Accepted February 02, 2005

A suspension bridge is being built over the Tacoma Narrows, Washington. The bridge will be placed on a structure mounted on two large concrete caissons, which will be exposed to strong currents. The piers are of rectangular section with chamfered edges in the upper portion. The caissons are being built at site while floating and moored in high currents. There are no known analytical methods or experimental data available on such structures at high Reynolds number. In order to determine the forces on the caisson due to current, a series of scaled physical model tests of one of the caissons was carried out. The forces on the caisson were measured in the presence of the existing bridge pier and the bottom contours of the Narrows were accurately modeled. The model scale was chosen as 1:100 and the tests were performed for the caisson at different drafts. This paper describes the test setup, and measurement system for a series of fixed caisson tests and demonstrates the consistency of the test data. The measured inline drag and transverse lift forces on the fixed caisson at different drafts are presented and the effect of the fluid velocity and flow vorticity on the frequency contents in the forces is discussed. The interaction effect of the neighboring existing pier on the current forces on the caisson is investigated. Since the measured forces were applied in the design analysis of the caissons, the scaling effect of the model test is also discussed. This paper is accompanied by two other papers, which form a group of three papers related to the project describing the current excitation on the caisson and the associated caisson responses. The other two papers in succession are by Chakrabarti (J. Offshore Mech. Arct. Eng., to be published) and Chakrabarti and McBride (J. Offshore Mech. Arct. Eng., to be published). The paper by Chakrabarti describes the numerical computation of the current forces on the caisson by a three-dimensional analysis, while the paper by Chakrabarti and McBride uses the information from these two papers to determine the motion response of the caissons and the mooring line tensions.

Copyright © 2005 by American Society of Mechanical Engineers
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Figure 1

Current flow facility at HRW

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Figure 2

Orientation of caisson and pier with respect to flow

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Figure 3

Setup showing “S” cells on top of caisson in place

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Figure 4

Time history and power spectrum of calibration current speed at 7 kts (3.6m∕s)

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Figure 5

Force time histories for the repeat runs—61 ft draft in 9 knot flood

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Figure 6

Force PSD’s for the repeat runs—61 ft draft caisson in 9 knot flood

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Figure 7

Drag and lift coefficients in ebb and flood flow versus Reynolds number

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Figure 8

X Force PSD’s for—(a) 143 ft 7 knot ebb and (b) 143 ft 7 knot flood

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Figure 9

Mean rigid body forces versus current angle: 143 ft draft in 9 knot current

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Figure 10

Current flow (right to left) past new caisson behind the roughened Pier

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Figure 11

Force time histories for smooth and rough cases—143 ft draft in 7.3 knot flood



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