Ayres Knowledge Center Scour Evaluation: Bridges with Unknown Foundations | Page 51

STRUCTURAL EVALUATION
DESCRIPTION
Bridge Number 794004 was constructed in 1959 and has undergone repairs and rehabilitation. The
bridge is 2,130’-10” long and has fifty-five (55) spans. The superstructure for the approach spans is
composed of a simple span prestressed concrete girder systems and the main span is a double leaf
bascule structure. Each intermediate pile bent has five (5) 18-inch square prestressed concrete piles.
According to the Bridge Plans, the design load for the intermediate bent piles was 45 tons per pile.
Seven (7) 18-inch square prestressed concrete piles support each end bent. According to the Bridge
Plans, the design load for the end bent piles was 45 tons per pile. End bent protection consists of
concrete seawalls and sand cement riprap.
Pile tip elevations were determined from the 2013 Nondestructive Parallel Seismic Testing Evaluation
Report. The remaining pile embedment and unsupported length for the intermediate bents were
determined at the existing conditions (using the 2011 Bridge Survey and the 6/14/2011 Field Visit
Channel Profiles) and at the calculated 100-year scour event. Bascule Pier No. 17 was chosen for the
analysis because its piles have the least amount of remaining embedment and the longest unsupported
length. At the 100-year scour event, the piles at Bascule Pier No. 17 would have scour up to 18.6 ft. and
have 69.7 ft. of remaining pile embedment.
LOAD CALCULATION
Vertical and lateral load calculation, performed in accordance with the requirements of the AASHTO
Standard Specifications for Highway Bridges (AASHTO), 17th Edition, used the following criteria:
x The Load Factor Design method was used to evaluate foundation stability. The Department
directive states that a load factor, J, and a live load coefficient, E, of 1.0 are to be used in Group1
of Table 3.22.1A of the AASHTO.
x Dead loads were calculated based on a tributary area of one span length.
x Live load was based on the HS20-44 truck for the stability analysis. An HS-20 truck was used to
determine the original pile design loads. The bridge is currently posted at 17 Tons.
x Impact was included.
x Stream pressure was calculated in accordance with Article 3.18.1 of the AASHTO and applied
only to the exposed pile surfaces. In accordance with the Department’s directive, debris
accumulation was not considered.
x The analysis did not include loads due to ship impact.
x Based upon results of the Phase 2 report, the water surface elevation submerges the seal, pile
cap, and a portion of the main pier wall during the 100-year flood. Therefore, buoyancy effects
were taken into consideration in the analysis.
Project No.:
62-0144.00
Project Name:
Title/Item:
Bridge No.:
794004
Main Street over Halifax River Bridge
Scour Evaluation Report – Stage 6 / Phase 3
Supplementary Report
Design By:
MES Date: 04/25/2013
Checked By: SML Date: 04/25/2013
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