An independent engineering firm hired to review design plans for a nearly $1 billion bridge in Texas has found what it said are more than 20 errors related to loading, foundations, substructure, superstructure and cables.

Work on the bridge had stopped in early January to address a potential design flaw related to the main pylons of the cable-stayed portion of the Sam Houston Tollway Ship Channel Bridge replacement project near Houston. The latest engineer's report was delivered by COWI North America Inc. to the Harris County Toll Road Authority (HCTRA) for review at a scheduled March 10 meeting.

A jump form system used to construct each pylon. Source: HCTRAIn a transmittal letter, HCTRA interim director Peter W. Key said that COWI's independent analysis identifies "21 areas of significant concern," with additional concerns and details summarized in the report.

In March 2019, HCTRA hired COWI to conduct a complete independent review of the Ship Channel Bridge engineering design performed by FIGG Bridge Engineers Inc.

This past January, COWI identified an issue related to the design of the curved portions of the pylon legs for the $962 million bridge. At the time, HCTRA said that "briefly pausing construction will allow for the engineer of record (FIGG) to design a solution, to be reviewed by COWI, and incorporated into construction before the work progresses."

HCTRA hired FIGG Bridge Engineers as part of a team to execute the bridge replacement project. The firm has served as Engineer of Record on seven concrete cable-stayed bridges built in the United States.

COWI considers the pile load-displacement relationship assumed for design inappropriate, significantly affecting the calculated loads in the piles and pile caps. Source: HCTRAThe project was bid and construction was awarded to Ship Channel Constructors, a joint venture between Traylor Brothers Inc. and Zachry Construction Corp. Their team includes T.Y. Lin International Group to provide construction engineering, a process that verifies a significant component of the overall design. The Texas county selected HNTB to provide construction management for the project.

In its most recent report to the HCTRA, COWI identified 21 areas of major concern across seven design areas.

Design parameters

• COWI considers the pile load-displacement relationship assumed for design inappropriate, significantly affecting the calculated loads in the piles and pile caps.
• COWI's estimates for pylon foundation settlement and tilt are significantly less than what was used by the contractor for construction analysis to develop the deck casting geometry and plan the erection process, including stay tensioning.

Loading

• Multiple changes to the wind loading used for design of the in-service bridge have been found necessary as a result of COWI's review. COWI considers the wind loading used for design unreliable. Multiple changes to the construction period wind loading have been found necessary as a result of COWI's review.
• HCTRA provided the original construction stage wind loading to the contractor for use in their construction analysis. COWI considers the construction wind loading used for construction analysis unreliable.
• Traffic load on the SB structure has not been considered in the contractor's analysis for northbound structure erection. COWI believes that such consideration is necessary.

Foundations

• Pile geotechnical capacities do not meet the project requirements at any of the foundations.
• Pylon pile cap structural capacities do not meet the project requirements for strength.

Substructure

• Pylon curved leg regions do not have the required capacity.
• Pylon leg cross-tie arrangements do not satisfy AASHTO-LRFD requirements.
• Pylon legs at deck level kinks require additional restraining reinforcing.
• Reinforcing at the lower pylon leg access door openings does not have the required capacity.
• Reinforcing in the pylon legs at transition zones from hollow to solid legs and adjacent to upper diaphragms does not meet the project requirements.
• Reinforcing at the connection of the pylon leg tops does not have the required capacity or details.

Superstructure

• Shear reinforcing in edge beams of the superstructure does not meet the project requirements.
• COWI was informed that the stay segments are being redesigned to add capacity for transfer of stay loads into the segment webs. Consequently, COWI did not complete a check for related aspects of the design.

Cables

• Slip resistance of stay cables in cradles in pylons does not meet the project requirements in-service or during construction.

Construction-related findings

• Construction wind loads provided for use by the contractor have been superseded.
• Settlement data provided for use by the contractor may not be accurate. Contingency plans are recommended.
• Cable slip checks provided by the contractor are not in accordance with the project requirements.
• Traffic load on the southbound deck while the northbound deck is being constructed needs to be considered in the contractor's analysis.
• Transverse local stress and strength checks under the segment lifter are required. Adjustments to the lifter support arrangement may be needed.

Last October, the National Transportation Safety Board (NTSB) said that FIGG Bridge Engineers made "significant errors in determining loads" for a pedestrian bridge that collapsed in March 2018 at Florida International University near Miami.

In particular, the NTSB said that FIGG chose the wrong interface shear demand value that led to a "severe underestimation" of demand. In addition, the firm chose the wrong load factor in calculating the permanent compression loading, which led to a "significant overestimation" of capacity.