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Update on the Nipigon River Bridge – Engineering Issues cause failure

Sep 22, 2016 @ 09:59

 

Today, Steven Del Duca, Minister of Transportation, and Michael Gravelle, Minister of Northern Development and Mines and MPP for Thunder Bay-Superior North issued the following statement:

“Earlier today, we visited the Nipigon River Bridge to provide an update on the investigation into what happened earlier this year and see first-hand how work is progressing.

On January 10, 2016, the Nipigon River Bridge was unexpectedly closed to traffic due to an issue with the tie-down connection on the northwest corner of the bridge. Following the closure, two firms were contracted to test the bolts from the bridge. The testing confirmed that the bolts broke due to overloading and not due to any flaw in the bolts themselves.

“South Girder Temporary Support”, graphic supplied by Ministry of Transport.

On February 26, 2016, the installation of a temporary support was completed and the Nipigon River Bridge reopened to two lanes of traffic.

The temporary support consists of a hold down system anchored to both the girder and the bridge foundation and will remain until the permanent retrofit is in place.

 

A thorough engineering analysis was then conducted by ministry bridge engineers and an independent engineering consultant to determine the cause of the malfunction. These analyses were conducted independently and reached the same conclusion.

Following the unexpected closure of the Nipigon River Bridge on January 10, 2016, two firms were contracted to conduct bolt testing. The testing confirmed that the bolts broke due to overloading.
A thorough engineering analysis was conducted to determine the cause of the malfunction of the tie-down connection on the northwest corner of the bridge. This analysis was undertaken by ministry bridge engineers and an independent engineering consultant with expertise in cable stayed bridges – Associated Engineering (Ont.). These analyses were conducted independently and at their conclusion reached the same findings.
The Investigation:
The two engineering groups were tasked with:
• Providing independent engineering advice relating to:
  – The design of the bridge and/or its components
  – The construction of the bridge and/or its components
• Providing independent findings related to the northwest tie-down connection. With a specific focus on:
• Review and analysis of the bearing and associated components
• Review of construction documentation for fabrication and installation of the bearings
• Structural analysis including computer modeling of the bearing and girder connection
• Compliance with the Canadian Highway Bridge Design Code (CHBDC)
The Findings:
Both engineering groups have now completed their work.
The engineering reviews found that there were three main factors that led to the malfunction: shoe plate flexibility, a lack of rotation in the bearing, and improper tightening of the bolts.
1. The Shoeplate – The review found that the shoe plate was too flexible, creating a “prying action” which increased the forces on the two outer rows of bolts. This additional force resulted in the bolt heads/nuts bending, stretching and eventually breaking.
2. The Bearing – The bearing did not rotate. The lack of rotation increased the forces being placed on the bolts, causing the bolts to break.
3. The bolts – Proper tightening keeps the forces in the bolts more consistent when the load on the bridge changes.
Other factors were also found, but their contribution to the cause was minimal. Neither cold
temperatures nor wind were contributing factors in the failure.

 

The engineering reviews found that three main factors led to the malfunction: first was the design of the shoe plate and its flexibility; second was a lack of rotation in the bearing that was constructed; and third was improperly tightened bolts attaching the girder to the shoe plate. When combined, these three factors produced the malfunction. Neither cold temperatures nor wind contributed to the closure.

Permanent Retrofit: The design of a permanent retrofit for the bridge is well underway and will be reviewed by Associated Engineering (Ont.).

The design criteria for the permanent retrofit included:
• Robust system with a built-in backup
• Minimize impact to the portion of the bridge already built
• Components can be inspected and maintained/replaced with minimal interruption to traffic
• Minimal aesthetic impacts
The solution chosen meets all of these criteria and consists of a linkage anchoring the girder to the bridge foundation. The design concept includes vertical bars placed at each girder end with pins at top and bottom of the bars, allowing longitudinal movement and rotations but preventing uplift.  This concept has been used on other on cable-stayed bridges, including ones in Quebec and Kentucky. The permanent retrofit has been reviewed by Associated Engineering (Ont.) and they have agreed it is appropriate for the Nipigon River Bridge. The permanent retrofit will allow the bridge to function safely for its intended useful life. The ministry is also developing an inspection manual specific to the Nipigon River Bridge. The permanent retrofit means that the existing bridge design will be modified. The south half of the bridge will be constructed using only the permanent retrofit – the original shoe plate and bearing will not be needed. The permanent retrofit will be installed on the existing north half of the bridge which will allow for the removal of the temporary support.

A new permanent retrofit design has been developed to address the issues identified by the engineering reviews. This permanent retrofit will ensure that the Nipigon River Bridge functions safely for road users throughout its intended useful life. This design will be reviewed by Associated Engineering (Ont.) and ministry bridge engineers and will also be applied to the south portion of the bridge, where construction is progressing well. Preliminary estimates for the initial repair work in February and the final repair to be implemented are between $8 and $12 million.

This fall, the province will also move forward with a formal route planning study and Environmental Assessment for an emergency detour route. This is expected to take approximately 18 months to complete. While our government has full confidence that the retrofit, once complete, will ensure that the bridge is safe for all users, this detour route would provide an alternate option for local residents and businesses in an emergency.

The safety of the travelling public is of paramount importance to our government, and we appreciate the ongoing patience and understanding of the people who use and depend on this bridge.”

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