Unexpected movement in supporting erection truss caused fall in segment of South Norfolk Jordan Bridge - recovery under way
By Peter Samuel
One of two steel trusses used to support precast concrete box girder segments during construction moved unexpectedly June 21 at the new South Norfolk Jordan Bridge causing a segment to fall about 50ft to the ground below, and other segments to move. No one was seriously hurt but it was an embarrassing incident which forced the postponement for a month or so of opening ceremonies, a 5K walk and 'bridge bash' that were long scheduled for this Saturday July 14.
The bridge segments are 52ft wide (the width of the deck) by about 7ft long in the direction of the roadway deck, with a depth in the center where the trapezoidal box shape gives them structural strength that varies between 7ft and 12ft depending on the span length.
On either side of the structural 'box' each segment has projecting 'wings' adding width to the deck.
With piers typically about 150ft apart, twenty or so box segments are used per span, supported initially by their 'wings' resting on the parallel erection trusses, the box sections hanging between.
The span is made permanent by winching steel cables the length of the span through ducts in the cell walls in a process called post-tensioning.
This heavy cabling ties the segments together after which the erection truss can be moved on for work on the next span.
In span by span construction as used in South Norfolk each span is self-sufficient, so the collapse in one span had no implications for other 30 or so spans already built. The whole bridge has 35 spans, only the central span at 375ft notably longer than the rest at 150ft to 200ft.
Accident cause "being further assessed"
Kevin Crum general manager at the bridge site tells they can't say why the erection truss moved.
"It is being further assessed," he says.
The iBeam construction camera picture 3:55pm June 21 just before the incident shows the truss ends hanging free and unsupported at their ends just as the segment loads are increasing.
Truss ends usually tied in at ends
No other diagrams or pictures that we can find show free unteathered erection truss ends in this kind of construction.
They all show spans under construction with the truss ends firmly tied in to the end pier and to one another.
So absent any more persuasive explanation of the incident you have to suspect the unusual free floating truss ends may have been the cause of the truss moving, and hence of the segment drop.
Anyway in the wake of the accident they ordered new precast segments to replace those damaged, and they ordered a new erection truss to replace the damaged one.
Just missed July 4 for spectacular demo
Then last weekend in the afternoon of Saturday July 7 demolition specialists brought down the damaged truss with simultaneous explosions at key points in the damaged structure.
(They should have done the demo three days earlier as part of Figg Engineering's contribution to July 4 festivities!)
Kevin Crum, General Manager of the South Norfolk Jordan Bridge is quoted in a statement on the demolition: "This was a planned and purposeful operation in the best interest of a strong safe bridge. We want to thank the city of Chesapeake for their outstanding support in maximizing public safety during this operation. We would like to especially thank the Police, Fire and Public Works Departments for their dedication and professionalism."
Crum is also quoted: "Safety has been a top priority throughout construction with more than 370,000 work hours logged to date, the project construction has an exceptional safety record."
Replacement truss going up
Having removed the damaged erection truss July 7 they are now putting up the replacement truss that has been delivered in sections this week.
Also replacement segments - the precast concrete box girder segments - are being made to replace those damaged in the drop incident June 21.
The new toll bridge is privately owned so the project sponsors bear the cost.
At the time of the segment drop they had all but 4 end spans on the approaches finished - 90% of the 5,350ft length of the bridge.
They still hope to open the grand new bridge some time in August.
BACKGROUND: Figg based out of Tallahasse FL is the leading engineering firm on segmental concrete box girder construction in the US and has a slew of incident-free projects to its credit. Box girder construction is increasing favored for its speed and economy, and aesthetically for its spare elegant lines.
earlier report for more background:
on box girder construction:
Figg engineer on various methods for box girder construction: