Building a bridge where the old one stands means closing the road for a year.
Building it next door and moving it means closing the road for a fortnight.
That trade is obvious once somebody says it out loud, and it is not obvious at all how you move the thing afterward.
Because you cannot lift it. Nothing lifts 22 million pounds.
What you can do is slide it, if the surface underneath is right.
So the whole job comes down to what it slides on.
Friction is the enemy and the lift is the answer
The answer to that turns out to have almost nothing to do with pushing harder.
A structure this heavy sitting on temporary supports is welded to the ground by friction.
Steel resting on steel with that much load on it will not move sideways for any force you could reasonably apply. Push harder and you buckle something before it budges.
So the first move is to take the weight off the supports entirely. Hydraulic shoes under the structure are pressurized until they raise it about 10 inches, and from that moment the bridge is standing on the shoes and nothing else.
Each shoe then sits on a greased track, and the surfaces are chosen so that a very small horizontal force overcomes what is left.
Load is the other half of it. Spread across enough shoes, no single point on the structure carries more than it was designed for.
Forty two of them went under this one, grouped at four places where the piers and abutments would eventually take the load.
The bridge never gets stronger for the move. It just gets carried differently.
What was actually being moved
The crossing carries an interstate through the eastern side of Pittsburgh, over a street and a rail line below.
The old structure ran 846 feet in three spans, and the replacement was assembled beside the live lanes while traffic underneath kept running.
Seven girder lines make it up, and the longest single girder is 126 feet.
The frames that transferred load into the shoes were built from seven steel pieces each, some weighing 80,000 pounds on their own.
The slide distance was 102 feet, sideways, into the space the demolished bridge had occupied.
Everything had to land within an inch from pier to abutment.
Seventeen days from closed to open
The old bridge closed to traffic on July 10 and was dropped by a controlled blast on July 16, using roughly 2,500 drilled holes for the charges.
The slide began on July 19 and took under ten hours from start to finish.
The new bridge opened on July 27, which is 17 days against the 25 the contract allowed.
The project came in around 95 million dollars, and the contractor earned about 2 million in hourly incentives for finishing early.
The skidding was supplied by a Dutch heavy transport firm, which called it the heaviest bridge it has ever moved in this country.
New micropiles under the substructure go down as deep as 30 feet, and about 5,000 trees and shrubs are going in around the site.
The comparison that needs its numbers
One line in the coverage of this move is true and gets used misleadingly.
The same firm slid the containment arch over the destroyed reactor at Chernobyl, and that is a real link rather than a marketing one.
The scale is not comparable though. That arch weighed about 36,200 tons, which is more than three times this bridge, and it travelled 1,083 feet rather than 102.
It moved on 116 skid shoes pulled by 56 strand jacks working together, and it was 361 feet tall.
So the shared element is the method and the contractor, not the size of the job.
Slides of this weight also happen more often than the coverage suggests, in the way that numbers around a major crossing or a heavy lift get reported as unique, as the engineering trade account of the move sets out.
Why the closure length is the real product
Nobody pays a premium for a bridge that happens to arrive sideways.
They pay for the road being shut for two and a half weeks instead of two summers, and every hour of that has a cost the state can put a number on.
That is why the incentive was written by the hour, and why finishing eight days early was worth two million dollars to the people doing it.
The method spreads because the arithmetic of closure keeps getting worse as traffic grows, as the department’s own project page explains the approach.
The skid shoes were never the expensive part of this job.
The closed road was.
