Bridging “Gaps” on the Manhattan Bridge
How to bridge 8’ wide gaps? This seems an easy task for structure engineers. However, it’s not so simple if these gaps are at the towers of a suspension bridge with a main span of 1,480 ft long and carries daily traffic of over 85,000 vehicles. CHI was retained by Skanska, the General Contractor, to design temporary roadway plates, i.e., temporary expansion joints, for the Manhattan Bridge when the eight (8) existing modular joints at towers and anchorages in the North Upper Roadway were being replaced, creating gaps in the roadway up to 8’ wide.
There are three main aspects that needed to be considered in the design of these temporary roadway plates: structural adequacy; driver safety; and construction efficiency.
For Structural Adequacy, the following design criteria were adopted:
Tandem axles that each weigh 25 ton in total, plus 75% impact loading
±1 ½” longitudinal movements for the short duration of construction
Secure connections to the existing decks for frictional and breaking forces
On the Driver Safety aspect, the design included:
Gentle 1:18 slop asphalt ramps on both sides
Low profile temporary roadway plates and bearings (4 3/8” in total thickness), as shown in the figure below
Durable anti-skid coating
The ¾” deck plate and C10 channels form a very shallow (3 ¾” tall) steel orthotropic deck, which is also very light, about 30 psf. Given this unique design, many considerations were given to its detailing, such as welding details and access, etc.
To improve Construction Efficiency, the following features were adopted:
Simple fabrication due to the use of plates and rolled sections, as well as fillet welds
Four individual roadway plates, each measured 5’ wide and 9’ long, were used for the two-lane roadway. Each plate weighs only about 1,500 lbs, which was easy to maneuver in the field
Anchor bolts securing the plates to the deck were easy to remove and reinstall
Asphalt ramps were easy to install and remove
The temporary roadway plates/expansion joints had been in service on the Manhattan Bridge for several months before decommissioning. They performed very well, providing safe passages for millions of cars and trucks.
Acknowledgements: New York City Department of Transportation (Owner); Skanska Civil USA (General Contractor); Unicorn Construction (Sub Contractor for expansion joint work)