The largest, and arguably most complex, bridge project on New Hampshire’s 20-mile I-93 Corridor Widening program is underway, as R.S. Audley replaces a 60-year-old span over the Interstate in Londonderry.

Audley’s $62 million contract (Project 14633D) with New Hampshire Department of Transportation (NHDOT) reconstructs and widens about 2 miles of I-93 northbound and southbound, including replacing the aging Route 102 Bridge. Among the construction items of NHDOT’s ambitious I-93 program is the replacement of 20 bridges and the rehabilitation or widening of 23 others. Of these 43 structures, the Route 102 Bridge crossing over both barrels of I-93 in Londonderry stands out not only for its proportions but its intricate geometry and innovative technology as well.

Designed by WSP Parsons Brinckerhoff (WSP), the Route 102 Bridge is being constructed adjacent to the existing bridge, which Audley will demolish once the new structure is operational. This allows traffic on the old structure to continue relatively unimpeded while construction takes place. 

 

Unusual Configuration

The new bridge is a two-span structure approximately 340 feet long by 112 feet wide. It is supported by comparatively small concrete abutments (A and B) that are shored by mechanically stabilized earth walls (MSE), and a pier located about 150 feet from abutment A and 190 feet from abutment B. Twelve, 6-foot-deep built-up plate girders support a concrete deck.

Bridge geometry is highly unusual, challenging steel fabricators and deck material suppliers as well as surveyors who lay out the structure at the site. This bridge is curved horizontally with a 1,750-foot radius, requiring the deck to have substantial superelevation to overcome the force that tends to make vehicles skid towards the outside of the curve. WSP called for the deck at the outside edge of the curve to be approximately 4.67 feet higher than the deck at the inside edge, producing a surface sufficiently angled inward so vehicles don’t have to rely on friction to keep them on the road.

This is good news for safety reasons, but shaping superelevation for this particular bridge brings more complexity to a structure already configured with an intricate design that includes a relatively steep 4 percent pitch, a skewed alignment, and girders with both horizontal and vertical curvature.

As for alignment, the new bridge crosses over both barrels of I-93 at a skew of about 20 degrees, so its abutments and pier are aligned in parallel with the I-93 travel lanes below but not perpendicular to the direction of Route 102 traffic flow.

And fabricators also have to fashion curved camber in the girders. Camber is the difference between the shape of the girder under full dead load at outside temperature, and its shape at no-load condition in fabrication temperature.Typically, steel plate girders are I-beams made up by welding together separate structural steel plates rather than rolled sections. Camber curvature is produced by reducing the length of girders’ web plates during the fabrication process to compensate for dead load deflections in the field. In this case, beam flanges are 1-inch thick and 20 inches wide, while webs consist of 5/8-inch-thick plate.

Casco Bay Steel is fabricating more than 1.5 million pounds of this structural steel, all of it metalized with zinc alloy and seal coated for better corrosion protection than provided by simply painting. Metallizing is the general name for the technique of coating metal on the surface of objects such as bridge members.

 

Precast Plus CIP With Stainless Rebar

The structural steel supports a composite bridge deck consisting of 3-1/2-inch-thick  precast concrete panels topped with 5-1/2 inches of cast-in-place concrete. Joseph P. Carrara & Sons of Vermont is supplying 439 deck panels totaling more than 32,000 square feet, while Redimix Companies of Maine and New Hampshire is providing more than 970 cubic yards of ready-mixed concrete for bridge components including footings, piers, pier caps, wingwalls, approach slabs, abutments and the deck overpour. Where concrete pumping is required it is performed by Northeast Concrete Pumping using Putzmeister pumps.

The cast-in-place deck overpour was initially 5-3/4 inches thick but workers then diamond-ground away about 1/4-inch of deck to produce an anti-skid surface. Asphalt pavement is not used because the pitch and curvature of the bridge is such that designers believe traffic would shove the asphalt pavement on the steep curved surface. 

Rebar corrosion was another concern of bridge designers, so they specified stainless steel reinforcing for the deck. Stainless steel reinforcing bars are experiencing increased use in concrete projects because of the material’s inherent properties, including corrosion resistance, depending on the particular type specified. For this bridge, designers specified alloy duplex 2205, said to be among the most corrosion resistant stainless steel reinforcement alloys on the market.

Barker Steel – a Harris Rebar company, is providing more than 228,000 pounds of stainless steel rebar for the bridge project.

 

Robot Runs Tedious Tying

With more than 32,000 square feet of deck overpour concrete required, numerous man-hours would be spent in tying the stainless rebar; however, Audley is avoiding this tedious, time-consuming process by letting a robot do the work.

Audley leased a TyBot autonomous rebar tying machine from TyBot LLC, a wholly owned subsidiary of Advanced Construction Robotics Inc., a Pittsburgh based robotics company. The machinerides along the existing concrete finishing screed rail support and identifies and ties rebar intersections. It handles up to #8 by #8 rebar intersections, bridge widths between 10 feet and 145 feet, and up to 12 percent grade and 12 percent superelevation.

According to Scott Stevens, P.E., Vice President of R.S. Audley, the machine has worked very well.

“This robotic rebar tying machine relieves our bridge group personnel from this tedious task, and frees them up for more complicated work,” Stevens said.

 

Ubiquitous Rock

NHDOT advertised Audley’s Contract 14633D in September 2016 and awarded the job to the Bow contractor in December of that year. The contractor started construction at the bridge site early in January 2017.  One of the first orders of business was excavating the roadway and bridge sites, which were marked by substantial ledge. Maine Drilling and Blasting (MD&B) was subcontracted to remove the rock. Brian Smith, Audley’s Project Manager, said that MD&B drilled and blasted an estimated 66,000 cubic yards of rock for the roadway portion, about 11,000 cubic yards of utility trench rock, and some 8,500 cubic yards of rock for the Route 102 bridge site. Smith noted that blasted rock was crushed by subcontractor Continental Paving using a Telsmith crusher. The rock was crushed to produce 1-1/2-inch stone for recycling on site as road base.

As noted by NHDOT, State Police rolling road blocks are utilized on I-93 for ledge blasting operations. Lane closures are allowed only at night. Truck crossings and alternating one-way traffic take place on local road crossings, and on Route 102, two lanes of traffic are maintained during daylight hours, with lane closures allowed during off-peak traffic hours at night. Barring any unforeseen circumstances, Audley is expected to complete this contract sometime in September 2020.            

Derek Moynihan is Audley’s Foreman for the challenging Route 102 bridge construction, while Scott Perry is the contractor’s Chief Surveyor for determining and confirming elevations and locations of hundreds of critical layout points.

 

Four Major Contracts

Audley has a second NHDOT contract, Project 14633B, that is part of the Salem-Manchester I-93 corridor widening. This one, awarded for a bid of approximately $49 million, rebuilds and widens another 3.5 miles of both I-93 barrels.  These two contracts are the southernmost of four major contracts currently reconstructing and widening a large portion of the  I-93 corridor project. The other two contracts are Alvin J. Coleman & Sons’ $46 million Contract 14633H, covering 3.3 miles of northbound and southbound I-93, and Weaver Brothers Construction’s $34 million Contract 14633I involving about 2 miles of both I-93 barrels. Weaver Brothers is also responsible for the final paving of the Interstate through the Exit 5 area.

 

Photos courtesy of Scott Stevens, Vice President, R.S. Audley Inc.