Bethlehem Steel: Truss Bridges Report pg2

The new Central Bridge, carrying US 27 over the Ohio River between Newport, Kentucky and Cincinnati, Ohio was designed using a team concept with Hazelet + Erdal, Inc., serving as lead consultant, in association with Burgess & Niple, Limited and Balke Engineers. This three-span continuous truss has spans of 574’, 850’ and 425’ for a total length of 1,849’. The 70’ constant depth trusses spaced 67’-0" center-to-center provide a usable deck width of 60’0" with concrete barriers on each side. An 8’-0" sidewalk cantilevers from the outside of the truss on each side of the bridge.

The substructure for the main spans consists of four piers with two of the piers in the water. The main river piers, Piers B and C, are two-column units with a heavy strut beam at the top. These piers were designed without webwalls in order to maintain an open view of the panoramic Cincinnati skyline. The anchor piers, Piers A and D, are founded on steel H piles driven to rock. Both river piers have tremie seals keyed 8’ into solid bedrock.

To minimize deck participation in truss strains, the bridge deck was divided into nine four- or five-span continuous units. Eight lines of W24x68 or W24x76 (Grade 50) stringers support the 8" thick composite reinforced concrete slab. Welded plate girder floorbeams (Grade 50) are spaced at 41’-0" in the north span and 42’-6" throughout the remainder of the bridge. With the exception of four floorbeams in the super elevation transition at the south end of the truss, the 1/2 inch thick floorbeam webs taper from 64 inches at the center to about 56 inches at the truss connections. On each side of the floorbeam, 1/2 inch thick transverse stiffeners have been welded to the web. The inspection walk access hole located at the center of each floorbeam measures 30 inches by 18 inches.

The stringer to floorbeam expansion bearings utilize elastomeric pads throughout the truss. These relatively maintenance free bearings are designed to allow movement without the usual problems of rust or "freezing up" associated with steel-to-steel bearings.

Data For Kentucky Longspan Steel Truss Bridges
Bridge Name	Structure Type	Span(s)	Deck Type	Deck Width	Bid Date	Approx. Cost/sq. ft.	Comments
Portsmouth	Cantilever Through Truss	509'-900'-509' (Plus 7-PC "I"-Beam, and 4-Plate Girder Approach Spans)	8" Conc. Slab	51'-3.5"	1/18/85	$125	4-Lane Alternate Cable Stayed
Central Bridge	Continuous Parallel Chord Through Truss	574'-850'-425' (plus 6-PC "I"-Beam Approach Spans)	8" Conc. Slab	83'-1"	11/15/91	$136	4-Lane Alternate Cable Stayed
Clays Ferry	Deck Truss	225'-2"-321'-8"- 448'-0"- 321'-8"- 225'-2" (plus 2-PC "I"-Beam Approach Spans)	8" Conc. Slab	125'-11.5"	10/22/93	$120	6-Lane No Alternate

Designers selected a standard Warren truss scheme to compliment the clean lines of the parallel chord design. All of the truss members are 24 inches wide with depth varying from 20 inches to 32 inches. Grades of steel used in the welded "H" and sealed box truss members vary from Grade 36 in lower stressed members and bracing to Grade 70W in the highest stressed members such as upper and lower chords. Most of the chord members and compression diagonals are sealed box members. Tensions upper chords, verticals, tension diagonals, laterals, sway frames and struts are primarily welded "H" members.

Demonstration Project Features
Innovative Use of Materials and Low Maintenance Features

The most unique feature of the new Central Bridge is the use of different strength steels to correspond to varying stress levels throughout the structure. Designers used AASHTO M270 Grade 70W, a high strength quenched and tempered, low alloy steel in areas subjected to high stresses and negative moments, and AASHTO M270 Grade 36 and Grade 50 in low and moderate stress level areas. Nearly 20% of the approximately 5,800 tons of structural steel used in the bridge in Grade 70W, 50% in Grade 50 and the remaining 30%, Grade 36. The new Central Bridge will be the first major truss bridge in the country to make use of Grade 70W steel. Bethlehem supplied plates for this bridge as follows: 654 tons of Grade 70W, 1,307 tons of Grade 50 and 367 tons of Grade 36.

Both special materials and low maintenance features were incorporated into the deck. Type K expansive cement is being used in the deck to minimize shrinkage cracks. Concrete using this type of cement expands against the internal reinforcement during the 7-day moist cure period and then shrinks uniformly to zero expansion/contraction, significantly reducing shrinkage cracks, the amount of road salt contaminated water reaching the reinforcing steel is minimized, thus prolonging the life of the deck.

The reduced number of drains in the deck at the north end of the bridge also minimizes maintenance requirements. Due to the recreational use of the area directly below the bridge, water could not be allowed to fall freely to the ground. By taking advantage of the relatively steep grade at the north end of the bridge, designers eliminated several deck drains and allowed the water to run along the curb to a double drain at the north end of the truss. Fewer deck drains means maintenance requirements to keep the drains open will be reduced. Use of neoprene and stainless steels in the expansion dam drains at Piers A and D also help reduce corrosion potential and subsequent maintenance.

The paint system has been designed for long life and low maintenance. The three coat system consists of an inorganic zinc primer, and epoxy intermediate coat provides the long life to the system and urethane finish coat provides the color and ultraviolet protection for the epoxy coat.

Using a parallel chord arrangement rather than a variable depth scheme facilitated erection. Elimination of the tall towers associated with variable depth trusses made unusually large erection equipment unnecessary. In addition, the similarity of the shapes and alignments of the steel pieces inherent too a parallel chord scheme allowed the contractor to establish relatively routine lifting procedures to expedite fit-up.

A variety of other techniques and materials were employed to facilitate construction. All of the bolts used throughout the structure were mechanically galvanized, minimizing blast cleaning of the truss joints before painting and improving corrosion resistance. In addition, the "turn of the nut" method was used on all shop and field bolts, speeding up the construction process. Other timesaving features on the bridge include the shop application of primer and the intermediate paint coat and stay-in-place forms used in the approach span decks.

Cost saving design and construction techniques were also applied to the substructure. Phase construction for the piers reduced the number of concrete column forms required. The deep pier strut beam was constructed in lifts rather then "full depth" to decrease the strength of falsework required.

The main piers were designed with individual footings, thus minimizing the size of the cofferdams and reducing the cost of these temporary structures.

Dynamic monitoring of the driving of the concrete piles originally planned for use in the Kentucky Approach substructure indicated unforeseen difficulties with the use of concrete piles and the plans were revised to specify steel bearing piles. This technique allowed engineers to recognize a potential problem and make necessary changes to fit the field conditions.

Consulting Engineer Associates:
Burgess & Niple, Ltd., Columbus, OH
Balke Engineers, Cincinnati, OH

Steel Fabricators:
Stupp Brothers Bridge and Iron Co., St. Louis, MO
Vincennes Steel Corporation, Vincennes, IN

Steel Erectors:
J. F. Beasley Construction Co., Grove City, OH
Armstrong Steel Erectors, Inc., Newark, OH

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�1998, Bethlehem Steel Corporation