Mac Hoke Bridge Replacement

After flood damage in 2020 and subsequent post-inspection closure, the 135’ through truss required replacement. The need was prioritized after a casualty train collision occurred on the 30-kilometer detour route. Steel was selected as the material of choice due to project remoteness, speed of construction, installation versatility, and aesthetical characteristics.

Project Challenges:

Reconstruction near the Historic Oregon Trail route presented many logistical challenges. Equipment access from the north was restricted by load posted bridges. All bridge components and equipment had to traverse the final 30 miles on well-maintained County gravel roads to the south side of the site. The rugged environment and lack of USACE permit 404 necessitated all work activity above the mean low lower water (MLLW). Migratory swallows also added a critical installation deadline.

Project Solutions:

Modular Truss units played a key role in the logistical transportation of the bridge to the site. Before shipment, 172,000 lbs of A709-50W were shop assembled per AISC best practices. An innovative slip fit camber disc and redundant pin connection allowed for ultra-fast field assembly, roughly 300 labor hours. The camber disc is engineered to provide 3 ½” of positive camber prior to loading; bridge deflection in use is near zero.

After delivery and erection, the bridge was launched across the Umatilla River on rollers. Longitudinal and lateral movement elements made it so the largest single piece of equipment on site was a 150-ton lattice boom crane. Contractor utilization of an underslung guided gantry was used to delaunch the old bridge and install the new. This concept negated the use of much larger cranage, thus saving the project upwards of $300k.

Project Results:

Administering separate contracts for the procurement of a Prefabricated Modular Steel Bridge prior to the Construction contract offered several advantages over the traditional design-bid-build procurement method. Prefab design allowed for industry innovation and speed of delivery. Construction costs of $2.1 million were split between 45% supply and 55% to remove and install the new bridge. The bridge arrived six months after order placement, and the bridge project was completed in three additional months.

Narrative:

The Weathering Steel bridge design aesthetically blends into the environment of the Echo Historic District. Resilient and tuff, the bridge and character of people in Umatilla County are well matched. A bluff outcropping gives passerby motorists a picturesque above-grade view of the bridge, the Cunningham Sheep Ranch, and the UP Railroad in the background. Nearby is a heavily utilized, 1907 constructed Two-Span railroad portal truss. Eight miles 8 miles upstream is another truss designed by Conde Balcom McCullough. McCullough, a leading 20th century bridge engineer, served as Oregon’s State Bridge Engineer from 1919 until his death in 1946. The new HL-93 rated bridge has significantly improved safety and mobility of farm equipment, accommodating all ODOT permit loads.

Sitework began after all bridge system components were delivered. First the contractor drove temporary piles outside of outside of MLLW and placed temporary crossbeam pile caps. Contractor then passed and set two sets of W36 x 210# x 120’ bolted steel diaphragm onto the crossbeams, slightly cantilevered and outside of the existing bridge footprint. Using Hillman mountable rollers, the 120’ underslung girder laterally slid underneath the old bridge. The same rollers were then moved from underneath to on top of the longitudinal 120’ underslung girder. Welded channel provided guides for the rollers to facilitate both the removal of the old bridge intact, as well as the launch of the new truss. Umatilla River runoff was high due to large snow pack the previous winter. Two weeks after the bridge opened to traffic on 5/3/23, migratory swallows returned and promptly began to nest.

The new bridge has a shallow depth less than 30” and an additional 13-foot span. This geometry provides greater freeboard, a major benefit to a previously flooded location. Due to the good condition of the existing abutments walls, which provide critical scour protection, great design care was put into having as little disturbance as possible. The truss twofold saved cost and impact to the approach road, keeping intact the scour wall as compared to a steel or concrete girder superstructure. Behind the old abutments, four 14” x

.500” pipe pile per abutment were driven, drilled, and filled with concrete. For a bridge project of this size to have only 9.5 yd³ of concrete is rare and further showcases the versatility of steel. On top of the pile is the abutment seat or pile cap of adjoined W14 x 82# x 25’. In addition to the superstructure, a modular backwall requirement was fulfilled by the awarded supplier. The backwall consisted of approx. 6,500 lbs of galvanized HSS tube, angle iron, and Steel Deck Institute forms. A ledge connection to the flange of the pile cap allows for field elevation adjustment of the backwall height, ensuring smooth riding transition on and off the structure. The bridge deck consists of 3” steel deck forms transversely welded to both stringers and floorbeams. The corrugations were filled with an aggregate base, compacted, and a 2½” average thick Hot-Mix Asphalt riding surface.

Umatilla County had previously implemented prefabricated modular steel procurement with great success. This alternative delivery method accelerates lead time, allows for iterative engineering thus reducing design cost. In many ways this is the Agile form of Project Management (PMI) in practice within the steel bridge world. With a desired span close to 150’, Umatilla County's RFP for a Prefabricated Modular Bridge was a bold departure from the status quo. Instead of dictating specific design elements, the county focused on the desired performance outcomes. This allowed the bridge system supplier to be innovative in their approach, ultimately resulting in a more cost-effective and efficient solution. The letting occurred during a very volatile steel market, and only one fixed price bid was received. The bridge system supplier was transparent that this was a new concept backed by tried-and-true fundamentals of the WWII Bailey Bridge. To protect itself, the County requested a Supplier’s Bond, which was provided. The financial instrument gave a security instrument to ensure accountability and success. The fixed bridge superstructure system, including design and load rating equated to $331/SF delivered.