Even after decades of use, engineering materials can find renewed longevity and fresh interest within the bridge and roadway design community. This has certainly been the case with structural plate. And although steel structural plate continues to be a viable and cost-effective material for bridges, culverts, and pipelines, aluminum is capturing the attention of civil engineers as an engineering material for infrastructure rehabilitation and reconstruction. The following three case studies highlight the varied use of aluminum structural plate and the benefits that the material affords highway and municipal engineers.
A multi-phase drainage-improvement project in Canfield, Ohio, is expected to alleviate local flooding by upsizing an existing 48-inch concrete pipe that no longer was adequate to provide the drainage relief needed.
Options precluded the use of larger-diameter pipe because of low burial depths over much of the project area. Under these restrictive conditions, the design engineer consulted a local Lane Enterprises, Inc., representative to determine if a solution could be found in a specialty product used for decades but virtually unknown by many of today’s designers — structural plate.
Offered in many shapes, some capable of exceeding a 50-foot span, structural plate is a versatile engineering material. Because steel and aluminum structural plate are available, the engineer can choose the material best suited to the application.
The initial phase of the Canfield project used a 14-foot, 8-inch by 4-foot, 1-inch aluminum structural plate box culvert with a full invert, which provided the additional hydraulic capacity needed without breaching the height restriction. Subsequent phases will primarily use a comparable 13-foot by 3-foot section. Fabrication of the full range of fittings — inlets, elbows, manholes, et cetera — was also managed by the structural plate manufacturer.
Structural plate is usually shipped to the site in curved plates and field assembled into its final shape by bolting. However, a requirement to backfill excavations at the end of each day, coupled with an installation time constraint, challenged conventional installation methods. With the plate manufacturing facility in close proximity to the job site, economy was achieved by having Lane Fabrication (a division of Lane) assemble the box culvert in 9-foot sections at its Pulaski facility, and ship the sections to the job site.
The lighter weight of aluminum structural plate created a significant handling advantage, especially compared with concrete box culverts. Contractor Foust Construction was able to install four or five of the 9-foot sections per day and stay ahead of an aggressive completion schedule.
At the same height as a 48-inch concrete pipe, the box culvert shape provided more than four times the cross-sectional area, significantly enhancing hydraulic capacity while also creating flood storage previously unrealized. With inclusion of stiffening elements, low-profile shapes can be constructed to withstand anticipated loading. Reinforcing ribs bolted to the corrugations along the top of the structure addressed concerns about high service loads at low burial depths.
Transportation agencies across the country must devote scarce resources toward the programs necessary to ensure aging bridges are appropriately maintained or replaced. The North Carolina Department of Transportation (NCDOT) has met this challenge with an innovative bridge maintenance replacement program that achieves the desired economy by minimizing design and construction costs using aluminum structural plate.
Categorically, small bridges include culverts of 54 inches in diameter and larger. Integrating an aluminum structural plate headwall into the selected culvert size and shape creates an installation-friendly bridge replacement solution. With no rebar schedules, mix specifications, pours, frequent testing, or cure times, aluminum structural plate headwalls are an excellent alternative to concrete.
Engineering costs are streamlined by incorporating design and resident engineering services into the material bid specification — the manufacturer is responsible for engineering design, drawings, and construction supervision. The headwalls are structurally engineered as a cantilevered retaining wall, with the design calculations and dimensions incorporated into a submittal subject to NCDOT approval.
Construction costs are mitigated by managing the bridge replacements as maintenance projects using NCDOT equipment and workers. The aluminum structures are easily managed, requiring no rental equipment or specially trained operators. A typical installation requires little more than an excavator and loader.
NCDOT maintenance workers assemble the structures onsite under manufacturer supervision. Assembly is usually completed on the same day. Ideally, the structure is completely assembled to form a one-piece, drop-in solution. In most cases, the road remains open during assembly. At times, one lane may be needed, requiring some minor traffic coordination. To expedite project completion, excavation can be concurrent with assembly.
Two-barreled structures can still be assembled and mobilized as an integrated unit. However, experience with headwalls several hundred feet long shows that segmented construction is to be expected, albeit minimized to the extent possible.
Once the bridge or segment is assembled, the unit can be hoisted and placed in the excavation. When the structure is properly set, the excavation can be backfilled and prepared for road restoration.
Depending on the design, there may be some finishing touches before placement of the road subgrade. Deadmen anchors are tied back from each headwall or the headwalls are directly connected via threaded rods and turnbuckles to further stiffen the headwall-culvert-backfill interaction system.
Once restoration is complete, the roadway may be immediately opened. Road closures typically range from one to four days.
Headwalls are fabricated and shipped with the culvert stub completely welded to the periphery, many with a skewed alignment to match the in situ road crossing. The barrel may be aluminum, aluminized steel, or in some instances, aluminum structural plate to accommodate larger spans.
At a Metro-North railroad culvert crossing in Naugatuck, Conn., the Connecticut Department of Transportation (ConnDOT) was tasked with rehabilitation of severely corroded twin concrete arches. The state of Connecticut purchased its portion of the New Haven Line from Penn Central in 1985, assigning ownership and maintenance responsibility to ConnDOT. Subsequent maintenance assessments of culvert crossings are now culminating into a number of rehabilitation projects to ensure the continued operation of this railway. Because of its size, condition, and location, the culvert crossing of the Waterbury Branch in the Borough of Naugatuck presented some interesting challenges.
The Waterbury Branch is located along the Naugatuck River, and this particular crossing has historically served as the outlet for the Long Meadow Pond Brook. As is commonly done to accommodate development, the stream was subsequently enclosed upstream of the crossing. In fact, a concrete vault at the upstream side of the crossing is under the three-story Uniroyal Rubber Factory Building.
With the river maintaining a water level several feet deep in the culverts and access crippled at the upstream opening, rehabilitation options were restricted. The solution was found in aluminum structural plate.
The existing concrete arches each had a 20-foot span and an open bottom. An 18-foot, 5-inch by 11-foot, 6-inch aluminum structural plate pipe arch shape was chosen for the reline material. The south culvert required 109 feet of relining; the north culvert required 111 feet.
Structural plate was shipped to the site in curved plates and field assembled into its final shape by bolting. With flow bypassed into the north culvert and a cofferdam constructed around the south culvert outfall, the south culvert was completely lined with structural plate in one day. A reversal of this arrangement facilitated installation of the north culvert.
Once the structural plate is assembled and the bolts are tightened, the pipe arch is precisely positioned with the use of alignment bolts. Alignment bolts around the periphery and along the entire length of the relining plate ensure the shape is properly situated within the host structure.
After the pipe arch has been completely aligned into its final position, preparations can be made to grout the annular space. The relining plate includes a number of grout plugs around the circumference and along the length to facilitate this process.
Because of the buoyant forces generated as the annular space is filled, additional bracing may be necessary to help counteract any associated movement. Grouting is typically staged into multiple lifts to lesson any flotation effects, with adequate setting time allowed between each pour.
The project included complete rehabilitation of the concrete headwall. This final touch brought the appearance of the culvert restoration to a “like new” condition.
Jerome S. Silagyi, P.E., is manager of technical services for Lane Enterprises, Inc., in Camp Hill, Pa. He can be contacted at email@example.com.