The state of U.S. infrastructure and need for funding to improve it has been highlighted through efforts of national engineering organizations such as the American Society of Civil Engineers’ Infrastructure Report Card and the American Council of Engineering Companies’ advocacy for robust infrastructure investment. These organizations and others have identified work on bridges as a significant need and provided data in the form of structural deficiency ratings to validate their findings (25 Years of U.S. Bridge Data Reveals Steady Condition Improvements, Big Challenges Ahead; /25-years-of-u-s-bridge-data-reveals-steady-condition-improvements-big-challenges-ahead).
In addition, agencies such as the Washington State Department of Transportation are confronted with a need to replace many culverts with short-span bridges as a result of its 2013 federal court injunction related to required improvement to waterways that impede aquatic organism (fish) passage.
The direct construction costs and societal impacts — delays, alternate routes, etc. — required to replace deficient culverts and short-span bridges are well known and infrastructure-replacement programs have been funded assuming these costs are fixed. Fortunately, industry has scrutinized this need and harnessed well-known engineering principles for soil-structure interaction analysis and design of culverts to develop corrugated structural steel plate buried bridge systems (buried bridges) that are installed for a fraction of the cost and with many other advantages compared with conventional short-span and medium-span highway bridge structures.
These reasons provided impetus for efforts by the Transportation Research Board (TRB) Technical Committee AFF70 Culverts and Hydraulic Structures that I chair to establish a buried bridges subcommittee tasked with addressing necessary research for the transportation industry. A 2014 TRB webinar introduced the concept of buried bridges and a follow-up 2016 webinar introduced the structural design methods, along with applications and case studies (http://www.trb.org/Calendar/Blurbs/174243.aspx). This article defines buried bridges, identifies typical applications, and provides guidance to engineers on the structural design and inspection of structural steel plate buried bridges.
A buried bridge is a buried structure supporting a roadway that derives its support from the interaction of the structure and surrounding soil with an unsupported span (bridge length) greater than 20 feet. Structural steel plate buried bridge shapes include round, ellipse, arch, or box geometries. The structural steel plate has common corrugation sizes of 6 by 2 inches for shallow-corrugated plate and 15 by 5-1/2 inches, 16 by 6 inches, or 20 by 9-1/2 inches for deep-corrugated plate. Plates are manufactured in various sizes and at required radii and are bolted together onsite. ASTM A761 provides material properties for plates, bolts, and nuts and ASTM A796 provides design information. Typical plate yield strengths range from 33,000 psi to 44,000 psi and typical zinc-coated plate thicknesses range from 0.111 inch to 0.505 inch.
Open-bottom buried bridges have a structural plate wall supported on footings and are surrounded by soil up to the roadway elevation, allowing placement of conventional flexible (hot-mix asphalt) or rigid (concrete) paving. The term “buried” is used to identify the need for design and analysis methods that consider static soil-structure interaction (SSI). The term “bridge” is used to distinguish these structures from culverts and to indicate that the unsupported spans exceed 20 feet, and in some cases approach 100-foot span, requiring the same safety considerations as conventional bridges. Buried bridges must be inspected at regular intervals not to exceed 24 months in accordance with the Federal Highway Administration (FHWA) National Bridge Inspection Standards (NBIS) 23 CFR 650.311 (a) – routine inspections.
Buried bridges are appropriate for the same applications as short-span and medium-span conventional highway and low-volume road bridges, including new installations, replacement of existing bridges, or for bridge rehabilitation. Specific applications include use for restricted site access and remote locations, staged construction and curved applications, wildlife and aquatic crossings, pedestrian access tunnels, emergency or temporary detours, as a single-span solution to daylight multi-cell culverts, and for cases of extreme live loadings such as heavy mine vehicles or runways.
Advantages relative to conventional bridges include reduced costs, accelerated design process with production of project specifications and drawings of two to three weeks total, rapid shop fabrication and reduced materials shipment needs, accelerated bridge construction (ABC) with one- to three-day installations, reduced onsite manpower and expertise for installation, adaptable architectural end treatments, improved environmental characteristics including resilience and sustainability by use of onsite materials for backfill, significantly reduced maintenance with no bridge deck and no expansion joint, and increased tolerance for foundation settlement, often allowing reuse of existing foundations.
Design includes evaluation of the function and site constraints to select buried bridge size and shape, evaluation of durability needs, appropriate SSI parameters, and specification/design of end treatments. Buried bridge size is typically specified by rise and span and is determined by the necessary hydraulic opening or roadway clearance envelope. The length is determined by the required crossing (roadway width), waterway alignment, and selected end treatments.
Buried bridge structural design guidance is provided in the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications, Section 12. Design combinations consider soil dead loads (weight of soil over the buried bridge) and vehicle live loads (AASHTO HL-93, similar to conventional bridges) as shown in AASHTO Section 3. For open-bottom arch sections, AASHTO Section 10 provides guidance for foundation design.
Live loads are assumed to be distributed over the height of the fill from the pavement surface to the top of the buried bridge structure such that the design live load patch applied to the top of the buried structure is larger than the load patch applied at the pavement surface, but with the same total magnitude. This approach is similar to distribution width calculations used for live loads on conventional bridges. Buried bridge structural design assumes a long bridge length (not span), allowing design of a 2D cross-section, similar to culverts. End treatments are designed independently from the buried bridge structure and may support skews and bevels, or provide for any selected architectural finish.
Buried bridge structural design of shallow-corrugated arches (AASHTO Section 12.7) is based on compression thrust in the wall and includes evaluation of the wall area yield, global buckling, capacity of the bolted seams, and minimum flexibility for safe installation and backfill. Design for thrust in each leg of the arch uses closed-form solutions, where demand thrust in each leg (each side) is calculated as half of the total soil load and live load on the arch. Wall area capacity is based on material yield times the wall area. Buckling capacity is determined using equations accounting for a soil-supported structure. Bolted seam compression strengths are determined in physical testing and published in ASTM A796 for the range of available corrugation sizes, plate thicknesses, and bolt diameters. Maximum allowable plate flexibility is calculated based on the wall flexural (bending) stiffness and is limited to values found to be safe in practice for corrugated plate long-span buried bridge structures.
Structural design of shallow-corrugated boxes (AASHTO Section 12.9) is based on evaluating bending due to the large radius (relatively flat) top plate. These structures often include circumferential stiffener elements (ribs) with added bending capacity and may also include buried concrete relieving slabs in the soil above the box to reduce the effects of concentrated live loads on the buried bridge. AASHTO provides equations to design these structures and has geometry limits (span and radius combinations) for the overall structure and for plate sections.
Structural design of deep-corrugated structures (AASHTO Section 12.8.9) is based on evaluating the interaction of compression thrust and bending, global buckling, and seam strengths and requires rigorous evaluation of the SSI using finite element analysis (FEA). Two-dimensional (2D) FEA is acceptable for most applications. The FHWA developed the computer program CANDE (Culvert Analysis and Design) specifically for design of buried culverts (of all spans) and it has the necessary soil and buried bridge material models to complete buried bridge designs. When conditions include significantly varying structure geometries, special loadings, or change in backfill conditions along the structure length, 3D FEA may be required to calculate the demand forces for the AASHTO design method.
Construction of buried bridges requires onsite inspection, similar to conventional bridges. Manufacturers and suppliers of the structural plate provide the installation and monitoring instructions, which typically include measuring structure deflection throughout the backfill and verifying backfill type and compaction. Buried bridges may be load rated using the same approach that is used for conventional highway bridges in combination with the use of 2D FEA SSI.
In service, these structures are inspected using methods developed for culvert systems (see Final Report for NCHRP 14-26, Development of the New Culvert and Storm Drain System Inspection Manual, J.L. Beaver and M.C. Richie, 2016), which is currently balloting through AASHTO for publication as the AASHTO Culvert and Storm Drain Inspection Guide.
Further information on application and design of buried bridges can be obtained from the National Corrugated Steel Pipe Association (http://ncspa.org), the Short Span Steel Bridge Alliance (www.shortspansteelbridges.org), or from TRB AFF70 (www.trb.org/AFF70/AFF70.aspx; Chair: Jesse L. Beaver).
Jesse L. Beaver, P.E., P.Eng., principal and vice president with Simpson Gumpertz & Heger (SGH; www.sgh.com), is a soil-structure interaction engineering consultant specializing in the performance of buried structures, structural materials, and bridge construction. Prior to joining SGH, he served as assistant state construction engineer, Bridge, with the Washington State Department of Transportation. He can be contacted at firstname.lastname@example.org.