To be or Not to Be… LRFD

That is the buried precast drainage and utility vault question.

By Eric Carleton, P.E.

In 2000, the Federal Highway Administration issued a memorandum stating the use of the load and resistance factor design method, commonly known as LRFD, for the design of bridges will be mandated for all federally funded bridge construction by 2007.¹ For most state departments of transportation and other agencies that relied on FHWA financial assistance, the massive bridge design change stopwatch started ticking.

What is LRFD?

This policy should have been expected since it was the culmination of an extensive 14-year research process started by state DOT bridge engineers and the National Cooperative Highway Research Program. In 1986, the initial project, NCHRP 20-7/31, “Development of Comprehensive Bridge Specification and Commentary,” commenced. The project included an in-depth synthesis of existing bridge design methods from all over the world along with analysis of gaps within the allowable stress design (ASD) and load factor design (LFD) methods described within the American Association of State Highway and Transportation Officials’ “Standard Specification of Bridges and Structures, 13th Edition.”

Importantly, another task was to develop an outline of what a new state-of-the-art design standard would require. This report and corresponding specification outline was presented to the bridge engineering community during the 1987 AASHTO Subcommittee on Bridges and Structures meeting. This provided direction for NCHRP project 12-33, “Development of Comprehensive Specification and Commentary,” which would ultimately develop the new design standard. The bridge engineering, inspection and design firm Modjeski and Masters, headed by John Kulicki, was awarded the work to write this specification and commentary. As described by Kulicki, this new proposed LRFD design method introduced “a new safer philosophy of safety” for bridge design.² This method enables the designer to apply respective design coefficients to both the load side and the structure side of the design equation. This change is based on research and statistical calibration depending on the type of structure and materials used, and implementation of a selected, acceptable reliability level for both the load and material.

Inside LRFD

Mathematically, the LRFD method is shown as:

Σηi γi Qi ≤ ɸ Rn
load side ≤ structure side

ηi = load modifier – relating to the product of ductility, redundancy and operational classification factors
γi = load factor – a statistically based multiplier applied to the force effects
Qi = force effect
ɸ = resistance factor – a statistically based multiplier applied to nominal resistance as outlined within the specification (Section 5 and 12, “Concrete Structures” and “Buried Structures and Tunnel Liners”)
Rn = nominal resistance

From the first published edition to the 8th Edition published in 2017, there has been continued research, calibration, deliberation, balloting and eventual adoption of LRFD for the various facets of bridge design beyond superstructures, including substructures and foundations.

Also included within FHWA’s aforementioned 2000 memorandum on LRFD design was the provision that read, “All new culverts, retaining walls, and other standard structures on which states initiate preliminary engineering after October 1, 2010, shall be designed by LRFD Specifications, with the assumption that the specifications and software for these structures are ‘mature’ at this time.” The implication is that LRFD would be the assumed design method for these structures in all DOT projects.

Precast concrete inlet and manhole drainage structures have proven structurally robust through decades of use. NPCA file photo.

Design complexities for standard structures

Although this method is considered to be a more refined analysis that incorporates the design considerations for complex bridge structures, it also adds design complexities to simple and reliable structures. Certainly, reinforced concrete pipe, box culvert and other standard precast concrete buried drainage structures have shown reliability in structural capacity and function through many decades of service. However, as stated within the FHWA requirements, culverts and other standard structures were to be designed per LRFD methods.

Within the LRFD specification, specific design guidelines are provided for buried structures within Section 12, “Buried Structures and Tunnel Liners.” Within Section 12.2 “Definitions,” a buried structure is described as “a generic term for a structure built by embankment or trench methods.” However, the section’s scope further narrows the focus of the chapter to: “Buried structure systems considered herein are metal pipe, structural plate pipe, long-span structural plate, deep corrugated plate, structural plate box, reinforced concrete pipe, reinforced concrete cast-in-place and precast arch, box and elliptical structures, and thermoplastic pipe and fiberglass pipe.”

Unfortunately, this scope leaves out an important segment of buried structures used on many, if not most, transportation projects: precast drainage manholes, inlets, catch basins and utility vaults. Consequently, this has created confusion within the design community as to the appropriate design method to employ when designing these types of precast concrete structures.

In the Jan.-Feb. 2015 issue of Precast Inc., Ron Thornton, P.E., executive director of the Precast Concrete Association of New York, wrote a compelling article titled, “Does the LRFD Standard Work for All Precast?”⁴ The answer is not necessarily clear. However, the good news is regardless of the method used to design precast drainage and utility structures, be it ASD, LFD or engineering judgment with LRFD, the finished product provides comparable structurally robust precast elements for the traffic and buried loading conditions encountered in modern transportation systems.

LRFD Concerns

Since public safety is not at risk as a result of using or not using the LRFD design method for buried precast drainage and utility structures, the primary issues at hand with the current AASHTO LRFD design method are:

1. National design homogeneity – The current design methodology of these specific buried precast structures can lead to various parameters and assumptions used to design the same precast structure in different states or regions. Though the different designs are still structurally sound, this can lead to economic consequences and is contrary to use of a national design standard.
2. Uniform design review acceptance – The current design methodology of these specific buried precast structures can lead some project submittal reviewers to reject this non-pipe design as noncompliant with FHWA policy. Other reviewers may believe these specific small structures fall outside the purview of LRFD requirements because they are excluded from the specification’s scope of work, or because they believe the LRFD design method is still within a maturing stage as described in the FHWA memorandum.

Precast inlet drainage structures waiting to be installed on a road project. NPCA file photo.

Design assistance and guidance

Many of the buried precast drainage structures in question are covered through various product standards by AASHTO or ASTM International. In 2010, Gary Munkelt, P.E., a consulting engineer with Gary K. Munkelt & Associates, authored an article published in the Summer 2007 Precast Solutions issue titled, “Code Breakers.”5 Included in the article was a description of the various ASTM standards for precast manholes, utility vaults and rectangular water and wastewater structures and the direct incorporation of these same standards by the AASHTO

Subcommittee on Materials (i.e. AASHTO M199, “Standard Specification for Precast Reinforced Concrete Manhole Sections,” and ASTM C478-15, “Standard Specification for Circular Precast Reinforced Concrete Manhole Sections”). In these product standards, in addition to minimum material requirements, are provisions for the structural design methodology to be employed if a physical proof-of-design test is not performed. The current precast drainage product structural design standards reference the up-to-date American Concrete Institute (ACI) 318 code, “Building Code Requirements for Structural Concrete and Commentary.”

To further assist the design engineer in applying the ACI 318 code to these precast structures, ASTM Committee C27 on Precast Concrete Products developed two standards: ASTM C890, “Standard Practice for Minimum Structural Design Loading for Monolithic or Sectional Precast Concrete Water and Wastewater Structures,” and ASTM C857, “Standard Practice for Minimum Structural Design Loading for Underground Precast Concrete Utility Structures.” These standard practices provide guidance with emphasis on live load distribution considerations, which are based on the 17th Edition of the AASHTO bridge specification and are still used in the current ACI 318 code. This live load information of an HS20-44 truck footprint and corresponding load distribution through the soil no longer follows the new LRFD live load provisions, which are described as HL93. HL93 loading incorporates the existing HS20 truck weight and footprint, but applies it as an axle load combined with a lane load. The use of an axle and added lane load, combined with multiple presence factors, makes excellent sense for the intended updated bridge design analysis. However, attempting to apply these expanded loading conditions can be problematic for many of the small precast concrete buried drainage structures or those placed within a roadway curb that can only be exposed to a single- or dual-tire footprint.

ASTM Committee C27 has begun work to provide guidance for designers wishing to comply with the LRFD specification for small buried precast drainage structures and vaults. The intent is for the work item to be developed into a standard practice: “Practice for Minimum Structural Design Loading for Monolithic or Sectional Precast Concrete Utility, Water, and Wastewater Structures Using AASHTO Load and Resistance Factor Design (LRFD).” This practice will provide live load design criteria along with a recommended load and resistance factors appropriate for these unique precast structures to fulfill the LRFD methodology.

Next Step: Inclusion in AASHTO

The clock is ticking to include the necessary design guidance language for these undesignated buried precast structures in the AASHTO LRFD specification. The AASHTO Committee on Bridges and Structures (COBS) has agreed to publish further specification updates in the AASHTO LRFD Bridge Design Specifications on a three-year cycle with interim revisions issued only for major corrections. Any proposed revisions for the 9th Edition will need to be prepared and submitted by the end of 2018 for proper consideration, review and balloting in 2019. After that date, the next bridge standard revision will be issued in 2022. Though seemingly an easy task, the specification revisions can be complicated as the language will require evaluations and approval from three subcommittees, T-13 Culverts, T-5 Loads and Load Distribution and T-10 Concrete Design. It will be incumbent upon the precast concrete industry to develop these proposed specification additions and to work with the AASHTO COBS to refine and put them on the 2019 ballot. Next year’s AASHTO COBS meeting will be in Montgomery, Ala., from June 23-27.

Eric Carleton, P.E., is NPCA’s director of codes and standards. He is an ASTM Award of Merit recipient and currently serves as vice-chairman of ASTM C13, Concrete Pipe.

Resources:

1. June 28, 2000, FHWA LRFD Memorandum, fhwa.dot.gov/bridge/062800.cfm
2. The AASHTO LRFD Bridge Design Specification – Past, Present and Future. John M. Kulicki, Ph.D., Modjeski and Masters, modjeski.com/file.ashx?id=45b932fd-3a9d-4294-9fa3-b3c4990a44a7
3. AASHTO LRFD Bridge Design Specifications, 8th Edition
4. “Does the LRFD Standard Work for all Precast?” Precast Inc. Ron Thornton, P.E.
5. “Code Breakers” Precast Solutions. Gary Munkelt, P.E.

Reference:

1 Multiple presence factors account for simultaneous loads from multiples trucks.