CHAPTER 1

Introduction

Background

As of January 1, 2020, all new and replacement bridge rails installed on the National Highway System are required to conform to the requirements stipulated in the Manual for Assessing Safety Hardware, 2nd edition (referred to herein as MASH 2016) (AASHTO 2016). Currently, however, the bridge rail guidance provided in the 2017 AASHTO LRFD [Load and Resistance Factor Design] Bridge Design Specifications (referred to herein as AASHTO LRFD BDS), 9th edition (AASHTO 2020a) reflects the requirements of the preceding crash-testing standard, NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features (Ross et al. 1993). The conversion from NCHRP Report 350 to MASH 2016 criteria is accompanied by near-universal increases to expected vehicle impact severities, which manifest as increased design loads and overturning moments. From the designer’s perspective, the conversion will generally require barriers with greater capacities and taller profiles.

No significant changes to Section 13, Railings, of AASHTO LRFD BDS have been made since their introduction in 1994 other than the source of crash-test criteria (AASHTO 1994). All editions of the AASHTO LRFD BDS have approached bridge traffic railing with performance-based criteria, where the determination of a traffic railing’s crashworthiness is solely based on physical crash tests.

AASHTO LRFD BDS Section 13 needs revision to reflect the updated MASH 2016 crash-testing criteria. Since the last revision, substantial research in testing, designing, and evaluating bridge rail systems has been performed. Growing numbers of full-scale crash tests and advances in computer simulation technology have resulted in confident estimates of design loads and minimum heights for bridge rail systems. Furthermore, these research efforts have yielded novel design practices incorporating more nuanced aspects of bridge rail design, such as the proper shape of the bridge rail’s traffic face.

To confidently revise AASHTO LRFD BDS Section 13, a comprehensive review of both the state of the practice and the state of the art for bridge rail testing, design, and evaluation was undertaken. The safety performance of bridge rail systems is multifaceted and depends on several aspects, including (1) the capacity of the rail, (2) the height of the rail, (3) the traffic-face profile of the rail, and (4) the design of the bridge deck overhang. During the review, archival and contemporary literature addressing these aspects were identified and evaluated for inclusion in Section 13.

Research Objective

AASHTO LRFD BDS Section 13 addresses the implementation, design, and evaluation of bridge rail systems. As such, this section includes bridge rail strength, height, and proportioning requirements corresponding to NCHRP Report 350 criteria. A primary goal of this research is to

identify alternative design values reflecting MASH 2016 criteria. However, since the implementation of the current AASHTO LRFD BDS Section 13, a multitude of research efforts in the roadside safety community have been performed in which MASH-compliant bridge rail designs have been demonstrated, new design methods for concrete bridge rails have been produced, and general design guidance not found in current standards has been provided. Therefore, a companion primary goal for revising AASHTO LRFD BDS Section 13 provides a unique opportunity to supplement the existing specifications with the substantial research performed over this time.

Evolution of AASHTO Bridge Rail Specifications

A review of historical AASHTO railing design specifications reveals that the treatment of railing can be divided into three distinct eras.

1931 to 1963

This era is noted for its comparatively low design forces and the use of curbs to complement rails for roadway departure safety. Design methodology was elastic with allowable stresses.

First Edition AASHO (American Association of State Highway Officials) Standard Specifications for Highway Bridges and Incidental Structures, 1931 (AASHO 1931)

Curbs were specified to be at least 9-in. tall with the face placed at least 6 in., and preferably 9 in., in front of railings. Curbs were to be designed for a minimum lateral force of 500 pounds per linear foot (plf) applied at the top of the curb. “Substantial” railings were required along each side of a bridge with the top of the railing to be a minimum of 36-in. above either the finished roadway surface when adjacent to a curb or the sidewalk surface when used with a sidewalk.

Railings were specified to be one of two classes, with the first class being for the protection of pedestrians on bridges in cities and villages and the second class for use on country bridges not subject to general pedestrian traffic.

Metal railings for the first class required lower and upper horizontal rails connected by a suitable web, with the clear distance between the top of the curb or sidewalk to the lower rail limited to a minimum of 6 in. Metal railings for the second class required a minimum of two horizontal rails of “an approved section.” Openings in concrete railings of the first class were to be proportioned in due regard to the safety of persons using the structure; no other specifications for concrete railing geometry were listed.

Loads to be resisted by railings, regardless of class, were a minimum horizontal force of 150 plf applied at the top of the railings and a minimum vertical force of 100 plf. No specific references to concrete railings were made. The design method specified was allowable stress.

Third Edition AASHO Standard Specifications for Highway Bridges, 1941 (AASHO 1941)

Consideration of the architectural aspects of the railings was introduced to “obtain proper proportioning of its various members and harmony with the structure as a whole.” Also new was the consideration for railings to not obstruct the view of passing vehicles, in consideration of safety and appearance.

Two classes of bridge railings were still listed, but these were now noted as “Roadway” railings and “Sidewalk” railings. Minimum rail heights were reduced for traffic railings to 30-in. above the roadway when adjacent to curbs. Sidewalk railings were limited to a minimum of 36 in. less half the width of the top rail’s width.

Loads applied to tops of curbs remained at 500 plf but not at a height to exceed 10 in. Curbs over 10-in. tall were to be stepped or sloped back such that vehicle tires could not come into contact with it. The term “safety curb” was introduced, and it was intended to provide additional width for “occasional pedestrian traffic.”

Loads for roadway railings were a minimum lateral force of 150 plf applied simultaneously with a minimum vertical force of 100 plf applied at the top of railing. If curbs were 10 in. or less in height, lower rails or rail webs were to be provided and designed for a minimum lateral force of 300 plf; if there was no lower rail element, the “web members” were to be designed for a minimum 300 plf applied at 21-in. above the roadway with the horizontal forces to be applied simultaneously. If the railing had only one horizontal rail, or it did not have a web, the forces specified for the lower rail were to be used for the top rail. This force was allowed to be reduced by 15 plf per inch of curb height above 10 in., but the reduction was limited to 150 plf.

Sidewalk railings were to be designed for the same loads listed for roadway railings unless through trusses, girders, or arches that separated the sidewalk from the roadway or if the curb width exceeded 24 in. In this case, only the forces for the top rail needed to be considered.

Sixth Edition AASHO Standard Specifications for Highway Bridges, 1953 (AASHO 1953)

Provision for smooth horizontal rail surfaces, “unbroken by the vertical posts or other projections,” was introduced for the first time. Roadway railings could be omitted from “low clearance” bridges if they were provided with “substantial” curbs, and the roadway width was at least the shoulder width but not less than the approach pavement width plus 12 ft. Guidance to what constitutes a substantial curb was not provided.

The two classes of railings, Roadway and Sidewalk, remained. The minimum roadway railing height was listed at 27 in. and a maximum height of 42 in. above the roadway adjacent to curbs. Sidewalk railings were still required to be 36-in. above the sidewalk, less one-half the width of the top rail but with an additional requirement that in no case could the rail height be less than 30 in.

Loads for railings remained essentially unchanged, with the exception that the horizontal load specified for lower rails was to be increased by 40 plf for each inch of curb height less than 9 in., up to a maximum of 500 plf.

1964 to 1988

This can be considered the beginning of the modern era in bridge railing design, with substantially increased design impact forces, acknowledgment of crash testing, and the importance of railing to guard rail transitions. Design methodology remained largely elastic with allowable stresses. Rails introduced in this era have proven themselves to be largely crashworthy for passenger vehicles.

Eighth Edition AASHO Interim Specifications to Standard Specifications for Highway Bridges, 1964 (AASHO 1964)

The 1964 Interim represents the first substantial rewrite of railing specifications by AASHO. It introduced the railing design objectives of “protection of the occupants of a vehicle in collision

with the railing” and “protection of other vehicles near the collision.” Recognition of bridge end transitions was made for the first time; however, no specific guidance was provided beyond avoiding exposed rail ends or sharp changes in railing geometry. The requirement in previous specifications that consideration be made to aesthetic features and proportioning of members was omitted.

The two classes of railings from previous versions were abandoned, but the concept of a traffic rail placed between traffic and a sidewalk was introduced. A separator rail was not mandatory, but a curb between a sidewalk and traffic was mandatory if no separator rail was provided.

Materials listed for traffic and pedestrian railing were listed as concrete, metal, timber, or a combination, with an additional requirement of 10% tested elongation to be provided with metal railing materials with traffic railings. Reference to the use and design of aluminum railing elements was introduced.

Minimum railing heights were listed as 27 in. and 36 in. for traffic and pedestrian railings, respectively, but indicated a preferred height of 42 in. for pedestrian railings.

The most notable change from previous specifications was a substantial increase in traffic railing loading and the introduction of a schematic depicting the apportionment of loads. The basic minimum horizontal design load was 10 kips, with the drawings depicting how this load was to be distributed to each railing element for various rail types. For concrete barrier railings, the load was specified to be distributed over a 5-ft railing length. An exemption from these load requirements was added as a footnote for railing configurations that had “been successfully tested by full-scale impact tests.” The 10-kip load was to be used with the allowable stress design method.

Pedestrian rail loads were specified to be 50 plf minimum, acting transversely and vertically simultaneously on each longitudinal member. Members located higher than 50 in. from the walkway were exempted from these load requirements. Loads at posts were specified to be 50 plf times the post spacing, applied at the center of gravity (c.g.) of the upper rail but not to exceed 60 in. above the walkway.

Tenth Edition AASHO Standard Specifications for Highway Bridges, 1969 (AASHO 1969)

This edition required for the first time a “smooth transition by means of a continuation of the bridge barrier, guard rail anchored to the bridge end, or other effective means” to guard against hazards at bridge ends. However, no specific guidance on achieving this was provided.

An important change to separator railing was made with the introduction of the term “urban expressway,” but what constituted an urban expressway was not provided. Separator rails were now required for urban expressways but could be omitted elsewhere.

Twelfth Edition AASHTO Interim Specifications to Standard Specifications for Highway Bridges, 1979 (AASHTO 1979)

Requirements for Bicycle Railings were introduced, with a minimum rail height listed at 54 in. with load requirements similar to those listed for pedestrian railings.

1989 to Present

Rails must now be crash tested to verify their adequacy prior to use, with crash-test criteria set for specific combinations of vehicle type, speed, and impact angle. Vehicles vary from small cars to large semi-tractor-trailer trucks. Design methods are now largely strength-based.

Summaries of AASHTO specifications where new concepts were introduced or substantial revisions were made are presented in the following sections. This summary is not intended to be comprehensive; rather, it is meant to highlight what are considered significant steps in the progression of railing design specifications. Likewise, subtle changes in design methodology, e.g., allowable stress method, are not captured.

Context is imperative with all historical perspectives. With this summary, it is helpful to keep in consideration the evolution of vehicle speed limits, the introduction of the Interstate highway system in the 1950s, the ever-increasing trend in vehicle-miles driven, social factors related to driving, etc., all of which are beyond the scope of this project.

AASHTO Guide Specifications for Bridges (AASHTO 1989)

This nonmandatory specification for bridge railings required crash testing to verify that railing met performance goals. Three performance levels were established with associated minimum heights and crash-test severity. Loads and analysis methods were established to assist in the design of crash-test specimens but were not mandatory.

Fourth Edition AASHTO LRFD Bridge Design Specifications with Interim Revision (AASHTO 2007)

In June 2000, the FHWA established a goal for all states to implement the AASHTO LRFD BDS for the design of all new bridges in which states initiated preliminary engineering after October 1, 2007. Modifications to existing structures could still be accomplished with either the AASHTO LRFD BDS (AASHTO 2007) or the AASHTO Standard Specifications for Highway Bridges (AASHTO 2002).

Crash testing was required for all new rails at one of five test levels as prescribed in NCHRP Report 350 (Ross et al. 1993). Deck overhang designs for railing impact loads could be qualified with the same crash testing. General guidance is provided for selecting a test level for bridge sites.

Similar to the 1989 Standard Specifications (AASHTO 1989), recommended loads, analysis, and design procedures are provided to develop crash-test specimens, including bridge deck overhangs, in an appendix to the Specifications. No mandatory minimum strength is provided other than successfully passing the appropriate crash-test level.