CHAPTER 5

Proposed Section 13 Updates

Proposed updates to AASHTO LRFD BDS Section 13 were presented in the literature review and synthesis described in the preceding chapters. A summary of key updates includes impact loads and load application parameters, railing geometry, analysis and design procedures for concrete barriers, addition of ground-mounted railings to supplement bridge deck overhang support requirements, and a new subsection addressing railing details.

The current Section 13 notes that its provided impact loads and analysis and design procedures are for crash-test specimens, but this has not been well-understood and applied in practice. Therefore, one proposed change to Section 13 is to increase emphasis and understanding that the provided impact loads and analysis and design procedures are for crash-test specimen design and that railing strength is performance-based in accordance with AASHTO MASH.

Section Organization

With overall improvement to Section 13 as a research objective, the proposed Section is reorganized mainly to move the design information—loading and structural analysis and design—into the main body of the specification. The current Section 13 has this information in an appendix. An appendix is retained, where the current loading and Test Level information related to NCHRP Report 350 is kept as a reference for legacy railings.

Provisions for pedestrian and bicycle railings are combined; previously, they were in their own subsections.

Geometry

Railing heights were updated for MASH Test Levels. Minimum railing heights for TL-1 and TL-2 decreased, with increases in minimum railing heights for TL-3 and TL-4. Railing heights for TL-5 and TL-6 remain unchanged.

Traffic-face geometry recommendations, intended to mitigate vehicle snagging, are revised per the recommendations made in NCHRP Research Report 1024 (Williams, Schulz, and Abu-Odeh 2022). These are generally less restrictive than those in the current Section 13.

Loads

Recommended impact loads in the proposed Section 13 trend higher than those in the current specifications. TL-4 and TL-5 introduce railing height as a variable in recommended impact loading, with taller railings having higher impact load values. These higher values for taller rails

better capture the tail-slap effect of the truck rear axles as a secondary or tertiary impact for TL-4 and TL-5, respectively.

Load application height recommendations in the proposed Section 13 trend lower than those in the current specifications. When considering both load and load application height, the overall change to load effects is minimal with the proposed Section 13.

Expected Material Strengths

There was a lack of data supporting higher material strengths due to material strain rates experienced with vehicular impact. However, with traffic railing design at the extreme event limit state, expected material strengths are allowed for use in design with values from the AASHTO Guide Specifications for LRFD Seismic Bridge Design with 2012 and 2014 Interim Revisions (AASHTO 2014). These expected material strengths are not mandated but offered as an option to mitigate perceived or observed conservatism.

Analysis and Design

All design provisions for traffic railings are emphasized for crash-test article design. This is not a departure from the current Section 13. A new proposed subsection is titled “Rail Crash-Test Article Design” as a strong reminder that traffic railing crashworthiness is performance-based, with approval criteria listed in AASHTO MASH.

Analysis and design for concrete barrier railings are revised to use a trapezoidal yield-line approach, as described in preceding chapters. This recommended approach better captures observed railing resistance and mitigates perceived or observed conservatism. Additionally, a new design check for punching shear in concrete barrier railings is added to the proposed Section 13.

New provisions for the design of energy-absorbing railings are added. Design and analysis recommendations for post-and-beam railings, metal railings mounted to concrete curbs or parapets, and wood railings are largely unchanged from previous Section 13 provisions.

Railing Supports

A new subsection to Section 13 is proposed, specific to railing supports. This is further divided into bridge-mounted railings, primarily focused on bridge deck overhangs and ground-mounted railings. The proposed provisions for bridge deck overhangs are from NCHRP Research Report 1078 (Steelman et al. 2023).

AASHTO LRFD BDS Section 11 provides specifications for MSE-type walls addressing the effects of traffic railing impacts when rails are mounted to them. Research on other ground-mounted railing applications, such as cast-in-place or block retaining walls, grade beams, or footings, is not substantial enough to develop specifications.

Railing Details

The proposed Section 13 includes a new subsection covering a wide range of railing designs and construction details and how they relate to the provision of safety and railing maintenance. One topic is railing orientation with respect to the roadway surface and superelevation. Research is not conclusive on this issue, but references are provided in the proposed Commentary to assist decision-making for rail construction details.

Reinforcement spacing is recommended to not exceed 6 in., in any direction, in concrete railing elements, to limit debris size from impacts. This recommendation is in the proposed commentary and is not mandatory. AASHTO MASH test criteria include an evaluation of debris and its potential for harm to traffic, pedestrians, or personnel. Debris observed at some railing impact sites has been substantial enough to be a real concern. No specific data were found to substantiate a specific reinforcement spacing limit or other potential mitigating factors, such as increased concrete thickness/mass, to address this concern. In the absence of this data, observations of multiple in-service railing impacts led to the recommended limit.

Another consideration of railing construction details that could alter impact performance is the presence of electrical conduit placed in concrete railings. The proposed Section 13 recommends these be included in crash-test articles if their use is anticipated in actual construction.