Proposed Modifications to AASHTO Culvert Load Rating Specifications (2019) / Chapter Skim
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From page 92... ...
92 C H A P T E R 4 Conclusions and Suggested Research Following are the conclusions and recommendations that can be drawn based on the review of the analysis results shown in this report and in the appendices: Recommendations – AASHTO LRFD Bridge Design Specifications Depth of live load, Article 3.6.1.2a Currently the AASHTO LRFD Specifications state: "For single span culverts the effects of live load may be neglected where the depth of fill is more than 8.0 ft and exceeds the span length; for multiple span culverts the effects may be neglected where the depth of fill exceeds the distance between inside faces of end walls." This provision requires consideration of live load until the depth exceeds the span; however, in the experience of some members of the research team the provision is often interpreted as ignoring live loads at depths of 8 ft and greater. At a depth of 8 ft, the live load (design tandem)
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93 The load is primarily transmitted as a thrust through the top slab, reacting with the soil on the far side of the culvert. The moments resulting from this pressure are small.
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94 Article 3.6.1.2.6a Revise the first paragraph of Article 3.6.1.2.6a-General in the Design Specifications as follows: The effects of live load may be neglected when the factored live load pressure at the surface of the culvert is less than 10% of the sum of the factored earth load plus factored live load pressure. Article 3.6.1.2.6a Revise 2nd paragraph Live load shall be distributed to the top slabs of flat top three- or four-sided concrete culverts, three-sided arch top concrete culverts or concrete arch culverts over the area calculated in this article, but not less than the dimensions calculated using the procedure specified in Article 4.6.2.10.
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From page 95... ...
95 Figure C4.6.2.10.2-1 – Single-Axle Transverse Distribution Through Fill Revise Article 4.6.2.10.3 Traffic traveling perpendicular to the span shall consider multiple lane loadings with the appropriate multiple presence factor. When traffic travels perpendicular to the span, wheel loads shall be distributed to the top slab as specified here: Perpendicular to the span: E = ((Ax -1)
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96 Revise Article C4.6.2.10.3 Add new paragraph: When vehicles travel perpendicular to the span, the wheel loads from adjacent axles (e.g. typical tandem and tridem axle configurations)
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97 This load need not be applied to culverts with a depth of fill over the top slab greater than 2 ft nor to concrete culverts with round tops or metal, thermoplastic or fiberglass culverts. Add new commentary: Article C3.11.6.4.1 Retaining walls have historically been designed considering a lateral live load surcharge pressure to represent the additional load applied by a vehicle located near the wall.
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From page 98... ...
98 installation methods. The data from construction documents, including culvert dimensions, materials and material properties, and installation methods should be confirmed during a visual inspection of the culvert and any discrepancies from the construction documents should be addressed.
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99 Table 6A.10.3.1-1 Modulus of Subgrade Reaction for Bedding Support of Rectangular Concrete Culverts Soil Range2 (pci) Rating Values3 (pci)
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100 on how to apply and distribute live loads to account for the three-dimensional load spreading that occurs as load is transmitted through the soil. Specific modeling and design assumptions that engineers should evaluate include the following.
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101 Continuous load scaling (CLS) – this feature permits a live load to spread longitudinally as it is transferred from the top of the culvert to the bottom slab.
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102 C6A.10.4 The approaching wheel load replaces the live load surcharge as more appropriate for culverts. 6A.10.5 – Limit States Culverts shall be load rated for the Strength I load combination for the design and legal loads and the Strength II load combination for permit loads.
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103 Table 6A.10.5-1 Limit States and Load Factors for Culvert Load Rating (Modified from current MBE Table 6A.5.12.5-1)
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104 6A.10.6-Resistance Factors Resistance factors for culverts shall be taken as specified in LRFD Design Article 12.5.5. 6A.10.7-Condition Factors Use of condition factors as presented in Table 6A.4.2.3-1 may be considered optional based on an agency's load rating practice.
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From page 105... ...
105 C6A.5.12.10.3a-Live Load Distribution Current specification Article 6A.5.12.10.3a with proposed changes listed below. Change 1- Replace deleted sentence with: Culverts where design for live load is not required per the LRFD Design Specifications Article 3.6.1.2.6a do not require rating for live loads.
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From page 106... ...
106 C6A.5.12.10.3b No change from current Article C6A.5.12.10.3b 6A.10.10.3c – Approaching Wheel Load Rectangular concrete culverts with less than or equal to 2 ft of cover shall be loaded with a lateral pressure distribution to produce the effects of a truck axle just before going over the culvert. This pressure shall be computed using Eq.
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From page 107... ...
107 Table C6A.10.10.3d-1 presents guidance on the conditions and locations where pavements are effective in reducing loads on culverts. Table C6A.10.10.3d-1 Pavement Effect in Distributing Live Load on Culverts Pavement thickness, in.
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From page 108... ...
108 6A.10.11 - Concrete Culverts 6A.10.11.1 Design for Shear The shear strength of culverts without prestressing and with less than 2.0 ft of cover that are performing well based on inspection can be evaluated with a modified approach to shear capacity. Use the General Procedure for shear strength in LRFD Design Specifications Article 5.7.3.4.2, substituting the following procedure to compute the strain in the reinforcement: 𝜀 | | .
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From page 109... ...
109 C6A.10.12 - Metal Culverts Metal culverts should only be rated after a field inspection has documented the culvert shape and condition. Metal Culverts should be analyzed for service and factored forces in accordance with the LRFD Design Specifications and appropriate provisions of this manual.
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From page 110... ...
110 Suggested Research In addition to the results of the research and recommendations provided in the previous section, the following are recommended follow-ups to the research of this project: – Updates to CANDE – The CANDE software was updated for this project and is included as part of the deliverable. While in some cases the analysis engine was updated, time and budget constraints prevented the CANDE graphical user interface from being updated.
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111 AASHTOWare BrDR Export Files (XML) The AASHTOWare BrDR software has the intrinsic capability to export bridge files (in this case culvert files)
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112 – CMD (.cmd) Format for import of LUSAS Modeller model files saved as command (CMD)
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