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73 NCHRP Project 15-29, âDesign Specifications for Live Load Distribution to Buried Structures,â investigated how surface live loads distribute through the soil and load various culvert structures. AASHTO Standard and LRFD Specifications dif- fer in how live loads are spread through fill onto culvert struc- tures. Standard Specifications apply surface point loads and spread loads at the rate of 1.75 times the culvert depth. The LRFD specifications apply live load through a tire footprint of 10 by 20 inches at the surface but attenuate with a lower coefficient (1.00 or 1.15 as a function of soil type) as the depth of fill increases. This project investigated how live loads spread with depth, as a function of soil and culvert type, using 3D numerical modeling. The numerical investigation included selection of appropriate software and soil models, verification of model predictions, and 3D analysis of about 830 buried culverts, including concrete arches, concrete pipes, concrete boxes, corrugated metal pipe, corrugated metal arches, and thermo- plastic pipe (profile wall), to provide a basis for developing SDEs for structural response. Proposed revisions to AASHTO design specifications were developed. First, the limit states and design methodologies were evaluated and compared for all culvert types included in the AASHTO specifications. Next, numerical values of the limit states from the numerical modeling were compared with the values resulting from the Standard and LRFD Spec- ifications. Finally, proposed SDEs were developed that pro- vided better correlation with modeling results. For all culvert types, the proposed SDEs included a culvert span-relatedterminthecalculationofthe load spread parallel to the culvert axis. Table 3-1 summarizes the proposed changes to each section for the six culvert types. Recommended changes to the AASHTO LRFD Design Specifications are presented in Section 3-2 and Appendix C. The recommended changes are limited to Specification Section 3, where the live load magni- tude is specified, and Specification Section 12, where structural responses are calculated. To understand the effect of the proposed SDEs on culvert de- signs, the critical structural responses were calculated and com- pared for Standard, LRFD, and proposed SDEs, for 248 culvert, depth, span, and soil combinations. For the project, the research team conducted extensive 3D modeling of the transfer of surface live loads to buried cul- verts. From the results, the research team has proposed SDEs that permit culvert design without modeling. However, many design situations are not addressed by the SDEs. In these sit- uations, 2D and 3D modeling may be used for design. Guidelines were developed for conducting 2D and 3D modeling. The 2D guidelines provide a means for selecting the surface load intensity to be applied to a 2D model in order to achieve approximately the structural response from a 3D model. Two-dimensional computer models have an inherent limitation when computing the effect of surface live loads. Because the models are 2D, the load spreading that occurs in the longitudinal direction, parallel to the axis of the culvert, cannot be correctly computed. The model represents a verti- cal slice through the real-world, 3D geometry. The resulting guideline for the surface pressure to be used for conducting 2D analyses is a two-parameter equation found to provide reasonable results for the structural response of reinforced concrete pipe, thermoplastic (profile wall) pipe, and corrugated metal pipe. The behavior of concrete boxes, concrete arches, and corrugated metal arches was found to be too variable to be adequately captured by these guidelines. 3D guidelines were developed that address software, live load application, representations of the pavement and the soil, model dimensions, element size, symmetry and boundary con- ditions, representations of the culvert structures, and the soil- culvert interface. The overall design and reliability margin of the proposed SDEs was assessed by computing statistics about the ratio of SDE design force to Standard design force and the ratio of SDE to LRFD design forces. For most design forces, the ratio of SDE to LRFD is between 0.9 and 1.1. Exceptions to these limits are C H A P T E R 4 Conclusions
the RCP crown moment at 0.888, the corrugated metal arch peak thrust at 1.460, and the reinforced concrete arch peak moment at 0.882. The range of design force ratios is generally larger for the SDE/Standard ratio. This reflects that the SDEs, like the LRFD design methods, spread the loads from a finite-size wheel patch (typically 20 by 10 inches), rather than a point load. Except for a few structure forces, the proposed SDEs do not significantly affect the design margin or reliability on average. However, the relatively large spread in the ratios means that there are some combinations of soil type, diameter, and depth where the SDEs are significantly different than the LRFD de- sign forces. Where there is a significant variation between the proposed SDEs and current practice, the differences are not randomâ rather the SDEs model behavior not captured in the current standards. For example, in metal pipe, the ratio gets larger as depth of fill decreases. As discussed earlier, this is the result of the high thrust noted in the crown of these pipes which occurs because of the low bending stiffness and high axial stiffness. The proposed SDEs reflect an improvement in the distri- bution of live load with depth and better culvert designs. 74
