Proposed Modifications to AASHTO Culvert Load Rating Specifications (2019) / Chapter Skim
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43 C H A P T E R 3 Findings and Applications This chapter summarizes the findings from the literature review, field testing, analysis, and the experiences of the research team and discusses application to practice. The summary of the outcomes of this chapter (specification recommendations)
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44 material stiffness for concrete structures where the compressive strength may differ from the target design strength and can also be impacted by cracking. Soil properties are also considered in the adjustments to the model as necessary to complete the calibration.
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45 Effects of Subgrade on Rating The effects of using the modulus of subgrade reaction to model the foundation support under box culverts are discussed in this section. Three different subgrade soils will be used for comparison purpose.
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46 Figure 40 – Vertical Soil Strain (in./in.) on Concrete Box Section The benefits of using springs is demonstrated in the form of rating factors from BrDR based on the analyses listed in Table 3.
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47 provide comparative plots of the HL93 inventory ratings for 200 kcf springs/no-springs. As expected, the benefit of the springs is minimal for depths of cover less than 2 ft and increasingly more significant as the depth increases and is greater with stiffer subgrade soils.
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48 Table 4 – Rating Factors – Models 1,2,3 – No Springs vs. 100 kcf Fill Depth (ft)
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49 Table 5 – Rating Factors – Models 1,2,3 – Caltrans Models – No Springs vs. 200 kcf Fill Depth (ft)
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50 Fill Depth (ft) Vehicle HL93-InvNoSprings HL93-OpNoSprings HL93Inv- 200 kcf Springs HL93Op- 200 kcf Springs Inv-Ratio*
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51 Table 6 – Rating Factors – Models 1,2,3 – No Springs vs. 400 kcf Fill Depth (ft)
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52 Figure 42 – Model 1 – HL93-Inventory Rating Factors vs. Fill Depth – No Spring vs.
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53 Figure 44 – Model 3 – HL93-Inventory Rating Factors vs. Fill Depth – No Spring vs.
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54 Figure 46 –Caltrans CS12x8-1952– HL93-Inventory Rating Factors vs. Fill Depth – No Spring vs.
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55 Design-Analysis Much of the Design-Analysis guidance was based on results presented in the following paragraphs. The conclusion from that study are presented as an agenda item in Appendix H of this report.
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56 reinforcing layouts consisted of u-shaped outside reinforcement extending into and lapping at the center of the top and bottom slabs and straight bars for inside reinforcement. The culvert was subjected to earth loads using a soil density of 120 pcf.
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57 Table 8 – BOXCAR Reinforcement (in^2/ft) Depth (ft)
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58 Design shear forces are reduced as the location of the critical sections moves to a distance "d" from the tip of the haunch. Design for Shear Capacity - Exceeding the shear capacity at zero cover was expected based on the culvert geometry and the concrete strength.
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59 In this study, CANDE was operated in an analysis mode where reinforcement areas are provided, and the output is the culvert response. For this comparison, the CANDE model incorporated reinforcement that approximated the BOXCAR analysis for 8 ft of fill without live load (see Table 10)
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60 Table 11 – Comparison of Factored Moments – CANDE vs BOXCAR, 8 Ft Fill (ft-k/ft) No Haunch – No LL No Haunch – LL Haunch CANDE BOXCAR CANDE BOXCAR CANDE BOXCAR Negative -12.9 -11.8 -13.9 -14.8 -7.6 -11.9 Top positive 10.0 11.6 12.7 14.0 9.2 10.5 Bottom positive 10.9 14.9 10.8 16.6 10.6 12.8 Note: The controlling moment for negative reinforcement occurs in the slabs for sections without haunches and in the walls for sections with haunches For the section without haunches, Table 11 shows comparable moments for the negative and top positive moments and substantially lower moment for the bottom positive moment which is a result of the load redistribution discussed above.
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61 Table 12 – BrDr Inventory Rating Factors Depth (ft) Load application No Haunch Loc./Limit With Haunch Loc./Limit 0 Distributed 0.93 Top/shear 1.14 Top/shear 2 Distributed 1.10 Top/flexure 1.19 Top/flexure 8 Distributed 1.81 Top/flexure 1.88 Bot/flexure 8 Bedding springs, 100 pci 1.86 Top/flexure 2.10 Bot/flexure 8 Bedding springs, 400 pci 1.99 Top/flexure 2.42 Top/flexure Review of Table 12 in light of the BOXCAR analyses and reinforcement level indicates the following: The rating factor at 0.0 ft cover and no haunches of less than 1.0 was expected.
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62 occurs in both the top and bottom slab. The difficulty in this approach for design is uncertainty about the foundation and backfill stiffness that will be achieved in the field.
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63 Culvert Load Distribution The proposed culvert load distribution modifications are shown in Appendix H Agenda Item (Subject: Live Load Distribution for Culverts)
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64 Caltrans Culvert Name, Year Built Fill Depth Vehicle Inv Rating Before Change Op Rating Before Change Inv Rating After Change Op Rating After Change Inv Ratio Before/ After* Op Ratio Before/ After CS10x8;10 1933-Rev 1.9 ft Cover HL-93 (US)
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65 Caltrans Culvert Name, Year Built Fill Depth Vehicle Inv Rating Before Change Op Rating Before Change Inv Rating After Change Op Rating After Change Inv Ratio Before/ After* Op Ratio Before/ After CS12x12;10 2002Rev 1.9 ft Cover HL-93 (US)
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66 Non-Rectangular Culverts Culverts other than box sections incorporate a range of shapes and sizes and are designed by a variety of methods, most developed by manufacturer's trade associations and then adopted by AASHTO. While design procedures for concrete box culverts have evolved, procedures for other types of culverts have remained largely unchanged since first added to the AASHTO Specifications.
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67 Model 7 3D Analysis Review Model 7 is a corrugated metal box culvert located in Attleboro, Massachusetts. The research team was able to coordinate with MASSDOT and the culvert contractor to instrument and test this culvert under construction with the intent that the effects of paving on the response of the culvert could be captured.
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68 Figure 51 - Instrumentation Locations
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69 Load Set 1 thru 7. Load Set 8 thru 15.
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70 Results Before and After Paving While the results presented in Appendix K and Appendix M document the selection of a 3-D modeling scheme and the corresponding results from Test 1 (prior to paving) , the results herein illustrate the differences in the stresses at each of the strain gauge locations as measured in the field.
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71 Figure 55 - Model 7 Before and After Paving: Test N1, Gauges 9-12 (Cluster 3) Figure 56 - Model 7 Before and After Paving: Test N1, Gauges 13-16 (Cluster 2)
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72 Figure 57 - Model 7 Before and After Paving: Test N1, Gauges 17-20 (Cluster 1) Figure 58 - Model 7 Before and After Paving: Test N2, Gauges 1-4 (Cluster 5)
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73 Figure 59 - Model 7 Before and After Paving: Test N2, Gauges 5-8 (Cluster 4) Figure 60 - Model 7 Before and After Paving: Test N2, Gauges 9-2 (Cluster 3)
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74 Figure 61 - Model 7 Before and After Paving: Test N2, Gauges 13-16 (Cluster 1) Figure 62 – Model 7 Before and After Paving: Test N2, Gauges 17-20 (Cluster 1)
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75 Figure 63 – Model 7 Before and After Paving: Test N3, Gauges 1-4 (Cluster 5) Figure 64 – Model 7 Before and After Paving: Test N3, Gauges 5-8 (Cluster 4)
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76 Figure 65 – Model 7 Before and After Paving: Test N3, Gauges 9-12 (Cluster 3) Figure 66 – Model 7 Before and After Paving: Test N3, Gauges 13-16 (Cluster 2)
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77 Figure 67 – Model 7 Before and After Paving: Test N3, Gauges 17-20 (Cluster 1)
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78 CANDE Models with and Without Pavement Several of the models in this study were analyzed in CANDE with varying fill heights and with and without pavement elements. These were analyzed using CANDE and the new features provided in the CANDE Tool Box.
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79 Figure 69 – Node Numbering for M7C1 Recommendations We conclude that live load ratings can be improved by including the effects of pavement. A 3-D model is not required to analyze live load response associated with a paved surface.
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80 Shear Capacity The shear capacity of reinforced concrete culverts was reviewed as part of this research. The following sections provide the reasoning for the changes recommend in Chapter 4 of this report related to the AASHTO MBE Article 6A.5.13.1.
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81 The PennDOT study concluded that the distribution from the AASHTO Standard Specifications (red dotted line) matched the distribution width for shear forces and was the most conservative.
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82 is provided in Appendix G These were run using AASHTOWare BrR using a revision to 6.8.3 of the software.
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83 Table 15 – Comparisons of Shear Rating Factors for Change in the Shear Capacity Calculation Bridge ID Fill Depth Critical Element (Before) Location (Before)
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84 Bridge ID Fill Depth Critical Element (Before) Location (Before)
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85 Bridge ID Fill Depth Critical Element (Before) Location (Before)
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86 Live Load Surcharge vs. Approaching Wheel Load This section summarizes the change related to the proposed culvert section in the LRFD 3.11.6.4.
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87 Figure 71 – Approaching Vehicle Lateral Pressure on Culvert Wall vs. Depth The FEM models show high pressure near the surface that reduces quickly with increasing depth of fill.
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88 For the existing specifications, the following values were input that are used in the calculation of the LS load in BrDR Lateral Earth Pressure Coefficient = 0.5 Surcharge Height = 2.0' Unit weight of Soil = 120.0 Pcf Note: The change made in BrDR was applying a revised LS (now referred to as AW) at all fill depths.
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89 Figure 72 – Lateral Pressure vs. Fill Depth (Proposed/Current)
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90 Table 17 – HL93 Inventory Controlling Rating Comparisons for the Change in LL Surcharge Culvert Cover Inv Rating HL93 (Before) Oper Rating Factor HL93 (Before)
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91 Culvert Cover Inv Rating HL93 (Before) Oper Rating Factor HL93 (Before)
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