Recommended LRFD Minimum Flexural Reinforcement Requirements (2010) / Chapter Skim
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48 CHAPTER 3 INTERPRETATION, APPRAISAL AND APPLICATION To evaluate the methods of prescribing minimum reinforcement, a parametric study was performed on four representative minimum reinforcement methods of the eight described in Section 2.4. The criterion for this evaluation includes reliability, as defined as providing a consistent level-of-safety for all concrete bridge members covered in the LRFD specifications, economy and ease-of-use, as described in Section 3.1.
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49 prescribed in the current LRFD specifications. Appropriate factors for flexural cracking and prestress improve economy and consistency.
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50 Table 4. Concrete member database structural dimension limits Bridge Types Span – L (ft)
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51 It should be noted that the Caltrans permits the use of concrete compressive strength of 3,600 psi for concrete slab bridges. Table 5.
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52 Table 7. Reinforced concrete box girder structure dimensions BRC1 BRC2 Units No.
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53 Cantilever cap-beams tend to be heavily reinforced, and as a result, minimum flexural reinforcement provisions rarely control. For integral bridges in California, an additional two-foot of width is required to confine the cap-beam to column joint region.
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54 Table 11. Precast prestressed I-girder dimensions PCI1 PCI2 Units No.
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55 Table 13. Precast prestressed box-beam dimensions PBB1 PBB2 Units No.
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56 Table 15. Span-by-span segmental bridge girder dimensions SBS1 SBS2 Units No.
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57 5. For the Modified LRFD method, the ratio of the maximum strength to the nominal strength Mo/Mn = fu/fy = 1/γ3= 1.5.
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58 Table 17. Minimum reinforcement ratios for rectangular reinforced concrete sections Method (min)
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59 7. For the Modified provisions, the minimum reinforcement is calculated based on the following: 1 = 1.6 (assuming that fcr = 7.5f'c (psi)
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60 3.1.2.2.2 (External) Unbonded Prestress Concrete Members Sections SBS1 and SBS2 feature unbonded tendons, and the moment capacity is based on the AASHTO equations for tendon prestress at ultimate.
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61 3.1.3 Cracking Moment (Mcr) The cracking moment Mcr is based on the computed minimum amount of reinforcement (or prestress)
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62 Mo – nominal moment at overstrength including the effects of strain hardening, as illustrated in Figure 1. Mo/Mcr - ratio of the nominal moment at overstrength to the cracking moment This last term is the effective factor of safety or brittleness ratio.
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63 prestressed concrete members. Therefore, no reserve strength is available beyond the nominal capacity.
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64 offers economy, where compression-controlled and transition-region sections are not subject to minimum reinforcement requirements. 3.2 PROPOSED REVISIONS TO THE AASHTO LRFD SPECIFICATIONS The following are the changes the project team recommends regarding minimum flexural reinforcement provisions in the LRFD specifications.
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65 the actual modulus of rupture could be as much as 50% greater than 0.24f'c, the 20 percent margin of safety could be lost. Using an upper bound is more appropriate in this situation.
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66 rc nc c dnccperccr fSS SMffSM 1)
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67 1.33 times the factored moment required by the applicable strength load combinations specified in Table 3.4.1-1. The provisions for Article 5.10.8 shall apply.
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68 negative moments. Increasing the quantity of reinforcement to meet minimum reinforcement provisions can adversely affect this ductility.
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69 Minimum reinforcement controls the number of prestress strands at the point of maximum positive moment in the side spans. It should be noted that it is not necessary to increase the jacking force, and thus the cracking moment.
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70 reinforcement provisions (Caltrans, 1989)
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71 3.3.3 Span-by-Span Segmental Bridge with External Tendons A two-span precast segmental bridge is the subject of this design example. The bridge is built using the span-by-span construction method.
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72 Figure 23. Span-by-span precast segmental bridge example details Analysis of this bridge was done using LARSA 4D.
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73 Figure 24. Span-by-span precast segmental bridge strength moment profiles 3.3.4 Balanced Cantilever Bridge with Internal Tendons A four-span precast segmental bridge using the cantilever construction method is the subject of this design example.
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74 Figure 25. Balanced cantilever precast segmental bridge example details The bridge is almost symmetric about centerline of Pier 8-3.
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75 proposed provisions considerably reduce the minimum required design moments (MFR)
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76 Figure 26. Concrete cap beam example details
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