LRFD Minimum Flexural Reinforcement Requirements (2019) / Chapter Skim
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Pages 85-97
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From page 85... ...
85 4.1 Introduction On the basis of the experimental and analytical investigation of the research described herein, modifications to the AASHTO LRFD Bridge Design Specifications, 8th edition (AASHTO 2017) , minimum flexural reinforcement provisions are proposed.
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From page 86... ...
86 4.3.1 Moment Resistance Greater than 1.33 Factored Moment Minimum reinforcement provisions are intended to reduce the probability of brittle failure by providing flexural capacity greater than the cracking moment. Article 5.6.3.3 of the AASHTO LRFD allows the designer to forgo this requirement if resistance (Mr)
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87 The flexural cracking stress of concrete members has been shown to significantly reduce with increasing member depth." Shioya et al.
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88 As shown, the average flexural cracking stress is below 0.24 f c′ (ksi) for all depth-specified models, except for one case.
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89 The full-scale test data, discussed previously, is based on compressive strength measured on the day of testing, and not based on the specified strength. Therefore, the statistical analysis is not indicative of the actual relationship between specified compressive strength and flexural cracking strength.
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90 represent four standard deviations above the mean flexural cracking strength because the requirement is based on the specified compressive strength rather than the mean compressive strength. The additional strength gain with age beyond the 28-day strength could also affect and lower this apparent large factor of safety.
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91 Tadros et al. demonstrated that long-term prestress loss due to creep, shrinkage, and relaxation can vary by as much as 30% from the mean value.
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92 Nebraska Department of Roads (2016) , and the Washington State DOT (2018)
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93 either rupture of the prestressed strand (esu = 0.04) or a peak compressive strain of 0.003.
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94 Figure 4-5. Ratio of required minimum reinforcement versus depth.
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95 LRFD. 5.4.2.6 Modulus of Rupture Unless determined by physical tests, the modulus of rupture, , for lightweight concrete with specified compressive strengths of up to 10.0 ksi and normal weight concrete with specified strengths up to 15.0 ksi may be taken as 0.24 where is the concrete density modification factor as specified in Article 5.4.2.8.
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From page 96... ...
96 LRFD. 5.6.3.3 Minimum Reinforcement Unless otherwise specified, at any section of a non-compression-controlled flexural component, the amount of prestressed and non-prestressed tensile reinforcement shall be adequate to develop a factored flexural resistance, Mr, greater than or equal to the lesser of the following at least equal to the lesser of: 1.33 times the factored moment required by the applicable strength load combination specified in Table 3-4.1-1; )
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From page 97... ...
97 For prestressing steel, 3 shall be taken as 1.0. The provisions of Article 5.10.6 shall apply.
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