High-Performance/High-Strength Lightweight Concrete for Bridge Girders and Decks (2013) / Chapter Skim
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From page 69... ...
69 Attachment A Proposed Changes to AASHTO LRFD Bridge Design Specifications
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70 5.4.3.2 – Creep The creep coefficient may be taken as: Ψ(t,ti) = 1.9kskhckfktdti-0.118 (5.4.2.3.2-1)
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71 5.4.2.4 – Modulus of Elasticity In the absence of measured data, the modulus of elasticity, Ec, for concretes with unit weights between 0.090 and 0.155 kcf and specified compressive strengths up to 15.0 ksi may be taken as: (5.4.2.4-1) where: K1 = correction factor for source of aggregate to be taken as 1.0 unless determined by physical test, and as approved by the authority of jurisdiction wc = unit weight of concrete (kcf)
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From page 72... ...
72 5.4.2.6 -- Modulus of Rupture Unless determined by physical tests, the modulus of rupture, fr ksi, for specified concrete strengths up to 15.0 ksi, may be taken as: • For normal weight and sand lightweight concrete: o When used to calculate the cracking moment of a member in Articles 5.7.3.4, 5.7.3.6.2, and 6.10.4.2.1 ................0.24√f′c o When used to calculate the cracking moment of a member in Article 5.7.3.3.2 ........................................................ 0.37√f′c o When used to calculate the cracking moment of a member in Article 5.8.3.4.3 ........................................................
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73 5.5.4.2 -- Resistance Factors 5.5.4.2.1 -- Conventional Construction Resistance factor φ shall be taken as: • For tension-controlled reinforced concrete sections as defined in Article 5.7.2.1 ..............................................................0.90 • For tension-controlled prestressed concrete sections as defined in Article 5.7.2.1 ..............................................................1.00 • For shear and torsion: Normal weight concrete ..........0.90 Sand lightweight concrete…....0.85 All lightweight concrete ..........0.70 • For compression-controlled sections with spirals or ties, as defined in Article 5.7.2.1, except as specified in Articles 5.10.11.3 and 5.10.11.4.1b for Seismic Zones 2, 3, and 4 at the extreme event limit state ..............................................................0.75 • For bearing on concrete .......................0.70 • For compression in strut-and-tie models ............................................................. 0.70 C5.5.4.2.1 In applying the resistance factors for tensioncontrolled and compression-controlled sections, the axial tensions and compressions to be considered are those caused by external forces.
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74 Previous editions of AASHTO LRFD have given a φ of 0.7 for shear and torsion of lightweight concrete. Research by Cousins, Roberts-Wollmann and Brown (2013)
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75 5.8.2.2 -- Modifications for Lightweight Concrete Where lightweight aggregate concretes are used the following modifications shall apply in determining resistance to torsion and shear. • Where the average splitting tensile strength of lightweight concrete, fct, is specified, the term √f′c in the expressions given in Articles 5.8.2 and 5.8.3 shall be replaced by: 4.7fct ≤ cf ′ • Where fct is not specified, the term 0.75√f′c for all lightweight concrete, and 0.85√f′c for sand lightweight concrete shall be substituted for √f′c in the expressions given in Articles 5.8.2 and 5.8.3.
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76 5.8.4 – Interface Shear Transfer – Shear Friction 5.8.4.1 – General The interface shear strength Eqs. 5.8.4.1-3, 5.8.4.1-4, and 5.8.4.1-5 are based on experimental data for normal weight, nonmonolithic concrete strengths ranging from 2.5 ksi to 16.5 ksi; normal weight, monolithic concrete strengths from 3.5 ksi to 18.0 ksi; sand lightweight concrete strengths from 2.0 ksi to 6.0 8.0 ksi; and all-lightweight concrete strengths from 4.0 ksi to 5.2 ksi.
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77 5.8.4.3 – Cohesion and Friction Factors The following values shall be taken for cohesion, c, and friction factor, µ: • For a cast-in-place concrete slab on clean concrete girder surfaces, free of laitance with surface roughened to an amplitude of 0.25 in: c = 0.28 ksi µ = 1.0 K1 =0.3 K2 =1.8 ksi for normal weight concrete =1.3 ksi for lightweight concrete • For normal weight concrete placed monolithically: c = 0.40 ksi µ = 1.4 K1 =0.25 K2 =1.5 ksi • For lightweight concrete placed monolithically, or nonmonolithically, against a clean concrete surface, free of laitance with surface intentionally roughened to an amplitude of 0.25 in.: c = 0.24 ksi µ = 1.0 K1 = 0.25 K2 = 1.0 ksi C5.8.4.3 The values presented provide a lower bound of the substantial body of experimental data available in the literature (Loov and Patnaik, 1994; Patnaik, 1999; Mattock, 2001; Slapkus and Kahn, 2004: Cousins, Roberts-Wollmann, and Brown (2013)
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78 5.9.5.4 – Refined Estimates of Time-Dependent Losses 5.9.5.4.1 – General For nonsegmental Prestressed members, more accurate values of creep-, shrinkage-, and relaxationrelated losses than those specified in Article 5.9.5.3 may be determined in accordance with the provisions of this Article. For precast pretensioned girders without a composite topping and for precast or castin-place nonsegmental post-tensioned girders, the provisions of Articles 5.9.5.4.4 and 5.9.5.4.5, respectively, shall be considered before applying the provisions of this Article.
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From page 79... ...
79 5.11.4 – Development of Prestressing Strand 5.11.4.1 – General In determining the resistance of pretensioned concrete components in their end zones, the gradual buildup of the strand force in the transfer and development lengths shall be taken into account. The stress in the prestressing steel may be assumed to vary linearly from 0.0 at the point where bonding commences to the effective stress after losses, fpe, at the end of the transfer length.
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