Design Criteria

Standard girder designs must meet the following requirements:

Add and drape strands in the order shown on the Prestressed I-Girder Non-Standard Designs (IGND) standard drawing. Draping strands is the preferred method to reduce tensile stresses at the end of the beam.

Straight strand designs with and without debonding are permitted as an alternate to draping provided stress and other limits noted below are satisfied.

Debonded strands must conform to Article 5.9.4.3.3 except as noted below:

The maximum debonding length is the lesser of: (a) one-half the span length minus the maximum development length; (b) 0.2 times the beam length; or (c) 15 ft.

Not more than 50% of the debonded strands, or 10 strands, whichever is greater, shall have the debonding terminated at any section, where section is defined as an increment (e.g., 3 feet, 6 feet, 9 feet).

Do not use both draped strands and straight strands with debonding to reduce tensile stresses within a beam.

Use hold-down points shown on the standard drawing IGD.

Strand stress after seating of chucks is limited to

0.75 f_{p u}

for low-relaxation strands.

Initial tension stress up to

0.24 λ {\sqrt{f^{'}}}_{c i}

(ksi) is allowed for all standard TxDOT I-girder sections.

Initial compression stress up to

0.65 {f_{c i}}^{'}

(ksi) is allowed.

Final stress at the bottom of girder ends need not be checked except when straight debonded strands are used or when the effect of the transfer length of the prestressing strand is considered in the analysis.

Final tension stress up to

0.19 λ \sqrt{f_{c}}'

(ksi) is allowed.

The required final concrete strength

(f_{c}^{'})

is typically based on compressive stresses, which must not exceed the following limits:

0.60 f_{c}^{'}

for stresses due to total load plus effective prestress.

0.45 f_{c}^{'}

for stresses due to effective prestress plus permanent (dead) loads.

0.45 f_{c}^{'}

for stresses due to Fatigue I live loads plus one-half of the sum of stresses due to prestress and permanent (dead) loads.

Tension stress up to

0.24 λ \sqrt{f_{c}^{'}}

is allowed for checking concrete stresses during deck and diaphragm placement.

'Use an effective strand stress after release of

0.75 f_{p u} - Δ f_{p E S}

Keep the end position of depressed strands as low as possible so that the position of the strands does not control the release strength. Release strength can be controlled by end conditions when the depressed strands have been raised to their highest possible position.

Use the General Procedure as provided by Article 5.7.3.4.2 to determine shear resistance. Do not use provisions of Appendix B5 of the AASHTO LRFD Bridge Design Specifications.

Calculate required stirrup spacing for #4 Grade 60 bars according to the Article 5.7. Change stirrup spacing as shown on IGD standard drawing for I-girders only if analysis indicates the inadequacy of the standard design.

Only apply the requirement in Article 5.7.3.5 from inside face of support to inside face of support. Do not calculate from the inside face of support to the end of the beam.

Replace Equation 5.7.4.5-1 with the following:

v_{u i} = \frac{V_{u l} Q_{s l a b}}{I_{g} b_{v i}}

where

Q_{s l a b}

is the first moment of the area of the slab with respect to the neutral axis of the composite section.

Take

b_{v i}

width of the interface, equal to the beam top flange width. Do not reduce bvi to account for prestressed concrete panel bedding strips.

Determine interface shear transfer in accordance with Article 5.7.4. Take cohesion and friction factors as provided in Article 5.7.4.4 as follows:

c = 0.28 ksi

µ = 1.0

K_{1}

= 0.3

K_{2}

= 1.8 ksi

Replace Equation 5.4.2.3.2-2 with the following:

k_{s}

= 1.45 - 0.13 (V/S) > 0.0

Compute deflections due to slab weight and composite dead loads assuming the girder and slab to have the same modulus of elasticity. Assume

E_{c} = 5000

ksi for girders with

f_{c}^{'}

< 8.5 ksi. Show predicted slab deflections on the plans although field experience indicates actual deflections are generally less than predicted. Use the deflection due to slab weight only times 0.8 for calculating haunch depth.

TxDOT standard I-girders reinforced as shown on the IGD standard drawings are adequate for the requirements of Article 5.9.4.4.

A calculated positive (upward) camber is required after application of all permanent (dead) loads.

Use the following equations to determine prestress losses:

Total prestress losses:

∆ f_{p T} = ∆ f_{p E S} + ∆ f_{p S R} + ∆ f_{p C R} + ∆ f_{p R}

Elastic shortening:

∆ f_{p E S} = \frac{E_{p}}{E_{c i}} f_{c g p}

, where

f_{c g p} = 0.7 f_{p u} A_{p s} (\frac{1}{A_{g}} + \frac{e_{p}^{2}}{I_{g}}) - \frac{M_{g} e_{p}}{I_{g}}

Shrinkage loss:

∆ f_{f p S R} = E_{p} (\frac{140 - H}{4.8 + f_{c i}^{'}}) 4.4 * 10^{- 5}

Creep loss:

∆ f_{f p C R} = 0.1 (\frac{195 - H}{4.8 + f_{c i}^{'}}) (\frac{E_{p}}{E_{c i}}) (f_{c g p} + 0.6 ∆ f_{c d})

where

∆ f_{c d} = - (\frac{M_{s d} e_{p}}{I_{g}})

Relaxation loss:

∆ f_{p R} = \frac{2 f_{p t}}{K_{L}} (\frac{f_{p t}}{f_{p y}} - 0.55)

Use of AASHTO LRFD Bridge Design Specifications 2004, 3rd Ed
., Article 5.9.5, “Loss of Prestress,” is also allowed (available from the Bridge Division). Other methods to determine prestress losses are not allowed.

Section 4: Pretensioned Concrete I Girders