751.23 P/S Concrete Double Tee

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751.23.1 General

751.23.1.1 Material Properties

Standard compressive strength for P/S Double Tee Girder shall be: \, f'_{ci} = 4.5 ksi
  \, f'_c = 6.0 ksi
Optional concrete strength shall be: \, f'_{ci} = 5.0 ksi
  \, f'_c = 7.0 ksi
Modulus of Elasticity, E_c = 33,000\ K_1 \ (w_c^{1.5}) \sqrt{f^'_c}
  f'c in ksi
  K1 = correction factor for source of aggregate
aa = 1.0 unless determined by physical testing
  w_c = 0.145 kcf\ \mbox{for}\ f'_c\le 5.0 ksi
  w_c = 0.140 + 0.001 f'_c\ \mbox{for}\ f'_c>5.0 ksi
Prestressing strand
Type of strand:
AASHTO M203 (ASTM A416) Grade 270
Uncoated, seven-wire, low-relaxation strand
Ultimate tensile strength, \, f_{pu} = 270 ksi
Yield strength, \, f_{py} = 0.9 f_{pu} ksi
Strand modulus of elasticity, \, E_p = 28500 ksi
For standard concrete:
  Strand diameter, \, d_{ps} = 0.5 in
  Strand area, \, A_{ps} = 0.153 in^2
For optional concrete:
  Strand diameter, \, d_{ps} = 0.6 in
  Strand area, \, A_{ps} = 0.217 in^2
Mild reinforcing steel
Minimum yield strength, \, f_y = 60.0 ksi
Steel modulus of elasticity, \, E_s = 29000 ksi

751.23.1.2 Girder and Slab Design

The cast-in-place slab is designed to carry only the live load of the bridge. One layer of steel is used in each direction of the cast-in-place slab with 1" clearance from bottom of slab. A 1" integral wearing surface is deducted from effective slab depth for design.

The double tee girder flange is designed to carry the load of the cast-in-place slab, the flange weight and a 50 lb./sq. ft. construction dead load. (or a 300 lb. concentrated construction load per OSHA).

When determining the section properties of the double tee girder the center of gravity is based upon the total area (excluding corner fillets) of the entire exterior or interior girder and not on the individual stems. Moment of inertia and dead loads to each stem are figured to the center of the flange between the stems. The non-composite dead load of the external double tee girder is divided according to the following sketch.

Image:751.23 section thru double tee.gif

The non-composite dead load of the interior double tee girder is divided evenly to each stem. A 9 lb./lin ft. dead load is added to the non-composite dead load of each stem for haunching. The non-composite dead load of C.I.P. slab, girder and haunch are based on simple spans from Ë to Ë Bearings.

The composite dead load of the safety barrier is divided 75% to the exterior stem of the exterior girder and 25% to the interior stem of the exterior girder. No dead load due to the barrier curb is added to the interior girder. The composite dead load for future wearing surface is 35 lb./sq. ft. or 3" of material. The composite dead load of safety barrier curb, and future wearing surface are based on continuous, composite spans from Ë to Ë Bearings.

The live load design is based on HL93. The live load distribution factors are figured separate for exterior and interior Double Tee Girders. The assumption, that if the bridge is widened in the future the exterior girder then becomes an interior girder should be investigated.

Composite sections are adjusted for the difference in modulus of elasticity of slab and girder by multiplying by the factor (Eslab/Egirder). A 1" integral wearing surface is deducted for effective slab design.

Negative moment reinforcement is based on the amount required for the entire roadway width and not the amount required for each stem. The continuous span lengths are based on the distance from the Ë Bearing at End Bents to the Ë Interior Bent. The area of reinforcing bars is provided by adding bars between the normal longitudinal bars.

Continuity is obtained at intermediate supports by extending and bending all strands to form interlocking right angle hooks and pouring a concrete diaphragm monolithic with deck slab encasing the prestressed girders with the flange cut back at diaphragm.

Camber is based on simple spans. A transformed moment of inertia is figured for the total exterior or interior double tee girder since the total girder is cambered at the same rate.

Maximum horizontal curvature for Double-Tee structures is 3^. Outside flange edge of exterior girders on curved bridges is to be curved.

751.23.2 Details

751.23.2.1 Girder Dimensions

Image:751.23 part plan of ps conc dbl tee girder.gif
Part Plan of P/S Conc. Dbl. Tee Girder

Image:751.23 section thru ps conc dble tee girder.gif
Section Thru P/S Conc. Dbl. Tee Girder

Image:751.23 section thru ps conc dble tee girder detail a.gif
Detail "A"

751.23.2.2 Vent Holes

Note: Use vent holes only on all stream crossing structures.

Image:751.23 double tee vent hole details.gif
Part Elevation of Girder Stem  
Part Section Near Vent Hole

751.23.2.3 Diaphragm Reinforcement

Image:751.23 double tee diaphragm reinf. for end bent int bent.gif
End Bent   Int. Bent

Image:751.23 double tee diaphragm reinf. for skew thru 30 deg skew over 30 deg.gif
Skew Thru 30 ø   Skew Over 30 ø

Image:751.23 double tee diaphragm reinf. for skew thru 30 deg skew over 30 deg notes.gif

751.23.2.4 Detail of Optional Construction Joint

Image:751.23 double tee optional constr joint.gif

Optional Const. Joint

Image:751.23 double tee optional constr joint sec aa.gif

Section A-A

* Place Const. Joint 6" from negative reinforcement in slab.

Note: Const. Joint placement will be determined by direction of slab pour.

751.23.2.5 Details for Curved Structures

Image:751.23 double tee curved structure detail.gif
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