2010-06-02_manual for bulb tee girders.pdf

PRECAST BULB TEE GIRDER MANUAL

Refer to Utah Department of Transportation (UDOT) Specification 03412S – Prestressed Concrete

And 03251S – Post Tensioning

PRECAST BULB TEE GIRDERS

Bridge Design and Detailing Manual 1 of 28 UDOT – April 29, 2010

TABLE OF CONTENTS

SECTION 1 – GENERAL INFORMATION ........................................................... 2

SECTION 2 – TYPICAL GIRDER SECTIONS ..................................................... 3

SECTION 3 – USE OF GIRDER WORKSHEETS.............................................. 13

SECTION 4 – SHEET CHECKLIST ................................................................... 19

SECTION 5 – TYPICAL REINFORCING AND CONCRETE PROPERTIES ..... 20

SECTION 6 – DESIGN ....................................................................................... 21

SECTION 7 – LIFTING DEVICES, HANDLING, AND STORAGE .................... 28



Section 1

GENERAL INFORMATION The purpose of this manual is to provide guidance with the design and detailing of Precast Prestressed Concrete Bulb Tee Girders. This manual discusses the design, detailing, fabrication, and handling of precast prestressed girder bridges. Precast girders are fabricated in a plant and then erected. There are two means of prestressing a concrete girder:

Pre-tensioning is accomplished by stressing strands to a predetermined tension and then placing concrete around the strands while the stress is maintained. The strands are released after the concrete has gained a specified strength. The concrete that has become bonded to the tendon is prestressed as a result of the strands attempting to relax to their original length. The strand stress is maintained during concrete placing and curing anchoring the strand ends to abutments that may be as much as 500 feet apart. The abutments and appurtenances used in the prestressing procedure are referred to as a pre-tensioning bed. Post-tensioning is a process where strands are placed within ducts that are cast into the precast beam and tensioned after the girder has been removed from the casting bed. Post tensioned girders are often used for long spans where shipping limitations preclude the use of pretensioned girders. Post-tensioned girders are often cast in two or more pieces that are connected in the field. The individual pieces of a post-tensioned girder often contain a prestressing strand in addition to the post-tensioning strand.



Section 2

TYPICAL GIRDER SECTIONS

Three families of bulb tee girders have been developed by UDOT. The girders are based on the Washington State DOT WF Series girders. These girders were developed in metric units. UDOT has standardized the section by converting the metric cross section to English units. The girder depths have been converted to “hard” English dimensions. The girder depths range from 42 inch to 98 inch in 8 inch increments. The three families of UDOT bulb tee girders are as follows:

Prestressed Bulb Tee Girders: These girders are used for medium to long span bridges with composite concrete decks. The girder designation for the prestressed bulb tee girders is “UBTXX” (Utah Bulb Tee). The XX number following the “UBT” designates the girder depth. The approximate span ranges for these girders are as follows:



BULB TEE GIRDERS (f'c=6.5 ksi)

60

80

100

120

140

160

180

8 9 10 11 12

Girder Spacing (FT)

Des

ign

Span

(ft)

UBT42UBT42

UBT50

UBT58

UBT66

UBT74

UBT82

UBT90

UBT98

Approximate Maximum Spans for Bulb Tee Girders f’c = 6.5 ksi



BULB TEE GIRDERS (f'c=8.5 ksi)

80

100

120

140

160

180

8 9 10 11 12

Girder Spacing (FT)

Des

ign

Span

(ft)

UBT42

UBT50

UBT58

UBT66

UBT74

UBT82

UBT90

UBT98

Approximate Maximum Spans for Bulb Tee Girders f’c = 8.5 ksi



Prestressed Bulb Tee Deck Girders: These girders are used for medium to long span bridges without composite concrete decks. The top flange of the girder becomes the deck of the bridge.

The use of decked bulb-tee girders is limited to roadways with an Average Daily Traffic (ADT) of 30,000 or less. Specify an HMA overlay with membrane over all girders of these types. The girder designation for the prestressed bulb tee deck girder is “UDBTXX” (Utah Deck Bulb Tee). The XX number following the “UDBT” designates the girder depth. The approximate span ranges for these girders are as follows:



DECK BULB TEE GIRDERS (f'c = 6.5 ksi)

80

100

120

140

160

180

40 50 60 70 80 90 100

Girder Size (in)

Des

ign

Span

(ft)

Approximate Maximum Spans for Deck Bulb Tee Girders f’c = 6.5 ksi



DECK BULB TEE GIRDERS (f'c = 8.5 ksi)

100

120

140

160

180

200

220

40 50 60 70 80 90 100

Girder Size (in)

Des

ign

Span

(ft)

Approximate Maximum Spans for Deck Bulb Tee Girders f’c = 8.5 ksi



Post-tensioned Bulb Tee Girders: These girders are used for long span bridges with composite concrete decks. These girders are more expensive than equivalent depth pre-tensioned girders and have only been developed for the larger sections. The girder designation for the post-tensioned bulb tee girders is “UBTXX-PT” (Utah Bulb Tee – Post Tensioned). The number following the “UBT” designates the girder depth. The approximate span ranges for these girders are as follows:



PT BULB TEE GIRDERS (f'c=6.5 ksi)

100

110

120

130

140

150

160

170

180

190

200

8 9 10 11 12

Girder Spacing (FT)

Des

ign

Span

(ft)

UBT66PT

UBT74PT

UBT82PT

UBT90PT

UBT98PT

Approximate Maximum Spans for Post-Tensioned Bulb Tee Girders f’c = 6.5 ksi



PT BULB TEE GIRDERS (f'c=8.5 ksi)

100

110

120

130

140

150

160

170

180

190

200

8 9 10 11 12

Girder Spacing (FT)

Des

ign

Span

(ft)

UBT66PT

UBT74PT

UBT82PT

UBT90PT

UBT98PT

Approximate Maximum Spans for Post-Tensioned Bulb Tee Girders f’c = 8.5 ksi



Typical girder worksheets have been developed for the families of bulb tee girders. Additional detailing will be required for typical bridges. The additional details will normally contain but is not limited to the following listed details:

1. Framing Plan View of each span 2. Typical Transverse Sections as needed 3. Special girder end details

See Section 3 for more information on detailing.



Section 3

USE OF GIRDER WORKSHEETS The drawings developed by UDOT represent worksheets. These are very similar to standards in that they depict standard beam sections and reinforcing. The only information not contained on the sheets is the patterns for prestressing strand and the spacing of the shear reinforcement away from the end regions. The reinforcement in the end regions is shown on the worksheets. A design table is included on the worksheet. This table needs to be filled in for each girder type on a project. The sheets can be inserted into the project plans once this is done. The following tables define the parameters for each girder input table:



Pretensioned Bulb Tee Girders

Span

The span number shown on the framing plan

Girder

The girder number shown on the framing plan

End 1 Type

The end type used for the left end of the girder: A for integral abutment designs B for cantilever abutment designs

End 2 Type

The end type used for the right end of the girder: A for integral abutment designs B for cantilever abutment designs

Shear Connection Type

The method used to connect the composite deck to the girder: W for welded studs R for reinforcing hoops

Theta 1 (degrees)

The skew angle at end 1 of the girder

Theta 2 (degrees)


Plan Length (feet)

This is the length of the girder measured along the grade of the girder taking into account the added length caused by the grade change measured from end to end

F’ci (ksi)

The minimum concrete strength at time of release

F’c (ksi)

The final minimum concrete strength

Harped No. of Strands

The number of strands that are harped or draped in the girder (This should always be an even number).

Harped Jacking Force (kips)

The specified jacking force for the harped strands

Straight No. of Strands

The number of strands that are not harped or draped in the girder (This should always be an even number).

Straight Jacking Force (kips)

The specified jacking force for the straight strands

Ecl (inches)

The distance from the bottom of the girder to the center of gravity of all strands near mid-span

Es (inches)

The distance from the bottom of the girder to the center of gravity of all straight strands

Ehcl (inches)

The distance from the bottom of the girder to the center of gravity of all harped strands near mid-span

Ehend (inches)

The distance from the bottom of the girder to the center of gravity of all harped strands at the girder end

Camber DR (inches)

The calculated camber at release of prestress

Camber D40 (inches)

The calculated camber at 40 days, which corresponds to the anticipated date of deck casting

Camber DF (inches)

The calculated final camber at 120 days after all dead loads are applied to the girder

Deck Offsets A1-A4 (inches)

The calculated distance from the top of the girder to the underside of the deck based on the D40 camber and the grade of the roadway (see diagram on sheet)



Pretensioned Deck Bulb Tee Girders

Span


Girder


Theta 1 (degrees)


Theta 2 (degrees)


Plan Length (feet)

This is the length of the girder measured along the grade of the girder taking into account the added length caused by the grade change measured from end to end

F’ci (ksi)

The minimum concrete strength at time of release

F’c (ksi)


Harped No. of Strands

The number of strands that are harped or draped in the girder (This should always be an even number).

Harped Jacking Force (kips)

The specified jacking force for the harped strands

Straight No. of Strands

The number of strands that are not harped or draped in the girder (This should always be an even number).

Straight Jacking Force (kips)

The specified jacking force for the straight strands

Ecl (inches)

The distance from the bottom of the girder to the center of gravity of all strands near mid-span

Es (inches)

The distance from the bottom of the girder to the center of gravity of all straight strands

Ehcl (inches)

The distance from the bottom of the girder to the center of gravity of all harped strands near mid-span

Ehend (inches)

The distance from the bottom of the girder to the center of gravity of all harped strands at the girder end

Camber Release (inches)


Camber 30 (inches)

The calculated camber at 30 days

Camber 120 (inches)




Post-Tensioned Bulb Tee Girders

Post-Tensioning Table Span


Girder


Min. Compressive Strength - Girder (ksi)

The minimum final concrete strength in the girder

Min. Compressive Strength - Closure (ksi)

The final minimum concrete strength in the girder splice closure pour

Number of Strands The total number of post-tensioning strands (22 maximum per 4” diameter duct)

Prestressing Load Jacking (kips)

The specified jacking force for the post tensioning strands.

Prestressing Load After Seating (kips)

The calculated force in the post-tensioning strand after release and seating at the anchorage.

Total Prestress Loss Pretensioning Mid-Segment (ksi)

The total loss of pretensioned strand prestress in the middle segment

Total Prestress Loss P/T Mid-Segment (ksi)

The total loss of post-tensioned strand prestress in the middle segment

E1 (inches)

The distance from the bottom of the girder to the center of gravity of all post-tensioning strand at the ends of the girder

E2 (inches)

The distance from the bottom of the girder to the center of gravity of all post-tensioning strand at the girder splice points

E3 (inches)

The distance from the bottom of the girder to the center of gravity of all post-tensioning strand at mid-span

Camber Release (inches)


Camber 30 (inches)

The calculated camber at 30 days

Camber 120 (inches)


Girder Schedule Span


Girder


F’ci (ksi)

The minimum concrete strength at time of release of pre-tensioning strand

F’c (ksi)


Theta 1 (degrees)


Theta 2 (degrees)


End Segment 1 Plan Length (feet)

This is the length of the girder end segment 1 measured along the grade of the girder taking into account the added length caused by the grade change measured from the girder end to the centerline of the field splice



Post-Tensioned Bulb Tee Girders End Segment 1 Straight No. of Strands

The number of strands in end segment 1 of the girder (This should always be an even number).

End Segment 1 Straight Jacking Force (kips)

The specified jacking force for the straight strands in end segment 1 of the girder

End Segment 1 C.G. Strands – E (inches)

The distance from the bottom of the girder to the center of gravity of all pretensioned strands in end segment 1

End Segment 1 D1 (40)

The calculated camber due to pretensioning at 40 days in end segment 1 (see diagram on standard sheet)

Mid-Segment Plan Length (feet)

This is the length of the girder mid-segment measured along the grade of the girder taking into account the added length caused by the grade change measured from the centerline of the field splice the centerline of the field splice

Mid-Segment Straight No. of Strands

The number of strands in the mid-segment of the girder (This should always be an even number).

Mid-Segment Straight Jacking Force (kips)

The specified jacking force for the straight strands in mid-segment of the girder

Mid-Segment C.G. Strands – E (inches)

The distance from the bottom of the girder to the center of gravity of all pretensioned strands in the mid-segment

Mid-Segment D2 (40)

The calculated camber due to pretensioning at 40 days in the mid-segment (see diagram on standard sheet)

End Segment 2 Plan Length (feet)

This is the length of the girder end segment 2 measured along the grade of the girder taking into account the added length caused by the grade change measured from the girder end to the centerline of the field splice

End Segment 2 Straight No. of Strands

The number of strands in end segment 2 of the girder (This should always be an even number).

End Segment 2 Straight Jacking Force (kips)

The specified jacking force for the straight strands in end segment 2 of the girder

End Segment 2 C.G. Strands – E (inches)

The distance from the bottom of the girder to the center of gravity of all pretensioned strands in end segment 2

End Segment 2 D3 (40)

The calculated camber due to pretensioning at 40 days in end segment 2 (see diagram on standard sheet)

D4 (40) Post-Tensioning Camber (inches)

The calculated camber due to post-tensioning at 40 days (see diagram on standard sheet)

Deck Offsets A1-A7 (inches)

The calculated distance from the top of the girder to the underside of the deck based on the cambers noted above and the grade of the roadway (see diagram on sheet)



Typical transverse cross frame details and deck shear connection details have been developed. These details can be added to the girder worksheets with minor revisions. Supplemental details will be required for some designs. These details may include but are not limited to the following:

1. Special end treatments and projecting end reinforcing 2. Prestressing strand extensions from the girder ends for beams made

continuous for live load 3. Special treatments for bearings 4. Geometric end treatments that differ from the standard worksheet 5. Any other details that differ from the standard worksheets



Section 4

SHEET CHECKLIST In addition to the girder worksheets, the following sheets should be included: Framing Plan View Accurate, measurable detail, with exceptions to enhance clarity.

1. Label and locate the control line at each substructure unit. Match the

terminology on the layout such as reference line, centerline, or profile grade line.

2. Show abutment numbers, bent number, or both. 3. Reference control dimensions at all working points such as the

intersection of the control line and the centerlines of bents and abutments. 4. Beam lines located and numbered. 5. Skew angles. 6. Diaphragm layout and locations 7. Label joint locations and type 8. Design Data.

Typical Transverse Sections Accurate, measurable detail, with exceptions to enhance clarity.

1. Dimensioned girder spacing 2. Control line location 3. Deck layout and thickness 4. Wearing surface type and thickness 5. Deck cross slopes

Supplemental Details Accurate, measurable details, with exceptions to enhance clarity. Final Checks

1. Comply with UDOT CADD Detailing Standards. 2. Check all substructure details and dimensions. 3. Double check bars in various details against the bars shown in the bar

table. 4. Ensure that the name and number of the bridge is same on all detail

sheets including layout. 5. Initial the sheet after back-checking corrected details.



Section 5

REINFORCING AND CONCRETE PROPERTIES Mild Reinforcement: Coat all mild reinforcement according to UDOT Specification Section 03211, Reinforcing Steel. Welded wire reinforcement will not be allowed in girders. Prestressing and Post-Tensioning Strand Use AASHTO M 203 Grade 270, low relaxation. Use 0.6 inch diameter prestressing strand for pretensioned girders. Use the same size strand in post-tension application as required for the post-tension anchorage assembly and the post-tension duct. Concrete Properties: Nominal 28-day concrete strength (f’c) for precast girders with a cast-in-place deck is 8,500 psi. This strength can be increased to up to a maximum of 10,000 psi with UDOT approval where higher strengths will eliminate a line of girders . Specify the final concrete design strength and show on the plans in the worksheet design table. Calculate the minimum concrete compressive strength at release (f’ci) for each prestressed girder in a bridge and show on the plans in the worksheet design table. Specify 4,000 psi and 6,000 psi. Arbitrarily increasing the release strength does not improve girder quality since the strength is only a temporary condition but it does increase girder cost because it needs to be kept in the bed longer. Include in the plans only the release strength that is required by the design. Limit the required compressive strength at release to 6,500 psi if possible for high strength concrete. Release strengths of up to 8,500 psi can be achieved with extended curing for special circumstances but there is a higher fabrication cost associated with extended cure times. Round the specified concrete strength at release to the next highest 100 psi.



Section 6

DESIGN Design bulb tee girders according to AASHTO LRFD Bridge Design Specifications except as noted otherwise. Design prestressed concrete bridges for allowable stresses and check for ultimate load capacity. Bridge end skew angle is often controlled by the roadway geometry. Limit this skew angle to 45 degrees for all prestressed girders. Requirements for Continuous Span Design: Design continuous girders according to AASHTO LRFD Article 5.14.1.4 "Bridges Composed of Simple Span Precast Girders Made Continuous". Use a more exact analysis as directed by UDOT for continuous structures consisting of a large number of girders. Use the same type and number of prestressed girders in each continuous superstructure segment for continuous structures. Girder type or number of girders can only be changed at expansion joints at piers if applicable. Composite Action: Design all prestressed bulb tee girder bridges for composite action.

Application of loads for design: Use the following sequence and method of applying loads in girder analysis:

1. Apply Girder Dead Load to the girder section. 2. Apply Diaphragm Dead Load to the girder section. 3. Apply Slab Dead Load to the girder section. 4. Apply Barrier, Overlay Dead Load, and Live Load to the composite

section. The dead load of one traffic barrier is divided among a maximum of three girders and this uniform load is applied to the composite section. The dead load of any overlay and live load plus impact is applied to the composite section.

Composite Section Properties: Minimum deck slab thickness is specified as 8¾ inches but may be thicker if girder spacing dictates. Use the full deck thickness for dead load computations if deck grinding is specified. Use the reduce deck thickness after grinding for section property calculations. Assume that the bottom of the slab is directly on the top of the girder for purposes of calculating composite section properties. This assumption may prove to be true at center of span when excess girder camber occurs. Do not use transformed steel areas for the calculation of section properties.



Use an increased dimension from top of girder to top of slab at centerline of bearing for plan dimensioning. This is called the “A” dimension. This dimension accounts for the effects of girder camber, vertical curve, slab cross slope, etc.

Diaphragm Requirements: Diaphragms used with prestressed girder bridges serve two purposes. The diaphragms provide girder stability for pouring the slab during the construction stage. The diaphragms act as load distributing elements during the life of the bridge. Standard diaphragm details and spacing are shown on the girder work sheets for prestressed girder bridges. Diaphragms that fall within the limitations stated on the typical detail drawings need not be analyzed. Perform special diaphragm designs when large girder spacing will be used or other unusual conditions exist. Geometry

1. Orient diaphragms parallel to skew for bridges that are skewed up to 20 degrees.

2. Orient diaphragms square to girders for bridges that are skewed over 20 degrees.

Grade and Cross Slope Effects: Large cross slopes require an increased girder pad dimension (A dimension) so the structure can be built. This effect is especially pronounced if the bridge is on a horizontal or vertical curve. Girder lengths may need to be modified to correct for added length along slope. This is more pronounced for longer span bridges on steep grades. The girder work sheets include a table for setting the gap between the deck slab and the top of the beam. It is important to note that the position of the girder top corner is affected by grade, girder camber, and tolerances. Account for these effects in the data entered into the worksheet tables. Curve Effect and Flare Effect: Curves and tapered roadways complicate the design of straight girders. Use the girder spacing at mid-span for bridges with minor curvature and flare. Prestressing Strand Patterns: Standard strand pattern for all types of UDOT prestressed girders are shown on the work sheets.

Straight Strands: The position of the straight strands in the bottom flange. Those strand positions and the girder flange sizes are shown on the girder work sheets.

Harped Strands: The harped strands are bundled at the 0.4 and 0.6 points of the girder length. The harped strands are bundled at the harping points. Bundles are limited to eight strands each. Eight and fewer harped strands are placed in a single bundle with the centroid normally three inches above the bottom of the girder. Strands in excess of eight are bundled in a second bundle with the



centroid six inches above the bottom of the girder. The strands are splayed to a normal pattern at the girder ends. The centroid of strands at both the girder end and the harping point may vary to suit girder stress requirements.

The slope of the harped strands cannot be steeper than eight horizontal to one vertical for all prestressed girders. Hold the harped strand exit location at the girder ends as low as possible while maintaining the concrete stresses within allowable limits.

Partially Debonded Strands: The strand may be debonded where it is necessary to prevent it from actively supplying prestress force near the end of a girder. Do not debond more than 25 percent of the total number of strands and do not debond more than 40 percent of the strands in any horizontal row. Distribute the debonded strands symmetrically about the centerline of the member. Keep the lengths of the debonded strand pairs symmetrically positioned and equal about the centerline of the member. Do not debond the exterior strands in each horizontal row. Double the development length specified in AASHTO LRFD section 5.11.4.2 where a portion or portions of a pre-tensioning strand are not bonded and where tension exists in the pre-compressed tensile zone. Debonding will normally require changes to standard shear reinforcement and confinement reinforcement. The details on the standard sheets are based on girders without debonding. Designers should modify the girder details or ad supplemental details for girder with debonding. Post-Tensioned Spliced Girders Post tensioning can be used to maximize the span length of a girder. Post tensioning can also be used to simplify girder shipping because the girder can be fabricated in three pieces and spliced together in the field. Spliced girder technology can be used to create multi-span bridges. The girder worksheets for spliced girders are limited to single span bridges. Many of the details included on the worksheets can be used for multi-span construction with additional detailing. The girders are spliced with reinforced concrete closure pours. The post-tensioning strand crosses these closure pours and provides the moment capacity at the splice. Match casting of these joints is not recommended because of the potential for end rotation due to camber effects. Only several sections of post-tensioned girders have been developed. Only use these sections for long span bridges that cannot be shipped as single pieces. The major difference between the pretensioned girders and the post-tensioned girders is the width



of the sections and the required end blocks. Two inches has been added to the girder to provide ample room for post tensioning ducts in the girder web. It is recommended that the design of the girder include both pretensioned strand and post-tensioning. The pretensioned strands in the end sections will aid in resisting shipping and handling stresses. The pretensioned strand in the middle section will also do this in addition to providing additional bending capacity at mid-span. Post-Tensioning Anchorage Systems There are numerous post-tensioning systems that combine a method of stressing the prestressing strands with a method of anchoring it to concrete. UDOT requires approval of all multi-strand and bar anchorages used in prestressed concrete bridges by testing or by a certified report, stating that the anchorage assembly will develop the yield strength of post-tensioning strand. UDOT approved anchorages are listed in the special provisions. Encase longitudinal prestressing stands in a galvanized, ferrous metal duct that is rigid and spiral. Maintain the required duct profile within a placement tolerance of plus or minus ¼ inch. Provide vents at high points and drains at low points of the tendon profile. Use ½ inch minimum diameter vents and drains made of standard steel or polyethylene pipe. The current AASHTO LRFD Bridge Design Specifications states:

The size of ducts shall not exceed 0.4 times the least gross concrete thickness at the duct.

Based on this provision, a girder with an 8.125 inch web could only use a maximum size duct of 3.25” diameter, which can only accommodate 12 strand (Note that the AASHTO Standards Specifications for Roads, Bridges and Incidental Construction does not have this requirement). This was found to be very restrictive and resulted in significant reductions in span length capacity. The Department investigated this situation and found that several states disregard this provision. Washington State Standards, which are the basis for the Utah Bulb Tee Standards uses a 4” duct for their girders. Utah has also used 4” ducts for several post-tensioned girders in the state. Based on this, the Department has decided to allow the use of 4” diameter duct, that can accommodate 22-0.6 inch diameter strand. Use ducts that are capable of withstanding at least 10 feet of concrete fluid pressure and have adequate longitudinal bending stiffness for smooth, wobble free placement. Use a radius of curvature of tendon ducts that is less than 20 feet except in anchorage areas where 12 feet may be permitted. Girder End Types: There are two end types shown on the girder sheets.



Integral abutment ends: Integral abutments have girder ends that are cast into the abutment stem to provide a fixed connection. The connection to the abutment is a fixed connection. The negative fixity moments are resisted by extending reinforcing from the bridge deck into the end closure pour. The positive moment are resisted by reinforcing bars passing through the girder web and the embedded girder end cast into the closure pour. Positive moment resistance generated by extending prestressing strand from the beam end is not normally used in Utah. Draw additional details on supplemental detail sheets if there is a need to provide more positive moment capacity. Cantilever Abutment Ends: The ends of girders supported on cantilever abutments do not have any reinforcing projecting from the girder ends.

Deflection and Camber: Accurate predictions of the deflections and camber are difficult to determine since modulus of elasticity of concrete, Ec, varies with stress and age of concrete. The effects of creep on deflections are difficult to estimate. An accuracy of 10 to 20 percent is often sufficient for practical purposes. The deflection and camber of prestressed members may be estimated by the multipliers as given in the following table for normal design and in lieu of methods that are more accurate.

Table 4

The following sections describe the different types of camber and deflection phenomenon:



Elastic Deflection Due to Prestress Force: The prestress force produces moments in the girder tending to bow the girder upward. Resisting these moments are girder section dead load moments. The result is a net upward deflection. A shortening of the girder occurs due to axial prestress loading. Slab Load Deflection: The load of the slab is applied to the girder section resulting in an elastic downward deflection. This deflection is offset by the screed camber that is applied to the bridge deck during construction. Composite Dead Load Deflection: Downward deflection due to composite dead loads such as traffic barriers, sidewalk, and overlay. Final Camber: The above slab dead load deflection may be accompanied by a continuing downward deflection due to creep. Many measurements of actual structure deflections have shown that the girder tends to act as though it is locked in position once the slab is poured. The deflection calculated at 120 days is sufficient for determining the final haunch dimensions to obtain a smooth riding deck surface.

Bearings for Precast Concrete Beams: Recesses and embedded steel plates in the bottom of precast concrete girder ends have been used in Utah. The recessed were used to provide a level surface that also restrains the elastomeric bearing from sliding. The steel plates can be beveled to provide a level bottom surface. There are issues with both of these approaches. The recesses reduce the cover of the prestressing strand and reinforcement at the beam end. This is undesirable especially for beam-ends that are exposed such as cantilever abutments. Beveled embedded plates can cause problems with conflicts with the prestressing strand. The girder form may need to be cut to allow the plate to pass through if the beveled plate is detailed as projecting from the bottom surface of the girder. The use of beveled elastomeric bearings is being considered by UDOT. Text and details for this approach are being considered at this time and will be included in future versions of this manual. Supplemental Girder Longitudinal Reinforcing: Section 5.8.3.5 of the AASHTO LRFD Bridge Design Specifications established requirements for longitudinal reinforcement. The reinforcement is required to resist the forces generated by the shear reinforcement internally in the girder. Preliminary girder designs used for the development of the standards revealed that the girder prestressing did not supply sufficient reinforcement to satisfy this article. Additional reinforcing steel has been added to the girder webs near the ends (below the draped strand). These bars are hooked to provide development near the girder end. Designers should use this reinforcing in their girder design calculations. For girders with extended reinforcing



(integral diaphragms), it may be possible to use this reinforcing by eliminating the internal hook and extending the bars into the diaphragm pour.



Section 7

LIFTING DEVICES, HANDLING, AND STORAGE

Lifting devices: The Contractor is responsible for locating and designing lifting locations. The Contractor will provide the spacing and location of the lifting devices on the shop drawings and calculate handling stresses. Lifting devices will be removed below the top surface of deck bulb tee beams after placement. Any divot or void at the lifting devices will have a heavy broom finish. Fill divots or voids with structural non-shrink grout after girder placement. Place grout high and ground to final elevation. Handling and Storage: The Contractor is responsible for girder handling and storage in such a manner that does not cause undue stress on the girder. The Engineer will inspect all girders and reject any defective elements. Replace any rejected elements at the Contractor’s expense. Contractor will be responsible for any schedule delays due to rejected elements.

2010-06-02_manual for bulb tee girders.pdf

Documents

precast girders

concrete girder

prestressed bulb tee

families of bulb tee

precast bulb tee girder

posttensioned girders

use of pretensioned

girder designation