Structures Design Guidelines | July 2004
4 - Superstructure - Concrete |

## 4.6 Post-Tensioned Box Girders

During preliminary engineering and when determining structure configuration, give utmost consideration to accessibility and to the safety of bridge inspectors and maintenance. Precast, pretensioned Florida U-Beams are exempt from special requirements for inspection and access.

### 4.6.1 Access and Maintenance (01/05)

A. Height: [2.5.2.6.3]

1.) For maintenance and inspection, the minimum interior, clear height of box girders is 6 feet.

2.) Proposed heights less than 6 feet require SDO approval. If structural analysis requires less than 5 feet box depth, consult the SDO and the District Structures and Facilities Engineer for a decision on the box height and access details.

B. Electrical:

1.) Design and detail in accordance with SDO Standard Drawings.

2.) Show interior lighting and electrical outlets spaced at not more than 50 feet.

C. Access:

1.) Design box sections with access doors located at maximum 300 feet spacing.

2.) Design entrances to box girders with in-swinging, hinged, solid doors. Design doors in diaphragms with in-swinging, hinged, 0.25-inch mesh screen doors. Equip all doors at abutments and entrances with a lock and hasp. Require that all locks on an individual bridge be keyed alike.

3.) Provide an access opening through all interior diaphragms. If the bottom of the diaphragm access opening is not flush with the bottom flange, provide concrete ramps to facilitate equipment movement.

4.) The minimum access opening is 32-inches wide x 42-inches tall. Indicate on plans that diaphragm access openings are to remain clear and are not to be used for utilities or other attachments. If utilities are required, provide additional areas or openings.

5.) Analyze access opening sizes and bottom flange locations for structural effects on the girder.

6.) Avoid entrance locations over traffic lanes and locations that will require extensive maintenance of traffic operations or that would otherwise impact the safety of inspectors or the traveling public.

D. Other Exterior Openings:

1.) Design each box girder with minimum 2-inch diameter ventilation or drain holes located in the bottom flange on both sides of the box spaced at approximately 50 feet or as needed to provide proper drainage. Place additional drains at all low points against internal barriers. Locate drains to accommodate bridge grade.

2.) Provide drains to prevent water (including condensation) from ponding near post-tensioning components, face of diaphragms, blisters, ribs and other obstructions. Show details on Contract Drawings. Include the following:

a.) Specify that drains may be formed using 2-inch diameter permanent plastic pipes (PVC with UV inhibitor) set flush with the top of the bottom slab.

b.) A small drip recess, ½-inch by ½-inch around bottom of pipe insert.

c.) Drains at all low points against internal barriers, blisters, etc.

d.) Drains on both sides of box, regardless of cross slope (to avoid confusion.)

e.) Vermin guards for all drains and holes.

f.) A note stating, “Install similar drains at all low spots made by barriers introduced to accommodate means and methods of construction, including additional blocks or blisters.”

3.) Require 0.25-inch screen on all exterior openings not covered by a door. This includes holes in webs through which drain pipes pass, ventilation holes, drain holes, etc.

4.) Design flexible barriers to seal openings between expansion joint segments of adjacent end units to prevent birds from roosting on the box end ledges. Barriers should be UV and weather resistant and easily replaceable.

E. Other Box Sections - Provide accessibility to box sections such as precast hollow pier segments in a manner similar to that for box girders, particularly concerning the safety of bridge inspectors and maintenance personnel. During preliminary engineering and when determining structure configuration, give utmost consideration to box girder accessibility and the safety of bridge inspectors and maintenance personnel. Due to the wide variety of shapes and sizes of hollow sections such as precast concrete pier segments, numerous site constraints and environmental conditions, each application will be considered on an individual, project-by-project basis. In all cases, contact the SDO for guidance in designing adequate inspection access and safety measures.

### 4.6.2 Prestress

A. Deck Slab:

1.) Detail all box girder deck slabs to be transversely post-tensioned.

2.) Where draped post-tensioning is used in deck slabs, consideration must be given to the final location of the center of gravity of the prestressing steel within the duct.

3.) Reduce critical eccentricities over the webs and at the centerline of box by ¼-inch from theoretical to account for construction tolerances.

### 4.6.3 Post-tensioning Anchorages

A. When temporary or permanent post-tensioning anchorages are required in the top or bottom slab of box girders, design and detail interior blisters, face anchors or other SDO approved means. Block-outs that extend to either the interior or exterior surfaces of the slabs are not permitted.

B. Provide continuous typical longitudinal mild reinforcing through all segment joints for Cast-in-place segmental construction.

C. Design and detail so that all future post-tensioning utilizes external tendons (bars or strands) and so that any one span can be strengthened independently of adjacent spans.

D. Detail anchor blisters so that tendons terminate no closer than 12-inches to a joint between segments.

E. Detail all interior blisters set back a minimum of 12-inches from the joint. Provide a “V”-groove around the top slab blisters to isolate the anchorage from any free water.

F. Transverse bottom slab ribs are not allowed. Design full height diaphragms directing the deviation forces directly into the web and slab.

G. Raised corner recesses in the top corner of pier segments at closure joints are not allowed. The typical cross section must be continued to the face of the diaphragm. Locate tendon anchorages to permit jack placement.

### 4.6.4 Design Requirements for Cantilever Bridges with Fixed Pier Tables (01/05)

A. Design Superstructures and Substructures to accommodate erection tolerances of L/1000 (where L is the cantilever length from center of pier to the cantilever tip.) Structure stresses shall be enveloped assuming a worst case condition (LA/1000 high on Cantilever A and Ln /1000 low on adjacent Cantilever B and vice-versa) assuming uncracked sections. Check the service limit state assuming these locked-in erection stresses, "EL" in LRFD Equation 3.4.1-2.

B. The service load stresses of the column and column-superstructure connection, including crack control of the column shall also be checked for both erection and final structure.

Commentary: Field correction for geometry control for framed bridges built in balanced cantilever can result in high stresses in both the superstructure and substructure. These stresses need to be accommodated for by the designer. The L/1000 value is consistent with the allowable erection tolerance per FDOT Specification 452. These requirements stipulate a CIP traveler segmental project would have the same erection tolerance requirements as a precast bridge.

###

### 4.6.5 Thermal Effects [5.14.2.3.5]

A. For detailing purposes, take the normal mean temperature from Table 6.2
.

B. For Seasonal Variation (expansion/contraction), refer to Table 6.2
for temperature ranges.

C. Show temperature-setting variations for bearing and expansion joints on the bridge plans.

### 4.6.6 Creep and Shrinkage [5.14.2.3.6]

Calculate creep and shrinkage strains and effects in accordance with LRFD using Relative Humidity of 75%.

### 4.6.7 Expansion Joints

A. At expansion joints, provide a recess and continuous expansion joint device seat to receive the assembly, anchor bolts, and frames of the expansion joint, i.e. a finger or modular type joint. In the past, block-outs have been made in such seats to provide access for stressing jacks to the upper longitudinal tendon anchors set as high as possible in the anchor block. Lower the upper tendon anchors and re-arrange the anchor layout as necessary to provide access for the stressing jacks.

B. At all expansion joints, protect anchors from dripping water by means of skirts, baffles, v-grooves, or drip flanges. Ensure that drip flanges are of adequate size and shape to maintain structural integrity during form removal and erection.

### 4.6.8 Construction Data Elevation and Camber Curve for Box Girders

A. General: Base Construction Data Elevations on the vertical and horizontal highway geometry. Calculate the Camber Curve based on the assumed erection loads used in the design and the assumed construction sequence.

B. Construction Data Elevations: Show construction data elevations in 3D space with "x", "y", and "z" coordinates. Locate the data points at the centerline of the box and over each web of the box.

C. Camber Curve: Provide Camber Curve data at the centerline of the box. Camber curve data is the opposite of deflections. Camber is the amount by which the concrete profile at the time of casting must differ from the theoretical geometric profile grade (generally a straight line) in order to compensate for all structural dead load, post-tensioning, long and short term time dependent deformations (creep and shrinkage), and effects of construction loads and sequence of erection.

### 4.6.9 Transverse Deck Loading, Analysis & Design (01/05)

A. The loading for the transverse design of box girders shall be limited to axle loads without the corresponding lane loads. Axle loads shall be those that produce the maximum effect from either the HL-93 design truck or the design tandem axles (LRFD 3.6.1.2.2 and 3.6.1.2.3, respectively). The Multiple Presence Factors (LRFD 3.6.1.1.2) shall also be included in the transverse design. The Tire Contact Area (LRFD 3.6.1.2.5) shall not be included in the transverse design of new bridges when using influence surface analysis methods to calculate fixed-end moments.

B. The prestressed concrete deck shall be designed for Strength I and Service I Load Combination excluding all wind effects. All analyses will be performed assuming no benefit from the stiffening effects of any traffic railing barrier.

Commentary:

The Tire Contact Area (LRFD 3.6.1.2.5) may be used when evaluating the transverse operating rating of existing prestressed concrete box girder decks.

### 4.6.10 Span-by-Span Segmental Diaphragm Details (01/05)

A. Design external tendons so that the highest point of alignment is at anchorages in diaphragm segment.

B. Arching tendons in diaphragm segments are prohibited.

C. Design tendon grout ports and vents so that they do not pierce the top slab of a structural section.

D. In aggressive environments or over water:

1. Split pier segments are not allowed.

2. Specify that pier segments be constructed as one continuous section of concrete. (Diaphragms may be cast as a separate pour.)

External Tendons in Pier Segments - (A) Proper anchorage at the high point and (B) Prohibited arching of tendon in diaphragm

### 4.6.11 Analytical Methods for the Load Rating of Post-tensioned Box Girder Bridges (01/05)

Load rating will be performed in accordance with the AASHTO LRFR procedures as modified by the Department and contained in the FDOT Appendix E.6.1 addition to LRFR. For general references, see Vol. 10A “Load Rating Post-Tensioned Concrete Segmental Bridges”. Volume 10A can be found on the Structures Design web site at the following address: www.dot.state.fl.us/structures/posttensioning.htm.