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Superstructure Design
General Recommendations

Terminology and Notation

LRFD refers to Load and Resistance Factor Design, a design methodology that makes use of load factors and resistance factors based on the known variability of applied loads and material properties. Bracketed "references" reference relevant sections of the AASHTO LRFD Bridge Design Specifications.

Limit States

TxDOT recommends the following limit states for design of bridge system components "Article 3.4.1":

Component Limit State
Prestressed concrete beams Strength I and IV and Service I and III. Fatigue and extreme limit states need not be checked. Live load deflection need not be checked.

Load Factors

TxDOT recommends the following permanent loads "Article 3.5": The engineer may reduce the maximum load factor for wearing surfaces and utilities "DW in Table 3.4.1-2" to 1.25.

Prestressed Concrete Design

Recent incidents of further Alkali Silica Reactions (ASR) have caused us to revisit mix designs and the attainable concrete strengths used in the fabrication of prestressed concrete products. The current special provision to Item 420 has greatly increased amounts of fly-ash used in beam production. It is anticipated that this will also result in slower strength gains which could have a negative impact on fabricator production and ultimately on girder costs.

Therefore, we limit release concrete strengths, f'ci, to a maximum of 6.0 ksi. Design concrete strength, f'c, is limited to a maximum of 8.50 ksi.

The following links are provided for additional reading on ASR:

Superstructure Phasing Guidance

Phased Construction Recommendations

Do not use span standard detail sheets for phased structures.

Geometric Constraints

When selecting a location for the phase line, consider the following items:

  1. Traffic needs and the placement of any temporary barriers. If the clear distance between the back of the barrier and the edge of the slab is less than 2 feet, anchor the barrier to the deck.
  2. When building next to an existing structure (such as for phased replacements), provide enough space between the existing structure and the new construction to accommodate splicing of the deck reinforcement, the portion of the beam that extends beyond the edge of slab, the portion of bent or abutment that extends past the beam edge, and form work.
  3. Do not place a phase line in the middle or at the edge of a precast panel.
  4. For adjacent slab or box beam superstructures, place the phase line at the edge of the beam.
  5. For U-beam and X-Beams, place the phase line along the top flange of the beam.  If the phase line is located along the top flange of the beam, the majority of the beam will be under the initial phase of construction.  Do not place the phase line closer than 6 1/2 inches from the beam edge for U-beams and 10 inches for X-Beams, to allow for the use of precast panels in the future phase. Alternately, consider placing the phase line between two beams. Treat the slab between the beam and the phase line as an overhang.  Do not allow the use of panels in this space.
  6. For TxGirders, place the phase line a minimum of 4 inches past the centerline of the girder, so that the horizontal interface reinforcement is cast into the initial construction phase of the slab. The phase line may be place up to 12 inches past the centerline of the girder and accommodate PCP placement in the next phase.
  7. If a full depth open longitudinal joint is used at the phase line, the bridge is considered 2 structures and should have 2 NBI numbers.
  8. Phased superstructures may require variable spacing of beams.

Pictorial representations of phase line locations for superstructures are provided in this document.

Structural Analysis

When designing the beams, consider all temporary loading such as temporary rails as permanent loads for that phase. Design beams so that they meet all requirements for all phases of construction.

The beam located under the phase line will have less dead load deflection than the other beams constructed at the same time. This beam will not deflect additionally when the remainder of the slab is cast, due to the added stiffness of the cured slab. When calculating haunch for the beam along the phase line, use the dead load deflection from the initial slab weight. Do not use the full dead load deflection due to the full slab weight (initial and final).

Consider lowering the bearing seat elevations of later phases to account for the potential for higher than predicted cambers. There is no way to adjust the roadway grade in subsequent phases to accommodate high camber girders.

Software

Use PGSuper for beam design. Model phasing in PGSuper by using separate files for each phase and the completed structure. Refer to PGSuper Design Guide for further guidance about using PGSuper for beam design. Alternatively, use this spreadsheet to calculate live load distribution factors and PSTRS14 for beam design.

Corrosion Protection Measures

In areas of the state where de-icing agents are frequently used during winter storms, it is recommended that additional corrosion protection measures be incorporated into the bridge design and details.

District-specific requirements are available for review.

Concrete Deck Slabs on Stringers

Materials

See Corrosion Protection Measures above for special considerations where deicing agents are used.

Geometric Constraints

Deck slabs less than 8 in. thick are not recommended with TxDOT's standard prestressed concrete panels because they are not as durable or as constructible and they do not provide enough practical room above a 4-in. panel.

Structural Analysis
  • Standard deck slab designs account for effects of a 2.5-in. asphaltic overlay (DW). Weight of asphaltic overlay is based on a unit weight of 0.140 kcf "Table 3.5.1-1".
  • Live load is HL93 plus dynamic load allowance, IM.
  • Account for extra concrete in permanent metal deck form flutes for custom deck slab designs for which prestressed concrete panels cannot be used.
Design Criteria
  • Space deck slab main reinforcing steel (transverse reinforcement) no less than 6 in. to take advantage of reduced lap length requirements, as described in "Article 5.11.2.1.3
  • Make overhangs the same thickness as the slab unless other factors override this preference. For example, use of HT railing requires a 10-in. overhang thickness.
  • See "Appendix A4: Table A4-1" for LL+IM moments for traditionally designed deck slabs. TxDOT standard deck slab designs use this method.
  • Take negative moment at 1/3 the flange width from beam centerline for concrete I beams, and 1/4 the flange width from beam centerline for steel girders "Article 4.6.2.1.6".
  • Effective slab width at transverse slab edges is approximately half the normal effective slab width "Article 4.6.2.1.4c". Standard thickened slab ends are theoretically overstressed, but they have been experimentally proven to have more than adequate strength.
  • Standard deck slab designs are based on Class 2 exposure conditions. "Article 5.7.3.4".

Software

No software is needed for the majority of deck slabs. For special cases, use RISA 3D, any suitable finite element program, or SLAB49.

Detailing
  • To account for reduced wheel load distribution at transverse slab edges, strengthen the slab by increasing its depth, as shown on the Thickened Slab End Details standard drawing.
  • The standard deck slab corner break dimension is 2 ft. 0 in when skew is more than 15 degrees. The corner break point must occur at least 1 in. and preferably 3 in. from the toe of any concrete parapet into which an expansion joint is upturned.
  • With simple-span construction, minimize expansion joints by creating multi-span units with the slab continuous over interior bents. At bents without expansion joints, locate a control joint or construction joint in the deck.
  • Distribution reinforcing steel in standard deck slabs, Bars D (bottom longitudinal reinforcement), is based on "Article 9.7.3.2".
  • Additional longitudinal reinforcing steel is required for continuous steel girders "Article 6.10.3.7". Adding one #5 bar in the top slab between each usual longitudinal bar meets this requirement.
Concrete Deck Slabs on U Beams (U40 and U54)

Materials

See Corrosion Protection Measures above for special considerations where deciding agents are used.

Structural Analysis

Consider using a normal overhang when conditions make the sloped overhang unsightly or difficult and expensive to construct. For the sloped overhang, the slope of the bottom face of the overhang may vary significantly when used with curved slab edges primarily because of the overhang distance varying along the length of the exterior U beam.

On a straight bridge slab edge, however, the slope of the bottom face of the overhang varies only because of the vertical curvature of the roadway surface and the camber and dead load deflection of the exterior U beam, thereby creating a more pleasing appearance.

Prestressed Concrete I Girders

Materials

For recommended concrete strengths, see "Prestressed Concrete Design" information under the "General Recommendations" section above.

Structural Analysis

  • You need not increase section properties of the girder to account for the transformed area of strands or mild steel.
  • For the calculation of live load distribution factors, modular ratio, n, may conservatively be taken as 1.0.

Design Criteria - For grade separation structures, use the same girder depth for the full length of structure for economies of scale and aesthetic reasons. Stream crossing structures may have different types and sizes of girders for purposes of economy. Optimize girder spacing in each span. Maintaining a constant girder spacing for the full length of structure is not necessary. Selection of the proper type girder for a span is a matter of economics; calculate relative costs using current average bid prices for girders and slab.

Software

Use PGSuper for girder design. Refer to PGSuper Design Guide for further guidance. Alternatively, use this spreadsheet to calculate live load distribution factors and PSTRS14 for girder design.

Detailing

On the plans for each design, show optional design parameters for maximum top flange stress, bottom flange stress, and ultimate moment due to all design loads. The fabricator will retain the option to use other strand arrangements, including straight strand patterns, stress relieved strand, or 0.6-in. diameter strand, if design parameters are satisfied by the prestress and concrete strength selected.

Prestressed Concrete U Beams (Types U40 and U54)

Materials

For recommended concrete strengths, see "Prestressed Concrete Design" information under the "General Recommendations" section above.

Geometric Constraints

  • U beams are not vertical but are rotated to accommodate the average cross slope of a given span. As a result, the depth of slab haunch at the left and right top edges of the beam may differ. Pay special attention to these beams in calculating the haunch values.
  • The preferred method for framing U-beam centerlines is at the top of the beam. This prevents spacing at the top of the beam from varying due to the cross slope of the beam and, thus, simplifies slab formwork dimensions for construction. Note beam spacings shown on the span details as being at the top of the beam, and in beam spacings shown on the substructure details, take into account the horizontal offset between the centerlines at the top and bottom of the beam.
  • The alternate method for framing U-beam centerlines is at the bottom of the beam. This method allows the U beams to be framed as vertical members whereby the beam spacings dimensioned on the span details and beam layouts match the beam spacings shown on the substructure details. However, if this method is used, call attention to the variable beam spacing at the top of the beam in the plans. A construction note is recommended on the span details stating, "Beam spacing shown is measured at bottom of beam. Beam spacing at top of beam may vary due to cross slope of U beams."
  • TxDOT's Bridge Division currently uses the Bridge Geometry System (BGS) software program to frame U beams. The latest version of BGS frames U beams using the alternate method. The BGS manual includes information on three framing options written specifically for U beams: Options 20, 21, and 22. These framing options help the designer calculate accurate slab haunch values, bearing seat elevations, and bearing pad taper reports for U beams under the alternate method.
  • Use the same minimum haunch value for all U beams in a given span if reasonable to do so.
  • Left and right bearing seat elevations are located at the intersection of the edges of bearing seats with centerline bearings. When calculating these elevations for each beam seat, be careful to apply the appropriate deduction at that elevation point - that is, the minimum deduction at the correct elevation point and the maximum deduction at the other elevation point. Typically, the minimum deduction and maximum deduction are each applied at diagonally opposite corners of a beam in plan view. See Prestressed Concrete U Beam Design Guide for information on calculating U-beam slab haunches. The information is tailored for use with BGS, but the principles behind the method remain the same.
  • Provide at least 2 in. from the end of the cap or corbel to the edge of the bearing seat.

Design Criteria

Software

Use PGSuper for beam design. Refer to PGSuper Design Guide for further guidance. Alternatively, use this spreadsheet to calculate live load distribution factors and PSTRS14 for beam design.

Detailing

  • A full-depth cast-in-place deck with permanent metal deck forms may be provided at the contractor's option. This optional form is shown on the Permanent Metal Deck Form standard drawing.
  • Use thickened slab ends at all expansion joints with non-inverted tee bents. See the Miscellaneous Slab Details standard drawing for details of thickened slab ends.
  • Do not show a detailed bill of reinforcing steel on production drawings. Instead, show a table of bar designations with sizes used in the slab as is currently done with I-beam structures. In addition, show a table of estimated quantities with the total reinforcing steel based on 3.7 lbs. per sq. ft. of bridge deck. This quantity includes the extra slab steel required over inverted-tee bents and in thickened slab ends.
  • If inverted-tee caps are used and are sloped to match the sloping face of the U beam, use a 4:1 slope normal to the centerline of the bent.
  • The actual cross slope of the U beams framing into the bent potentially complicates construction of the bent cap and need not be considered. Try to extend the ends of the inverted-tee bents about 6 in. past the bottom edge of the exterior U beam. This extension allows for a more defined break between cap and beams, and the contractor is unlikely to set the beams perfectly in line with the end of the cap. The Miscellaneous Slab Details for Inverted-Tee Bents standard drawings show overhang details using this configuration over the inverted-tee bent caps.
  • Use slab dowels to provide lateral restraint when constructing U beams with inverted-tee bents. These dowels are located at the top of the inverted-tee stem and are in a slotted pipe to allow for expansion and contraction of the unit. Typically, only one dowel is placed at the centerline of every beam 1 ft. from the centerline of the bent. Slab dowels need to be placed on only one side of the centerline of the bent. The criteria for locating slab dowels within units are similar to the method used for locating dowels within concrete I-beam units.
  • A left and right bearing seat elevation is given for each U-beam bearing seat location. Bearing seats for U beams are level perpendicular to the centerline of the bent but slop uniformly between the left and right bearing seat elevations. This allows the bearing pads to taper in one direction.
  • Include a Bearing Pad Taper Report sheet in the plans summarizing bearing pad tapers to be used by the fabricator. See Prestressed Concrete U-Beam Design Guide for information on the calculation of bearing pad tapers for U beams.
Prestressed Slab Beams and Decked Slab Beams

Materials

For recommended concrete strengths, see "Prestressed Concrete Design" information under the "General Recommendations" section above.

Geometric Constraints

  • Limit skew to 30 degrees. Larger skews may result in beam twist and uneven bearing on the pads.
  • The requirement to bevel the bearing pads to match the beam slope on the Elastomeric Bearing Details sheet will not result in parallel pad and beam surfaces for skewed bridges. The actual calculations and fabrication of pads for each particular skewed case is complex. Given the small area of the pads, experience with box beams and the nearly parallel surfaces, the pads should be able to deform sufficiently to accommodate the mismatches.
  • When both a vertical curve and skew exist, a complex planar relationship develops between the skewed bottom of the slab beam, bearing pad, and bent or abutment cap: a stepped bearing seat arrangement on the caps is required.
  • Except for the T411 and C411 railings, no adjustment is needed to individual reinforcing bars embedded into the slab beam to account for the effects of vertical curve. The vertical curve requires the slab to be thicker either at the ends of the beam or at midspan. Theoretically, each embedded bar should protrude from the beam a different amount. However, in the most extreme case (VC length = 600 ft., tangent slopes = -5%, 5%, and span length = 50 ft.), the maximum variation of the profile grade line from a straight line drawn between top of slab at adjacent bents is only 5/8 in. This is not significant enough to warrant complicating the detailing, fabrication, and installation of the railing reinforcing.

Design Criteria

Software

Use PGSuper for beam design. Refer to PGSuper Design Guide for further guidance. Alternatively, use this spreadsheet to calculate live load distribution factors and PSTRS14 for beam design.

Prestressed Concrete Double-Tee Beams

Materials

For recommended concrete strengths, see "Prestressed Concrete Design" information under the "General Recommendations" section above.

Design Criteria

Software

Use PGSuper for beam design. Refer to PGSuper Design Guide for further guidance. Alternatively, use this spreadsheet to calculate live load distribution factors and PSTRS14 for beam design.

Prestressed Concrete Box Beams (B20, B28, B34, and B40)

Materials

For recommended concrete strengths, see "Prestressed Concrete Design" information under the "General Recommendations" section above.

Geometric Constraints

  • A three-pad system is currently used with box beams. Typically, the forward station end of the beam sits on a single elastomeric bearing pad while the back station end sits on two smaller pads.
  • Box beams are fabricated using a two-stage monolithic casting. The bottom slab is cast in the first stage, and the sides and top are cast in the second stage while the slab concrete is still plastic. In addition, cardboard void forms are no longer permitted. All interior voids must be formed with polystyrene. Void drain holes are installed at the corners of the bottom slab during fabrication.
Design Criteria:
  • Standard drawings do not support spread box beam configurations.
  • Use a cast-in-place reinforced concrete slab rather than an ACP overlay on box beam bridges. The slab should have a 5-in. minimum thickness, typically at the center of the span (or at center of bearing in situations such as sag vertical curves).
  • Avoid slab overhangs. Choose box beams and gap sizes so that the edge of the slab corresponds to the edge of the top flange of the exterior beams.
  • Box beams are not appropriate for use on curved structures and should be avoided on flared structures. The complexity of the geometry required to frame the bridge increases dramatically as the degree of curvature exceeds 1 or 2 degrees.
  • Use 5-ft. boxes as exterior beams when the roadway width requires a combination of both 4-ft. and 5-ft. boxes.
  • Do not use dowels for lateral restraint. Provide lateral restraint by 12-in.wide by 7-in. tall ear walls located at the ends of each abutment and interior bent cap. Provide a 1/2-in. gap between the the ear wall and the outside edge of the exterior beam.
  • Provide longitudinal restraint at interior bents only when a continuous unit exceeds four spans in length, or when the beam grade exceeds 6%. Provide longitudinal restraint at interior bents only.
  • Use beam hold-downs at water crossings when the superstructure could be subjected to pressure flow. The hold-downs are typically placed at the center of the joint between the exterior beam and the first interior beam on both sides of the structure. The hold-downs may be moved to the second interior joint for heavily skewed bridges (approximately 25 to 30 degrees). A minimum gap of 1.5 in. is required at a joint where a hold-down is located.
  • Bearing seats are not used with box beams. The pads sit directly on top of the cap. Provide top-of-cap elevations at the points coinciding with the outer edge of the exterior boxes at the centerline of bearing. Also provide elevations at any intermediate points along the cap, at the centerline of bearing, where either a change in cap slope or change in cap elevation occurs.
  • Box beams are not vertical but either parallel the roadway surface when the cross slope is constant or are rotated to the average cross slope of a span in a transition area. Because there are no bearing seat build-ups, the top of the cap must be sloped to match the rotation of the beams.
  • Provide a minimum of three elevation points for unskewed spans with an even number of box beams and a constant housetop profile: one at the outside edge of each of the exterior beams and a third point at the center of the middle joint. Provide four elevation points for spans with an odd number of beams: one at the outside edge of each exterior beam and one at the center of each joint on either side of the middle beam.
  • Framing is complicated in cross-slope transition areas and skewed bridges. Orient the beams to minimize the variation in slab thickness both longitudinally and transversely along the span. This may require stepping the cap at some joints so that adjacent beams not only have a different slope but also sit at a different elevation. Elevation points may be required as often as every joint in some situations. The forward half of an interior bent cap may have a different elevation than the back half at some locations.
  • Recommended Span Lengths

Software

Use PGSuper for beam design. Refer to PGSuper Design Guide for further guidance. Alternatively, use this spreadsheet to calculate live load distribution factors and PSTRS14 for beam design.

Design Resources

For additional information on LRFD bridge design as implemented by TxDOT, consult the following resources:

Design Examples and Spreadsheets

Concrete Deck Slabs on Stringers

Prestressed Concrete I Beams and I Girders

Prestressed Concrete U Beams

Prestressed Slab Beams, Decked Slab Beams and Double-Tee Beams

Prestressed Concrete I Beam and U Beam Research Study 0-4751

Prestressed Concrete Box Beams

Cast-in-Place Concrete Slab Spans

Straight Plate Girders