Central Atlantic Bridge Associates
RSS


Rules of Thumb for Bridge Girder Designs


Skip Navigation LinksPreliminary Design Aids : Rules of Thumb for Bridge Girder Designs


Rules of Thumb for Bridge Girder Designs
(Updated 12/27/2005)

Download as Word Document

The following guidelines are offered to help guide the designer produce economical bridge designs using current LRFD specifications. Where we provide maximum or minimum values these values are not to be construed to be technical limits but are limits that generally provide for the most economical bridge designs.

Economical Girder Design for Prestressed Concrete Bridges
  • Use the fewest number of beams that will work for the given span and required beam under-clearance.
  • Use the shallowest girder that will reasonably work for the given span and beam spacing.
  • For multi-span bridges it is typically more economical to use the maximum span and the deepest girder that will meet under clearance requirements. For example: for a 1,200 foot structure the optimum solution will probably be 8 spans of 28/96 I-Beams at 150 feet.
  • First try a straight strand pattern design for I-Beams and Box Beams. For the mid-range of a given section straight strand designs will almost always be more economical, even though they will require 5 to 10% more strands than a draped design.
  • In the event the fascia beam controls the design, as is most often the case with LRFD, and requires a few more strands than the interior beams, make all beams with the same strand pattern so they can be produced in the same bed.
  • When designing multiple structures for the same project an attempt should be made to use the same beam section and if possible the same straight strand pattern throughout the project. This will be more economical than optimizing girder designs for 5 to 10 foot differences in span length. In most cases using the same beam size throughout a structure will be more economical than using a shallower beam for slightly shorter spans.


Girder Types
  • Always verify shipping - especially for longer beam lengths


Box Beams
  • 48 inches or 36 inches wide and from 17 to 66 inches deep.
  • (48 inches is the preferred width - use 36 inch width only when 48 inch width does not work).
  • Spans up to 120 feet.
  • Adjacent box beams with grouted shear keys can help if under clearances are tight.
  • Beam widths are fixed at 48 inches and 36 inches.
  • Do not mix widths unless absolutely necessary.
  • Non-standard depths can be accommodated.
  • Non-standard slab and wall thickness can also be accommodated.


Bulb Tee Girders
  • Typical span length is 100 to 170 feet
  • Use Bulb Tee sections from PCEF Standards.
  • Available with either a 42 or 48-inch top flange width.
  • Use the same beam type (bottom bulb or top flange width) for the entire job.
  • Flange and bottom bulb dimensions are fixed.
  • For very large projects or where absolutely necessary the web depths could be modified with special, prior approval from the owner.
  • Bulb Tee beams can be a reasonable alternative to spread box beams.


I-Beams
  • Typical span length is 100 to 160 feet.
  • Use AASHTO sections.
  • Flange and bottom bulb dimensions are fixed.
  • For very large projects or where absolutely necessary the web depths could be modified with special, prior approval from the owner.

Back to Top

Girder Type Selection
  • Non-standard and AASHTO I-Beams should only be considered for repair or rehabilitation projects that used these beams in the past.
  • The Bulb Tee girders should be the preferred beam: it has standard beam depths in 8 inch increments.
  • For Bulb Tee girders the preferred top flange width is 48 inches, however the bridge geometry might require the 42 inch top flange.


For example:
  • When the horizontal curve would cause the drip notch to interfere with the top flange of the beam or cause the maximum overhang to exceed the allowable
  • When the scupper would be located in the top flange of the beam.
  • When using the Bulb Tee girders use the same beam type: do not mix bottom bulb depths (7 and 9 inches) or top flange widths (42 or 48 inches) for the entire project.
  • Box-Beams with a 48 inch width should be used for adjacent beam configurations.

Back to Top

Other Design Considerations

Concrete
  • Maximum release strength = 6,800 psi.
  • Maximum 28 day strength = 8,000 psi.

Strand
  • 1/2 inch diameter SPECIAL 270 Ksi Low-relaxation is most common today.
  • Area = 0.167 in2 (but for big beams 6/10 inch strand is preferred).
  • Jacking stress is 75% of GUTS (33.818 Kips per strand).

Strand Patterns
  • Straight strand patterns are preferred. The main reasons for this are safety aspects and versatility offered for production scheduling. If the only costs to consider were for the prestressed concrete beams, we would suggest going to a deeper beam size to utilize straight strands. However, when the entire bridge/project is considered, this may not be a desirable option. Components affecting this may include shipping issues, crane capacities, substructure design, bearing pads, waterway opening, roadway profile, etc.
  • To achieve the most economical design option, we offer the following for designers to consider during prestressed beam design:
  • Try to utilize straight strands with the current maximum f'ci and f'c values permitted by the owner (in most instances 6,800 psi and 8,000 psi respectively) It is our experience that in most cases, straight strands with debonding will work in lieu of draped strands.
  • When a design with straight strands with debonding is not possible: Try to utilize draped strands with the current maximum f'ci and f'c values permitted by the owner.

Back to Top
Draped Strands
  • Draped strands can be broken into groups so that single point capacities are not exceeded.

(CGend – CGhold down) * (Prestress Force (Kips))
Hold down force =
(Distance from hold down to end of Beam (inches))


The following are suggested maximums per location:
Box Beams
Maximum hold down force for each drape point is 15 Kips per side
I-Beams and Bulb Tee girders
Maximum hold down force for each drape point is 40 Kips
Unbonding
Viable alternative to draping -- especially for box beams.
Maximum 25% of strands 3 feet and longer
Crack Control
unbonding (6 and 36 inch) -- See local owner limits.
Continuity
Designers should verify constructability when utilizing continuity reinforcement projecting from the bottom flange with respect to strand pattern and stud locations. This may be of greater concern when beveled sole plates or pot bearings are required.
Back to Top
Bearings
Design beams using a dapped bearing area for longitudinal slope. Maximum dap depth governs.
Minimum dap depth = 1/8 inch (1/4 inch preferred).
Daps require the raising of strand by total depth of dap to maintain clearance.
In Box Beams daps require thicker bottom slabs (use 1/2 inch increments), and sometimes thicker top slabs to compensate.
Never change geometry of the top or bottom flange for Bulb Tee girders or AASHTO I-Beams!
Verify potential strand locations if strand pattern is raised to accommodate a dapped bearing area.
Free Assistance
If you would like to discuss any specific situations please call any of the CABA member companies
Member Companies
Back to Top


Skip Navigation LinksPreliminary Design Aids : Rules of Thumb for Bridge Girder Designs