Preliminary Design Aids : Rules of Thumb for Bridge Girder Designs

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Rules of Thumb for Pennsylvania Bridge Girder Designs
(Updated 7/26/2004)


The following guidelines are offered to help guide the designer produce economical bridge designs using current PENNDOT 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 Pennsylvania 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.

I-Beams

• Typical span length is 100 to 160 feet.
• Use AASHTO sections from PENNDOT BD652.
• PENNDOT I-Beam sections are not in general use.
• Flange and bottom bulb dimensions are fixed.
• For very large projects or where absolutely necessary the web depths could be modified with prior approval from PENNDOT.

PA Bulb Tee Girders

• Typical span length is 100 to 170 feet.
• Use PA Bulb Tee sections from PENNDOT BD652.
• 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 prior approval from PENNDOT.
  PA Bulb Tee beams can be a reasonable alternative to spread box beams.

Girder Type Selection

• The PA I-Beams should only be considered for repair or rehabilitation projects that used these beams in the past.
• The AASHTO I-Beams should be the preferred beam: it has standard beam depths in 6 inch increments.
• The PA Bulb Tee girders should only be used when the AASHTO I-Beams do not achieve the required span or achieve a limited beam depth required: standard beam depths are in 8 inch increments.
• For PA 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 PA 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.

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
• Jacking stress is 75% of GUTS (33.818 Kips per strand).
• Use a maximum prestressing force of 3,000 kips.

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 PENNDOT (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 PENNDOT (6,800 psi and 8,000 psi respectively)

Draped Strand Patterns

• 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 - 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 exempt) -- See Publication 408 for limits

Continuity

• PENNDOT DM-4, Section 5.14.1.2.7a – General
• This requires the designer to design for the more critical condition of full continuity or the complete loss of continuity at the diaphragms over the interior supports. A full continuity design option may only be used if there is a clear economic advantage to be gained. The best way to achieve this is to first design the structure as a fully continuous bridge. Then, using the resulting beam size, design the structure using the simple span option. If all that needs to be done is the addition of strands to the girder that should be the preferred solution. If the result is a deeper girder or an additional girder line, this should become the justification for utilizing full continuity and should be submitted as part of the TS&L submission including details for the positive moment steel connection.
• 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.

Bearings

• Design beams using a dapped bearing area for longitudinal slope. Maximum dap depth governs. See DM-4.
• 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 AASHTO I-Beams or PA Bulb Tee girders!
• 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 member companies .

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Preliminary Design Aids : Rules of Thumb for Bridge Girder Designs