Docstoc

DESIGN OF PRESTRESSED POST-TENSIONED BOX No. 5.1

Document Sample
DESIGN OF PRESTRESSED POST-TENSIONED BOX No. 5.1 Powered By Docstoc
					                         SECTION V – PRESTRESSED CONCRETE

PRESTRESSED CONCRETE BEAM DETAILS                                                      No. 5.01
The following criteria shall be adhered to in the design and detailing of PSC beams:
   1. Detail all straight strands in 2 inch (51 mm) patterns, center to center, beginning 1” (25
      mm) on each side of the centerline of the beam.
   2. Diaphragm bars - Detail all block-outs for diaphragm bar nuts and washers as shown on
      the Bridge Office standard sheet and in the Bridge Office cell. Diaphragm bars shall be
      located to avoid interference with draped strands and utilities.
   3. Release strengths - All Plans shall show a minimum design release strength of 4500 psi
      (30 MPa).
   4. Skewed ends - Girders on skewed bridges shall have only the top flange skewed. See the
      PSC Beam detail sheets.
   5. Cracking at girder ends - Provide the minimum amount of reinforcement at girder ends as
      required by AASHTO Article 9.22. Lifting devices shall be embedded in the girder ends.
      The lifting devices extend to within 4” (100 mm) of the bottoms of girders so that the
      weight of the girder will not cause tension stresses at the junction of web and top flange.
   6. Detail girders to 1/16” (1 mm) increments and state on the Plans that lengths given are
      horizontal dimensions for in-place girders and that the fabricator shall adjust those
      lengths for grade and fabrication effects, such as shrinkage and elastic shortening.
   7. The Designer should consider the fabrication tolerances for prestressed beams when
      positioning the bearing area so that the unreinforced areas at the ends of the beams are not
      put in bearing.
   8. Prestressed beam fabricators are able to achieve high concrete strengths with reliability.
      Therefore, the following criteria are in effect:
      a) 28 day concrete strengths up to 8000 psi (55 Mpa) may be used. Approval for
      strengths up to 10000 psi (69 Mpa) which result in a reduction in the number of beams
      may be requested from the front office.
      b) Release strengths up to 7500 psi (52 Mpa) may be used. There is an advantage to
      keeping the release strength at 5500 psi (38 Mpa) or below since fabricators can then strip
      their forms after 18 hours, resulting in very efficient production. 6000 psi (41 Mpa) may
      be obtained almost as easily in the summer, but in the winter it becomes more difficult
      and can result in adding a day to each production cycle. The designer should attempt to
      keep the release strength at or below 5500 psi (38 Mpa), but should not use an
      uneconomical design to achieve this. Approval for release strengths higher than 7500 psi
      which result in a reduction in the number of beams may be requested from the front
      office.
      c) 28-day strengths shown on the plans should be divisible by 200 psi or 500 psi. Metric
      28-day strengths should be given in whole MegaPascals. Release strengths shown on the
      plans should be in even 100 psi (0.5 Mpa) increments.
   9. Prestressed beams should usually be designed with the allowable tension as specified in
      the AASHTO Specifications. These are 6(f’c)1/2 for normal exposure and 3(f’c)1/2 for
      severe exposure. Normal exposure shall be used for all counties except the coastal
      counties, where severe exposure shall be used. The coastal counties are Chatham, Bryan,
    Liberty, McIntosh, Glynn and Camden. The severe exposure criteria shall apply to any
    bridge in a coastal county or over the county line of a coastal county. The allowable
    tension for which the beams are designed should be shown in the General Notes under
    Design Data if it is the same for all beams. Otherwise, the Design Data should reference
    the beam sheets, and the allowable tension shown should be shown on the beam sheets.
10. a) Do not approve shop drawings showing the web stirrups inside the strands in the web.
    b) Shop drawings may show a few small diameter holes through the top of the web at
    each end for hold-downs for shipping. This is acceptable.
11. When detailing prestressed concrete beams, provide a minimum of 2” (50 mm) from the
    end of the beam to the edge of the bearing pad. There is a ¾” (19 mm) chamfer at the
    end of the beam and a ¾” (19 mm) tolerance on the length of the beam. A distance less
    than this to the edge of the bearing pad leaves no room for error in construction.
12. Due to the difficulty in setting Bulb-T beams, increase the size of the slot at the
    expansion end to 6” X 1.5” X 7” deep (150 mm X 38 mm X 175 mm deep).
13. Fabricators of PSC beams sometimes redesign beams to use two hold-downs instead of
    one to reduce the hold-down force. The two hold-downs are usually placed close together
    so that the effect on the design is minimal. If there are no other changes in the beam
    design, the fabricator will not be required to provide redesign calculations provided that
    the hold-downs are within 3’-0” (1 m) of the designed location of the single hold-down.
14. Fabricators of PSC beams often show pick-up points on the beams further from the end of
    the beam than is shown on the detail sheet, particularly with Bulb-T beams. In the future,
    we will approve shop drawings when the pick-up points are within 1.5 times the beam
    depth of the end of the beam.
15. When checking shop drawings for prestressed concrete beams, please be certain that the
    chamfers on the bottom of the beam at the ends are not larger than ¾” (19 mm) for Type I
    through Type IV beams and not larger than 1” (25 mm) for Type V and Bulb-T beams.
16. Use ½” diameter special low-relaxation strands where possible. When necessary, .6”
    diameter low-relaxation strands may be used.
17. All prestressing strands shall be the same size and type within any one PSC beam.
18. When designing or checking shop drawings for PSC beams with straight strands (without
    draped strands), insure that there are no strands in the bottom of the beam inside the
    stirrups where they will conflict with the dowel bar chase. The strand pattern should be
    arranged so that there are no strands inside the stirrups in the bottom 7” (175 mm) of the
    beam at the end of the beam.
19. Dimensions for PSC beams should be given to the sixteenth of an inch. The dimensions
    from centerline bent or BFPR to centerline of bearing should be calculated, then
    subtracted from the beam length to get the bearing-to-bearing length. The lengths from
    centerlines of bearing to the ends of the beam should be added to this to get the beam
    length. If the beam length contains a sixteenth of an inch, the half-length shown should
    be rounded to a sixteenth or an eighth. This will provide for the most accurate dimension
    for the beam length and for the bearing-to-bearing length.
20. When designing Bulb-T beams (except fascia beams), reducing the force in the top
    strands to 10k (45 kN) may reduce the number of strands in the beam. This should
    always be checked, and the number of strands reduced if possible. Only a reduced force
    of 10k (45 kN) should be used.
   21. When checking shop drawings for PSC beams (all types), make sure the fabricator does
       not reduce the 18-hour age requirement for releasing strands. This requirement can be
       found in Section 865.2.01.B.6 of the Standard Specifications.

PRESTRESSED CONCRETE BEAMS -
 PRELIMINARY DESIGN                                                                   No. 5.02
See Fig 5-01a  5-01h for plots of Beam Spacings vs. Design Spans. These charts are to assist
the Bridge Designer in selecting preliminary PSC beam spacings. Note that tension is allowed.
Concrete beams shall be utilized where possible when laying out bridges.

BRACING FOR EXTERIOR TYPE I BEAMS                                                     No. 5.03
Due to problems with exterior beams rotating during deck pours, the following note shall be
placed on the deck section sheet for bridges with Type I or Type I Mod beams or fascia beams
where no diaphragms are present:
     The Contractor shall provide bracing between the exterior beam and the first interior beam
   until the deck has been poured and the overhang forms removed. All costs for designing,
   providing, installing and removing bracing shall be included in price bid for Lump –
   Superstructure Concrete.

COPING FOR BULB-T PSC BEAMS                                                           No. 5.04
When designing using Bulb-T prestressed concrete beams, allow 1½” (40 mm) of coping in
addition to what is needed for vertical curve, cross-slope, camber and dead load deflection.
Camber and deflection of these beams is often erratic, resulting in little or no coping.

SAFETY RAIL INSERTS ON PSC BEAMS                                                      No. 5.05
At the request of the contractors, PSC beam fabricators have been showing pipes in the tops of
PSC beams to support posts for safety railings on the beams. This detail is not acceptable since
these pipes hold water which freezes and causes splits in the beams. An alternative detail which
has been approved and which remains acceptable is to use a reinforcing bar extending out of the
beam over which the post is placed. This bar should be cut off or bent into the deck steel after
the safety rail is no longer needed.

PRECAST PRESTRESSED CONCRETE BOX BEAMS                                                No. 5.10
   1. At bridge sites involving the crossing of deep railroad cuts or streams of substantial width
      with shallow water (floating equipment not feasible), prestressed concrete boxes longer
      than 40 feet should not be used inasmuch as (1) site conditions would prohibit the placing
      of lifting equipment in the area below the bridge, and (2) it is not feasible to erect the
      boxes from the bridge deck. If spans longer than 40 feet are to be erected from above,
      then the bridge must be designed to accommodate the construction procedure and the
      erection equipment rather than the design traffic load.
   2. When designing using the 1’-5” x 4’-0” (432 x 1219 mm) 2-cell prestressed box beam,
      the details shown in Fig. 5-02 shall be used as a guide in order to obtain a maximum
      consistency in our detailing. The designer shall be guided by this Figure not only as to
      the geometric configuration of the concrete section, but also as to the size of non-
      prestressed strands, details for dowel chases, transverse threaded bars, and details for
      deck drains.

POST-TENSIONED BOX GIRDER BRIDGES -
 SEGMENTAL CONSTRUCTION ALTERNATE                                                       No. 5.12
When a segmental construction alternate is to be allowed on post-tensioned box girder bridges,
the following note shall be included in the Bridge Plans:
      Segmental Construction - Proposals for construction by segmental methods may be
      submitted for consideration as an alternate to the method shown in the plans. All
      proposals shall include a set of construction drawings and complete design calculations.
      All proposals shall conform to the latest AASHTO Specifications except that no tension
      stress in the concrete after losses shall be allowed. All proposals shall also conform to the
      Georgia DOT Specifications, and will be subject to approval by the State Bridge Engineer.
      If approved for use, the alternate segmental construction methods shall be at no extra cost
      to the Department and with no increase in contract time.

POST-TENSIONING SHOP DRAWINGS                                                           No. 5.13
The Bridge Designer shall make the approved post-tensioning shop drawings a permanent,
reproducible part of the bridge file by having the reproducibles of the shop drawings placed in
the bridge plans. Also, the Bridge Designer shall include in the file any pertinent calculations
relative to the post-tensioning operation.

PRESTRESSING STRAND DATA                                                                No. 5.14
Data for prestressing strands for use on PSC beam details sheets shall be as follows:
                            English                                   Metric
   Type         (in.)      A*s (in2)   Pjack (k)       (mm)       A*s (mm2)      Pjack (kN)
    ½”          .5           .153       30.983           12.7        98.71          137.8
½” Special       .5           .167      33.818           12.7        107.74          150.4
  0.6”          .6           .217       43.943           15.2       140.00          195.5

             Pjack = .75 A*s f’s   Low-relaxation strands, f’s = 270 ksi ( 1860 MPa)
OPEN GRATE ACCESS DOORS                                                                No. 5.15
Open grate access doors shall be used on concrete box girders carrying water mains. Every bay
which has a water main should have this type access door at each end of the bridge so that if the
water main leaks or breaks, the water will run out of the bay.

DESIGN AND REDESIGN OF PSC BEAMS                                                       No. 5.16
In the design or redesign of PSC beams, use of the transformed area of bonded reinforcement to
calculate section properties per Article 9.17.1 of the AASHTO Specifications is not allowed.
                                    PSC Beam Charts
               Beam Spacing (X-Axis) versus Maximum Design Span (Y-Axis)

Assumptions

1.   Beam Spacing – 4.0' to 10.0' in .25' increments
2.   Number of Lanes = 3, constant
3.   Number of Beams = 6, constant
4.   Slab was design using BRSLAB99 with 2½" cover
5.   Maximum Design Span was determined using BRPSBM1
        a. Live Load: HS20 with Impact
        b. Concrete Properties
                 i.  Ec = [(145)1.5](33)[(f'c).5]
                ii. Dead Load = 150 pcf
               iii. Initial Tension in the Beam (SIT) = 6(f'ci).5
               iv.   Final Tension in the Beam (SIF) = 6(f'c).5
        c. Distribution Factor for Deflection = 1.000
        d. Composite Slab Properties – Depth of Coping (DF)
                 i.  0.000, Type I MOD through Type IV PSC Beams
                ii. 1.000, Bulb Tees
        e. Non-composite Dead Loads (NCDL) – Coping
                 i.  [(1.5)(Top Flange Width)/144](.150), Type I through Type IV PSC Beams
                ii. [(3)(Top Flange Width)/144](.150), Bulb Tees
        f. One Diaphragm at Midpoint for all spans greater than 40'-0"




                                       Figure 5-01a
Type I MOD PSC Beam




                               70.0




                               65.0




                               60.0
  Maximum Design Span (feet)




                               55.0




                               50.0




                               45.0




                               40.0




                               35.0
                                  4.00       5.00                  6.00             7.00                  8.00                   9.00             10.00
                                                                           Beam Spacing (feet)

                                         f'c = 5.0 ksi, f'ci = 4.5 ksi    f'c = 6.0 ksi, f'ci = 5.0 ksi          f'c = 7.0 ksi, f'ci = 6.0 ksi
                                         f'c = 8.0 ksi, f'ci = 7.0 ksi    f'c = 9.0 ksi, f'ci = 8.0 ksi          f'c = 10.0 ksi, f'ci = 9.0 ksi

                                              All strands are ½“ diameter low relaxation strands each stressed to 33,818 pounds.


                                                                           Figure 5-01b
Type II PSC Beam
                                  85.0




                                  80.0




                                  75.0




                                  70.0
     Maximum Design Span (feet)




                                  65.0




                                  60.0




                                  55.0




                                  50.0




                                  45.0




                                  40.0
                                     4.00       5.00                  6.00               7.00                    8.00                 9.00               10.00
                                                                                Beam Spacing (feet)

                                              f'c = 5.0 ksi, f'ci = 4.5 ksi      f'c = 6.0 ksi, f'ci = 5.0 ksi          f'c = 7.0 ksi, f'ci = 6.0 ksi
                                              f'c = 8.0 ksi, f'ci = 7.0 ksi      f'c = 9.0 ksi, f'ci = 8.0 ksi          f'c = 10.0 ksi, f'ci = 9.0 ksi

                                            All strands are ½“ diameter low relaxation strands each stressed to 33,818 pounds.


                                                                              Figure 5-01c
Type III PSC Beam
                                  115.0




                                  110.0




                                  105.0




                                  100.0




                                   95.0
     Maximum Design Span (feet)




                                   90.0




                                   85.0




                                   80.0




                                   75.0




                                   70.0




                                   65.0




                                   60.0
                                      4.00         5.00                  6.00              7.00                    8.00                 9.00               10.00
                                                                                   Beam Spacing (feet)

                                                f'c = 5.0 ksi, f'ci = 4.5 ksi      f'c = 6.0 ksi, f'ci = 5.0 ksi          f'c = 7.0 ksi, f'ci = 6.0 ksi
                                                f'c = 8.0 ksi, f'ci = 7.0 ksi      f'c = 9.0 ksi, f'ci = 8.0 ksi          f'c = 10.0 ksi, f'ci = 9.0 ksi

                                             All strands are ½“ diameter low relaxation strands each stressed to 33,818 pounds.


                                                                                Figure 5-01d
Type IV PSC Beam
                                  135.0




                                  130.0




                                  125.0




                                  120.0




                                  115.0
     Maximum Design Span (feet)




                                  110.0




                                  105.0




                                  100.0




                                   95.0




                                   90.0




                                   85.0




                                   80.0




                                   75.0
                                      4.00         5.00                  6.00              7.00                    8.00                 9.00               10.00
                                                                                   Beam Spacing (feet)

                                                f'c = 5.0 ksi, f'ci = 4.5 ksi      f'c = 6.0 ksi, f'ci = 5.0 ksi          f'c = 7.0 ksi, f'ci = 6.0 ksi
                                                f'c = 8.0 ksi, f'ci = 7.0 ksi      f'c = 9.0 ksi, f'ci = 8.0 ksi          f'c = 10.0 ksi, f'ci = 9.0 ksi

                                             All strands are ½“ diameter low relaxation strands each stressed to 33,818 pounds.


                                                                                Figure 5-01e
54” Bulb Tee Beam
                                  145.0



                                  140.0



                                  135.0



                                  130.0



                                  125.0



                                  120.0



                                  115.0
     Maximum Design Span (feet)




                                  110.0



                                  105.0



                                  100.0



                                   95.0



                                   90.0



                                   85.0



                                   80.0



                                   75.0



                                   70.0
                                      4.00         5.00                  6.00              7.00                    8.00                 9.00               10.00
                                                                                  Beam Spacing (feet)

                                                f'c = 5.0 ksi, f'ci = 4.5 ksi      f'c = 6.0 ksi, f'ci = 5.0 ksi          f'c = 7.0 ksi, f'ci = 6.0 ksi
                                                f'c = 8.0 ksi, f'ci = 7.0 ksi      f'c = 9.0 ksi, f'ci = 8.0 ksi          f'c = 10.0 ksi, f'ci = 9.0 ksi

                                                              All strands are .6“ diameter low relaxation strands.
                          The 4 top flange strands are stressed to 10,000 pounds each and all remaining strands are stressed to 43,943 pounds each.
                                                                                Figure 5-01f
63” Bulb Tee Beam
                                  160.0



                                  155.0



                                  150.0



                                  145.0



                                  140.0



                                  135.0



                                  130.0
     Maximum Design Span (feet)




                                  125.0



                                  120.0



                                  115.0



                                  110.0



                                  105.0



                                  100.0



                                   95.0



                                   90.0



                                   85.0



                                   80.0
                                      4.00         5.00                  6.00              7.00                    8.00                 9.00               10.00
                                                                                   Beam Spacing (feet)

                                                f'c = 5.0 ksi, f'ci = 4.5 ksi      f'c = 6.0 ksi, f'ci = 5.0 ksi          f'c = 7.0 ksi, f'ci = 6.0 ksi
                                                f'c = 8.0 ksi, f'ci = 7.0 ksi      f'c = 9.0 ksi, f'ci = 8.0 ksi          f'c = 10.0 ksi, f'ci = 9.0 ksi

                                                              All strands are .6“ diameter low relaxation strands.
                          The 4 top flange strands are stressed to 10,000 pounds each and all remaining strands are stressed to 43,943 pounds each.
                                                                                Figure 5-01g
72” Bulb Tee Beam
                                  170.0



                                  165.0



                                  160.0



                                  155.0



                                  150.0



                                  145.0



                                  140.0
     Maximum Design Span (feet)




                                  135.0



                                  130.0



                                  125.0



                                  120.0



                                  115.0



                                  110.0



                                  105.0



                                  100.0



                                   95.0



                                   90.0
                                      4.00         5.00                  6.00              7.00                    8.00                 9.00               10.00
                                                                                   Beam Spacing (feet)

                                                f'c = 5.0 ksi, f'ci = 4.5 ksi      f'c = 6.0 ksi, f'ci = 5.0 ksi          f'c = 7.0 ksi, f'ci = 6.0 ksi
                                                f'c = 8.0 ksi, f'ci = 7.0 ksi      f'c = 9.0 ksi, f'ci = 8.0 ksi          f'c = 10.0 ksi, f'ci = 9.0 ksi

                                                              All strands are .6“ diameter low relaxation strands.
                          The 4 top flange strands are stressed to 10,000 pounds each and all remaining strands are stressed to 43,943 pounds each.
                                                                                Figure 5-01h

				
DOCUMENT INFO