Specified tolerances for sweep or horizontal curvature of a prestressed box beam or I -beam are listed in the Office of Structural Engineering's standard drawing for I beams. I-beams are 1/8 inch every 10 feet with a maximum of 1 inch.
Box beam tolerances are 3/8-inch maximum for box beams 40 feet or less, 1/2-inch maximum for box beams 40 to 60 feet, and 5/8-inch maximum for box beams greater than 60 feet.
Camber is based on the design plan requirements and is generally within a tolerance of a 1-inch maximum. Box beams also have a maximum side-by-side differential camber of 1/2 inch. The side-by-side differential may override the 1-inch maximum camber tolerance for an individual member. The side-by-side differential camber is not checked by laboratory prestressed inspectors as the beams are not installed in their final position.
Phased construction can cause unique problems with camber. Camber is time dependent: as the members get older they will gain camber. If all beams are manufactured for a bridge at the same time but only half are shipped for an initial phase of construction there is a good probability that the second phase beams will have a higher camber than the first-phase beams. For box and I beam structures this additional camber may be able to be absorbed by thinning the haunch or variable depth deck. Field loading of the field-installed beams to reduce the camber is a method some Contractors have used to deal with the camber growth. The best methods are for the Contractor to order the beams early enough so none of the beams are erected and loaded before they are six months or older, or to coordinate their and the fabricator’s schedules so that the fabricated age of the beams at time of erection is within 30 days of each other for all phases.
When prestressed members arrive on the site, they should be inspected for damage and quality of fabrication as thoroughly as time and conditions permit. Inspection should include areas that look like they were patched and cracking. Fine cracking at the ends of the beams, whether box beams or I beams, is not unusual and will tighten under erection and dead loading of the members. Cracking in other locations is not usual or acceptable. Any prestressed members should have a certification document (TE-24) with the shipment. Do not accept un-documented members. Notify the District Engineer of Tests or the Office of Materials Management Cement and Concrete Inspection Section.
Erection components for prestressed box beam members (i.e., tie rods, splices, nuts, etc.) will be considered included under and covered by the fabricator’s certification document. For I-beams the embedded components will be covered under the fabricator’s certification document. If galvanized cross-frames are used for prestressed I-beams, the prestressed fabricator will probably not supply a certification document; they will be provided by a steel fabricator with separate certification documentation (TE-24).
The nature and extent of any damage which may have occurred because of loading, transit, or unloading should be noted and reported to the Director along with the identifying piece mark or member. If corrective work is obvious, the Contractor should be advised immediately so that the responsible party will be notified and correction can be performed in the most advantageous location. For help on possible corrections, patching repairs, etc., contact the Cement and Concrete section of the Office of Material Management.
Special care should be taken when cutting shipping tendons which are sometimes necessary to allow transport of AASHTO Type 4 modified beams. The location of these tendons as well as the sequence and means of removal are determined by the fabricator. The relaxation in the beam that occurs when the tendons are “released” can induce stress cracking in the beam.
Prestressed members, if stored on the site, will be supported off the ground on blocking at their design bearing points. Ensure that members are stored in a true vertical position.
Box beams should have had a silane treatment on the exterior beams or they also may be delivered with a colored coating (generally epoxy urethane) on the exterior members. I-beams do not require a silane sealer and will generally not be coated at the fabricator’s yard
A final check must be made of the elevation of bearing seats on the piers and abutments before erection of prestressed members is scheduled to begin. If bearing seats are found that need correction, it must be performed in the manner and to the tolerances described in the section entitled, Bearing Seats, in Section 511 of this manual.
The erection plan submitted by the Contractor should be reviewed with his representative in charge. The purpose of this review is to ensure that ODOT’s interpretation of the plan is concurrent with the Contractor’s intended course of action. Methods and equipment approved for erection of members must be used in handling during transportation to the bridge site and unloading.
The erection drawings, usually the “E” sheets of the approved shop drawings, will typically be used to locate the members on the bridge and may give special instructions for the erector to follow.
Deviations from the submitted erection procedure will not be permitted. If the erector proposes deviations in the procedure that appear to have merit, they must be referred to the Engineer(s) whose stamp is on the submitted plan for review and sign off prior to use.
The specifications require that the Contractor submit an erection procedure for structural concrete members. If there is railroad involvement, the PE stamped plan needs to be reviewed and approved by the appropriate railroad. Otherwise, the plan must have two PE stamps. For additional requirements, refer to C&MS Item 501.05.
Typical items that should be included in the submitted erection procedure are:
1. A drawing of the complete framing plan showing each girder or beam section by “piece mark” and numbered in the order of proposed erection. A print of the erection sheet of the shop drawings may be used.
2. The number of pieces and load capacity of erection equipment to be used and method of lifting members.
3. Methods and details for supporting the first beams or girders at the abutments and piers in each unit.
4. Cross-frame installation.
Use Erection Checklist Form CA-S-20.
Keyways should be grouted after erection of box beams. Generally, plastic rope or jute is installed into the bottom of the keyway to block the grout from flowing out. Ensure that the installation is done properly. Box beam keys have failed because of improper jute installation. Grout should meet the material requirements of the Office of Structural Engineering's standard box beam drawings. OMM has an approved list of grout materials. The manufacturer’s mixing instructions are required and should ensure that the grout is properly mixed, vibrated into the joints, cured, and sampled for testing. Grouting should not be allowed if there is construction traffic or erection still going on. The grout can be cracked by the vibration and deflection movements and make the keyways worthless. The design of the structure counts on the grout in the shear keys.
Do not allow traffic on the deck before the grout has obtained the required strength. This includes construction traffic.
Galvanized cross-frame for prestressed I-beams should not be tightened until the adjacent beams are set. While bolts are high-strength, the connections are not friction type. The bolts should be well tightened, but the turn-of-the-nut method of installation is not required. If there are cross-frame alignment problems, do not allow I-beam field drilling until the Contractor has a method approved by the Engineer to determine where the reinforcing and the prestressing strands are and how to avoid drilling into it.
Do not allow the Contractor to elongate or enlarge slots in the steel cross-frames.
Records must be on file for the following items:
1. Shop inspected prestressed members documented by a TE-24.
2. Shop drawings per C&MS 501.04.
3. Bearing seat inspection.
4. Elastomeric bearings accepted.
5. Document condition of beams on delivery.
7. Grout mixed per manufacturer’s directions.
8. Make samples for testing.
9. Document vertical offset in adjacent beams per standard drawing PSBD-2-07.