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For long-span concrete continuous steel bridges, continuous bridges or steel-continuous beam composite bridges (hereinafter collectively referred to as continuous beam bridge), the prestressing technology is both a structural means and a combination of construction methods to form a complete set Prestressed construction method based on segmental construction. As the span of these bridges increases, the length of the prestressing strands of the prestressed steel strands becomes longer and longer due to the large number of segments in the superstructure during the process of pouring the cantilever segments by segment, More prone to broken wire, sliding wire, the actual lack of elongation and other issues, the entire structure of the quality and construction period plays an increasingly decisive role, so its prestressed construction quality should be raised to a special location, the correct choice The construction process and adopt strict quality control measures to ensure the quality of prestressed construction.
Longitudinal steel beam using large tonnage 12 × d 15.24 mm ~ 15 × d 15.24 mm low relaxation and high strength prestressed steel strand, VLM15-9 ~ 15 post-tensioned prestressed system, the longitudinal prestressed beam is arranged flat Vertical curved combination of space curve, the use of metal bellows into the hole, both ends of the tensile steel up to 101.2 m. During the prestressing construction of the main bridge box girder, the problems that are easy to occur are analyzed. Some effective control measures are taken during the construction and good results are obtained. The conclusions are as follows, which can provide reference for similar prestressed construction
1 prone to the cause of the problem analysis and control measures
Prestressed steel strand tension, in order to achieve the design of the expected effective stress, requiring the use of tension control, elongation control of the double check measures, when the actual tension to design tension, the actual elongation deviation Should be in the range of -6% to + 6%, and the number of filaments or filaments per strand is controlled to 1 filament. The number of filaments or filaments in each section shall not exceed 1% of the total number of filaments in that section. In the long strand tensioned and anchored, due to various reasons, prone to problems are: individual strand wire, broken wire; with the increasing strand length, the actual deviation of the elongation of the value From positive to negative trend, the greater the length of the beam, the greater the negative deviation, even exceeding the allowable negative deviation of -6%.
1.1 slide the reason and handling
Stranding reasons for more wire, the general reasons are: stranded wire adhesion of oil, sand or mortar; strand hit the spark welding
Or for wire welding machine, resulting in changes in its mechanical properties, anchoring clip after the heat loss of anchor; clips and anchor anchor hole between the dust or other inclusions; flare pipe burr or concrete residue; limit plate Of the limit slot depth; horn end of the bellows and the existence of angled, the connection between the two are not straight; anchor plate, the clip hardness uneven deformation; unloading the oil too fast too fast. Filament usually occurs during anchoring or within 20 min of anchoring, sometimes during the stretching or half a day to one day after the end of the tensioning. After the completion of the tension should be promptly marked from the strand on the eye-catching mark or anchor before and after the exposure of the length of the strand to determine whether the strand appeared in the sliding wire, if the overall sliding wire should be replaced tool clip or clamp anchor Board, and then strands of steel strands to pull, if the amount of retraction is relatively large, it is likely that the work of the performance of the clip is not satisfactory or damaged, the need to anchor the beam for the anchor, identify the cause after the installation of new Work anchor clip and then re-tensioned. If there are individual strands slippery wire, the use of YDC270 single tension jack for tensioning, or re-anchor after loading new work anchor clips re-tension in place. The project upstream bridge TB5-4 steel strand in the anchor occurred when the 2 strand sliding wire, using YDC270 jack back anchor, re-install the new anchor clip to re-pull in place.
1.2 broken wire reason and handling
Strand breakage occurs, the general reasons are: strands of uneven material or serious corrosion; strand hit the welding sparks or welding wire used, its mechanical properties change; severe cross strand near the anchor hole ; Anchors deviate from the anchorage plate at the mouth, strand skewed, or flare end and bellows there is a larger angle, resulting in stress concentration of individual wires; clip hardness and strand does not match; Provisions of calibration, or hydraulic gauge failure, resulting in excessive tension.
Broken wire occurs, the commonly used approach: First, to improve the control of other strands of tension as a compensation; but need to be aware that in any case the maximum tensile stress shall not exceed the ultimate tensile strength of strand 0.8 times the standard value . Second, change the beam, that is, the unloaded steel wire bundle, back anchor, for beam, re-stretched to the design stress. Third, if necessary, enable the spare beam. For long-span prestressed concrete girder bridges, there are often reserve channels reserved for design. When the broken wires are serious, spare beams can be used.
