Box girder tensioned steel plate-UHPC shear reinforcement structure and construction and design method

Abstract

The application discloses a kind of box girder tension steel plate-UHPC shear reinforcement structure and construction and design method, belong to bridge engineering reinforcing technical field.The application is aimed at the problem that existing concrete box girder is easy to debond and age after steel bonding reinforcement, actively reinforcing and secondary reinforcing are carried out in combination with prestressed tension steel plate and UHPC.The structure includes concrete box girder web, oblique steel plate, L-shaped steel pressing strip, connecting plate, tension anchoring plate, tensioning piece, structural adhesive and UHPC layer.The construction method expands the steel plate after anchoring by heating, prestress is applied by shrinkage during cooling, and then structural adhesive is poured and UHPC layer is poured.The design method provides calculation formula of tension control stress, effective prestress and shear bearing capacity.The application realizes active prestressing of steel plate, overcomes the aging defect of steel bonding adhesive, improves the durability and crack resistance of the structure, and reduces the life cycle cost.

Description

Technical Field

[0001] This invention relates to the field of bridge engineering structural reinforcement technology, and in particular to a box girder tensioned steel plate-UHPC shear reinforcement structure and its construction and design method. Background Technology

[0002] Concrete box girder bridges with spans of 10-50 meters constitute a significant proportion of in-service bridges due to their simple structure, well-defined stress distribution, and ability to be prefabricated in standardized factories. However, early construction often had lower load standards, resulting in a large number of early concrete box girders facing structural aging and performance degradation after prolonged service. Rapidly increasing traffic flow and heavy overloading place higher demands on the safety and reliability of bridge structures. Web cracking is the most common defect in box girder bridges. After numerous concrete box girder bridges experienced web cracking, steel plates were bonded for reinforcement to improve the stress distribution on the web and ensure structural safety. However, traditional steel-bonded reinforcement schemes have the following shortcomings: First, under the coupled effects of overload, fatigue, ultraviolet radiation, and hot-dry cycles, the organic structural adhesive degrades, peels off, and detaches, causing the reinforced steel plate to fail to share the load with the main structure. Second, the web cracks that had already formed before reinforcement were not effectively closed, making it difficult to prevent external moisture and chloride salts from corroding the internal steel bars. Third, the external steel plate requires regular application of anti-corrosion coatings, resulting in high maintenance costs. Fourth, the reinforced steel plate is exposed, and its strength degrades significantly under extreme high temperatures such as fires, resulting in insufficient fire resistance. Fifth, the box girder has already cracked and damaged before reinforcement, so the reinforced steel plate cannot share the dead load, and the structural stiffness is not significantly improved. Sixth, the steel-bonded reinforcement has a prominent visual facade, which may cause social panic and does not meet urban aesthetic requirements.

[0003] Ultra-high performance concrete (UHPC) is an advanced cement-based composite material prepared by optimizing particle size distribution and incorporating steel fibers and reactive powders. It possesses superior mechanical properties, durability, and corrosion resistance. Prestressed reinforcement can seal cracks and adjust the stress state of the structure. This invention combines prestressed tensioned steel plates with UHPC for active reinforcement. Based on the existing situation of steel-bonded reinforced concrete bridges, it fully utilizes the steel plates used in the first steel-bonded reinforcement as shear reinforcement. By adding L-shaped steel strips, anchor bolts, and anchoring connectors, prestress is applied using a torque wrench. Combined with steel plate studs and web concrete reinforcement as shear connectors, and post-cast UHPC as the structural stress-bearing and protective layer, the crack resistance and ultimate bearing capacity of the box girder are significantly improved, achieving maintenance-free operation and reducing the total life-cycle cost of the bridge. Summary of the Invention

[0004] The technical problem to be solved by this invention is to overcome the problem that the debonding of steel plates in steel-bonded reinforced concrete box girders cannot prevent web cracking. Based on the existing steel plate bonding reinforcement of box girder webs, this invention proposes a steel plate debonding treatment and enhanced anchoring technology, designs a prestressing application structure for bonded steel plates and a UHPC reinforcement and repair method, and proposes a corresponding prestressing and shear bearing design method for reinforced beams.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0006] This invention provides a box girder tensioned steel plate-UHPC shear reinforcement structure, including a concrete box girder web, a diagonally attached steel plate, an L-shaped steel strip, a connecting plate, a tensioning anchor plate, a tensioning member, structural adhesive, and a UHPC layer.

[0007] Specifically, the L-shaped steel strip is pressed onto the upper end of the inclined steel plate and fixedly connected to the top plate of the concrete box girder web. This structure effectively strengthens the anchorage of the inclined steel plate at the top, prevents warping and detachment of the steel plate ends, and enhances the overall load-bearing capacity. The connecting plate is fixed to the lower end of the inclined steel plate, and the tensioning anchor plate is fixed to the lower part of the concrete box girder web and located below the connecting plate, providing a stable anchoring foundation for prestressing tensioning. The tensioning member passes through the connecting plate and the tensioning anchor plate to tension the inclined steel plate to apply prestress. By actively applying prestress, the steel plate can participate in the stress distribution, close existing cracks, and improve the structural stress state. The structural adhesive is filled between the inclined steel plate and the concrete box girder web to achieve full-length bonding between the steel plate and the concrete, ensuring that the two work together. The UHPC layer covers the web of the concrete box girder and the outer side of the inclined steel plate, providing not only a durable protective layer to prevent metal oxidation, but also participating in structural stress through its excellent mechanical properties, thereby improving the overall shear bearing capacity.

[0008] Preferably, the outer surface of the inclined steel plate is provided with shear studs, and the surface of the concrete box girder web is provided with a roughened surface. The UHPC layer forms an integral load-bearing structure with the inclined steel plate and the concrete box girder web through the shear studs and the roughened surface, respectively. This structure forms a reliable force transmission path of concrete box girder-UHPC reinforcement layer-reinforcement steel plate, ensuring that all components work together and fully utilizing the mechanical and durability properties of UHPC.

[0009] Preferably, the tensioning member is one of finely rolled threaded steel bars, high-strength steel wire bundles, or steel strands, which can be flexibly selected according to project needs and construction conditions. Furthermore, the L-shaped steel strip is provided with stiffening ribs and is fixedly connected to the web and top plate of the concrete box girder by bolts. The stiffening ribs enhance the rigidity and load-bearing capacity of the L-shaped steel strip, while the bolted connection ensures reliable anchoring and convenient construction.

[0010] This invention also provides a construction method for a box girder tensioned steel plate-UHPC shear reinforcement structure, used to apply prestress to obliquely attached steel plates of existing concrete box girders. The existing concrete box girder has obliquely attached steel plates on its web. The method includes the following steps: fixing a connecting plate at the lower end of the obliquely attached steel plate and fixing a tensioning anchor plate at the lower part of the concrete box girder web; heating the obliquely attached steel plate to cause it to expand and elongate; inserting and anchoring a tensioning member through the connecting plate and the tensioning anchor plate; stopping heating and allowing the obliquely attached steel plate to cool and shrink naturally, applying prestress to the obliquely attached steel plate through the tensioning member. This method utilizes the physical properties of thermal expansion and contraction of steel, achieving prestress application to the steel plate without large tensioning equipment. It is simple to construct, low in cost, and the heating process also softens and removes the original deteriorated structural adhesive, achieving two benefits in one step.

[0011] Preferably, applying prestress to the inclined steel plate further includes: tightening the nut of the tensioning member to a predetermined torque using a torque wrench while it is heated to apply initial prestress; the natural cooling and shrinkage process applies secondary prestress to the inclined steel plate. The initial prestress value can be precisely controlled using a torque wrench, while the secondary prestress is automatically generated by cooling and shrinkage. The two are superimposed to form a stable prestress system, effectively improving the utilization rate of the steel plate.

[0012] Preferably, after applying prestress, the method further includes: injecting structural adhesive between the inclined steel plate and the web of the concrete box girder, and casting a UHPC layer on the outer side of the concrete box girder web and the inclined steel plate. Injecting the structural adhesive transforms the unbonded prestressed steel plate into a bonded prestressed steel plate, reducing subsequent prestress loss; casting the UHPC layer provides durable protection and participates in structural stress, achieving integrated reinforcement and protection.

[0013] The present invention also provides a method for designing the above-mentioned box girder tensioned steel plate-UHPC shear reinforcement structure, comprising the following steps:

[0014] S1. Determine the tension control stress of the obliquely bonded steel plate. The calculation formula is:

[0015] ;

[0016] in, The axial force applied to the inclined steel plate when tightening the nut of the tension member is expressed by the formula. Sure, The torque applied to the tensioner The torque coefficient, The nominal diameter of the tensioning member; This represents the temperature difference before and after heating. , , , These are the nominal cross-sectional area, elastic modulus, coefficient of thermal expansion, and design strength value of the obliquely bonded steel plate, respectively. This formula comprehensively considers the prestress generated by mechanical tension and temperature difference effects, has a clear physical meaning, and can guide the reasonable determination of construction parameters to ensure that the stress of the steel plate does not exceed the design strength.

[0017] S2. Calculate the effective prestress of the diagonally bonded steel plate. The calculation formula is:

[0018] ;

[0019] in, This refers to the prestress loss caused by deformation of the anchoring system. This formula represents the prestress loss caused by steel relaxation. It takes into account prestress loss during construction, accurately assessing the actual effective prestress remaining in the steel plate after reinforcement, thus providing a basis for subsequent load-bearing capacity calculations.

[0020] S3. Calculate the shear capacity provided by the diagonally attached steel plate. The calculation formula is:

[0021] ;

[0022] in, The angle between the obliquely attached steel plate and the beam axis. The formula represents the stress hysteresis strain. It considers both the contribution of the applied initial prestress and the stress hysteresis effect on the steel plate caused by the unloadable self-weight, and can accurately assess the actual shear contribution of the steel plate in the reinforcement system.

[0023] Preferably, the stress hysteresis strain Calculated using the following formula:

[0024] ;

[0025] in, and These are the standard values ​​of shear force on the cross-section before and after reinforcement, respectively. The yield strain is defined as 0.0015 to 0.0025. This formula, by measuring the ratio of shear force changes before and after reinforcement, can reasonably estimate the strain lag of the steel plate under live load, making the calculation results more consistent with the actual stress state.

[0026] Preferably, the method further includes the step of calculating the total shear capacity of the structure, and the calculation formula is:

[0027] ;

[0028] in, This is the structural importance coefficient. This is the design value for shear force. and The shear bearing capacity provided by the original box girder concrete and the original box girder stirrups, respectively. The shear capacity provided to the UHPC layer. This formula is based on the principle of linear superposition, and the physical meaning of each resistance contribution is clear, allowing for a comprehensive assessment of the overall shear safety of the reinforced structure.

[0029] Preferably, the shear bearing capacity provided by the UHPC layer Calculated using the following formula:

[0030] ;

[0031] in, The shear strength utilization factor of UHPC. Let U be the cross-sectional area of ​​the UHPC layer in the web. and These represent the initial crack strength and ultimate tensile strength of UHPC, respectively. This formula fully considers the tensile properties of UHPC material, using the average of the initial crack strength and ultimate tensile strength as the representative strength, which can reasonably evaluate the shear contribution of the UHPC reinforcement layer.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. This invention effectively improves the long-term maintenance and durability of existing steel-bonded bridges by making full use of the existing inclined steel plates on the web for active prestressed reinforcement, thus avoiding the stubborn problems of insufficient strength of the steel-bonded adhesive and long-term degradation of bonding performance.

[0034] 2. By heat-treating the steel plate with a hot air gun or other means, the original aged structural adhesive can be removed without damage. In addition, the steel plate can be actively prestressed by temperature difference and tightening the bolts with a wrench in combination with the anchoring device. No large tensioning equipment is required, and the construction is simple.

[0035] 3. UHPC material has high mechanical strength and good durability. By casting a UHPC reinforcement layer, cracked web cracks are repaired, and a reliable force transmission path and durable protective layer are constructed.

[0036] 4. The design method provided by this invention takes into account the damage to the original structure, the prestress and stress hysteresis of the steel plate, and the shear contribution of UHPC, which can guide the rational design of structural reinforcement and improve the structural safety and reliability. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, wherein:

[0038] Figure 1 This is a schematic diagram of a box girder tensioned steel plate-UHPC shear reinforcement structure;

[0039] Figure 2 This is a schematic diagram of the cross-section of the reinforced box girder;

[0040] Figure 3 yes Figure 2 Enlarged schematic diagram of section A in the middle;

[0041] Figure 4 This is a schematic diagram of a prestressed tensioning and anchoring structure for steel plates.

[0042] Figure 5 This is a schematic diagram of the assembly structure of the tension anchor plate and the connecting plate;

[0043] Figure 6 This is a schematic diagram of the steel plate structure and stud arrangement of the web of a concrete box girder;

[0044] Figure 7 This is a diagram of the shear capacity analysis and calculation model for inclined sections.

[0045] Legend:

[0046] 1. Concrete box girder web; 2. L-shaped steel strip; 3. Bolt; 4. Stiffening rib; 5. Diagonally attached steel plate; 6. Shear stud; 7. Tensioning anchor plate; 8. Connecting plate; 9. Structural adhesive; 10. Roughened surface; 11. UHPC layer; 701. Anchor plate steel frame; 702. High-strength anchor bolt; 703. Tensioning end nut; 704. Tensioning component; 801. Connecting plate steel frame; 802. Fillet weld; 803. Fixed end nut. Detailed Implementation

[0047] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0048] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature.

[0049] Example 1: Box Girder Tensioned Steel Plate-UHPC Shear Reinforcement Structure

[0050] like Figures 1 to 6 As shown, this embodiment provides a box girder tensioned steel plate-UHPC shear reinforcement structure, including a concrete box girder web 1, a diagonally attached steel plate 5, an L-shaped steel strip 2, a connecting plate 8, a tensioning anchor plate 7, a tensioning member 704, structural adhesive 9, and a UHPC layer 11.

[0051] The inclined steel plate 5 is an existing component used during the initial bridge reinforcement; its continued use in this embodiment does not result in steel waste. In actual engineering projects, it can also be replaced with a new, thicker, higher-strength, high-toughness steel plate.

[0052] The L-shaped steel strip 2 is made of high-strength steel and is pressed onto the upper end of the inclined steel plate 5, and is fixedly connected to the top plate of the web 1 of the concrete box girder by bolts 3. Preferably, the L-shaped steel strip 2 is provided with stiffening ribs 4, which are formed by on-site welding to enhance the rigidity and load-bearing capacity of the L-shaped steel strip 2. This structure can effectively strengthen the anchorage of the inclined steel plate 5 at the top, prevent the end of the steel plate from warping and detaching due to stress, and enhance the overall structural performance and durability. During construction, the L-shaped steel strip 2 can be bent and adjusted on-site to ensure that the two sides of the adhesive are pre-pressed and aligned. Then, the stiffening ribs 4 are welded on-site for local reinforcement, and the L-shaped steel strips 2 are spliced ​​longitudinally to form a continuous longitudinal structure to ensure uniform stress distribution.

[0053] Further specific combinations Figure 4 and Figure 5 As shown, the connecting plate 8 is fixed to the lower end of the inclined steel plate 5. Specifically, the three sides of the connecting plate steel frame 801 are welded together by fillet welds 802 to ensure that the strength of the weld joint is greater than the strength of the steel plate base material. The tension anchor plate 7 is fixed to the lower part of the concrete box girder web 1 and located below the connecting plate 8. The anchor plate steel frame 701 is chemically connected to the concrete box girder web 1 by high-strength anchor bolts 702 to form a reliable anchoring foundation.

[0054] The tensioning member 704 is inserted into the connecting plate 8 and the tensioning anchor plate 7 to tension the inclined steel plate 5 to apply prestress. Preferably, the tensioning member 704 is one of fine-rolled threaded steel bars, high-strength steel wire bundles, or steel strands. In this embodiment, PSB785 fine-rolled threaded steel bars with a nominal diameter of 18mm are preferred. The connecting plate 8 and the tensioning anchor plate 7 are respectively provided with a fixed end nut 803 and a tensioning end nut 703 that cooperate with the fine-rolled threaded steel bars. The double nut interlocking achieves anti-loosening, enhances anchoring reliability, and reduces prestress loss.

[0055] The structural adhesive 9 is filled between the inclined steel plate 5 and the concrete box girder web 1. It is a heat-resistant and aging-resistant high-performance structural adhesive to achieve a prestressed structure with full-length bonding between the steel plate and the concrete, ensuring that the two work together, while preventing the steel plate from continuing to shrink and creep, and stabilizing the prestressed system.

[0056] The outer surface of the inclined steel plate 5 is provided with shear studs 6, which are fixed by welding with an arc screw welding machine. The surface of the concrete box girder web 1 is provided with a roughened surface 10, which is roughened by high-pressure water gun, roughening machine and other equipment to expose the coarse aggregate of ordinary concrete. The UHPC layer 11 covers the outer side of the concrete box girder web 1 and the inclined steel plate 5. It is a low-shrinkage, early-strength and fast-hardening UHPC. It forms an integral load-bearing structure with the inclined steel plate 5 and the concrete box girder web 1 through the shear studs 6 and the roughened surface 10, respectively. This structure forms a reliable force transmission path of concrete box girder-UHPC reinforcement layer-reinforcement steel plate, ensuring that the components work together. At the same time, the UHPC layer covers all steel and bolt structures, preventing metal exposure and oxidation, providing long-term stable and reliable durable protection, and significantly improving the fire resistance of the structure.

[0057] Example 2: Construction method of box girder tensioned steel plate-UHPC shear reinforcement structure

[0058] This embodiment provides a construction method for a box girder tensioned steel plate-UHPC shear reinforcement structure, used to apply prestress to the inclined steel plate of an existing concrete box girder. It is applicable to the structure described in Embodiment 1 and includes the following steps:

[0059] S1. Before reinforcing the box girder, a full-span scaffold should be erected for temporary support as needed, or other measures should be taken to ensure the box girder's safety. The surface of the web 1 of the concrete box girder requiring reinforcement should be treated. For ordinary concrete surfaces that do not require steel plates or structural components, a roughening process 10 should be performed using a high-pressure water gun, roughening machine, or other equipment, exposing the coarse aggregate of the ordinary concrete. For areas with peeling protective layer or rusted reinforcing bars, the roughening depth can be increased to expose the original stirrups. L-shaped reinforcing bars can also be drilled and inserted into the interface to enhance the bond and force transfer between the web and the UHPC layer 11.

[0060] S2. Weld a connecting plate 8 to the lower end of the inclined steel plate 5, and install a tension anchor plate 7 at the lower part of the concrete box girder web 1. Specifically, the surface of the inclined steel plate 5 is first ground to remove protective paint, loose rust, and dust, and then shear-resistant short studs 6 are evenly arranged and welded. The connecting plate 8 is welded to the lower part of the inclined steel plate 5, and fillet welds 802 are applied to the three sides of the connecting plate steel frame 801 to ensure that the strength of the weld joint is greater than the strength of the steel plate base material. The tension anchor plate 7 is constructed at the bottom of the box girder web, and the anchor plate steel frame 701 is connected to the box girder through high-strength anchor bolts 702, completing the connection construction of the tension anchor.

[0061] S3. Heating the diagonally bonded steel plate 5 to cause it to expand and elongate. Specifically, using tools such as an industrial-grade hot air gun, the diagonally bonded steel plate 5 is heat-treated to soften and detach the original adhesive, removing the original deteriorated structural adhesive. When the diagonally bonded steel plate 5 is heated to the expected average temperature and remains stable, the steel plate expands and elongates longitudinally.

[0062] S4. The tensioning member 704 is inserted into the connecting plate 8 and the tensioning anchor plate 7 and anchored. Under heating conditions, the nut of the tensioning member 704 is tightened to a predetermined torque using a torque wrench to apply initial prestress. In this embodiment, for the high-strength precision-rolled threaded steel used for tensioning prestressing, the fixed-end nut 803 and the tensioning-end nut 703 are tightened to a predetermined torque value using a torque wrench to apply the first portion of prestress to the inclined steel plate 5. A second nut and washer are screwed into the tensioning and fixed-end nuts, and the double-nut interlocking prevents loosening, enhancing the anchorage of the prestressed threaded steel.

[0063] S5. Stop heating and allow the inclined steel plate 5 to cool and shrink naturally. Apply secondary prestress to the inclined steel plate 5 through the tensioner 704. During the natural cooling process of the steel plate, annealing can release residual stress and enhance performance. Under the constraint of the anchor plate connection force transmission system, the inclined steel plate 5 generates a second part of temperature prestress, thereby compacting and tightening the diagonal cracks in the web, effectively improving the crack resistance and load-bearing capacity of the box girder. This step does not require large prestressing tensioning equipment and is simple to construct.

[0064] S6. After applying prestress, structural adhesive 9 is injected between the inclined steel plate 5 and the concrete box girder web 1. Specifically, after cooling, a new heat-resistant and aging-resistant high-performance structural adhesive 9 is injected under pressure using a special injection device, densely filling the gaps between the inclined steel plate 5, the L-shaped steel strip 2, and the concrete box girder, forming an effective bond between the steel plates. After the structural adhesive 9 hardens, it can prevent the inclined steel plate 5 from continuing to shrink and creep, stabilizing the effective stress after tensioning the prestressed system.

[0065] S7. After injecting structural adhesive 9, pour a UHPC layer 11 on the outside of the concrete box girder web 1 and the inclined steel plate 5. The UHPC layer 11 is poured on the surface of the web using formwork. Low-shrinkage, early-strength, and fast-hardening UHPC is employed, which, under self-compacting flow, can repair relatively coarse cracks in the web concrete, covering the steel plate, anchoring system, and other metal components to form the outer protective structure of the box girder. The UHPC layer 11, through the shear studs 6 on the steel plate and the roughened concrete surface 10, forms an integral load-bearing structure with the original structure, achieving crack resistance and durability protection for the concrete box girder.

[0066] Example 3: Design Method of Box Girder Tensioned Steel Plate-UHPC Shear Reinforcement Structure

[0067] This embodiment provides a method for designing or evaluating the box girder tensioned steel plate-UHPC shear reinforcement structure described in Embodiment 1, including the following steps:

[0068] S1. Determine the tension control stress of the inclined steel plate 5. The calculation formula is:

[0069] ;

[0070] in, The axial force applied to the inclined steel plate 5 when tightening the nut of tension member 704 is determined by the formula. Sure, The torque applied to tension member 704, The torque coefficient, The nominal diameter of the tensioning member 704; This represents the temperature difference before and after heating. , , , These are the nominal cross-sectional area, elastic modulus, coefficient of thermal expansion, and design strength value of the inclined steel plate 5, respectively. This formula comprehensively considers the prestress generated by mechanical tensioning and temperature difference effects. Based on the strength of the inclined steel plate and the structural reinforcement requirements, a reasonable tensioning control stress can be determined, thereby clarifying the torque and heat treatment temperature difference of the tensioning prestressing screw.

[0071] S2, Calculate the effective prestress of the obliquely attached steel plate 5. The calculation formula is:

[0072] ;

[0073] in, The prestress loss caused by deformation of the anchoring system is expressed by the formula. calculate, This refers to the shrinkage value caused by slippage of the anchor nut during tensioning and cooling processes. The effective length between the two ends of the prestressed tensioning anchorage system; The prestress loss caused by steel relaxation is expressed by the formula. calculate, The prestress loss coefficient caused by the relaxation of the prestressed steel plate is 0.050 for single tensioning and 0.035 for over-tensioning. After the UHPC layer 11 is cast, the structural adhesive 9 has hardened, and the shrinkage and creep of UHPC are relatively small. The effective prestress of the inclined steel plate 5 can be maintained for a long time, and the stress performance under normal use can be calculated accordingly.

[0074] S3, Calculate the shear bearing capacity provided by the inclined steel plate 5. The calculation formula is:

[0075] ;

[0076] in, The angle between the obliquely attached steel plate 5 and the beam axis is... This formula considers the stress hysteresis strain caused by secondary forces. It takes into account both the contribution of the initial prestress and the stress hysteresis effect of the steel plate caused by the unloadable self-weight, and can accurately assess the actual shear contribution of the steel plate in the reinforcement system.

[0077] As a preferred option, stress hysteresis strain Calculated using the following formula:

[0078] ;

[0079] in, and These are the standard values ​​of shear force on the cross-section before and after reinforcement, respectively. The yield strain is taken as a value ranging from 0.0015 to 0.0025, with a preferred value of 0.002 in this embodiment. This formula, by multiplying the ratio of shear force change before and after reinforcement by the load distribution coefficient, can reasonably estimate the strain hysteresis of the steel plate under live load.

[0080] S4. Calculate the shear capacity provided by UHPC layer 11. Calculated using the following formula:

[0081] ;

[0082] in, The shear strength utilization factor for UHPC can be 0.7. The cross-sectional area of ​​UHPC layer 11 in the web; and These represent the initial tensile crack strength and ultimate tensile strength of UHPC, respectively. This formula fully considers the tensile properties of UHPC material and can reasonably assess the shear contribution of the UHPC reinforcement layer.

[0083] S5. Calculate the total shear capacity of the structure. The calculation formula is as follows:

[0084] ;

[0085] in, This is the structural importance coefficient. This is the design value for shear force. and These represent the shear bearing capacities provided by the original box girder concrete and the original box girder stirrups, respectively. Based on the principle of linear superposition, the physical meaning of each resistance contribution is clear, allowing for a comprehensive assessment of the overall shear safety of the strengthened structure. Its analytical calculation model is as follows: Figure 7 As shown.

[0086] As a preferred option, the shear bearing capacity provided by the original box girder concrete and the original box girder stirrups... Calculate using the following formula:

[0087] ;

[0088] in, The cracking damage influence coefficient is 1.0 when no diagonal cracks appeared before the original beam was reinforced, 0.835 when the crack width is 0.2mm or less, and 0.78 when the crack width is 0.4mm. The value is 1.1 for the compression flange influence coefficient, and 1.0 for T-shaped and I-shaped beams. The prestress influence coefficient is 1.25 for cases where the cracks have been repaired and closed after reinforcement and the bending member is under full compression; otherwise, it is 1.0. The shear span ratio is set to 1.5 when it is less than 1.5 and 3 when it is greater than 3. This represents the tensile strength of ordinary concrete. This refers to the width of the ordinary concrete web of the original box girder; This is the original effective height of the box girder; This refers to the spacing between the stirrups; This refers to the cross-sectional area of ​​the stirrups; This is the design value for the tensile strength of the stirrups.

[0089] Application Examples

[0090] Taking a 30m span concrete box girder as an example, the bridge was constructed to low standards in its early stages, resulting in a thin web. The shear capacity of the web from the bridge supports to one-quarter of the span was insufficient. After reinforcement with diagonally bonded steel plates, the steel plates debonded, and the web continued to crack. The box girder is 1600mm high, with a single-sided ordinary concrete web thickness of 240mm and a concrete strength grade of C50. The diagonally bonded steel plates are made of Q355qD steel, 8mm thick and 100mm wide, with an angle of 45° to the beam axis. According to the shear analysis mechanical model of the inclined section, there are 6 diagonally bonded steel plates that have passed through the critical diagonal cracks (3 on each side of the box girder's web).

[0091] S1: Calculate the tension control stress during prestressing. Each inclined steel plate is paired with two PSB785 precision-rolled threaded steel bars with a nominal diameter of 18mm. Set the torque applied by the torque wrench. With a torque coefficient of 0.15, axial prestress is generated by a tie rod consisting of two precision-rolled threaded steel bars and a steel plate using a torque wrench.

[0092] ;

[0093] Temperature difference before and after heat treatment of diagonally bonded steel plates steel plate elastic modulus Take 206 GPa, coefficient of thermal expansion Take 12×10 -6 / ℃, the design value of the tensile strength of the steel plate is 305MPa.

[0094] Tension control stress of diagonally bonded steel plates

[0095] ;

[0096] Tension control stress of fine-rolled threaded steel bars

[0097] ;

[0098] S2: Calculate the effective prestress of the steel plate after tensioning and stress loss. Based on actual measurements, the deformation of the prestressed fine-rolled threaded steel anchor, the rebar retraction, and the retraction caused by the joint can be taken as 0.4 mm. The effective length of the prestressed system is 1980 mm. Therefore, the prestress loss of the prestressed steel plate due to anchor deformation, rebar retraction, and joint stress is calculated. The prestress loss coefficient caused by prestressed steel plates and reinforcing bars during the early relaxation process is taken as 0.050. Therefore, the first-stage loss caused by creep relaxation over time is... The effective stress of the diagonally attached steel plate was calculated as follows: The effective stress of the prestressed fine-rolled threaded steel bar was calculated as follows: .

[0099] S3: Calculation of shear strength of UHPC-reinforced concrete box girder with inclined steel plate. The original box girder has a compression flange. Before reinforcement, the existing crack width does not exceed 0.2mm. The entire section is under compression. The cracking damage influence coefficient is taken as 0.835, the compression flange influence coefficient is taken as 1.1, and the prestressing influence coefficient is taken as 1.25. The shear span ratio of the verification section is greater than 3, and the formula is taken as 3. The design value of the tensile strength of C50 concrete is 1.89MPa. The total width of the double web is 480mm, the effective height is taken as 1500mm, and the stirrups in the web have a total of four limbs with a nominal diameter of 10mm and a spacing of 150mm. The design value of the tensile strength is taken as 330MPa. Then the remaining bearing capacity provided by the original ordinary concrete and stirrups is:

[0100] ;

[0101] Before reinforcement, the standard shear force generated by all dead loads (including the wet weight of the reinforcement layer) on the box girder was 1780 kN. After reinforcement, the standard shear force of the section (including all dead loads and live loads) was 2189 kN. The yield strain was taken as 0.002. Therefore, the stress hysteresis strain of the steel plate is:

[0102] ;

[0103] According to the mechanical model of shear resistance analysis of inclined sections, there are 6 inclined steel plates passing through the critical inclined crack (3 on each side of the web of the box girder). The shear bearing capacity provided by the inclined steel plates is:

[0104] ;

[0105] The web UHPC reinforcement layer is 50mm thick and 1250mm high, reinforcing both sides. The initial tensile crack and ultimate tensile strength are taken as 4.6MPa and 5.2MPa respectively, based on design values. The strength utilization factor is taken as 0.7. Therefore, the shear bearing capacity provided by the UHPC is:

[0106] ;

[0107] The structural importance coefficient is taken as 1.1. Under the most unfavorable working condition, the design value of the maximum shear force at the verification section is 2433 kN. Therefore:

[0108] ;

[0109] The design assessment indicates that the bearing capacity verification meets the requirements.

[0110] By employing the structure and construction method of this invention, a prestressed tensioning and anchoring system was established and realized. This fully utilizes the strength of the secondary reinforcing steel plate material, achieving the dual benefits of mechanical properties and durability protection of UHPC material. The shear design method provided accurately calculates the shear bearing capacity of the beam section after reinforcement, proving that the reinforcement scheme meets the safety and requirements of shear design.

[0111] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for designing a box girder tensioned steel plate-UHPC shear reinforcement structure, the box girder tensioned steel plate-UHPC shear reinforcement structure comprising: Web of concrete box girder (1); A diagonally attached steel plate (5) is attached to the surface of the web (1) of the concrete box girder. L-shaped steel strip (2) is pressed onto the upper end of the inclined steel plate (5) and fixedly connected to the top plate of the web plate (1) of the concrete box girder. A connecting plate (8) is fixed to the lower end of the inclined steel plate (5); Tensioning anchor plate (7) fixed to the lower part of the web (1) of the concrete box girder and located below the connecting plate (8); Tensioning member (704), which is inserted between the connecting plate (8) and the tensioning anchor plate (7), is used to tension the inclined steel plate (5) to apply prestress; Structural adhesive (9) is filled between the inclined steel plate (5) and the web of the concrete box girder (1); and, A UHPC layer (11) covering the outside of the web (1) of the concrete box girder and the inclined steel plate (5); The construction method of the box girder tensioned steel plate-UHPC shear reinforcement structure is characterized by the following steps: A connecting plate (8) is fixed at the lower end of the inclined steel plate (5), and a tensioning anchor plate (7) is fixed at the lower part of the web plate (1) of the concrete box girder. The obliquely attached steel plate (5) is heated to cause it to expand and elongate; The tensioning member (704) is inserted into the connecting plate (8) and the tensioning anchor plate (7) and anchored. solid; Stop heating and allow the inclined steel plate (5) to cool down and shrink naturally. Apply prestress to the inclined steel plate (5) through the tensioning member (704). The design method for the box girder tensioned steel plate-UHPC shear reinforcement structure includes the following steps: S1. Determine the tension control stress of the obliquely attached steel plate (5). The calculation formula is: ； in, The axial force applied to the inclined steel plate (5) when tightening the nut of the tensioning member (704) is determined by the formula. Sure, The torque applied to the tensioning member (704) The torque coefficient, The nominal diameter of the tensioning member (704); This represents the temperature difference before and after heating. , , , These are the nominal cross-sectional area, elastic modulus, coefficient of thermal expansion, and design strength of the obliquely attached steel plate (5); S2. Calculate the effective prestress of the obliquely attached steel plate (5). The calculation formula is: ； in, This refers to the prestress loss caused by deformation of the anchoring system. This refers to the loss of prestress caused by the relaxation of steel. S3. Calculate the shear bearing capacity provided by the inclined steel plate (5). The calculation formula is: ； in, The angle between the obliquely attached steel plate (5) and the beam axis is... Stress hysteresis strain is considered to account for the effects of secondary forces.

2. The method according to claim 1, characterized in that, Stress hysteresis strain Calculated using the following formula: ； in, and These are the standard values ​​of shear force on the cross-section before and after reinforcement, respectively. The yield strain is defined as 0.0015 to 0.0025.

3. The method according to claim 1, characterized in that, It also includes the step of calculating the total shear capacity of the structure, and the calculation formula is: ； in, This is the structural importance coefficient. This is the design value for shear force. and The shear bearing capacity provided by the original box girder concrete and the original box girder stirrups, respectively. Shear capacity provided for the UHPC layer (11).

4. The method according to claim 3, characterized in that, The shear bearing capacity provided by the UHPC layer (11) Calculated using the following formula: ； in, The shear strength utilization factor of UHPC. The cross-sectional area of ​​the UHPC layer (11) in the web is... and These represent the initial tensile crack strength and ultimate tensile strength of UHPC, respectively.

5. The method according to claim 3, characterized in that, The outer surface of the inclined steel plate (5) is provided with shear studs (6), and the surface of the concrete box girder web (1) is provided with a roughened surface (10). The UHPC layer (11) forms an integral load-bearing structure with the inclined steel plate (5) and the concrete box girder web (1) through the shear studs (6) and the roughened surface (10).

6. The method according to claim 1, characterized in that, The tensioning member (704) is one of finely rolled threaded steel bars, high-strength steel wire bundles or steel strands; the L-shaped steel pressure strip (2) is provided with stiffening ribs (4) and is fixedly connected to the web plate (1) and top plate of the concrete box girder by bolts (3).

7. The method according to claim 1, characterized in that, Applying prestress to the inclined steel plate (5) further includes: applying initial prestress by tightening the nut of the tensioning member (704) with a predetermined torque using a torque wrench while it is heated; and applying secondary prestress to the inclined steel plate (5) during the natural cooling and shrinkage process.

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