1. The span design of prestressed T-beam bridge must be considered in the design. China has compiled the standard design of simply supported beam bridges in post-tensioned concrete, with standard spans of 25m, 30m, 35m and 40m. But at present, a prestressed T-beam bridge with a span of 50m has been designed in the expressway built in our province.
2. The height-span ratio of prestressed T-beam should be selected within the range of114-1/25 in the design of equal section. With the increase of span, the smaller value is taken, and with the decrease of the number of T-beams, the larger value is taken. In order to arrange the section economically and reasonably and save the quantity of prestressed reinforcement, the efficiency index P of T-beam section should be greater than 0.5. Increasing the width of flange plate can effectively improve the section efficiency index.
3. In order to meet the layout requirements of the steel strand bundle and bear the large supporting reaction and shear force at the end, the beam rib should be widened into a horseshoe shape when designing the end of the prestressed T-beam, and the horseshoe shape area should not be less than 10%-20% of the total section, so as to prevent the horseshoe shape from longitudinal cracks during construction and transportation. The web near the fulcrum of the beam end and the horseshoe shape should be designed to be equal in width at the height of the beam of about1m.. At the same time, it should be noted that the horseshoe-shaped part should not be too high or too large, thus reducing the centroid of the section and the ability to offset the dead weight of the T-beam.
4. The reinforcement of prestressed T-beam includes the arrangement of prestressed reinforcement and non-prestressed reinforcement. The layout principles of prestressed tendons are as follows: the center of gravity of prestressed tendons should be as low as possible on the premise of ensuring the thickness of protective layer at the bottom of beam and keeping prestressed tendons at the boundary of cable; While meeting the structural requirements, the prestressed tendons should be as close to each other as possible to make the members compact. Bending all the prestressed tendons to the beam end can reduce the friction loss, but the beam end has more prestress. The reinforcement amount of prestressed steel bars must be obtained through strict calculation. According to the reinforcement principle of reinforced concrete, the main reinforcement at the bottom of T-beam, stirrups, horizontal distribution reinforcement, prestressed tendon positioning reinforcement, anti-cracking reinforcement during tension, anti-shrinkage reinforcement and erection reinforcement are added to the reinforcement of non-prestressed reinforcement. Here, it is especially required to set steel mesh in the widening range of beam end to prevent concrete from cracking due to tension.
5. Prestress calculation of prestressed steel beam. At present, most prestressed steel strands adopt ASTMA4 16-98 standard 270-grade low relaxation steel strand, with standard strength Ryb= 1860MPa, nominal diameter 15.24mm and nominal area 140mm2. The elastic modulus is 1.95* 105MPa. The tension control stress is 0.75 times of the standard strength, that is, 0.75 *1860 =1395 MPa. The tension control force is (1395 *140)/1000 =195.3kn. Therefore, the tension control force p of each steel beam is 195.3* the number n of steel strands of the steel beam. Tensile elongation value is calculated according to straight line segment and curve segment. Straight line segment: first calculate the straight line segment of prestressed steel strand, and then calculate it according to the following formula:
L straight line =P*( 1-e-kl)/Ay*Eg
P- tension (KN)
Ay—— cross-sectional area of prestressed steel strand (mm2)
Eg-elastic modulus of prestressed steel strand (MPa)
K—— the influence coefficient of local deviation per meter of tunnel on friction (see Technical Specification for Bridge Construction).
U—— Friction coefficient between prestressed reinforcement and tunnel wall (see technical specification for bridge construction).
L—— linear length of prestressed reinforcement (m)
Curve segment: divide all curve segments in the arrangement of prestressed tendons and tendons, and calculate according to the following formula:
l song = p * l * {[ 1-e-(KL+uθ)]/(KL+uθ)}/ay * eg
Each symbol has the same meaning as a straight line segment.
Finally, the calculated elongation of straight line segment and curve segment is summarized as the total elongation:
L total =∑L straight line +∑L curve
When tensioning, it is necessary to ensure that the concrete strength reaches more than 90% of the design strength. Prestressing adopts single-end tensioning and two-end tensioning. Tension at both ends shall be carried out at the same time. The stress and elongation of each stage are recorded, and the actual elongation of steel beam is calculated, and compared with the theoretical elongation, the error value is controlled within 6%. Moreover, it is not allowed to slip or break the wire beyond the value specified in the specification (the specification stipulates that the broken wire or broken wire of each strand of steel strand shall not exceed1; The total number of broken wires in each section shall not exceed 65438+ 0% of the total number of steel wires in this section).
After the tensioning of prestressed tendons is completed, hole grouting should be carried out as soon as possible. Grouting label shall be carried out according to the design, with one-time grouting, pressure 1.0MPa, and grouting sequence in the hole shall be carried out according to the design. Press in from one end of the channel to the other end, and make it full into slurry. The consistency of the cement slurry flowing out must be the same as that at the press-in end, and meet the specified requirements.
6. Prestressed anchor sealing must be carried out in strict accordance with the specifications and design requirements. The label of anchor sealing concrete must be the same as or even higher than that of T-beam concrete.
Key points of operation of prestressed T-beam bridge in construction practice
Main construction procedures of prestressed T-beam bridge:
1. First of all, the selection of prefabrication site of T-beam should meet the requirements of prefabrication, hoisting and transportation of T-beam. And should be able to meet the convenience of material transportation and construction.
2. Strictly control the quality of each process in T-beam prefabrication. Include steel bar manufacture and installation, prestressed steel beam manufacture and installation, raw materials used in concrete pouring, mix proportion, quality control, post-tensioned prestress application control, anchor sealing control, grouting material control, mix proportion, grouting technology and grouting pressure.
3. Control of appearance quality of T-beam and treatment of appearance quality defects after concrete formwork removal.
4. Safety precautions for storage of finished T-beam and safety precautions for hoisting T-beam. T-beam must be stored in a flat site where the bearing capacity of foundation meets the requirements and there is no landslide or collapse around it; When hoisting, the safety operation procedures for hoisting must be observed, and the T-beam can only be transported out after it is reinforced firmly; During the installation process, the operation of the bridge erecting machine and the positioning of the T-beam should strictly abide by the hoisting safety operation procedures, and pay attention to safety supervision to prevent safety accidents; T-beam is located on the temporary support, and its axis, elevation and inclination must meet the specifications and design requirements.
5. After the installation of T-beam is completed, the steel bars of diaphragm, wet joint and continuous section shall be welded firmly according to the design and specification requirements. Complete the concrete pouring of diaphragm, wet joint and continuous section as soon as possible (remember to embed the negative moment steel beam of bridge deck through each continuous section first during construction). In the construction of these parts, it is necessary to strengthen the technical disclosure of safety construction and supervise the implementation of safety measures.
6. After the above concrete reaches the required strength, the system transformation of the bridge will be carried out. When the T-beam is hoisted, it shall be installed on the temporary support. According to the requirements of several spans and one joint in the design (generally three spans and one joint or four spans and one joint), solid piers, continuous piers and redundant piers are poured. After the strength of concrete meets the design requirements, all temporary supports will be removed, so that the whole bridge can complete the system conversion and meet the design requirements (if the bridge deck construction is carried out without system conversion first, the bridge deck elevation caused by system conversion after bridge deck construction will not meet the design and specification requirements). Then start the construction of negative bending moment prestress on the bridge deck. The purpose of negative bending moment prestress on bridge deck is to prevent bridge deck cracking caused by negative bending moment at pier top. In the construction of negative bending moment of bridge deck, special attention should be paid to the application of prestress and pipeline grouting.
7. After the construction of negative bending moment of bridge deck is completed, chisel the bridge deck and construct the bridge deck and guardrail. During bridge deck construction, the elevation and smoothness of bridge deck should be strictly controlled. Elevation should be controlled by level, and only one leveling point can be used for a bridge. The flatness shall be detected with a 3m ruler, and the error value shall not exceed the design and specification requirements. In the process of bridge deck construction, we must also pay attention to the roughening and cleaning of the bridge deck, so that the bridge deck and pavement asphalt have good adhesion.
8. During guardrail construction, attention should be paid to the curve and alignment of the route, and the pile number can be controlled within 5- 10m during construction lofting to adjust the alignment of the route. The elevation of the guardrail should also be strictly controlled, and it is strictly forbidden to have obvious ups and downs at the top of the guardrail. It is very important to control the appearance quality of guardrail. After the concrete pouring of guardrail is completed, the bubble holes and local pits can be smoothed with dry cement ash, and polished with emery cloth if necessary.
9. After all the above processes are completed, the completion acceptance can be put forward. Pavement asphalt and other ancillary facilities shall be laid by the pavement unit.
To sum up, the design and construction of highway bridges and the combination of theory and practice of highway bridges are inseparable from the principle of economy and rationality. As long as the basic knowledge is solid, you can give full play to your potential in practical operation. We also strive to innovate in practice, accumulate experience and constantly improve and improve ourselves.
(Reprinted)