The bridge is set with a pre camber, is it necessary to set a reverse arch?

In bridge engineering, pre-arching and reverse arching are relative. Ordinary bridges should be set with pre-camber, and bridges with prestressed structures should be set with reverse arches. Just set the value according to the construction drawing.

For ordinary bridges, after the bridge is completed, due to the action of cars, pedestrians and other loads, there will be a downward displacement, which is also called deflection, which will cause the actual elevation of the bridge deck to be inconsistent with the elevation at the time of design. In order to eliminate or reduce the degree of inconsistency, it is necessary to set up a pre-arch, that is, let the bridge be raised to a certain height in the process of construction, so that the elevation of the completed bridge deck is one value higher than the design elevation, so that after pedestrians and vehicles get on the bridge, the height of the bridge deck can be basically consistent with the design elevation. The amplitude of this pre-camber is called the pre-camber value.

For bridges with prestressed structures, the bridge deck will slowly arch upwards under the action of prestressing, and after a period of use, the bridge deck will be higher than the design elevation. At this time, in order to make the elevation of the bridge deck meet the requirements of use, it is necessary to let the bridge be lowered to a certain height in the process of construction, which is the opposite of the pre-arch, which is called the reverse arch, and its amplitude is called the reverse arch value.

The precamber is the reverse camber. As long as you follow the construction drawing, you can set the value. It is mainly due to the fact that the second phase or the car load is larger.

The reverse camber is due to the upward deflection caused by the prestressed tendons, so when calculating the precamber, the reverse camber value should be subtracted to adjust the precamber required to adjust the elevation.

I don't understand what you mean, I haven't heard of the reverse arch.

The precamber is to achieve the design line type after the bridge is completed, and the pre-elevation amount is lofted in the case of insufficient construction loading.

The load, temperature, and geometric nonlinearity of the arch before and after the bridge are taken into account, and it is an indispensable close to accurate estimate of the combination of comprehensive factors.

The following factors need to be considered:

1. The disturbance caused by the superstructure itself and half of the live load after the bracket is removed.

2. Elastic deformation of the bracket under load.

3. Inelastic deformation of the bracket under load.

4. The inelastic settlement of the support foundation under the load.

5. Disturbance caused by concrete shrinkage and temperature change.

Precamber is the amount of correction that counteracts the deflection of beams, arches, trusses and other structures under load, and is reserved in the opposite direction of displacement during construction or manufacturing.

Determinating factors: the elastic deformation caused by the scaffold after bearing the construction load, the deflection caused by the shrinkage and creep of the concrete of the super-static structure, the plastic deformation caused by the extrusion of the rod joint and the compression of the unloading equipment, the elastic settlement of the scaffold foundation after loading, and the precamber setting of the bottom formwork of the beam, plate and arch.

Setting the pre-supply degree can offset the deflection of the production number of picos under the self-weight of the component, and ensure that the deflection of the concrete component under long-term load is controlled within the requirements of the normal service limit state.

In the construction of long-span floors or roof beams and slabs, in order to improve the perception of viewing angles and correct the settlement of self-weight, measures must be taken to increase the height of the mid-span in advance. This is the so-called precamber, or arching.

In short, because the self-weight of the component or support is taken into account and the man, machine, and material walking load in the construction, the reserved camber must be calculated, which is the precamber.

Because the strand of the post-tension beam is concave, the tension force at both ends of the beam will produce an upward camber, if the reverse arch is not set in advance, the post-tension beam will crack, and the bridge deck is not smooth.

The basic process of bridge construction includes:

Project approval stage, feasibility study stage, preliminary design stage, construction drawing design stage, bidding stage, construction preparation stage, acceptance stage. At the same time, the various stages are closely related.

Precamber calculation.

1) Computational model. The camber change of the 35 m mid-span t beam in the prefabrication, lifting and storage stages of excavation was taken as the research object of the judgment wheel, and the beam body was subjected to prestress and self-weight.

Assuming that the beam body is of equal cross-section, the known conditions of the beam are as follows: the whole section of the beam body is equipped with 30 bundles of steel hinges, divided into 3 channels, the number of bundles of steel hinges in the upper, middle and lower 3 channels is added to , the diameter of the steel hinge is J= mm, the nominal cross-sectional area is 140 mm, the modulus of elasticity EP=, and the standard strength FPK=1 860 MPa;

The control stress δcon = 395 MPa, the concrete strength grade is C50, the elastic modulus is EC=, the standard value of the cubic compressive strength of the concrete during tension is fcu=40 MPa, the self-weight of the beam is Q= Kn m, and the calculated span is L= M.

2) Cross-section geometric features. After calculation, the geometric characteristics of the mid-span section are as follows: the distance from the center of gravity of the steel hinge line to the lower edge of the cross-section y1=; Convert the distance from the center of gravity of the section to the lower edge of the section y0= m; Convert cross-section moment of inertia i = 27 m4 ; The distance from the center of gravity of the net section to the lower edge of the section yn = m; Net cross-sectional moment of inertia in= 33 m.

3) Deflection f1 due to self-weight. The mid-span cross-section deflection f1 can be calculated according to the formula of general material mechanics, i.e.: f1=5 48 ml2 b0 (1) in equation (1):

m—mid-span bending moment under beam weight; b0—the bending stiffness of the whole section, b0=, is the stiffness reduction factor. Substituting the relevant values yields f1= cm.

4) Reverse camber f2 resulting from prestressing. When the prestress is applied to the beam, the prestress produces an eccentric prestress in the beam, and the beam produces a reverse arch f2. In this example, the steel bundles in the beam are arranged as two straight lines sandwiched by a circular curve with a radius of 5 000 m.

I am happy to answer for you: how is the precamber of the box girder of the bridge calculated? A: Hello, dear! <>

The camber is calculated. If the deflection is less than L 1600 (L is the length of the span beam), if it is greater than L 1600, the precamber needs to be set, and the live load of the dead load +1 2 is generally considered. Once the maximum deflection is obtained, the precamber value is assigned according to the parabolic shape.

The precamber of the fulcrum (pier top) is 0Thank you for your patience. If my answer is helpful to you, please give a thumbs up (in the lower left corner), I look forward to your like, your efforts are very important to me, and your support is also the motivation for my progress.

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Answer]: A2021 Textbook P343 2020 Textbook P324 2019 Jiaoqing Xincai P318

Common key points of quality control in the construction of bridge superstructure: (1) Strength control of simply supported beam concrete of stuffy tank simply supported beam bridge. Control of precamber. Position control of support inserts.

When the girder is installed, the height difference between the beam finch cover wheel and the beam is controlled. Control of the mounting type and direction of the bearing. Cast-in-situ concrete quality control between beams and slabs.

Quality control of expansion joint installation.