RU2715788C1 - Self-stressing method of reinforced concrete beam - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention relates to construction, namely, to self-stressing method of reinforced concrete beam used in ceilings and coatings of buildings. Proposed method comprises installation of lower belt out of steel section with anchor joints and laying monolithic concrete of upper belt by formwork. For period of hardening of concrete to retain deflection of beam from shrinkage of concrete temporary supports are installed in middle of span, which are removed after stabilization of internal moments from shrinkage, values of which are determined by proposed mathematical dependence. To increase bearing capacity and rigidity of beam before concrete placement in middle of span temporary post is installed – support, monolithic concrete is laid, after shrinkage of concrete (approximately two weeks later), internal stresses create internal forces and moment of reverse sign, which leads to formation of self-stressed beam. At that value of internal moments from concrete shrinkage is determined by mathematical dependence. Proposed method for self-stressing of beams or flooring in comparison with known solutions of pre-stressing of beams and floors provides for simplified manufacturing, absence of tensioning devices, reduced consumption of materials. Produced pre-stressed concrete beam or flooring has high carrying capacity, stiffness and reliability.

EFFECT: technical result of invention consists in improvement of beam carrying capacity.

1 cl, 4 dwg

Description

The invention relates to the field of construction and can be used in monolithic beams, ceilings of residential and public buildings.

Known methods for prestressing a steel-concrete beam containing a steel I-beam profile, anchor rods and flexible reinforcement with prestressing devices on both sides of the profile and concrete filling (see utility model patent No. 155802 Е04С 3/294, 2015)

The disadvantages of this method of beam prestressing are the presence of additional prestressing devices, the inability to evenly distribute the prestressing forces. This leads to additional consumption of material on the tensioning devices, to overload one branch of the prestressing system relative to the other and reduces both the total bearing capacity of the beam and increases the consumption of material.

The prototype of the invention is a method of prestressing the ceiling, including laying non-metallic bar reinforcement in the beams of a hollow section and mesh in the slab part, followed by monolithic concrete with expanding cement (see RF patent No. 17801 EV 05/23, publ. In Bulletin No. 8 03/13/2018 .).

The disadvantages of this method are: inefficient use of the strength properties of concrete in the beam (concrete filling of the zone below the neutral axis), the relatively high cost of expanding concrete and non-metallic reinforcement.

The invention is aimed at increasing the overall bearing capacity of a steel-concrete beam on traditional concrete and steel reinforcement due to its prestressing by redistributing the internal forces arising from the shrinkage of concrete.

The result is achieved in that in a steel-concrete beam comprising a steel profile and monolithic concrete having anchor ties, according to the invention, during the hardening period of concrete, temporary support-supports are installed in the middle of the span, which are removed after the time of alignment of internal moments from concrete shrinkage has passed.

In FIG. 1 shows a steel-concrete beam, consisting of a steel profile and cast concrete; in FIG. 2 - beam with temporary support-support, in FIG. 3 - diagrams of moments at the stages of loading and self-tension of the beam; in FIG. 4 - change of stress values in a steel profile with a single-span beam scheme and with the installation of a temporary support - supports.

Steel-concrete beam consists of monolithic concrete 1, steel profile 2 and anchor ties 3 (Fig. 1).

The method is performed in the following sequence. The steel profile 2 is first installed on the supports 4, then the temporary support-backup 5 and the monolithic concrete 1 is laid. When the concrete hardens, the concrete shrinks. After stabilization of the concrete shrinkage process, approximately two weeks later, the temporary support 5 is removed. Internal stresses create internal forces N _b and moment M _{b of the} opposite sign (in contrast to the moment of self-weight of the steel profile and monolithic concrete, which leads to the formation of a self-stressed beam. The value of the internal moment of self-stress is determined by the formula M = N _b y _b + M _b , where N _b , M _b is the internal longitudinal force and the moment from the concrete shrinkage stress σ _b , N _b is the shoulder of the internal force N _b .

In FIG. 3a shows a plot of moments for a steel beam 2 from its own weight in the presence of a temporary support, in FIG. 3b is a plot of the weight of freshly laid concrete 1, in FIG. 3c is a diagram of moments in the process of shrinkage of concrete 1, in FIG. 3g - the final plot of the moments after removing the temporary support-backup 5.

Numerical calculations show for one temporary support for a beam with a span of 600 cm, from a steel profile 130, the thickness and width of the upper belt made of concrete B208 and 120 cm, respectively, the maximum moment values during self-tension decrease 2.15 times, and the stresses in the stretched steel belt the profile by 1.5 times, and in the compressed belt by 11.9 times, and when two temporary supports are installed at equal distances, the stresses in the stretched shelf of the profile decrease by 1.47 times, the moment value decreases by 3.25 times.

This method of self-tension (prestressing) allows you to evenly distribute internal forces from shrinkage of concrete, increases; general bearing capacity, beam rigidity and reliability of the self-stressed beam.

The prestressed steel-concrete beam has increased rigidity, bearing capacity and reliability, as well as reduced material consumption of the composite beam.

Claims (1)

A method of self-tensioning a steel-concrete beam, including installing a lower belt of steel profile with anchor ties and laying on the formwork of monolithic concrete of the upper belt, characterized in that for the period of concrete hardening to delay the deflection of the beam from concrete shrinkage, temporary supports in the middle of the span are installed, which are removed after stabilization of internal moments from shrinkage, the values of which are determined by the formula M = N _b y _b + M _b , where N _b , M _b are the internal longitudinal force and the moment from the concrete shrinkage stress σ _b , y _b is the shoulder All Strength N _b .

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