Calculate The Tensile And Elongation Value Of Prestressed Steel Strand
There are two types of pre-tension construction, namely the pre-tensioning method and the post-tensioning method. The pre-tensioning method is to first tension the pre-stress and then carry out the reinforced concrete construction. It is generally a straight line and is easy to calculate. It can be used as a post-tensioning method to calculate without pipe friction. . The tensioning process of the tensioning equipment has two types: overall tensioning and single tensioning. In post-tensioning construction, the components are poured first, the joints should be pre-buried before pouring, and then the beam tensioning should be carried out. In the post-tensioning method prestressed tendons are arranged in a combination of straight and curved lines. Taking the beam and slab as an example, the positive bending distance in the middle of the span mainly resists the load, and the negative bending distance generated when the beam end resists the load. System conversion is carried out by combining straight and curved. Since the average stress of different linear intervals is very different, in order to ensure the accuracy of the calculation, it is necessary to calculate the elongation separately and then accumulate it.
In the construction of prestressed tendons with tensioning machines, in order to ensure the quality of construction, in addition to stress control, the specification requires that the elongation value is also used for checking so that the difference between the actual elongation value and the theoretical elongation value is controlled within Within ±6%, so the calculation of the elongation value before tension is very important.
According to the construction specification, △L=△L1+△L2+△L3……+△Li, △L is the theoretical elongation value of the working length of the prestressed tendons. For the calculated elongation value △L of a linear interval, the calculation formula is:
Theoretical elongation: △L=PP*X/(AP*EP)
Among them, PP is the average tensile force of the i-th section. N, △Li is the working length of the i-th section. cm, AP is the cross-sectional area of the prestressed tendons. mm2, and EP is the elastic modulus of the prestressed tendons.
The formula for calculating the average tensile force under the anchor is: PP=P ×(1-e-(KX+μθ))/(KX+μθ)
Among them, P is the tensile force at the tensile end of the prestressed anchor steel. N, L is the length of the tunnel from the tensile end to the calculated section. m, and θ is the sum of the angles from the tensile end to the tangent of the tunnel part of the calculated section curve. For a circular curve, it is the central angle of the section. If the channel is bent in the vertical and horizontal planes at the same time, θ is the vector sum of the two-way bending angle. rad, K is the influence coefficient of the local deviation of the channel 1m on the friction, μ Is the friction factor between the prestressed tendons and the wall of the hole.
In the calculation process, the calculated stress value of each section interval cannot use the average tensile stress under the anchor. Instead, it uses the residual effective tensile force value after overcoming the friction resistance from the tensile end to the 1-i section. It varies with the area. The segment increases and decreases. In order to reduce the friction loss, the following measures are adopted during the tensioning process: (1) Both ends are tensioned to reduce the value of θ and the pipe length L; (2) Over tensioning is used, and the tensioning process is: 0 →Initial stress (usually 10%σk) →103%σk and holds for 5minσk.