What are the methods for applying prestress?

As is well known, in ANSYS, prestressed concrete analysis (bonded) can be performed using the equivalent load method and the solid reinforcement method. The so-called equivalent load method involves applying the effect of the tendons as loads to the concrete structure ； The so-called solid-reinforcement method involves using solids to simulate concrete, while using links to simulate reinforcement bars. 1 Advantages and disadvantages of the equivalent load method: The advantage is that modeling is simple; it is not necessary to consider the specific location of the reinforcement bars, allowing for direct modeling with a straightforward mesh division ； It is relatively easy to determine the overall effect of the structure under prestress. Its main drawback is that: ① The equivalent load method does not take into account the distribution and direction of the force exerted by the rebar on the concrete; since the force exerted by the rebar varies in different locations, the equivalent load method is unable to account for this ； The horizontally uniform component is not taken into account. ②It is difficult to simulate certain linear reinforcement patterns; for example, the commonly used pattern of straight (short) + curved + straight (very long) + curved + straight (short) is cumbersome to equate, and may not be reasonable. ③It is difficult to obtain an accurate reflection of the stress distribution in the structure; otherwise, the loads must be applied at the locations of the stress members, which in turn reduces the convenience of modeling. ④The interaction under external loads is difficult to consider, and it is not possible to determine the stress increase in the tendons under such loads. ⑤The tensioning process cannot be simulated. ⑥It is not possible to simulate the factor of uneven stress across the tendon caused by stress loss. Its biggest drawback is: it’s thick! The results obtained differ significantly from the actual situation! I recently carried out some practical calculations, and upon comparison I found that the results differed significantly from the actual values (possibly due to special cases); therefore, caution and verification are necessary when using this method. 2 Advantages and disadvantages of the physical reinforcement method: Concrete and reinforcement bars are divided into separate units, and prestress can be simulated using the cooling method or the initial strain method. The method for reducing temperature is relatively simple; it also allows for the simulation of tendon loss, and only elements and real constants are required ； The initial strain usually does not take prestress loss into account; otherwise, the actual constants for each element would be different, resulting in a large amount of work. It can eliminate the disadvantages of the equivalent load method. But the modeling workload seems to be larger. New Year greeting card 2-----The ANSYS processing procedure for the solid stress bar method in prestressed concrete analysis. There are two approaches: one is the volume segmentation method, and the other is the independent modeling and coupling method. 1 Volume division method: A plane is formed by using a working plane and a force tendon line, and the volume is divided using this plane; thereafter, a line on the resulting volume is defined as the force tendon line. By continuing this process of division, many complex bodies and multiple force member lines are ultimately formed; these are then divided into individual units, to which prestress, loads, and boundary conditions are applied before solving the problem. This method is based on the processing of geometric models; the geometric model is unified, the positions of the force members are accurate, and the solution results are precise. However, when the shape of the force members is complex, modeling becomes particularly difficult. 2 Independent modeling coupling method: The basic idea of this method is to create geometric models for the solid and the stress bars separately, dividing them into elements respectively; thereafter, coupling equations are used to connect the stress bar elements with the solid elements. This approach is based on the processing of finite element models. The basic steps are as follows: ① Create a geometric model of the entity (without considering rebar) ； ②Establish a geometric model of the force tendon line (without considering the presence of a solid) ； ③Divide the geometric model into elements according to certain requirements (which are also independent of each other at this point) ； ④Select all force tendon lines ； ⑤Select the nodes related to the aforementioned force members (nsll command), and define the selection set ； ⑥Store the aforementioned force tendon nodes in an array ； ⑦Select all nodes and remove the node set in ⑤ (which are all nodes except those of the force-transferring members) ； ⑧Search for all the nearest entity node numbers using the force tendon node array, and store them in the array ； ⑨The coupling force bars are coupled one-by-one with the nearest nodes (using the cp command); the cpintf command cannot be used as it may couple other nodes and can easily result in no coupling. ⑩ Select all, apply boundary conditions and loads, and then the solution can be obtained. This method is particularly simple for modeling, and the coupling process is also straightforward (though some familiarity with APDL is required). The downside is that when the meshing of solid elements is not dense enough, the positions of the force bar nodes may shift slightly, but the error remains within an acceptable range! This method is a good approach for dealing with situations where the force bars have complex shapes and a large number of them.

①Cable tensioning method: A system of cables is arranged within the structural framework, and by using jacks to tension the ends of these cables, unloaded stress is created within the structure, which brings benefits. It is a process that is widely used both domestically and internationally and has matured technology. However, anchor heads are required at the cable ends for fixation, increasing material consumption. Moreover, tensioning equipment and similar items increase the construction costs. ②Support displacement method: In continuous beams and indeterminate structures, artificial displacement of the supports (vertical or horizontal) is induced; by changing the designed position of the supports, internal forces can be adjusted, the peak bending moment can be reduced, and the cross-sectional area of the structure can be decreased. This method can save on additional materials such as steel cables and anchor heads, as well as the tensioning process, and is suitable for projects with good foundation conditions. ③Elastic deformation method: Under elastic deformation conditions, the members and plates that make up a structure are combined into a single unit. After the external forcing is removed, a beneficial prestress generated by the restoring force appears within the structure. This method is widely used in factory manufacturing processes to produce prestressed components for supply to the market. ④Manual simple method: Used for medium and small spans where the required tension is not high. It involves manual techniques such as tightening nuts to tension the tie rods, or using bolts with opposite threads to apply lateral force to the cables in order to generate tension. These methods are simple and feasible, easy to implement, and suitable for use in a wide range of areas. 　

Methods of applying prestress: The methods of applying prestress can be divided into two main categories based on the sequence relative to the fabrication of the component: pretensioning and post-tensioning. Based on the method of tensioning the rebar, it can be divided into mechanical tensioning and electrothermal tensioning. In the post-tensioning method, depending on the construction techniques used, it can further be categorized into conventional post-tensioning, self-anchoring post-tensioning, unbonded post-tensioning, electrothermal post-tensioning, etc.