A secondary tension anchor is an anchor system consisting of a fixed end anchor, prestressed steel strands, prestressed pipes, and a secondary tension anchor.

the term

Anchorage system of secondary tensioning low-retraction steel stranded vertical short cable

An anchor system composed of several parts, such as a fixed-end anchor, a prestressed steel strand, a pre-stressed pipe, and a secondary tension anchor. The fixed-end anchor is located at the lower end and the tensioned end is tensioned twice. The pulling anchor is located at the top, and the bellows is arranged in the web of the box girder bridge in a vertical state. After secondary tensioning of the vertical prestressing tendons, the effect of low retraction and efficient anchoring of the short cable is achieved.

Secondary tension

For the prestressed bundle of the same steel strand, first, according to the traditional tension method of the clip anchor, the first tension-releasing-clamping of the clip anchor bar is completed, and then the anchor cup of the bundle bar is stretched as a whole. Pull to control the stress. The lower end surface of the anchor cup is 5 to 13 mm away from the backing plate. After holding the load, twist the support nut to the backing plate side to eliminate the gap. Then, return the jack to the oil and release the anchor cup. In situ, the force plate at this time is theoretically non-retractive anchoring, thereby eliminating the stress loss caused by the retraction of the anchorage during the first tension and release. This prestressed construction process is referred to as a secondary tension anchor.

Vertical prestressed anchor system

It consists of fixed end anchors, prestressed steel bars, prestressed pipes, and tensioned end anchors. The fixed end anchors are located at the lower end, the tensioned end anchors are at the top, and the bellows is arranged vertically on the belly of the box girder bridge. Inside the slab, the prestressed anchor system is realized by tensioning it to realize the reinforcement of anchors.

Anchor

In a post-tensioned prestressed concrete structure or component, a permanent anchoring device used to maintain the tension of the prestressed tendons and transmit it to the concrete.

Secondary tension anchor

The utility model relates to a new type of anchor, which is provided with a thread on the outer edge of an anchor cup of a traditional clip-type circular anchor, and a support nut is arranged on the outer periphery of the anchor cup to connect with the outer thread of the anchor cup and can realize secondary tension.

working principle

After the first tension and anchoring of the same tendon, the anchor cup and the tendon are stretched again, and the anchor cup is 5 to 13 mm away from the pad. After the load is held, the support nut is moved to the pad side. The twist is used to eliminate the gap between the lower end surface of the anchor cup and the backing plate. Finally, the jack is oiled and released, and the anchor cup is locked in place. At this time, the ribs are theoretically retracted and anchored, eliminating the first Stress loss due to anchorage retraction during sub-tensioning.

Development background and working principle of secondary tension anchor

Development background

1. Overview of the status and typical diseases of long-span concrete beam bridges

Prestressed concrete beam bridges (including continuous beam bridges, continuous rigid frames, and continuous rigid frame continuous composite systems) are popular in the engineering community for their structural rigidity; smooth driving; relatively low cost; and simple maintenance. "At present, more than 20 continuous rigid frame bridges with a span of more than 200m have been built and are under construction in China. There are more than 100 prestressed concrete beam bridges with a span between 100 and 200m. There are total spans in the world. There are 18 super-long-span continuous rigid-frame bridges over 240m in total, 13 of which are in China, accounting for 72% of the world total. However, in recent years, large-span prestressed concrete beam bridges have generally encountered various problems during the construction process or the use phase. Different kinds of concrete cracking, long-term deflection and other diseases, these diseases pose a threat to the durability of the bridge and the safety of operation "[1]. In [2], the author investigated more than 180 prestressed concrete box girder bridges in China, and summarized the types and distribution of cracks. Among them, the occurrence ratio of web cracks was as high as 86%. Due to the existence of web cracks, the structural stiffness decreased. , Resulting in increased deformation. Literature [3] According to the load test of Kishwaukee.River Bridge, it was found that due to the existence of cracks in the box girder web, the shear stiffness of the crack zone structure was reduced by 50-55%.

According to a large number of investigations and analysis, vertical prestress is the most effective technical method to reduce the main tensile stress and overcome the diagonal cracks in the web. At present, a large number of diagonal cracks in the web of active large-span box girder bridges in China are mainly due to vertical prestress. In the design process, insufficient consideration was given to spatial effects. In addition, the YGM anchor system of the finished rolled threaded reinforcement used in the vertical prestress has structural defects and the prestress construction cannot effectively monitor the quality of the applied prestress. Short, almost no effective prestress can be established "[1].

After further analysis and research on the "pre-rolled rebar YGM anchor system" for vertical prestress, the structure has the following fatal flaws:

1. The strength of the finished rolled rebar is low, and the absolute value of the prestressed tensile extension is very small (especially the short bundle is only a few millimeters). Under the same tension and retraction value, the proportion of prestress loss is large, and the short bundle Prestress loss is terrible (some bridges have a vertical effective prestress loss of up to 60% compared to the vertical prestress tension control force [6]).

2. Although the bridge gauge stipulates that the retraction value of YGM fine-rolled rebar anchors with nuts is 1mm, actual tests show that "there is a loss of retraction of the rebar when it is stretched: the gap between the thread and the nut on the rebar and the deformation is about 2mm, and The angle between the contact surface of the nut and the backing plate and the axis of the steel bar at an angle of 45 ° causes an actual loss of about 4 mm "[5]. The actual retraction losses far exceed the specifications.

3. In actual engineering, the phenomenon that the fine-rolled rebar is pulled and broken sometimes occurs, and even extreme tensile construction is completed until the bridge is opened to traffic. More than 30 fine-rolled rebars break and break the bridge deck auxiliary layer. The finished rolled anchors protrude from the bridge deck (there are also some rare tendon fracture accidents after the bridge is opened to traffic). "Once the vertical fine-rolled rebar is broken, it cannot be remedied and is very harmful" [5].

4. The precision rolled threaded YGM anchor is a rigid cable. During construction, the installation accuracy of the anchor nut, prestressed thick steel bar, and backing plate is very high. Otherwise, the anchor nut may not be screwed in place during deployment. It is an important reason that the permanent stress of the structure is extremely difficult to ensure stability and prone to random changes.

5. Although the YGM anchor system of fine-rolled threaded steel bars has been used for more than 20 years, it lacks complete construction acceptance procedures. Due to the structure itself, technical management and supervisors cannot monitor and determine whether the construction is in compliance (or Meet) design requirements. Design, construction, supervision and management personnel in various links have no confidence in the quality of the prestressed construction, and are very uneasy.

6. "At present, vertical prestressing generally has poor grouting quality problems, mainly due to a, grouting failure; b, grouting is difficult to play the role of adhesive grip, domestic and foreign investigations on prestressed concrete bridges show that , The denseness of the pipeline grout is almost a common problem, and it is endless. "[5]

Development background and working principle of secondary tension anchor

2.Eradication measures for cracks in web girder bridges

The cracks in the webs of box girder bridges are mainly caused by the fact that the vertical prestress is not enough to overcome the main tensile stress and cause the webs to crack. Through a large number of solid bridge investigations, it is found that the vertical prestressed construction does not meet the design requirements and the vertical permanent stress is usually smaller than the main tensile stress. More importantly, the construction irregularities or deviations occur whether the construction party, the supervisor, Neither the designer nor the owner can monitor the quality of the vertical prestressed construction, resulting in a foreseeable risk-web cracking.

In view of the aforementioned shortcomings of the YGM anchor system for precision rolled rebar, the majority of bridge research, design, and construction workers have made a lot of improvements to the finished rolled rebar, such as: adopting secondary tension to establish a more comprehensive construction management system and strengthen On-site management, improved design calculations, multiplying the formula for calculating stress by a reduction factor of 0.6 in the new bridge gauge "JTGD62-2004" to overcome the problems of large vertical prestress loss and extremely unstable compressive stress. Certain effects, but the problem of cracks in the web of the box girder bridge is still not fundamentally solved.

Professor Shao Xudong, doctoral supervisor of Hunan University, applied a new thinking and presided over the development of the "secondary tensile low-retraction steel strand vertical prestressed anchoring system", which completely innovated the vertical prestressed anchoring structure and gave full play to high strength and low relaxation steel. The advantages of stranded tendons, using their flexible cables, high elongation, low tensile control stress (not easy to produce plastic deformation), innovative anchor structure, creatively proposed the secondary tensioning of stranded tendons (traditional steel Stranded clip anchors are not allowed to be tensioned twice. Overcoming the problem of large shrinkage loss of clip anchors, at the same time, the vertical prestressed tension has been successfully implemented to facilitate the quantitative monitoring of the quality of the tension construction and eliminate the vertical The common problem of poor pre-stressed grouting quality.

Through the web stress field test, web shear limit load test and real bridge test, it is shown that the "secondary tensile low-retraction steel strand vertical prestressed anchoring system" can greatly reduce the short and medium beam prestressed tendons. The loss of tension greatly improves the vertical prestressing efficiency and the shear safety of the structure, and the vertical actual permanent stress can stably meet the design requirements to avoid web cracking.

The secondary tensile low-retraction steel strand vertical prestressed anchoring system instead of the fine-rolled rebar anchoring system (take 15-3G instead of φ32 fine-rolled rebar as an example) can greatly improve the actual vertical pre-stress level (a single beam is actually permanent) The prestress has been increased from more than 300 kN to more than 520 kN), and the amount of prestressed steel has been reduced by 50%. It is very convenient to monitor the construction quality of the prestressed beams that have been constructed, ensuring that the vertical permanent stress does not change randomly and is very stable. ,reliable. It completely solves the problems of improper grouting in the channel (no porosity in the channel), incomplete compaction of the grout, and difficulty in bonding and gripping of the grout, realizing dense and full grouting in the channel.