Application of prestress technology in reconstruction project?

The application of prestress technology in reconstruction project has great research value, and every application detail reflects the combination of professional knowledge and actual situation, which is very critical. Zhong Da Consulting will explain the application of prestress technology in reconstruction project.

The auditorium of Southeast University is a landmark building of the school, which was built in 1930s. After inspection, some structural members have not met the safety level required by the code, so it is necessary to strengthen the auditorium, and transform the second floor of the east wing of the auditorium into a large-space lecture hall, named Chunhui Hall. In this way, it is necessary to extract the bottom four 300mm×300mm columns Z 1~Z4 (Figure 4- 18- 1) to meet the building requirements. The plane size of this building is17.28m×11.60m, which belongs to the mixed structure of internal frames.

However, the change of structural force transmission path after column extraction increases the external load on the external longitudinal wall and its foundation; In addition, due to the uneven settlement of the original soft foundation (R≤60kN/㎡) caused by the newly-built Lidong teaching building in l964, obvious oblique cracks appeared in the north and south external walls of the east end of Chunhui Hall. Therefore, in the process of reconstruction, the external longitudinal wall and foundation must also be strengthened accordingly.

Chapter 1 Reinforcement and Reconstruction Scheme

Upon inspection, the section size of the original inner frame beams L 1, L2, L3 and the secondary beams l 1', L2' of Chunhuitang is 300mm×400mm, and the width of the surrounding load-bearing wall accounts for bw=370mm. The concrete strength grade is C20, and the design value of yield strength is Grade I steel. Y=2 10N/m㎡。 In this regard, there are many schemes for reinforcement and transformation, but there are mainly the following three schemes:

Scheme 1 does not change the primary and secondary relationship of the original beam in principle, and directly strengthens beams L 1 and L2 as the main beams. However, L 1' and L2' are still secondary beams, which transfer the floor load to L3, then to l 1 and L2, and finally to the foundation through the north-south load-bearing longitudinal walls, but this makes the new load on the longitudinal walls at both ends of beams l 1' and L2' too concentrated. The load transfer process is shown in Figure 4- 18-2(a).

In the second scheme, the original beam L 1, L2 and the secondary beam l 1', L2' are all strengthened into the main beam, and the beam L3 is changed into the secondary beam. In this way, the new load on the outer longitudinal wall is evenly distributed and the force is reasonable. The load transfer process is shown in Figure 4- 18-2(b).

In the third scheme, the original floor beams L 1, L2, l 1', L2' and L3 are strengthened and transformed into a cross-beam structure, so that the floor load is evenly transmitted to the longitudinal and transverse external walls, but the construction of the reinforced beams is heavy and difficult. The load flow is shown in Figure 4- 18-2(c).

In practical engineering, considering the workload, construction difficulty and the uniformity of redistributed load of each scheme, the second scheme was finally adopted. The concrete reinforcement method is as follows: the folded high-strength prestressed steel strand bundles symmetrically arranged on both sides of the beam are used for reinforcement, and at the same time, the prestressed tendons at both ends of the reinforced beams L 1, L2, l 1' and L2' are embedded in the ring beam (770mm×400mm) around the longitudinal wall to meet the requirements of the building facade. After tensioning, C30 concrete is used to block the local gap on the outer ring beam and wrap the steel strand bundle to achieve the purpose of fire prevention and corrosion prevention, as shown in Figure 4- 18-3.

In addition, on the one hand, considering that the load of the external longitudinal wall increases and the lateral stiffness of the building weakens after the column is dismantled, the external longitudinal wall is strengthened by combining the unilateral reinforced fine stone concrete splint wall with the reinforced concrete buttress column (Figure 4- 18-4). On the other hand, because the foundation is weak and the load on the foundation increases due to the change of force transmission path, the method of pressure grouting jet grouting pile combined with beam lifting is adopted to strengthen the foundation under the external wall (Figure 4-4). The jet grouting pile is symmetrically arranged on the center line of the wall, with four buttresses and two windows.

Chapter II Design and Calculation of Reinforcement and Reconstruction

After the renovation of Chunhui Hall, an interior wall with a height of 3.5m will be added at L2 of the second floor beam. In order to reduce the self-weight, a lightweight wall is adopted, and 25mm mortar is plastered on both sides of the wall. The main beams strengthened by this method can be divided into two categories: L2 ′ and L 1 with walls, L2 and L1′ without walls. In the reinforcement design of beams L 1, L2, l 1' and L2', the whole concrete section of the original beam is regarded as the compression zone of the reinforced beam, and the role of the steel bars of the original beam is no longer included, but only regarded as a part of the safety reserve of the reinforced beam. In addition, due to the prestressed reinforcement method, in the process of strengthening beams L 1, L2, l 1' and L2' before pulling out the column, the equivalent load generated by prestress can balance the static load on the original beam, thus eliminating the compressive stress on the column and facilitating the removal of the column. The specific design calculation method is as follows:

1 Loads and internal forces of section beams L 1, L2, l 1' and L2'

1. After removing the bottom four columns, beams L 1, L2, l 1' and L2' can all be simplified as simply supported beams. The standard values of static and live loads on the beam are expressed by D.L., and Figure 4- 18-6 is the calculation diagram of the strengthened beam.

2. The internal force of the control section can be calculated from the calculation diagram of the strengthened beam: bending moment m1= 623 kn m, shear force v1= 215knl1'; The bending moment L2 ′ of the beam is 739 kN·m, and the shear force V2 is 255kN.

The estimation of prestressed reinforcement in the second quarter

1570Ф j15 steel strand is selected for prestressed reinforcement, and JM 15-3 clip anchorage is selected for anchorage. Tension control stress: σcon 1=0.65 for beams L 1, L2 and L 1'? Ptk =1020n/mm ㎡, σcon2=0.56 is the beam L2 ′? Ptk=879N/m㎡。 According to the formula Ap=M×(PPR)∕(0.9σps×hop), the area of prestressed reinforcement is estimated, and the results are listed in Table 4- 18- 1.

Where: PPR-degree of prestress, l.0 is because the bending effect of the original beam reinforcement is not considered;

σPS—— the ultimate stress design value of prestressed reinforcement, which is approximately σ con

Section 3 Calculation of Bearing Capacity of Prestressed Reinforced Beams

Prestress loss σ1; The prestress loss σ2 mainly includes friction loss σ 12, anchorage loss σ 1 1, relaxation loss σ 14 and concrete compression and creep loss σ 15. See table 4- 18-2 for the calculation results.

Ultimate stress design value σps of prestressed reinforcement: calculated according to formula σ PS = (11.2) (σ PE+500-1400β 0), and the results are listed in Table 4- 18-2.

Ms and Vs: According to the mechanical equilibrium conditions of ∑N=0 and ∑M=0, it is easy to obtain the flexural and shear capacity of the strengthened beam. The final result is shown in Table 4- 18-2. Comparison of Ms∕M and vs/ with design bending moment m and design shear force v

Design and calculation of external longitudinal wall and foundation: calculated according to the corresponding reinforcement specification, which is omitted here.

Chapter III Main Construction Techniques

1. Construction sequence: foundation reinforcement → load-bearing wall reinforcement → prestressed beam reinforcement → column demolition. Among them, the key is that the reverse arch behind the prestressed reinforced beam effectively facilitates the removal of the column.

2. Using the high-efficiency inclined drilling technology of impact drill pipe equipped with air compressor, the inclined hole penetrating through the prestressed beam is quickly punched on the ring beam. Prestressed steel strand adopts single thread. Three Ф j15 steel strand bundles are anchored by JM 15-3, and two Ф j15 steel strand bundles are also anchored by JM I 5-3. However, when tensioning, a Ф j15 short steel strand with a length of about 1.2m should be inserted into the third anchor hole. The tensioning method is: two thousand Halls are obliquely symmetrical at both ends of the beam, and one end is tensioned synchronously. In order to reduce the friction loss, a sliding thin iron sheet is placed between the steel strand and the steel box support pad during tensioning.

3. Use thick steel plates (h=35mm) across the beam head to transfer the anchoring force of prestressed tendons to the beam end, so as to avoid the damage of ring beams on both sides of the beam end due to weakening the cross section by drilling. In addition, the anchorage is completely buried in the outer ring beam, which does not affect the appearance of the building facade.

4. As shown in Figure 4- 18-7, the ring beam between the southern end of the beam L 1 and L2 is changed from the original rectangular section (dotted line) of 770mm×400mm to the trapezoidal section, and there is just a bunch of prestressed tendons passing through the ring beam in Ll and L2, and the inclination direction is opposite to the trapezoidal slope direction. This has brought great difficulties to the construction. Due to the tight construction period at that time, the concentrated stress of the anchorage here was very large. After comparing various schemes, high-strength concrete with special formula was poured in the shaded area of Figure 4- 18-7. After the length reaches 60cm.2d, the strength of concrete exceeds C30. When tensioning, this end is used as the anchor end to reduce the peak stress concentration. The construction results show that this measure not only ensures the quality, but also ensures the construction period.

5. Finally, taking advantage of the low tensile strength of concrete, four columns were demolished in half a day by using the prestressed fast breaking technology.

Chapter IV Conclusion

After the renovation, Chunhui Hall has become a spacious lecture hall with a total use area of 200.4㎡, and the effect is very remarkable.

The field test results are as follows: (1) The average total camber of the strengthened beam is 4.5 mm; ; The equivalent load produced by prestress in the tension stage is equal to the tensile force acting on the column, and these results are in good agreement with the design values.

Practice has proved that this prestressed reinforcement method can not only externally reinforce ordinary beams, but also be effectively applied to building renovation and demolition projects, and its popularization and application prospects and comprehensive economic benefits are self-evident.

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