Construction method of multi-layer profile steel enclosing purlin in steel pipe pile cofferdam

Abstract

The invention discloses a construction method of a multi-layer profile steel enclosing purlin in a steel pipe pile cofferdam, which comprises the following steps of: 1) respectively welding and fixing two ends of a main beam on the top surface of the cofferdam, and respectively completing the connection of a perforation jack, a finely-rolled twisted steel bar suspender, a limiting steel bar nut and the profile steel enclosing purlin at the bottommost layer; and (2) the plunger of the perforating jack falls back synchronously for multiple times, and the profile steel circuit purlin on the bottommost layer is driven to descend for multiple times through the finish-rolled twisted steel suspender till the profile steel circuit purlin on the bottommost layer descends in place and is welded and fixed to the lower portion of the cofferdam. And (3) multiple times of circulation are conducted, and positioning of multiple layers of profile steel enclosing purlins is completed one by one. The construction risk and the construction cost are greatly reduced, the profile steel enclosing purlins can be more conveniently positioned in the steel pipe pile cofferdam, and the safety of the steel pipe pile cofferdam is ensured. According to the method, the construction risk is remarkably reduced, the construction convenience is greatly improved, and the construction quality of the profile steel enclosing purlin can be checked more conveniently.

Description

Technical Field

[0001] The invention relates to the technical field of steel structure cofferdam construction, and more specifically, to a construction method of a multi-layer steel purlin used for supporting in a steel pipe pile cofferdam. Background Art

[0002] At present, when constructing a foundation pit on land or shallow tidal flats, the steel pipe pile cofferdam is constructed first, and then a drainage well is built for drainage. Next, the first layer of steel purlins used as internal supports are assembled and welded. Multiple steel purlins are assembled and welded in sections to form the first layer of steel purlins. The first layer of steel purlins are then positioned and welded to the inner side of the upper part of the steel pipe pile cofferdam. Then, earth excavation is carried out. Then, the second to Nth layers of steel purlins are assembled, welded and positioned in the steel pipe pile cofferdam. The construction is repeated many times until the bottom of the foundation pit, and then the bottom concrete construction is carried out.

[0003] The above construction method has the following defects: it is only applicable to construction sites with good hydrogeological conditions and has poor adaptability. When the assembly and welding construction from the second layer of steel purlin to the Nth layer of steel purlin is carried out directly in the foundation pit, the crane operator cannot directly see the operation, which requires a high level of technical skills for the crane operator, is very difficult to operate, has many operation procedures, and lasts for a long time. The construction efficiency is low, the cost is high, and there are major safety hazards. Since the reliable support of the steel pipe pile cofferdam by the multi-layer steel purlin cannot be completed in time, there is a risk of plastic deformation of the steel pipe pile cofferdam under stress and even leakage and sudden overturning. Welding construction workers need to work deep in the foundation pit for a long time, the construction conditions are poor, the welding quality is difficult to guarantee, and acceptance is difficult. Summary of the invention

[0004] The purpose of the invention is to provide a safe and efficient construction method for multi-layer steel purlins in a steel pipe pile cofferdam.

[0005] The present invention is achieved through the following technical solutions:

[0006] A construction method for multi-layer steel purlins in a steel pipe pile cofferdam comprises the following steps:

[0007] 1) At the top of both sides of the completed steel pipe pile cofferdam, cut the tops of several pairs of equally spaced steel pipe piles to the same horizontal position, and use them as pairs of supporting steel pipe piles. Weld and fix several evenly distributed T-shaped rib plates in the upper ports of the pairs of supporting steel pipe piles to complete the preparation for erecting the main beam;

[0008] 2) Weld and fix multiple temporary support brackets at intervals to a temporary fixed position at a distance L above the highest water level on the inner upper part of the steel pipe pile cofferdam to ensure that each layer of steel purlins is not affected by the tide during the assembly process;

[0009] 3) The crane on the operating platform outside one side of the steel pipe pile cofferdam hoists the three-piece I-beam assembly segments that have been welded at the processing site, including the longitudinal beam segments, the cross beam segments and the diagonal bracing segments, into the temporary support brackets in sequence, and the operator stands on the three-piece I-beam assembly segments to complete the assembly and welding of the bottom-layer steel purlin; the support points on the lower side of the bottom-layer steel purlin are respectively against the corresponding first-layer temporary support brackets;

[0010] 4) The lower sides of both ends of the main beam welded by I-beams whose length is greater than the width of the steel pipe pile cofferdam are welded and fixed to the top surfaces of the corresponding supporting steel pipe piles to complete the erection and construction of the main beam;

[0011] 5) Support the bottom of the perforated jack on the upper pads at both ends of the main beam respectively, the upper pads are welded and fixed on the upper sides of the two ends of the main beam respectively, the lower ends of the fine-tied threaded steel bar hanger rods respectively vertically downward through the corresponding perforated jacks, upper pads, upper side of the main beam, lower side of the main beam and the lower pads welded and fixed on the lower side of the lower side of the main beam, and the upper end of the fine-tied threaded steel bar hanger rods passes through the perforated jacks; as the initial position, the upper limit steel bar nuts are respectively screwed on the upper ends of the fine-tied threaded steel bar hangers, and the middle limit steel bar nuts are respectively screwed on the middle part of the fine-tied threaded steel bar hangers, and are located between the upper side of the main beam and the lower side of the main beam; the lower end heads of the fine-tied threaded steel bar hangers respectively vertically pass through the bottom layer of steel purlins, and the lower support steel bar nuts are respectively screwed on the corresponding lower end heads of the fine-tied threaded steel bar hangers, and respectively rest on the lower side of the bottom layer of steel purlins;

[0012] 6) Each perforated jack is filled with oil at the same time, and each plunger rises synchronously by the plunger stroke H, and then the upper limit steel bar nuts on the fine threaded steel bar hanger are screwed down respectively, so that the upper limit steel bar nuts move down and respectively abut against the top surface of the corresponding perforated jack plunger. At this time, the bottom steel purlin remains stationary;

[0013] 7.1) When there are no obstacles under the water surface, according to the welding and fixing position of the bottom steel purlin, weld multiple intervals of the bottom support brackets along the inner side of the bottom of the steel pipe pile cofferdam in the water;

[0014] 7.2) When there are obstacles under the water surface, weld multiple temporary support brackets at intervals along the inner side of the lower part of the steel pipe pile cofferdam on the upper side of the water surface;

[0015] 7.3) Cut off the temporary support brackets on the inner side of the upper part of the steel pipe pile cofferdam respectively, and the bottom steel purlin is supported on the corresponding plunger of the perforated jack through the upper limit steel nut on the upper end of each threaded steel bar hanger; then the perforated jacks return oil at the same time, and the plungers are respectively lowered by a stroke H, and the threaded steel bar hangers synchronously drive the bottom steel purlin to descend by a stroke H, and the middle limit steel bar nut moves down until the lower side of the middle limit steel bar nut is respectively against the lower side of the main beam; then screw up the upper limit steel bar nut, so that the distance between the bottom surface of the upper limit steel bar nut and the top surface of the plunger of the perforated jack is the plunger stroke H, at this time the bottom steel purlin remains stationary, and the weight of the bottom steel purlin has been supported on several main beams through multiple middle limit steel bar nuts;

[0016] 8) Repeat steps 6) and 7.3) multiple times except cutting off the temporary support bracket on the inner side of the upper part of the steel pipe pile cofferdam, so that the bottom steel purlin is lowered multiple times by the plunger stroke H until the bottom steel purlin is stably supported on the bottom support bracket, completing the lowering positioning of the bottom steel purlin in step 7.1) under the working condition of no obstacles under the water surface;

[0017] Or first lower the bottom steel purlin several times by the plunger stroke H, and support the bottom steel purlin in step 7.2) where there is an obstacle under the water surface on the temporary support bracket; then close the connecting valve and start the water pump to pump water until the bottom of the obstacle is exposed, and use a blasting machine to break and level the rock layer that hinders the support construction; after all obstacles are removed, weld multiple intervals of the bottom support brackets along the inner side of the bottom of the steel pipe pile cofferdam according to the welding fixed position of the bottom steel purlin; repeat steps 6) and steps several times. In step 7.3), except for cutting off the temporary support bracket on the inner side of the upper part of the steel pipe pile cofferdam, the bottom steel purlin is lowered by the plunger stroke H several times until the bottom steel purlin is stably supported on the bottom support bracket, thus completing the lowering positioning of the bottom steel purlin in step 7.2) under the working condition of obstacles under the water surface; then, the bottom steel purlin is welded and fixed to the inner side of the steel pipe pile cofferdam, and then the finely tied threaded steel bar hanger, the upper limit steel bar nut, the middle limit steel bar nut and the lower support steel bar nut are respectively disassembled;

[0018] 9) Similarly, after multiple cycles, the welding, lowering and welding of the Nth to third layers of steel purlins are completed in sequence; after each layer of steel purlin construction is completed, the connecting valve is opened to release water into the steel pipe pile cofferdam to submerge the completed layer of steel purlins, so that the water pressure in the steel pipe pile cofferdam is close to balance, ensuring the safety of the steel pipe pile cofferdam;

[0019] 10) At the installation positions of the first layer of steel purlin and the second layer of steel purlin, a plurality of first layer supporting corbels and second layer supporting corbels arranged at intervals are welded along the inner side of the steel pipe pile cofferdam; then the process of step 3) is repeated respectively, and the hoisting of the first layer of steel purlin and each segment of the first layer of steel purlin and the assembly and welding of the first layer of steel purlin and the second layer of steel purlin are completed in turn, and finally, the welding and fixing operations of the two layers of steel purlin and the inner side of the upper part of the steel pipe pile cofferdam are respectively performed, thereby completing the welding and lowering positioning and welding and fixing construction operations of the multi-layer steel purlin in the steel pipe pile cofferdam.

[0020] Furthermore, the three-piece I-beam is formed by welding three layers of I-beams stacked up and down.

[0021] Furthermore, in step 5), vertical ribs are respectively welded between the upper side of the main beam and the lower side of the main beam on both sides of the point where the fine-rolled threaded steel bar hanger rod passes through.

[0022] Furthermore, in step 2), L≥50 cm. In step 6), the plunger stroke H=35-45 cm.

[0023] Furthermore, in step 5), at the initial position, the distance between the upper limit steel bar nut and the top surface of the plunger and the distance between the middle limit steel bar nut and the lower side of the main beam are both less than or equal to the plunger stroke H.

[0024] The present invention does not need to use multiple large-scale lifting equipment to hoist the steel purlin as a whole. It only needs to use multiple through-hole jacks to lower the steel purlin weighing hundreds of tons to the required installation height in the steel pipe pile cofferdam after multiple settlements, which greatly reduces the construction risk and construction cost, and is more convenient for positioning the steel purlin in the steel pipe pile cofferdam. After each layer of steel purlin construction is completed, the present invention releases water into the steel pipe pile cofferdam to submerge the completed layer of steel purlin, and then carries out the construction of the next layer of steel purlin, so that the water pressure inside and outside the steel pipe pile cofferdam is close to balanced, which significantly reduces the construction risk and ensures the safety of the steel pipe pile cofferdam. The present invention allows the welding and forming of each layer of steel purlin to be carried out at the top of the steel pipe pile cofferdam, which greatly improves the convenience of construction and is also more convenient for checking the construction quality of the steel purlin.

[0025] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of several main beams being installed in place;

[0027] Figure 2 for Figure 1 AA section view;

[0028] Figure 3It is a schematic diagram of the connection between the perforated jack, the finely tied threaded steel bar hanger and the bottom steel purlin;

[0029] Figure 4 for Figure 3 A magnified view of part I;

[0030] Figure 5 This is a schematic diagram of the completion of the multi-layer steel purlin construction;

[0031] Figure 6 This is the process diagram for lowering the steel purlin. DETAILED DESCRIPTION

[0032] The present invention is further described below in conjunction with the accompanying drawings and an embodiment of a steel pipe pile cofferdam for the water intake and water intake tunnel project of the second phase expansion project of Zhejiang Energy Station No. 2 Power Plant. The project is located on the coast, and the original terrain is a structure of half bare rock and half silt. The maximum excavation depth of the foundation pit is 24.15m, the maximum head difference is 22.3m, and 5 layers of steel purlin internal supports are set in the steel pipe pile cofferdam, with elevations of +3.3m, -1.7m, -5.7m, -9.7m, and -13.7m respectively.

[0033] like Figure 1 to Figure 6 As shown, this embodiment includes the following steps:

[0034] 1) At the top of both sides of the completed steel pipe pile cofferdam 10, the tops of 6 pairs of equally spaced steel pipe piles are cut to the same horizontal position, respectively serving as paired supporting steel pipe piles 101, and several evenly distributed T-shaped rib plates 102 are welded and fixed in the upper ends of the paired supporting steel pipe piles 101 to improve the supporting strength of the upper ends of the supporting steel pipe piles 101, thereby completing the preparation for erecting the main beam 1.

[0035] 2) A plurality of temporary support brackets 2 arranged at intervals are welded and fixed to a temporary fixed position at a distance L above the highest water level on the inner upper part of the steel pipe pile cofferdam 10 to ensure that each layer of steel purlins is not affected by the tide during the assembly process.

[0036] 3) If Figure 2 As shown, the crawler crane 30 on the operating platform 20 outside the right side of the steel pipe pile cofferdam 10 hoists the three-piece I-beam assembly segments, including the longitudinal beam segments, the cross beam segments and the diagonal bracing segments, which have been welded at the processing site, onto the temporary support bracket 2 in sequence, and the operator stands on the three-piece I-beam assembly segments to complete the assembly and welding of the bottom-layer steel purlin 35. The lower support points of the bottom-layer steel purlin 35 are respectively against the corresponding first-layer temporary support bracket 2.

[0037] 4) The lower sides of both ends of the main beam 1 formed by welding I-beams whose length is greater than the width of the steel pipe pile cofferdam are respectively welded and fixed to the top surfaces of the corresponding supporting steel pipe piles 101 to complete the erection and construction of the main beam 1.

[0038] 5) If Figure 4 As shown, the bottom of the perforated jack 4 is supported on the upper pads 11 at both ends of the main beam 1, and the upper pads 11 are welded and fixed to the upper sides of both ends of the main beam 1. The lower ends of the finely tied threaded steel bar hangers 5 are vertically downward and successively pass through the corresponding perforated jack 4, the upper pads 11, the upper side 12 of the main beam, the lower side 13 of the main beam, and the lower pads 14 welded and fixed to the lower side of the lower side 13 of the main beam, and the upper ends of the finely tied threaded steel bar hangers 5 pass through the perforated jack 4. Figure 4 In the initial position shown, the upper limit steel bar nuts 15 are screwed on the upper ends of the finely tied threaded steel bar hangers 5, and the middle limit steel bar nuts 16 are screwed on the middle of the finely tied threaded steel bar hangers 5, and are located between the upper side 12 of the main beam and the lower side 13 of the main beam. The lower ends of the finely tied threaded steel bar hangers 5 vertically pass through the bottom steel purlin 35, and the lower support steel bar nuts 17 are screwed on the corresponding lower ends of the finely tied threaded steel bar hangers 5, and are respectively against the lower side of the bottom steel purlin 35.

[0039] 6) If Figure 6 As shown in step a of the present invention, each perforated jack 4 is simultaneously fed with oil, and each plunger 41 rises synchronously by a plunger stroke H, and then as shown in FIG. Figure 6 As shown in step b, the upper limit steel bar nuts 13 on the fine-tied threaded steel bar hanger rods 5 are respectively screwed downward, so that the upper limit steel bar nuts 13 are moved downward and respectively abut against the top surfaces of the plungers 41 of the corresponding perforated jacks 4, and the weight of the bottom layer of steel purlins 35 is dispersedly supported on the plungers 41 of the 12 perforated jacks 4 and remains stationary.

[0040] 7.1) When there are no obstacles under the water, Figure 5 As shown, according to the welding and fixing position of the bottom steel purlin 35, a plurality of interval-arranged bottom support brackets 21 are welded respectively along the inner side of the bottom of the steel pipe pile cofferdam 10 in water.

[0041] 7.2) When there are obstacles under the water surface, multiple temporary support brackets 2 are welded at intervals along the inner side of the lower part of the steel pipe pile cofferdam 10 on the upper side of the water surface.

[0042] 7.3) Cut off the temporary support brackets 2 on the inner side of the upper part of the steel pipe pile cofferdam 10, such as Figure 4 and Figure 6 As shown in step b, the bottom steel purlin 35 is supported on the corresponding plunger 41 of the perforated jack 4 through the upper limit steel nut 15 at the upper end of each threaded steel bar hanger rod 5. Figure 6As shown in step c, each perforated jack 4 returns oil at the same time, each plunger 41 drops by a stroke H, each threaded steel bar hanger rod 5 synchronously drives the bottom steel purlin 35 to drop by a stroke H, and the middle limit steel bar nut 16 moves down until the lower side of the middle limit steel bar nut 16 abuts against the lower side of the main beam 13. Then screw the upper limit steel bar nut 15 upwards, so that the distance between the bottom surface of the upper limit steel bar nut 15 and the top surface of the plunger of the perforated jack is the plunger stroke H. At this time, the bottom steel purlin 35 remains stationary, and the weight of the bottom steel purlin 35 has been supported on the 6 main beams 1 by 12 middle limit steel bar nuts 16.

[0043] 8) Repeat steps 6) and 7.3) several times except cutting off the temporary support bracket 2 on the inner side of the upper part of the steel pipe pile cofferdam 10, so that the bottom steel purlin 35 is repeatedly lowered by the plunger stroke H until Figure 5 As shown, the bottom steel purlin 35 is stably supported on the bottom support corbel 21, completing step 7.1) the lowering and positioning of the bottom steel purlin 35 under the obstacle-free working condition under the water surface.

[0044] Or first lower the bottom steel purlin 35 by the plunger stroke H several times, and support the bottom steel purlin 35 of step 7.2) under the condition of obstacles under the water surface on the temporary support bracket 2. Then close the connecting valve, start the water pump to pump water until the bottom of the obstacle is exposed, and use the blasting machine to break and level the rock layer that hinders the support construction; after all obstacles are removed, according to the welding fixed position of the bottom steel purlin 35, a plurality of intervals are welded along the bottom inner side of the steel pipe pile cofferdam 10. Repeat steps 6) and 7.3) several times except cutting off the temporary support bracket 2 on the upper inner side of the steel pipe pile cofferdam 10, so that the bottom steel purlin 35 is lowered by the plunger stroke H several times, until the bottom steel purlin 35 is stably supported on the bottom support bracket 21, and the lowering positioning of the bottom steel purlin 35 of step 7.2) under the condition of obstacles under the water surface is completed. Next, the welding and fixing operation of the bottom steel purlin 35 and the inner side of the steel pipe pile cofferdam 10 is carried out, and then the fine-tied threaded steel bar hanger 5, the upper limit steel bar nut 15, the middle limit steel bar nut 16 and the lower support steel bar nut 17 are disassembled respectively.

[0045] 9) Similarly, after multiple cycles, the fourth layer of steel purlin 34 and the third layer of steel purlin 33 are welded, lowered into place, and welded and fixed to the inner side of the steel pipe pile cofferdam 10. After each layer of steel purlin construction is completed, the connecting valve is opened to release water into the steel pipe pile cofferdam 10 to submerge the completed layer of steel purlin, so that the water pressure in the steel pipe pile cofferdam is close to balance, ensuring the safety of the steel pipe pile cofferdam 10.

[0046] 10) Figure 5As shown, at the installation positions of the first layer of steel purlin 31 and the second layer of steel purlin 32, a plurality of first layer support brackets 211 and second layer support brackets 212 are welded at intervals along the inner side of the steel pipe pile cofferdam 10. Then, the process of step 3) is repeated respectively, and the hoisting of each segment of the first layer of steel purlin 31 and the first layer of steel purlin 32 and the assembly and welding of the first layer of steel purlin 31 and the second layer of steel purlin 32 are completed in sequence, and finally, the first layer of steel purlin 31 and the second layer of steel purlin 32 are welded and fixed to the inner side of the upper part of the steel pipe pile cofferdam 10, thereby completing the welding, lowering, positioning and welding and fixing of the five layers of steel purlins in the steel pipe pile cofferdam 10.

[0047] like Figure 2 and Figure 3 As shown, the three-piece I-beam used for the steel purlin is made of three layers of I-beams laid down and welded together, and has high strength.

[0048] like Figure 4 As shown, in step 5), vertical ribs 18 are respectively welded between the upper side 12 of the main beam and the lower side 13 of the main beam on both sides of the point where the fine-rolled threaded steel bar hanger rod 5 passes through, so as to prevent the main beam 1 from plastic deformation after being loaded.

[0049] In step 2), L≥50 cm. In step 6), plunger stroke H=40 cm.

[0050] like Figure 4 and Figure 6 As shown, in step 5), at the initial position, the distance between the lower side of the upper limit steel bar nut 15 and the top surface of the plunger 41 and the distance between the lower side of the middle limit steel bar nut 16 and the lower side edge 13 of the main beam are both smaller than the plunger stroke H, ensuring that within the effective stroke of the plunger 41, the 12 plungers 41 and the 12 middle limit steel bar nuts 16 can respectively effectively withstand the steel purlin weighing hundreds of tons, thereby improving the reliability of the present invention.

[0051] In addition to the above embodiments, the present invention may also have other implementation modes. All technical solutions formed by equivalent replacement and equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A construction method for multi-layer steel purlins in a steel pipe pile cofferdam, characterized in that: The following steps are involved: 1) At the top of both sides of the completed steel pipe pile cofferdam, cut the tops of several pairs of equally spaced steel pipe piles to the same horizontal position, and use them as pairs of supporting steel pipe piles. Weld and fix several evenly distributed T-shaped rib plates in the upper ports of the pairs of supporting steel pipe piles to complete the preparation for erecting the main beam; 2) Weld and fix multiple temporary support brackets at intervals to a temporary fixed position at a distance L above the highest water level on the inner upper part of the steel pipe pile cofferdam to ensure that each layer of steel purlins is not affected by the tide during the assembly process; 3) The crane on the operating platform outside one side of the steel pipe pile cofferdam hoists the three-piece I-beam assembly segments that have been welded at the processing site, including the longitudinal beam segments, the cross beam segments and the diagonal bracing segments, into the temporary support brackets in sequence, and the operator stands on the three-piece I-beam assembly segments to complete the assembly and welding of the bottom-layer steel purlin; the support points on the lower side of the bottom-layer steel purlin are respectively against the corresponding first-layer temporary support brackets; 4) The lower sides of both ends of the main beam welded by I-beams whose length is greater than the width of the steel pipe pile cofferdam are welded and fixed to the top surfaces of the corresponding supporting steel pipe piles to complete the erection and construction of the main beam; 5) Support the bottom of the perforated jack on the upper pads at both ends of the main beam respectively, the upper pads are welded and fixed on the upper sides of the two ends of the main beam respectively, the lower ends of the fine-tied threaded steel bar hanger rods respectively vertically downward through the corresponding perforated jacks, upper pads, upper side of the main beam, lower side of the main beam and the lower pads welded and fixed on the lower side of the lower side of the main beam, and the upper end of the fine-tied threaded steel bar hanger rods passes through the perforated jacks; as the initial position, the upper limit steel bar nuts are respectively screwed on the upper ends of the fine-tied threaded steel bar hangers, and the middle limit steel bar nuts are respectively screwed on the middle part of the fine-tied threaded steel bar hangers, and are located between the upper side of the main beam and the lower side of the main beam; the lower end heads of the fine-tied threaded steel bar hangers respectively vertically pass through the bottom layer of steel purlins, and the lower support steel bar nuts are respectively screwed on the corresponding lower end heads of the fine-tied threaded steel bar hangers, and respectively rest on the lower side of the bottom layer of steel purlins; 6) Each perforated jack is filled with oil at the same time, and each plunger rises synchronously by the plunger stroke H, and then the upper limit steel bar nuts on the fine threaded steel bar hanger are screwed down respectively, so that the upper limit steel bar nuts move down and respectively abut against the top surface of the corresponding perforated jack plunger. At this time, the bottom steel purlin remains stationary; 7.1) When there are no obstacles under the water surface, according to the welding and fixing position of the bottom steel purlin, weld multiple intervals of the bottom support brackets along the inner side of the bottom of the steel pipe pile cofferdam in the water; 7.2) When there are obstacles under the water surface, weld multiple temporary support brackets at intervals along the inner side of the lower part of the steel pipe pile cofferdam on the upper side of the water surface; 7.3) Cut off the temporary support brackets on the inner side of the upper part of the steel pipe pile cofferdam respectively, and the bottom steel purlin is supported on the corresponding plunger of the perforated jack through the upper limit steel nut on the upper end of each threaded steel bar hanger; then the perforated jacks return oil at the same time, and the plungers are respectively lowered by a stroke H, and the threaded steel bar hangers synchronously drive the bottom steel purlin to descend by a stroke H, and the middle limit steel bar nut moves down until the lower side of the middle limit steel bar nut is respectively against the lower side of the main beam; then screw up the upper limit steel bar nut, so that the distance between the bottom surface of the upper limit steel bar nut and the top surface of the plunger of the perforated jack is the plunger stroke H, at this time the bottom steel purlin remains stationary, and the weight of the bottom steel purlin has been supported on several main beams through multiple middle limit steel bar nuts; 8) Repeat steps 6) and 7.3) multiple times except cutting off the temporary support bracket on the inner side of the upper part of the steel pipe pile cofferdam, so that the bottom steel purlin is lowered multiple times by the plunger stroke H until the bottom steel purlin is stably supported on the bottom support bracket, completing the lowering positioning of the bottom steel purlin in step 7.1) under the working condition of no obstacles under the water surface; Or first lower the bottom steel purlin several times by the plunger stroke H, and support the bottom steel purlin in step 7.2) where there is an obstacle under the water surface on the temporary support bracket; then close the connecting valve and start the water pump to pump water until the bottom of the obstacle is exposed, and use a blasting machine to break and level the rock layer that hinders the support construction; after all obstacles are removed, weld multiple intervals of the bottom support brackets along the inner side of the bottom of the steel pipe pile cofferdam according to the welding fixed position of the bottom steel purlin; repeat steps 6) and steps several times. In step 7.3), except for cutting off the temporary support bracket on the inner side of the upper part of the steel pipe pile cofferdam, the bottom steel purlin is lowered by the plunger stroke H several times until the bottom steel purlin is stably supported on the bottom support bracket, thus completing the lowering positioning of the bottom steel purlin in step 7.2) under the working condition of obstacles under the water surface; then, the bottom steel purlin is welded and fixed to the inner side of the steel pipe pile cofferdam, and then the finely tied threaded steel bar hanger, the upper limit steel bar nut, the middle limit steel bar nut and the lower support steel bar nut are respectively disassembled; 9) Similarly, after multiple cycles, the welding, lowering and welding of the Nth to third layers of steel purlins are completed in sequence; after each layer of steel purlin construction is completed, the connecting valve is opened to release water into the steel pipe pile cofferdam to submerge the completed layer of steel purlins, so that the water pressure in the steel pipe pile cofferdam is close to balance, ensuring the safety of the steel pipe pile cofferdam; 10) At the installation positions of the first layer of steel purlin and the second layer of steel purlin, a plurality of first layer supporting corbels and second layer supporting corbels arranged at intervals are welded along the inner side of the steel pipe pile cofferdam; then the process of step 3) is repeated respectively, and the hoisting of the first layer of steel purlin and each segment of the first layer of steel purlin and the assembly and welding of the first layer of steel purlin and the second layer of steel purlin are completed in turn, and finally, the welding and fixing operations of the two layers of steel purlin and the inner side of the upper part of the steel pipe pile cofferdam are respectively performed, thereby completing the welding and lowering positioning and welding and fixing construction operations of the multi-layer steel purlin in the steel pipe pile cofferdam.

2. The construction method of multi-layer steel purlin in steel pipe pile cofferdam according to claim 1, characterized in that: The three-piece I-beam is formed by welding three layers of I-beams stacked up and down.

3. The construction method of multi-layer steel purlin in steel pipe pile cofferdam according to claim 1, characterized in that: In step 5), vertical ribs are respectively welded between the upper side of the main beam and the lower side of the main beam on both sides of the point where the fine-rolled threaded steel bar hanger passes.

4. The construction method of multi-layer steel purlin in steel pipe pile cofferdam according to claim 1, characterized in that: In step 2), L≥50cm.

5. The construction method of multi-layer steel purlin in steel pipe pile cofferdam according to claim 1, characterized in that: Step 6) The plunger stroke H = 35 ~ 45cm.

6. The construction method of multi-layer steel purlin in steel pipe pile cofferdam according to claim 1, characterized in that: Step 5) At the initial position, the distance between the upper limit steel bar nut and the top surface of the plunger and the distance between the middle limit steel bar nut and the lower side of the main beam are both less than the plunger stroke H.

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