Construction method for backfilling superposed beam at first-layer rigid floor slab
Through phased construction and prefabricated beam lateral retaining wall technology, the quality and efficiency of backfill construction in basement-free buildings are solved, the construction quality and efficiency are improved, the structural stress performance is optimized, and it is suitable for a variety of building scenarios.
Patent Information
- Application Number
- CN202510530688.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-25
- Publication Date
- 2025-07-11
AI Technical Summary
Traditional construction methods In buildings without basements and rigid floors on the first floor, the quality of backfill soil construction is difficult to ensure, with quality hazards, low efficiency and high cost, which affects the structural stress and the quality of steel joints.
The staged construction method is adopted, including casting columns to the top elevation and overlapping layer of prefabricated beams and casting of rigid floor slabs. Prefabricated beams are used as lateral retaining walls to ensure the compactness of backfill soil and the integrity of structural connections, and the use of micro-expanded concrete and interface adhesives to enhance the bonding strength.
It improves construction quality and efficiency, reduces steel bar damage, optimizes structural stress performance, reduces cost and energy consumption, and is suitable for a variety of building scenarios.
Smart Images
Figure CN120291654A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of building construction, and particularly relates to a construction method for a backfill soil composite beam at the rigid floor of the first floor, which is particularly applicable to building or structural construction projects without a basement but with a rigid floor on the first floor. Background Art
[0002] With the acceleration of the urbanization process and the rapid development of the construction industry, more and more non-traditional design schemes are encountered. In the foundation treatment process of general buildings, after the foundation earthwork is excavated, the construction of the foundation slab can be carried out on the ground. However, the traditional construction method will face the problem that the quality of the backfill soil cannot be guaranteed, or the quality of the completed structure will be affected due to the backfill soil. In the face of the design scheme of combining no basement and a rigid floor on the first floor, to ensure the construction quality of the backfill soil under the floor, the existing traditional methods are as follows:
[0003] 1. First, build brick molds on both sides of the structural beams on the first floor to ensure that the brick molds are used as retaining walls to define the backfill range during backfill. After the backfill is qualified, then pour the beams and rigid floor on the first floor to complete the construction of this part. The problem with this construction method is that the construction efficiency of the brick molds is low and the cost is high, which is not applicable to the current industry background that pursues lean construction as a whole.
[0004] 2. First, backfill the soil in a large area, then measure and set out the positions of the beams, re-excavate the soil here, and then construct the beams and rigid floor on the first floor. The problems with this construction method are as follows: First, a construction joint is artificially left at the beam-column connection node (at the complex stress area of the column top and beam bottom, or at the beam-column joint), which has a greater impact on the structural stress; second, the large-area backfill soil will affect the quality of the steel bar joints of the columns or the steel bar joints of the beams; third, the process of soil excavation - backfill - re-excavation is inefficient, and there are also cost problems.
[0005] 3. First, complete the construction of concrete components except for the rigid floor, then backfill the soil, and finally complete the construction of the rigid floor. The problem with this construction method is that the combination of the floor and the beams on the first floor is affected, the steel bar joints of the floor interfere with the backfill soil construction, and the backfill soil construction has a serious impact on the quality of the steel bar joints. Similarly, if the method of post-implanting joint steel bars is used, the anchorage level between the floor and the beam will still be reduced.
[0006] Due to the above reasons, the traditional construction method will have greater quality hidden dangers and is not economically feasible when dealing with the design scheme of combining no basement and a rigid floor on the first floor. Therefore, it is urgent to develop a more scientific and efficient construction method to deal with it. Summary of the Invention
[0007] In view of the above-mentioned deficiencies in the traditional construction methods in the prior art, the purpose of the present invention is to provide a method for constructing composite beams with backfill soil at the rigid floor slab on the first floor. The method has a reasonable design, solves the construction problem of backfill soil under the rigid floor slab, improves the construction quality, efficiency and economy, and is applicable to the field of building construction technology, particularly to buildings or structural construction projects without a basement but with a rigid floor slab on the first floor.
[0008] In order to achieve the above object, the present invention adopts the following technical solutions:
[0009] A method for constructing a composite beam by backfilling soil at the first-floor rigid floor slab, the method comprising two stages of construction, namely: the first stage: pouring the column to the top elevation of the precast beam; the second stage: completing the pouring of the composite layer and the rigid floor slab;
[0010] 1) The first stage includes: steel bar binding, formwork support, and concrete pouring. The construction process of steel bar binding is as follows: after the independent foundation construction is completed, the column steel bars are bound, and the precast beam steel bars are bound simultaneously. The main bars of the column and the reserved steel bars of the column are connected with straight threaded sleeves. According to the design requirements of the "Concrete Structure Design Code", the length of the longitudinal bars of the precast beam extending into the column width should meet the standard anchoring requirements. The horizontal projection length of the longitudinal bars of the precast beam is ≥0.4l abE， l abE is the basic seismic anchorage length of the tensile reinforcement, and the vertical length of the projected downward bending part of the longitudinal reinforcement of the precast beam is ≥15d, where d is the diameter of the longitudinal reinforcement of the beam; the construction process of formwork support is: use wooden formwork to support the column formwork, use wooden formwork to install the side formwork of the precast beam, and the allowable deviation of the beam top elevation is ±3mm; the construction process of concrete pouring is: pour concrete to a position 10mm above the precast beam top elevation, and complete the continuous pouring of concrete in the beam-column node area and the precast beam;
[0011] 2) The second stage includes: backfill soil treatment, joint surface construction, superposition layer and rigid floor pouring; the construction process of backfill soil treatment is as follows: backfill soil in layers, thickness of each layer of soil ≤300mm, mechanical compaction to a density ≥95%, backfill soil to the design elevation and then level it, lay the rigid floor bottom cushion according to the requirements of the construction drawing, and use prefabricated beams as lateral retaining walls during the backfill process to avoid lateral movement of the soil, and simultaneously monitor the verticality of the columns to prevent deviation; the construction process of joint surface construction is as follows: mechanical roughening of the top surface of the prefabricated beam, the roughening depth ≥6mm, the exposed area of aggregate ≥70%, and the application of interface adhesive, the amount of adhesive ≥0.4kg / m 2 The construction process of casting the superimposed layer and the rigid floor slab is as follows: tie the shear steel mesh of the superimposed layer according to the construction requirements of the construction plan, and firmly tie it with the reserved steel bars of the prefabricated beams. The superimposed layer and the floor slab use micro-expansive concrete with the same strength grade of C30 or above and cast them at one time, vibrate them to make them dense, and cover them with film for maintenance.
[0012] As a further optimization of the above scheme, in the first stage, the second stage can be carried out only when the following conditions are met at the same time:
[0013] ① The elevation deviation of the column top is ≤±3mm, and the axis displacement is ≤5mm; the verticality deviation of the column is ≤H / 1000, and ≤15mm, where H is the column height; ② The bottom formwork can be removed only when the compressive strength of the concrete is ≥15MPa after 3 days of curing, and the compressive strength is ≥75% of the design value after 7 days, and the compressive strength of the concrete is ≥ the design value after 28 days.
[0014] As a further optimization of the above scheme, in the second phase, the second phase construction can be completed only when the following conditions are met at the same time:
[0015] ①Use every 500m 2 The density of backfill soil is tested by the 6-point ring knife method, and the qualified rate of the test needs to be equal to 100%; ② The compaction coefficient of backfill soil is ≥0.95, and the moisture content is controlled within the range of ±2% of the optimal moisture content; ③ The thickness deviation of the superimposed layer is ≤±5mm, and the thickness deviation of the steel bar protective layer is ≤+3mm / -2mm; the thickness deviation of the floor slab is ≤+8mm / -5mm, and the surface flatness is ≤4mm / 2m.
[0016] As a further optimization of the above scheme, the second stage also includes: using a total station or a level to monitor the composite beams and rigid floor slabs in real time, requiring the maximum deflection of the composite beams and rigid floor slabs to be ≤L / 400 and ≤15mm, where L is the span of the beam; in the first and second stages, the concrete durability needs to be tested after pouring, and the chloride ion diffusion coefficient of the concrete needs to be ≤3.0×10 -12 m 2 / s; the carbonation depth of concrete detected by phenolphthalein reagent method needs to be ≤5mm.
[0017] As a further optimization of the above scheme, the height of the precast beam should meet at least 50% of the sum of the height of the precast beam and the thickness of the superimposed layer, wherein the thickness of the superimposed layer is at least greater than 100 mm, the concrete strength grade is not lower than C30 and not lower than the original construction design requirements, the stirrups of the precast beam should all extend into the superimposed layer, and the length of the straight section of each limb intervening in the superimposed layer should be greater than 10d, where d is the diameter of the stirrup.
[0018] The construction method of the composite beam with backfill soil at the rigid floor slab of the first floor of the present invention has the following beneficial effects:
[0019] 1. Solved the quality defects caused by traditional construction methods
[0020] Through the phased construction of composite beams, problems such as contamination of steel bar joints and improper placement of construction joints caused by the cross-continuity of backfill and structural construction in traditional methods are avoided, the weak links in the structure are fundamentally eliminated, and the integrity and safety of beam-column nodes and floor slab connections are ensured.
[0021] 2. Significantly improve construction efficiency and economic benefits
[0022] The process of prefabricated beams being manufactured in advance and the superimposed layers being poured later reduces redundant processes such as repeated excavation and backfilling of earth, brick formwork and masonry, and significantly shortens the construction period. It also saves material and labor costs, which is in line with the concept of lean construction.
[0023] 3. Ensure the quality of backfill construction
[0024] The backfill soil is completed and compacted before the floor construction. There is no need to rely on temporary retaining structures (brick molds). The lateral support of the completed structure can be fully utilized to ensure that the backfill density meets the standards and reduce the risk of subsequent settlement.
[0025] 4. Optimize structural stress performance
[0026] Through the combined design of prefabrication and cast-in-place of composite beams, combined with mechanical verification, it is ensured that the bending and shear bearing capacity of the beams meet the original design requirements, while avoiding the hidden danger of reduced structural bearing capacity caused by leaving construction joints in complex stress areas in traditional methods.
[0027] 5. Highly collaborative construction process
[0028] The phased construction logic of prefabrication → backfill → pouring of the overlapping layer is clear, the process is closely connected, and cross-operation interference is reduced, which facilitates on-site management and quality control. It is especially suitable for rigid floor construction scenarios in buildings without basements.
[0029] 6. Wide application scope and strong scalability
[0030] This method is not only applicable to buildings without basements, but can also be extended to other special design scenarios where backfill soil is required under the structure (such as semi-basements, equipment floors, etc.), and has high engineering applicability.
[0031] 7. Significant environmental benefits
[0032] Reduce the number of earth excavation and backfilling, reduce dust pollution and energy consumption, and meet the requirements of green construction. BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Attached Figure 1 It is a structural schematic diagram of the first stage of the construction method of the backfill composite beam at the first-floor rigid floor slab of the present invention.
[0034] Attached Figure 2It is a structural schematic diagram of the second stage of the construction method of the backfill composite beam at the first-floor rigid floor slab of the present invention. DETAILED DESCRIPTION
[0035] The following is combined with Figure 1-2 The construction method of the backfill composite beam at the first floor rigid floor of the present invention is described in detail.
[0036] A method for constructing a composite beam by backfilling soil at the first-floor rigid floor, characterized in that the method comprises two stages of construction, namely: the first stage: pouring the column to the top elevation of the precast beam; the second stage: completing the pouring of the composite layer and the rigid floor;
[0037] 1) The first stage includes: steel bar binding, formwork support, and concrete pouring. The construction process of steel bar binding is as follows: after the independent foundation construction is completed, the column steel bars are bound, and the precast beam steel bars are bound simultaneously. The main bars of the column and the reserved steel bars of the column are connected with straight threaded sleeves. According to the design requirements of the "Concrete Structure Design Code", the length of the longitudinal bars of the precast beam extending into the column width should meet the standard anchoring requirements. The horizontal projection length of the longitudinal bars of the precast beam is ≥0.4l abE， l abE is the basic seismic anchorage length of the tensile reinforcement, and the vertical length of the projected downward bending part of the longitudinal reinforcement of the precast beam is ≥15d, where d is the diameter of the longitudinal reinforcement of the beam; the construction process of formwork support is: use wooden formwork to support the column formwork, use wooden formwork to install the side formwork of the precast beam, and the allowable deviation of the beam top elevation is ±3mm; the construction process of concrete pouring is: pour concrete to a position 10mm above the precast beam top elevation, and complete the continuous pouring of concrete in the beam-column node area and the precast beam;
[0038] 2) The second stage includes: backfill soil treatment, joint surface construction, superposition layer and rigid floor pouring; the construction process of backfill soil treatment is as follows: backfill soil in layers, thickness of each layer of soil ≤300mm, mechanical compaction to a density ≥95%, backfill soil to the design elevation and then level it, lay the rigid floor bottom cushion according to the requirements of the construction drawing, and use prefabricated beams as lateral retaining walls during the backfill process to avoid lateral movement of the soil, and simultaneously monitor the verticality of the columns to prevent deviation; the construction process of joint surface construction is as follows: mechanical roughening of the top surface of the prefabricated beam, the roughening depth ≥6mm, the exposed area of aggregate ≥70%, and the application of interface adhesive, the amount of adhesive ≥0.4kg / m 2 The construction process of casting the superimposed layer and the rigid floor slab is as follows: tie the shear steel mesh of the superimposed layer according to the construction requirements of the construction plan, and firmly tie it with the reserved steel bars of the prefabricated beams. The superimposed layer and the floor slab use micro-expansive concrete with the same strength grade of C30 or above and cast them at one time, vibrate them to make them dense, and cover them with film for maintenance.
[0039] In the first stage, the second stage can only be carried out when the following conditions are met at the same time;
[0040] ① The elevation deviation of the column top is ≤±3mm, and the axis displacement is ≤5mm; the verticality deviation of the column is ≤H / 1000, and ≤15mm, where H is the column height; ② The bottom formwork can be removed only when the compressive strength of the concrete is ≥15MPa after 3 days of curing, and the compressive strength is ≥75% of the design value after 7 days, and the compressive strength of the concrete is ≥ the design value after 28 days.
[0041] In the second phase, the construction of the second phase can be completed only when the following conditions are met at the same time:
[0042] ①Use every 500m 2 The density of backfill soil is tested by the 6-point ring knife method, and the qualified rate of the test needs to be equal to 100%; ② The compaction coefficient of backfill soil is ≥0.95, and the moisture content is controlled within the range of ±2% of the optimal moisture content; ③ The thickness deviation of the superimposed layer is ≤±5mm, and the thickness deviation of the steel bar protective layer is ≤+3mm / -2mm; the thickness deviation of the floor slab is ≤+8mm / -5mm, and the surface flatness is ≤4mm / 2m.
[0043] The second stage also includes: using a total station or a level to monitor the composite beams and rigid floor slabs in real time, requiring the maximum deflection of the composite beams and rigid floor slabs to be ≤L / 400 and ≤15mm, where L is the span of the beam; after the concrete is poured in the first and second stages, the durability of the concrete needs to be tested, and the chloride ion diffusion coefficient of the concrete needs to be ≤3.0×10 -12 m 2 / s; the carbonation depth of concrete detected by phenolphthalein reagent method needs to be ≤5mm.
[0044] The height of the precast beam should be at least 50% of the sum of the height of the precast beam and the thickness of the superimposed layer, wherein the thickness of the superimposed layer is at least greater than 100mm, the concrete strength grade is not less than C30 and not less than the original construction design requirements, the stirrups of the precast beam should all extend into the superimposed layer, and the length of the straight section of each limb intervening in the superimposed layer shall be greater than 10d, where d is the diameter of the stirrup.
[0045] Taking a certain construction project as an example, it is further confirmed that the above technical solution of the present invention can achieve beneficial effects.
[0046] (1) Overview of the original construction plan
[0047] ① According to the design blueprint requirements, backfill soil is required under the first rigid floor slab. The project department initially decided to carry out the construction work according to the following plan: concrete frame beam and column construction → reserved rigid floor slab steel bars → backfill soil and compact → floor slab cushion construction → rigid floor slab construction.
[0048] ②In the small-scale test area, it was found that the reserved floor slab steel bars seriously interfered with the backfill operation. Some of the reserved steel bars were damaged by the compaction machinery and needed to be re-implanted. Some of the floor slab steel bars were curled up. After the lime soil backfill was completed, the curled-up steel bars were re-bent to the designed position, which had a significant impact on the quality of the steel bars. During the specific process,
[0049] a. Construction of concrete frame beams and columns: First, complete the overall pouring of the first-floor frame beams and columns, and the beam top elevation is flush with the designed rigid floor slab top elevation.
[0050] b. Reserved rigid floor slab steel bars: Embed double-layer and two-way floor slab steel bars (Φ10@150) at the beam top.
[0051] c. Backfill construction: Backfill 3:7 lime soil in layers (each layer thickness ≤ 300mm), and compact it with an 18t vibrating roller (the number of rolling passes ≥ 6). After backfilling to the designed elevation, lay a 100mm-thick C15 concrete cushion.
[0052] d. Rigid floor slab construction: Bind the additional steel bars on the upper part of the floor slab (Φ12@150), and lap them with the reserved steel bars (the lap length ≥ 500mm). Pour the C30 concrete floor slab integrally (thickness 120mm), and cure it with film for ≥ 7 days.
[0053] (2) Contradictions and quality defects
[0054] The exposed length of the reserved steel bars generally exceeds 1m, forming a dense horizontal obstacle (spacing 150mm), which seriously hinders the operation space of the backfill machinery. The rolling of the roller causes 12% of the reserved steel bars to be bent and deformed (the radius of curvature ≤ 2d), and 36 steel bars are broken in the test area (200m 2 )). After backfilling, 23% of the steel bars are skewed by 30° - 60°, and manual hammering is required for resetting, resulting in a rib wear rate ≥ 40%. The repeated bending of the steel bars affects the mechanical properties and causes the yield strength of the steel bars to decrease. The broken steel bars need to be post-implanted (drilling depth 15d = 150mm, injecting glue for anchoring), and the tensile and pull-out performance is lower than the anchoring strength of the embedded steel bars.
[0055] (3) Optimization of the two-stage segmented construction process
[0056] After discovering the problems in the traditional construction plan, the technical solution of the present invention is adopted and the construction is carried out according to the following steps:
[0057] ①The first stage (coordinated construction of columns and precast beams):
[0058] Adopt the process of pouring the column body to the elevation of the top of the precast beam. Through the collaborative force design of the precast beam and the cast-in-place concrete column, the problem of spatial conflict between the backfill soil and the rigid floor construction is solved, and the construction progress of this step is smoothly promoted. During the construction, the steel bar binding of the precast beam and the stirrup densification (spacing ≤ 100mm) in the core area of the column top node are completed synchronously to form a rigid support system.
[0059] ② The second stage (construction of the composite layer and the rigid floor):
[0060] After the backfill soil, the composite layer is poured. Through the triple strengthening technology of chiseling + interface agent + slightly expanded concrete, the interface shear strength ≥ 2.0 MPa, which is 33% higher than the traditional process.
[0061] (4) Comprehensive benefits
[0062] The backfill boundary is formed through the precast beam to eliminate the problem of exposed reserved steel bars. The steel bar damage rate is reduced from 8.7% in the original plan to 0.5%, and the repair amount of the later implanted steel bars is reduced by 98%. The comprehensive benefits are significantly improved, realizing the overall improvement of quality, cost, construction period and safety.
[0063] The above are only the embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification of the present invention, or directly or indirectly applied in other related technical fields, shall be included in the patent protection scope of the present invention by the same token.
Claims
1. A construction method for a backfill soil composite beam at the first-floor rigid floor slab, characterized in that, This construction method includes two-stage construction, namely: The first stage: Pour the column to the elevation of the top of the precast beam; The second stage: Complete the casting of the composite layer and the rigid floor slab; 1) The first stage includes: steel bar binding, formwork erection, and concrete pouring. Among them, the construction process of steel bar binding is as follows: After completing the construction of the independent foundation, bind the column steel bars and simultaneously bind the precast beam steel bars. The main bars of the column are connected to the reserved steel bars of the column body by straight thread sleeves. According to the design requirements of the "Code for Design of Concrete Structures", the length of the longitudinal bars of the precast beam extending into the column width range should meet the specification anchorage requirements. Among them, the horizontal projection length of the longitudinal bars of the precast beam ≥ 0.4l abE， l abE is the basic seismic anchorage length of the tension steel bar, and the vertical section length of the downward bending part of the longitudinal bars of the precast beam ≥ 15d, where d is the diameter of the beam longitudinal bars; the construction process of formwork erection is as follows: Use wooden formwork to erect the column formwork and use wooden formwork to install the side formwork of the precast beam. The allowable deviation of the beam top elevation is ±3mm; the construction process of concrete pouring is as follows: Pour the concrete to a position 10mm above the top elevation of the precast beam, and complete the continuous pouring of the concrete in the beam-column joint area and the precast beam; 2) The second stage includes: backfill treatment, joint surface construction, casting of the composite layer and rigid floor slab; among which, the construction process of the backfill treatment is as follows: backfill the soil to the designed elevation according to the backfill requirements of laying lime soil in layers, with the thickness of each layer of lime soil ≤ 300 mm and mechanically tamping it to a compactness ≥ 95%, level it after backfilling, lay the bottom cushion layer of the rigid floor slab according to the requirements of the construction drawing. During the backfilling process, use the precast beam as a lateral retaining wall to prevent the lateral displacement of the soil mass, and synchronously monitor the verticality of the column to prevent deviation; the construction process of the joint surface construction is as follows: mechanically roughen the top surface of the precast beam, with the roughening depth ≥ 6 mm and the exposed area of the aggregate ≥ 70%, apply the interface adhesive, and the dosage of the adhesive ≥ 0.4 kg / m 2 , the construction process of casting the composite layer and rigid floor slab is as follows: tie the shear-resistant steel bar mesh of the composite layer according to the construction requirements of the construction plan, tie it firmly with the reserved steel bars of the precast beam, use micro-expansion concrete with the same strength grade above C30 for the composite layer and the floor slab and cast it at one time, vibrate it densely and cure it by covering with a film.
2. The construction method of the backfill soil composite beam at the first-floor rigid floor slab according to claim 1, characterized in that: In the first stage, the second stage can be carried out only when the following conditions are met simultaneously; ① The deviation of the column top elevation ≤ ±3mm, the axial displacement ≤ 5mm; the deviation of the column verticality ≤ H / 1000 and ≤ 15mm, where H is the column height; ② The bottom formwork can be removed only when the compressive strength of the concrete reaches ≥ 15MPa after 3 days of curing, ≥ 75% of the design value after 7 days, and ≥ the design value after 28 days of the concrete.
3. The construction method of the backfill soil composite beam at the first-floor rigid floor slab according to claim 1, characterized in that: In the second stage, the second-stage construction can be completed only when the following conditions are met simultaneously: ① Use the ring knife method to measure 6 points every 500m to detect the compactness of the backfill soil, and the qualified rate of the detection needs to be equal to 100%; ② The compaction coefficient of the backfill soil ≥ 0.95, and the water content is controlled within the range of the optimal water content ± 2%; ③ The deviation of the thickness of the superimposed layer ≤ ± 5mm, the deviation of the steel bar protection layer thickness ≤ + 3mm / - 2mm; the deviation of the floor slab thickness ≤ + 8mm / - 5mm, and the surface flatness ≤ 4mm / 2m. 2 4. The construction method of the backfill soil composite beam at the first-floor rigid floor slab according to claim 1, characterized in that: The second stage further includes: using a total station or a level to conduct real-time monitoring on the composite beam and the rigid floor slab, and requiring that the maximum deflection of the composite beam and the rigid floor slab ≤ L / 400 and ≤ 15 mm, where L is the beam span; after the concrete is poured in the first stage and the second stage, durability testing of the concrete is required. When using the electric flux method to detect the chloride ion diffusion coefficient of the concrete, it needs to be ≤ 3.0×10 -12 m 2 / s; when using the phenolphthalein reagent method to detect the carbonation depth of the concrete, it needs to be ≤ 5 mm.
5. A construction method for a backfilled soil composite beam at the first-floor rigid floor slab according to claim 1, characterized in that: The height dimension of the precast beam should meet at least more than 50% of the sum of the height of the precast beam and the thickness of the composite layer. Among them, the thickness of the composite layer is at least greater than 100mm, the concrete strength grade is not lower than C30 and not lower than the original construction design requirements. The stirrups of the precast beam should all extend into the composite layer, and the straight-line segment length of each limb extending into the composite layer is greater than 10d, where d is the diameter of the stirrup.