Reinforcement method for existing reinforced concrete floor against punching shear for eVTOL apron

Abstract

This invention discloses a method for reinforcing existing reinforced concrete floor slabs for eVTOL (Emergency Virtual Transport Operator) landing pads against punching shear, belonging to the field of civil engineering reinforcement technology. The method includes: first, calculating the equivalent dynamic impact load based on eVTOL takeoff and landing parameters and the existing floor slab condition; second, using a modified punching shear failure angle model based on stiffness gradient, sequentially calculating the modified punching shear angle, quantitatively designing the thickness of the upper ultra-high performance concrete (UHPC) layer and the amount of the lower fiber-reinforced polymer (FRP) layer and the anti-peeling anchor density; finally, implementing a composite reinforcement construction method of "upper layer and lower layer" according to design specifications. This method significantly improves the punching shear resistance of the existing floor slab by introducing a high-stiffness UHPC layer in the upper part of the existing floor slab to expand the punching shear cone angle, and by setting an FRP layer and anti-peeling anchors in the lower part to limit crack propagation. Through the synergistic effect of the upper and lower layers, it achieves rapid reinforcement and renovation of low-altitude aircraft landing pads.

Description

Technical Field

[0001] This invention relates to the field of reinforcement and renovation technology of existing building structures in civil engineering, and in particular to a method for punching shear reinforcement of existing reinforced concrete floor slabs for eVTOL helipads, specifically for urban air mobility (UAM) infrastructure construction. Background Technology

[0002] With the development of urban air mobility (UAM), utilizing existing building rooftops to construct eVTOL (eVTOL) vertical takeoff and landing (VTOL) fields has become a trend. However, existing reinforced concrete (RC) slabs are primarily designed for static loads and are ill-suited to withstand the high-frequency, high-energy point impact loads during eVTOL takeoff and landing. In particular, the small ground contact area of ​​the eVTOL landing gear makes the slabs highly susceptible to brittle punching shear failure.

[0003] Faced with this structural safety challenge, traditional reinforcement methods are proving inadequate. Currently, commonly used punching shear reinforcement methods in the engineering field mainly include bonding carbon fiber reinforced polymer (CFRP) sheets and adding steel beam supports. While bonding CFRP sheets alone performs excellently in flexural reinforcement, it has limitations in punching shear resistance. CFRP primarily improves punching shear resistance indirectly by enhancing the properties of the concrete in the tension zone, contributing little to punching shear failure dominated by shear cracks. Adding steel beam supports can effectively alter the force transmission path, but this usually requires sacrificing the clear height of lower floors and involves the vertical transportation and on-site welding of large steel components. For existing buildings already in use, this presents significant construction disruptions and is extremely difficult to implement. Therefore, there is an urgent need for a composite reinforcement method based on scientific quantitative design that can significantly improve punching shear resistance and is easy to construct. Summary of the Invention

[0004] This invention aims to solve the above-mentioned problems and provides a method for reinforcing existing reinforced concrete floor slabs against punching shear in eVTOL helipads. This method determines the material usage of "upper UHPC for punching shear resistance + lower FRP for bending resistance" through a quantitative design calculation process, and achieves anti-peeling and synergistic stress distribution through a targeted gradient anchoring structure.

[0005] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for reinforcing existing reinforced concrete floor slabs facing eVTOL helipads against punching shear includes the following steps:

[0007] Step S1: Obtain the geometric and material parameters of the existing reinforced concrete floor slab through testing and assessment, as well as the maximum takeoff weight and maximum descent speed of the eVTOL aircraft to be parked, and calculate the design value of the equivalent impact load per wheel. ;

[0008] Step S2: Set the initially selected parameters of the UHPC layer, calculate the stiffness modulus ratio n between the UHPC layer and the existing concrete layer, and determine the modified punching failure angle accordingly. ;

[0009] Step S3: Based on the modified punching failure angle , use the variable-angle projection model to check the punching shear bearing capacity, determine the theoretical thickness through iterative calculation, and take the larger value in combination with the shear resistance construction requirements as the design thickness of the UHPC layer. ;

[0010] Step S4: Calculate the bottom plate moment notch under the impact load, and determine the cross-sectional area of the lower FRP layer accordingly. ;

[0011] Step S5: Determine the core encryption zone radius R and the targeted gradient distribution density of the anti-peeling anchoring component according to the impact center position and the landing gear wheelbase characteristics.

[0012] Step S6: According to the design outputs of Steps S3 to S5, implement the composite reinforcement construction of "pouring UHPC on the upper part, pasting FRP on the lower part, and implanting anchor bolts", and complete the maintenance and waterproof repair.

[0013] Furthermore, in Step S2, the calculation formula for the modified punching failure angle is:

[0014]

[0015] Where, is the elastic modulus of the UHPC layer; is the elastic modulus of the existing concrete layer; is the stiffness ratio influence coefficient, and its value-taking rule is:

[0016] When n ≤ 1.4, = 4°; when 1.4 < n ≤ 1.7, = 5°; when n > 1.7, = 6°.

[0017] Furthermore, in Step S3, the design thickness of the UHPC layer satisfies the following punching shear bearing capacity equation:

[0018]

[0019] Where, is the remaining punching shear bearing capacity of the existing floor slab, is the interface collaborative working coefficient; is the design value of the axial tensile strength of UHPC; The critical section perimeter is taken as the design thickness, which is the larger of the theoretically calculated value that satisfies the above equation and the minimum construction thickness, and the minimum construction thickness is not less than 50mm.

[0020] Furthermore, in step S4, the cross-sectional area of ​​the FRP layer... Calculate using the following formula:

[0021]

[0022] in, The design bending moment under impact load; This represents the remaining flexural bearing capacity of the existing floor slab. The strength utilization factor of FRP; The elastic modulus of FRP; For effective design of tensile strain in FRP; This is the effective lever arm of the reinforced composite section.

[0023] Furthermore, in step S5, the targeted gradient anchoring design specifically involves: setting the radius R of the core densification zone centered on the projected landing point of the eVTOL, satisfying R ≥ 0.5 × L. max +500 mm, where L max The maximum landing gear wheelbase is used; an XY bidirectional orthogonal grid is used in the core densification zone, and the anchor point density is higher than that in the outer conventional zone; the upper shear key and the lower anti-peeling anchor are vertically aligned in the plane position, and the deviation is controlled within 20mm.

[0024] Furthermore, the UHPC reinforcement layer uses ultra-high toughness cement-based composite materials, whose performance indicators include: steel fiber volume content ≥2.0%, using straight steel fibers with a tensile strength of not less than 2000MPa or hooked steel fibers with a tensile strength of not less than 1500MPa; the elastic modulus of the UHPC layer ≥45 GPa, design value of axial tensile strength of UHPC ≥6.7 N / mm 2 .

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] 1. The high elastic modulus of UHPC and the crack bridging effect of steel fibers in this invention form a rigid arching effect, effectively inhibiting the propagation of diagonal cracks, forcing the punching shear failure surface to become gentler, and significantly improving the punching shear bearing capacity. This invention introduces a modified punching shear failure angle for the design calculation of this effect, realizing the quantitative design of this effect.

[0027] 2. This invention establishes an interface anti-peeling system based on "horizontal targeted gradient distribution + vertical alignment". Addressing the stress field characteristics of a large center and small periphery in impact loads, the anchors in the horizontal direction are distributed with a targeted gradient; the steel anchors at the UHPC / concrete slab interface and the FRP anchors at the FRP / concrete slab interface are vertically aligned. Compared to the traditional uniform anchoring method, this method densifies the anchoring in the stress-concentrated core area, effectively resisting instantaneous peeling caused by shock waves; the upper shear key and the lower anchors are vertically aligned in space, working together to bear the force, significantly improving the integrity and ductility of the structure.

[0028] 3. This invention proposes a complete mathematical model from "load equivalence" to "thickness iteration" and then to "reinforcement calculation". This makes the reinforcement design based on evidence, avoiding the roof overload caused by piling up materials, and ensuring the structural safety under extreme impact loads, thus realizing the rapid reinforcement and renovation of rooftop helipads.

[0029] 4. This invention adopts a composite reinforcement method of "top-laying and bottom-attaching", which has mature construction technology and can realize the rapid reinforcement and renovation of rooftop helipads with minimal impact on the normal use of existing buildings. Attached Figure Description

[0030] Figure 1 This is a flowchart of the reinforcement design calculation for this invention;

[0031] Figure 2 This is a schematic cross-sectional view of the composite reinforcement structure of the present invention;

[0032] Figure 3 This is a comparative schematic diagram of the modified punching failure cone mechanism of the present invention;

[0033] Figure 4 This is a plan view of the targeted gradient anchoring arrangement of the present invention. Detailed Implementation

[0034] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0035] Example: The roof of a commercial center was converted into an eVTOL parking apron with a maximum takeoff weight of 2.5t.

[0036] Phase 1: Quantitative Design Calculation

[0037] Basic parameter acquisition and setting:

[0038] Before conducting quantitative reinforcement design, the existing structure and design target parameters were determined through testing and assessment: Existing floor slab current parameters: Concrete strength grade: presumed to be C25 (existing concrete layer elastic modulus) =28,000 N / mm², design value of axial tensile strength of existing concrete layer =1.27 N / mm²). Cross-sectional dimensions: Slab thickness h = 120 mm, concrete cover thickness c = 20 mm, effective cross-sectional height h0 = 100 mm. Damage assessment: Considering material aging and micro-crack damage after long-term service of the roof structure, a reduction factor for punching shear capacity is set. =0.90.

[0039] Aircraft and Apron Design Parameters: Design Model: Maximum Takeoff Weight (MTOW) of 2500 kg, landing gear type: tricycle. Single Wheel Contact Area: Based on the deformation characteristics of aircraft tires under high impact conditions, the equivalent contact area of ​​a single wheel is taken as a 200mm × 200mm square (i.e., 0.04m²). Landing Gear Geometry: The main landing gear track is approximately 2.8m~3.0m. Reinforcement Scope: The effective load-bearing area of ​​the TLOF (Touch-Off Zone) is set at 15m × 15m (covering 2×2 column spans) to accommodate mainstream eVTOL aircraft with a D value not exceeding 15m.

[0040] Performance indicators of reinforcement materials: UHPC (Ultra-High Performance Concrete): Elastic modulus of the UHPC layer =45,000 N / mm², design value of UHPC axial tensile strength =7.5 N / mm² (including steel fiber reinforcement effect). Experimental materials and parameter definitions: Ultra-high performance concrete (UHPC) was used as the key load-bearing material in this embodiment. To ensure the repeatability of the reinforcement effect, the material preparation and parameter testing standards are defined as follows: Matrix mix proportion and fiber characteristics: P.II52.5 silicate cement, silica fume, ultrafine mineral powder, and quartz sand were used as the matrix, with a water-cement ratio controlled below 0.18. The key reinforcing material was copper-plated straight steel fiber with a diameter of 0.2 mm and a length of 13 mm, with a volumetric admixture of... =2.0% (i.e., approximately 156 kg of steel fiber per cubic meter of concrete). Performance index testing standards: Compressive strength: Using 100mm×100mm×100mm cube specimens, the average measured compressive strength... =135.6MPa, the material grade is determined to be UC120. The measurements were taken using a 100mm×100mm×300mm prism specimen. =46.2GPa (meets the design requirement of ≥45GPa). Basis for design value: Given that this invention is applied to structural reinforcement engineering, the relevant strength design values ​​are determined based on the current technical specifications for ultra-high performance concrete: UHPC axial tensile strength design value. Based on the measured standard value of axial tensile strength =9.4MPa, using the material partial factor commonly used in structural design. =1.40 will be reduced: =9.4 / 1.40=6.71N / mm². Values ​​used in the calculations of the example. =6.7 N / mm². FRP (carbon fiber reinforced polymer): High-strength Grade I 300g carbon fiber cloth is used, with an elastic modulus of 6.7 N / mm². =230,000 N / mm², design value of tensile strength =1600N / mm², nominal thickness of a single layer =0.167mm.

[0041] Detailed design calculation process:

[0042] Step S1: Load dynamic equivalent calculation:

[0043] Based on the relevant principles of building structural load codes and civil helicopter flight site technical standards, and considering the most unfavorable operating conditions, it is assumed that in the event of attitude instability leading to a single-point hard landing, the entire impact load is borne by a single main landing gear. Calculation formula:

[0044] ;

[0045] The standard gravity value for maximum takeoff weight is taken as 9.81 m / s², according to engineering specifications. =2500kg×9.81N / kg=24,525N=24.525kN. (Dynamic Amplification Factor (DAF): Based on the recommended value of 1.5, and considering the risk of falling without hydraulic buffer support and emergency fall, a safety factor is introduced, and the final value is 3.0. (Load partial factor): Based on the limit state design requirements for variable load control in the unified standard for building structure reliability design, a value of 1.5 is taken. Calculation results: =1.5×3.0×24.525kN=110.36kN (rounded down to 110.4kN).

[0046] Step S2: Existing resistance assessment and stiffness parameter calculation:

[0047] Review of punching shear resistance of existing floor slabs The calculation parameters were determined based on the formulas in the concrete structure design code and the reliability assessment standards for civil buildings. (Section Height Influence Coefficient): According to the 2015 version of the standard, since the floor slab thickness h=120mm<800mm, take... =1.0. (Design value of axial tensile strength of concrete): C25 concrete, take 1.27 N / mm² from the table. (Critical section perimeter): taken as the distance from the edge of the load-bearing surface. At the 1 / 2 position. = 4×(200 + 100) = 1200 mm. (Effective depth of the cross-section): 120 mm - 20 mm (cover) = 100 mm. (Reduction coefficient of existing structure efficiency): According to the provisions of Section 6.3 of GB50292 - 2015 regarding the safety appraisal of components: Current actual measurement: The average carbonation depth of this floor slab is 5 mm (not reaching the surface of the steel bars), and there are non - load - bearing dry - shrinkage cracks with a width < 0.2 mm at the bottom of the slab. Rating determination: According to Table 6.3.2 of the standard, the bearing capacity item is rated as bu level (no measures need to be taken, but the safety is slightly lower than the requirements of the current code). Coefficient value: Considering the deterioration of the existing concrete materials and the decline in the collaborative working performance under long - term load effects, the efficiency reduction coefficient = 0.90. Calculation process: = 0.9×(0.7×1.0×1.27×1200×100) = 96,012 N ≈ 96.0 kN. Notch determination: Resistance notch = 110.4 - 96.0 = 14.4 kN. Conclusion: The bearing capacity of the existing floor slab is insufficient (the notch is about 13%), and it belongs to the brittle failure mode. Anti - punching reinforcement must be implemented. Stiffness gradient and modified punching angle ( ) calculation, using the punching angle correction model based on stiffness gradient proposed by the present invention. Stiffness modulus ratio (n): = 45000 / 28000 ≈ 1.61; Gradient influence coefficient (k) value - taking rule: Determine the value of k according to the stiffness modulus ratio n: When n ≤ 1.4, k takes 4°; When 1.4 < n ≤ 1.7, k takes 5°; When n > 1.7, k takes 6°. Modified punching angle ( ) calculation: Given that n ≈ 1.61, which falls into the interval (1.4, 1.7], so k = 5°. = 45° - k×n = 45° - 5°×1.61 ≈ 37°.

[0048] Step S3: Iterative design of the UHPC reinforcement layer thickness:

[0049] Initial calculation of the theoretical thickness ( ): Based on the resistance notch calculated in Step S2 = 14.4 kN. Set the initial selected thickness = 30 mm, and substitute it into the anti - punching formula for trial calculation: Trial - calculation parameters: Interface coordination coefficient = 0.6; Tensile strength of UHPC = 6.7 N / mm² (obtained according to the technical specification of ultra - high - performance concrete); Trial - calculation critical perimeter : Estimated according to the 45° punching diffusion angle, ≈4×(200+30)=920mm; punching angle factor cot(37°)≈1.327. Trial calculation of resistance: =0.6×6.7×920×30×1.327=147,210N≈147.2kN. Judgment: 147.2kN>14.4kN. Conclusion: Even using conservative material strength design values, a theoretical thickness of only 30mm is sufficient to meet the stress balance requirements. (Construction thickness correction) Although 30mm meets the stress requirements, it is modified according to the shear connection construction principle: shear pin height (h) stud ): Select 35mm high studs to ensure grip strength; protective layer thickness (c cover ): Meets fire-resistant and durability requirements, with a 15mm protective layer reserved. Final value: =max(30,35+15)=50mm. Shear contribution of UHPC layer ( Final review: Based on the revised physical thickness of 50mm, calculations were performed according to the definition of the critical punching shear section in the concrete structure design code. Critical perimeter ( Calculation: Take the distance from the edge of the load-bearing surface. The perimeter at point 2 / 2 (i.e., 50 / 2 = 25mm). Since the contact surface of a single wheel is a 200mm × 200mm square, the side length of the critical section is: =200 + 2 × (50 / 2) = 250 mm. Critical perimeter =4 × 250 = 1000 mm. Verification calculation: =0.6×6.7×1000×50×1.327=266,727N≈266.7kN; Total bearing capacity verification: =96.0 + 266.7 = 362.7 kN; Safety verification: =362.7 / 110.4≈3.29 Conclusion: Safety factor K>1.5, which fully meets the punching shear design requirements.

[0050] Step S4: FRP layer flexural reinforcement design:

[0051] Design input parameters: Bending moment requirement ( Design value of maximum bending moment at the bottom of plate under single-point impact condition, based on finite element analysis of plate and shell. =36.6 kN·m. Existing resistance ( The remaining flexural bearing capacity is verified based on the existing floor slab. =21.6 kN·m. Reinforcement gap (ΔM): ΔM = 15.0 kN·m. Cross-sectional dimensions: Existing plate thickness 120 mm, UHPC overlay 50 mm, total height after reinforcement H = 170 mm. Carbon fiber usage calculation: , For effective FRP design, the tensile strain is set to a design control value of 0.01. The effective lever arm of the reinforced composite section is approximately 120 (original plate) + 50 (UHPC) - 10 (resultant force point offset) = 160 mm. Calculated value: =(15,000,000) / (1.0×230,000×0.01×160)=40.8mm²; 300g carbon fiber cloth is selected (single layer thickness t_f=0.167mm). Actual area: One layer is laid within every 1000mm of board width, providing an area of ​​167mm². Verification: 167mm²>40.8mm², meeting the design requirements.

[0052] Step S5: Targeted anchoring distribution design:

[0053] To prevent interface peeling caused by shock waves, a gradient anchoring scheme was designed based on the landing gear geometry. Core reinforcement zone calculation: Considering that the main track width of mainstream 2.5t eVTOL aircraft is typically between 2.5m and 3.0m, and to further cover extreme eccentric landing conditions and the size requirements of future larger aircraft, the core reinforcement zone is defined as a circular area with a radius R = 2000mm (i.e., a diameter of 4.0m) centered on the parking position, ensuring that the landing gear always falls within the high anti-peeling performance area. Anchoring density output: Core area (r ≤ 2000mm): High-density anchoring is used. Within the circular coverage area of ​​2000mm radius, an XY bidirectional orthogonal grid arrangement is used, with the grid spacing between the shear keys (upper part) and the anti-peeling anchors (lower part) set at 150mm × 150mm. Outer area (r > 2000mm): Only subjected to fuselage aerodynamic downwash pressure, the spacing is widened to 300mm × 300mm, arranged in a staggered pattern. Alignment Requirements: During construction, ensure that the deviation δ of the upper and lower anchor points on the plane projection is less than 20mm, forming a vertical constraint mechanism similar to a tie bolt. In this embodiment, the steel shear key is an inverted L-shaped rebar (compliant with the design code for reinforced concrete structures), and the FRP anti-stripping fan-shaped anchor is a standard fan-shaped anchor with 300g carbon fiber cloth. Both are commercially available standard products, and their model selection only needs to meet the gradient density and vertical alignment requirements of this invention; no additional customization is required.

[0054] Phase Two: Construction and Implementation

[0055] Step S6: Construction:

[0056] Surface preparation: Mark a 15m × 15m reinforcement work area (i.e., the TLOF effective load-bearing zone) on the roof structure surface. Remove the existing waterproof layer, insulation layer, and leveling layer within this area, and remove the loose concrete protective layer until the coarse aggregate of the structural layer is exposed. Mechanically roughen the surface to ensure a roughness range of not less than 6mm. Subsequently, strictly following the gradient density distribution requirements of the core area (R≤2000mm, spacing 150×150mm) and the outer area (R>2000mm, spacing 300×300mm) determined in step S5, implant the upper inverted L-shaped steel shear keys (conventional shear reinforcement, implantation depth ≥10d, where d is the diameter of the reinforcement bar, meeting the requirements of the concrete structure reinforcement design code).

[0057] UHPC casting: Cast UHPC with a design thickness of 50mm and a steel fiber volume fraction of 2%. Before initial setting, the top surface is artificially roughened.

[0058] FRP Bonding and Anchoring: After the concrete base layer at the bottom of the slab has been ground, cleaned, and primed, a layer of 300g carbon fiber cloth is fully bonded in both directions. Following the "upper and lower alignment" principle determined in step S5, the lower anchoring point position is determined by laser positioning during construction, ensuring that the deviation is controlled within 20mm. The lower FRP anti-peeling fan-shaped anchors (common commercial products, with an anchor plate and carbon fiber cloth bonding area ≥50cm², meeting the basic requirements for anti-peeling) are then implanted into the surface of the carbon fiber cloth.

[0059] Waterproofing, Insulation, and Corrosion Protection Construction: Flexible Sealing of Joints: Polyethylene foam rods (backing material) are filled into the pre-reserved grooves at the edge of the UHPC, and single-component polyurethane building sealant (or silicone weather-resistant sealant) is injected to form a "rigid-flexible" waterstop. Membrane Overlap Restoration: After restoring the insulation layer, the roof waterproof membrane is re-laid. The new waterproof layer must cover the entire UHPC area, and the overlap width with the existing waterproof layer must be no less than 250mm. The overlap should be fully bonded using hot air welding or a special adhesive. FRP Protective Coating: To prevent aging of the FRP resin on the underside of the board and to meet fire resistance requirements, a polymer cement mortar fire-retardant coating with a thickness of no less than 2mm is applied to the surface of the carbon fiber cloth, or two coats of acrylic polyurethane weather-resistant topcoat are sprayed.

[0060] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0061] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for reinforcing existing reinforced concrete floor slabs against punching shear for eVTOL helipads, characterized in that, Includes the following steps: Step S1: Obtain the geometric and material parameters of the existing reinforced concrete floor slab through testing and assessment, as well as the maximum takeoff weight and maximum descent speed of the eVTOL aircraft to be parked, and calculate the design value of the equivalent impact load per wheel. ; Step S2: Set the initially selected UHPC layer parameters, calculate the stiffness modulus ratio n between the UHPC layer and the existing concrete layer, and determine the corrected punching shear failure angle accordingly. ; Step S3: Based on the corrected punching failure angle The punching shear capacity was checked using a variable-angle projection model. The theoretical thickness was determined through iterative calculations, and the larger value was selected as the design thickness of the UHPC layer based on the shear resistance requirements. ; Step S4: Calculate the bottom bending moment notch of the slab under impact load, and determine the cross-sectional area of ​​the lower FRP layer accordingly. ; Step S5: Based on the impact center location and landing gear wheel track characteristics, determine the radius R of the core reinforcement zone and the target gradient distribution density of the anti-peeling anchoring component; Step S6: Based on the design output of steps S3 to S5, implement the composite reinforcement construction of "pouring UHPC on top, attaching FRP underneath and implanting anchors", and complete the curing and waterproofing repair.

2. The method for reinforcing existing reinforced concrete floor slabs against punching shear for eVTOL helipads according to claim 1, characterized in that, In step S2, the corrected punching failure angle is... The calculation formula is: ； in, The elastic modulus of the UHPC layer; The elastic modulus of the existing concrete layer; The stiffness ratio influence coefficient is determined according to the following rules: When n ≤ 1.4, = 4°; when 1.4 < n ≤ 1.7, = 5°; when n > 1.7, = 6°.

3. The method for reinforcing existing reinforced concrete floor slabs against punching shear for eVTOL helipads according to claim 1, characterized in that, In step S3, the UHPC layer is designed to have a thickness of... The following punching shear capacity equation must be satisfied: ； in, The remaining punching shear capacity of the existing floor slab, This is the coefficient for interface collaboration. This is the design value for the axial tensile strength of the UHPC. The critical section perimeter is taken as the design thickness, which is the larger of the theoretically calculated value that satisfies the above equation and the minimum construction thickness, and the minimum construction thickness is not less than 50mm.

4. The method for reinforcing existing reinforced concrete floor slabs against punching shear for eVTOL helipads according to claim 1, characterized in that, In step S4, the cross-sectional area of ​​the FRP layer Calculate using the following formula: ； in, The design bending moment under impact load; This represents the remaining flexural bearing capacity of the existing floor slab. The strength utilization factor of FRP; The elastic modulus of FRP; For effective design of tensile strain in FRP; This is the effective lever arm of the reinforced composite section.

5. The method for reinforcing existing reinforced concrete floor slabs against punching shear for eVTOL helipads according to claim 1, characterized in that, In step S5, the targeted gradient anchoring design specifically involves setting the radius R of the core densification zone, centered on the projected landing point of the eVTOL, such that R ≥ 0.5 × L. max +500 mm, where L max The maximum landing gear wheelbase is used; an XY bidirectional orthogonal grid is used in the core densification zone, and the anchor point density is higher than that in the outer conventional zone; the upper shear key and the lower anti-peeling anchor are vertically aligned in the plane position, and the deviation is controlled within 20mm.

6. The method for reinforcing existing reinforced concrete floor slabs against punching shear for eVTOL helipads according to claim 1, characterized in that, The UHPC reinforcement layer is made of ultra-high toughness cement-based composite material, and its performance indicators include: steel fiber volume content. ≥2.0%, using straight steel fibers with a tensile strength of not less than 2000MPa or hooked steel fibers with a tensile strength of not less than 1500MPa; the elastic modulus of the UHPC layer ≥45 GPa, design value of axial tensile strength of UHPC ≥6.7 N / mm 2 .

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