Keywords

1 Introduction

With the increase of water consumption in cities and towns, the harmless and resourceful treatment of domestic sewage can effectively reduce pollution and improve the utilization of water resources. Urban sewage treatment plant and the corresponding pipeline network construction investment is huge, and sewage treatment system has gradually changed from large-scale centralized to small-scale decentralized, in which integrated sewage treatment equipment has been widely applied and developed. The main structure of the integrated sewage treatment equipment is the water tank, which is divided into several sewage treatment areas inside the tank and arranged with the corresponding sewage treatment equipment. The steel plate tank has the advantages of high strength, light weight, easy construction, easy maintenance, etc. It is suitable for temporary integrated sewage treatment equipment [1].

At present, there is no special specification for the design and manufacture of steel plate water tank, the manufacturer is mostly designed and manufactured according to experience, although some practical engineering problems can be solved, but the bearing capacity of the steel plate water tank, the mechanical response of each part of the tank under load, the possible failure mode of the tank and the overall and local structural analysis and design of the tank are not thoroughly studied. In this paper, the mechanical properties of the steel plate water tank under the state of stress are analyzed by finite element calculation. The structural design criterion of steel plate water tank is proposed, and based on this criterion, the structural design of the steel plate water tank used in integrated wastewater treatment equipment is carried out to verify the safety of the tank structure [2].

2 Project Profile and Structure Selection

2.1 Project Profile

The water tank structure design for the above-ground integrated sewage treatment equipment with a capacity of 4.0 m3/h, the rectangular steel tank is 3500 mm long, 3500 mm wide and 5300 mm high, and the design working water level is 5000 mm. The structural safety level is Grade II, the design service life is 25 years, the seismic protection category is the standard protection category, the equipment is suitable for the I and II sites with seismic protection intensity of 8° and all kinds of sites in the area of 7 degrees and below 7° [3].

2.2 Structure Selection

The main body of the water tank structure is a box-shaped thin-walled structure composed of tank plates and reinforcing elements welded together [4]. The horizontal and vertical reinforcement ribs set in the outer wall of the tank, the tank plate is divided into smaller cells to increase the stiffness and reduce the deformation, in addition, the reinforcement ribs placed in the outer wall is also convenient for the inner wall of the tank for the anti-corrosion and maintenance. According to the distribution characteristics of hydrostatic pressure, the horizontal stiffening ribs take the distribution method of upper sparse and lower dense. The vertical stiffening ribs take equal spacing distribution, the spacing in the horizontal direction is 500 mm. Set up two rows of tension rods inside the tank in the vertical distribution corresponding to the horizontal stiffening ribs, in order to reduce the span of the horizontal stiffening ribs, so that the cross-sectional force of the horizontal stiffening ribs is more uniform [5]. Three internal partitions are arranged inside the water tank to separate different functional partitions.

3 Failure Modes and Design Criteria

Failure judgment basis and design criteria are the prerequisites for structural design. Based on the existing engineering cases of rectangular steel plate water tank and theoretical derivation, several structural failure modes of rectangular steel plate water tank and the corresponding structural design criteria are proposed [2].

3.1 Overall Slippage or Overturning of the Water Tank Structure

As a single structure placed on the ground, the water tank is subjected to wind load, relying only on the bottom of the box and the ground sassafras force to resist the horizontal force generated by the wind load. In the empty tank state, the pressure of the structure on the ground is reduced, resulting in a reduction in the maximum static friction force, which may lead to the phenomenon of overall structural slippage. In addition, when the height to width ratio of the tank structure is large, the wind area of the structure is relatively larger, and the overall structure may overturn the phenomenon.

When the tank structure overall slippage or overturning, may produce damage to the equipment and external pipe line, in order to prevent the structure from such failure, in the design of the structure, with reference to the stability design of the independent foundation, the tank structure as a whole to resist horizontal sliding and overturning test. If the test is passed, it means that the water tank structure can be directly rested on the ground and will not slip or overturn as a whole. Conversely, the support should be designed to connect the bottom of the tank structure with the ground to resist the forces that cause the overall slip and overturning of the structure in the horizontal direction.

3.2 Tank Plates Tear Damage

In the case of ensuring the welding quality and the strength of the stiffening ribs, the water tank plates are divided into grids by the stiffening ribs, and the plates are subjected to greater hydrostatic pressure near the bottom, and are more likely to be torn by excessive deformation. When the plate is in the elastic range, the plate will not tear and will not bulge outward significantly, which can ensure the normal operation of the water tank [6].

In the design of the water tank plate, should ensure that the water tank plate parts are basically within the elastic range. Calculate the deflection value of the plate under normal use and the equivalent force strength of the plate under the load carrying capacity limit state through finite element calculation. If the deflection value of the plate is less than b/100 and the equivalent force strength on the plate is less than the yield strength of the material, the plate meets the design requirements. Conversely, the thickness of the plate should be adjusted and the calculation should be performed again until the design guidelines are met.

3.3 Bending Damage to the Stiffening Ribs or Fracture of the Internal Tension Rods

The water tank is a typical thin-walled box-type structure, and its ability to bear internal pressure is very weak. The stiffening ribs and internal tension rods act as the structure skeleton to bear the force from the tank plates, while restraining the plates from excessive lateral deformation. The vertical stiffening ribs are designed as flexural members, the horizontal stiffening ribs are designed as tensile-bending members, and the internal ties are designed as axial tension members.

When the stiffening ribs are damaged by bending or internal tension rod fracture, the adjacent plate will assume a greater range of water pressure, thus producing a greater local equivalent force strength and more likely to occur plate tearing damage. In the design of the stiffening ribs and internal tension rods, the elastic design criterion is adopted, and the calculation is carried out by finite element software, and the steel structure calibration of the stiffening ribs and internal tension rods is applied to Chinese standards to obtain the stress ratio of the rods under different working conditions. If the stress ratio of the rods is less than 1.0, the rods meet the design requirements [7]. Conversely, the structural dimensions of the rods should be adjusted and the calculations should be performed again until they meet the requirements.

4 Anti-slip and Anti-Overturning Test

4.1 Anti-slip Test

The structural anti-slip test is performed by the following equation.

$$ K_1 = \frac{\mu N}{F} $$
(1)

In the formula: K1 is the horizontal sliding safety factor, requiring K1 ≥ 1.2; N is the sum of the vertical forces acting on the structure; F is the sum of the horizontal forces acting on the structure; μ is the friction coefficient of the structure base plate and foundation, taken as 0.6.

According to Table 1, it can be concluded that the water tank structure can meet the anti-sliding requirements under the full water working condition. However, under the wind load, the tank structure in the empty tank state cannot meet the requirements of anti-sliding.

Table 1 Results of horizontal sliding resistance test
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4.2 Anti-overturning Test

The structural overturning test is performed by the following equation.

$$ K_2 = \frac{y}{e_0 } $$
(2)

In the formula: K2 is the overturning stability coefficient, requiring K2 ≥ 1.5; y is the distance from the center of gravity of the structure to the side of the maximum pressure, y is taken as 1.75 m in this project; e0 is the eccentricity distance of the combined external forces, e0 = M/N.

According to Table 1, it can be concluded that the water tank structure can meet the requirements of overturning resistance under the working condition of full water. Under the wind load, the tank structure in the empty tank state cannot meet the overturning resistance requirements.

4.3 Support Design

Comprehensive consideration, in order to ensure the stability of the structure in the state of empty tank, the bottom of the vertical rib is connected with the foundation. The nodal reaction force is extracted for the design of the support articulation node, and the design pullout force of the support node is 5 kN and the shear force is 13 kN. The design of the support is shown in Fig. 1.

Fig. 1
A drawing of the design of the support. It includes an anchor tendon inside the foundation. Tank plates and vertical stiffening ribs are on the foundation.

Support design drawing

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5 Finite Element Calculation Analysis of Steel Plate Water Tank Structure

5.1 Modeling of Water Tank Structure

Using SAP2000 finite element calculation software for structural design, considering the force characteristics and geometric structural features of the tank plates and tank reinforcement elements, the thin shell unit is applied to establish the tank plates, and the frame unit is applied to establish the stiffening ribs and tension rods. The stiffening ribs, ties and plates are made of Q355 steel [8]. The transverse stiffening ribs are box-type 100 × 80 × 4 section, the vertical stiffening ribs are rectangular 70 × 6 section, and the tension rods are rectangular 50 × 4 section. The thickness of the tank plate is 5 mm, taking into account the rusting effect of sewage on the steel plate, the corrosion margin must still be reserved in the case of anti-corrosion coating. In the structural finite element analysis, the thickness of the tank steel plate is 4 mm.

For simplicity, and taking into account the actual form of connection between the components, the following assumptions are made in the finite element model: The bottom of the vertical stiffening ribs are hinged to the foundation to simulate the restraint of the support on the tank. Each node of the bottom plate is equipped with a support that only constrains downward displacement to simulate the support of the ground to the tank. The stiffening ribs are solidly connected to the box plate. Since the steel plate is thin, the joint action of the stiffening ribs and the plate is not considered in the design. The horizontal and vertical stiffening ribs are solidly connected to each other and together form the skeleton of the water tank structure. Taking into account the calculation accuracy and calculation speed, the plate is divided into 100 mm × 100 mm calculation units, and the junction between the plate and the bar is processed as a local subdivision unit. The finite element model is shown in Fig. 2.

Fig. 2
An illustration presents the finite element model. The water tank comprises vertical and horizontal ribs all around, plate and bar are inside the tank.

Water tank finite element model

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5.2 External Force Datum

In the finite element calculation, D denotes the constant load; SS denotes the hydrostatic pressure under full water working condition; Wx and Wy denote the wind load in X-direction and Y-direction respectively; Ex-d and Ey-d denote the seismic action in X-direction and Y-direction in the empty box state respectively. Ex-s and Ey-s denote the seismic action in X-direction and Y-direction under full water working condition respectively. The structure is symmetrical, the effect of wind load and earthquake in X and Y negative directions on the structure is the same as the effect in positive direction. According to Load code for the design of building structures and Structural design code for special structures of water supply and waste water engineering, load combinations are carried out to consider the effects of different working conditions on the structure [9, 10].

  • Self-weight of the structure: Automatically accounted for by the software.

  • Water load: The tank structure only considers the role of internal lateral hydrostatic pressure. The standard value of lateral water pressure at the bottom of the equipment is 53.55 kPa. The hydrostatic pressure is triangularly distributed along the height of the wall plate.

  • Wind load: Calculate the wind load borne by the water tank structure in accordance with Load code for the design of building structures. The basic wind pressure is taken as 0.75 kN/m2 [9].

  • Seismic action: When considering the seismic effect of the tank structure, the effect of internal water storage cannot be ignored. This design calculates the seismic action according to Article 6.2 of Code for sesismic design of outdoor water supply, sewerage, gas and heating engmeering [11].

6 Water Tank Structure Analysis and Design

6.1 Structural Analysis and Design of Tank Plates

The tank plate in the working condition mainly bears the hydrostatic pressure inside the tank, considering the restraint effect of the stiffening ribs on the plate, the force form of the plate is bi-directional support of the bending member. The displacement cloud diagram and stress cloud diagram of the tank plate are shown in Fig. 3. and Fig. 4, respectively.

Fig. 3
An illustration presents the design of tank plates. It highlights the displacement concentration on the walls of the water tank with different colors.

Displacement cloud diagram of tank plate

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Fig. 4
An illustration presents the design of tank plates. It highlights the stress concentration on the walls of the water tank, mostly below its mouth.

Stress cloud diagram of tank plate

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The calculation results show that the maximum span deflection of the tank plate is 3.65 mm in the limit state of normal use, which is less than b/100 = 5 mm, and the plate meets the stiffness requirement. In the limit state of bearing capacity, the maximum tensile stress of the box plate is 357 MPa and the maximum compressive stress is 150 Mpa. There are small areas in the plate where the stress exceeds the material strength design value of 305 MPa, mainly at the connection with the stiffening ribs there is stress concentration, but does not exceed the material strength limit value of 470 MPa, the plate meets the strength requirements [8].

6.2 Structural Analysis and Design of Tank Strengthening Members

The tank reinforcement member acts as the structural skeleton and assumes the role of the load transferred by the slab in the slave range. The displacement cloud diagram of the tank strengthening member and the stress ratio of the member are shown in Fig. 5. and Fig. 6, respectively.

Fig. 5
An illustration presents the design of a tank strengthening member. The water tank skeleton is made up of vertical and horizontal ribs with low and medium values. A color gradient scale of 0.8 to 11.2 is beside the skeleton.

Displacement cloud diagram of the member

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Fig. 6
An illustration presents the stress ratio of the strengthening member. The water skeleton is made up of horizontal and vertical ribs, which are between 0 and 0.7 values. A color gradient scale of 0 to 1 is beside the skeleton.

stress ratio of the member

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The calculation results show that the maximum spanwise deflection of horizontal stiffening ribs is 0.31 mm, which is less than b/500 = 1 mm in the limit state of normal use. The maximum spanwise deflection of vertical stiffening ribs is 0.53 mm, which is less than b/500 = 1 mm. The stiffening ribs meet the stiffness requirements. The maximum stress ratio of the tank reinforcement member is 0.855 in the load carrying capacity limit state. The tank reinforcement member meets the strength requirement [8].

In summary, the stiffness and strength of the water tank panels and tank strengthening members meet the design requirements, and the design scheme is feasible.

7 Conclusion

In this paper, the structural design of the steel plate water tank used in integrated sewage treatment equipment is carried out, the force performance of the tank structure is analyzed, the failure mode of the steel plate water tank and the corresponding design guidelines are proposed, and design ideas are provided for the structural design of the steel plate water tank. SAP2000 software is applied to the finite element modeling calculation of the steel plate water tank structure, and the force performance analysis and structural failure judgment of the tank as a whole, the tank plate and the skeleton of the tank structure are carried out respectively. After calculation and analysis, the structural design of the steel plate water tank is carried out according to the design criteria proposed in this paper. Due to the light weight of the steel plate water tank structure, it is necessary to design the support at the bottom of the tank to resist the overall slip and overturning of the structure. By analyzing the structural mechanical properties such as stress level and displacement, suitable member sizes are selected to ensure the stiffness, strength and stability of the structure and meet the design requirements. The structural design method of the water tank can provide reference for other similar projects.