Critical stresses arising in the destruction of a floating reinforced concrete pontoon

The article discusses the stress-strain state of a reinforced concrete floating pontoon caused by critical stresses before its destruction. The investigation was motivated by the destruction of the pontoon, which occurred under extreme wind and wave impacts. A method for determining wind, wave and current loads is presented. The investigation was carried out on finite element computer models. As a result of the investigation, critical efforts and stresses were obtained, causing the destruction of the pontoon. The main conclusion of the conducted investigation is the possibility of using a floating reinforced concrete pontoon in waters with limited values of wind, wave effects and loads from the current.


Introduction
The article discusses the stress-strain state of a floating reinforced concrete pontoon, which arose as a result of a storm of 2% coverage (occurs once every 50 years) at a wind speed in the bay of 46 m / s and a wave height of 2.5 m. It is actual to determine the limiting values of wind and wave loads that cause critical stresses in a reinforced concrete pontoon, which must be considered when choosing the type of pontoon to prevent its destruction.
A review of scientific and technical literature showed the absence of a solution to this specific problem in the literary sources. When determining the wave, wind and flow loads acting on the pontoon, we used the dependencies developed based on many years of field investigations (Shaitan VS, VV Shuleikin, VF Gushchin), used in modern regulatory documents of the Russian Federation on hydraulic engineering, as well as conclusions and recommendations of modern investigators (Pilyaeva SI., Kantarzhi IG., Mordvintseva KP. and others).
The purpose of this investigation is to determine the critical forces and stresses destroying a reinforced concrete floating pontoon. The objectives of the investigation are to identify the optimal values of wind, wave and flow loads, at which it is possible to use the considered constructive solution of a reinforced concrete pontoon.

Methods
The method for studying the stress-strain state of a reinforced concrete floating pontoon was based on the method for determining wind, wave and current loads set forth in the current regulatory document Code of Rules 38.13330. 2018. Loads and impacts on hydraulic structures (wave, ice and from ships).
The calculation recommendations contained in this Code of Rules were obtained based on numerous field investigations of previous years [1,2,3] and modern field investigations, laboratory [4] and numerical investigations [5], which make it possible to clarify existing and develop new calculation methods, as well as specify the loads acting on hydraulic structures, including the wave loading of piers [6,7,8,9].
It should be noted that the reliability of the design of hydraulic engineering facilities, including piers made of reinforced concrete pontoons, is associated with taking into account all the loads and impacts on them [10,11].
When performing the calculation, the article considers the extreme loading of the pontoon [12,13]. Loads on a reinforced concrete pontoon (Fig. 1a) were calculated for real conditions at a wind speed of 46 m/s, and a wave height of 2.5 m, at which the pontoons collapsed with pollution of the bay by crumbling parts of polystyrene foam blocks (Fig. 1b) placed in the body of the ribbed reinforced concrete slab of the pontoon and ensuring its flotation.
In [14], the definition of operational loads for reinforced concrete pontoon of river piers is given. The conditional load on the pontoon associated with the stay of people on it, cargo moved to the pontoon, etc., is considered. It is assumed that the ship at the pier will interact with the pontoon [15]. Different intensities of wind and wave loading can be expected for pontoons of the river and sea piers [16].
As you know, even load-bearing reinforced concrete structures of heated buildings can collapse from moisture and temperature changes as a result of prolonged construction of a building or a break in its construction [17]. In this case, the following defects of reinforced concrete structures may occur, leading to a decrease in the design bearing capacity. Cracks and delamination of concrete appear due to repeated alternating exposure to negative and positive temperatures, corrosion of steel embedded parts and reinforcement outlets, corrosion of frameworks and reinforcement meshes due to water seepage into the thickness of structures. Corrosion of concrete also takes place, that is, the process of destruction of its structure due to the dissolution and washout of the constituent parts of the cement stone by water, the formation and crystallization of hardly soluble substances in the pores of the concrete, the destruction of cement stone by substances contained in water and air; damage to concrete by fungi, mosses, plants. Reinforced concrete pontoons, during their operation, constantly interact with the aquatic environment; therefore, the described damage manifests in them to a greater extent.

Results and Discussion
In [14], the structural solution of a reinforced concrete pontoon is described in detail. The stress-strain state of the pontoon will depend on its constructive solution, including the features of its fastening [18].  The case under consideration refers to storms of 2% probability with a wind speed in the bay of 46 m/s. When determining the external loads acting on a floating pontoon, the methodology outlined in Code of Rules 38.13330.2018 was used, developed based on the results of many years of field research carried out by hydraulic scientists, and summarized in the Code of Rules. The formulas given below for determining the longitudinal and transverse forces from the effects of wind, waves and from the current, as well as the formulas for auxiliary empirical coefficients, are indicated in Code of Rules 38.13330. 2018. Loads and impacts on hydraulic structures (wave, ice and from ships. The article provides links to the dependencies, graphs and tables required for the calculation, taken from this Code of Rules. The calculated values of the transverse Q w and longitudinal N w components of the wind force on floating objects are determined by the formulas (Section 6.2): V q and Vn are, respectively, the transverse and longitudinal components of the anemometric wind speed, m/s, taken following a real event; ξ is a coefficient depending on the largest horizontal dimension α h , m, transverse or longitudinal silhouettes of the surface part of a floating object, taken according to Table 8.
When the object is parked for a long time (groups 3 and 4 of Table 7), the coefficient ξ = 1; γ f = 1.4 is the coefficient of reliability for wind load, adopted following Code of Rules 20.13330. 2016. Loads and Impacts.
Sail areas are determined taking into account the areas of shielding barriers located on the windward side (Appendix I).
Loads  Figure 3 shows the geometry of the longitudinal and cross-section of the pontoon slab, indicating the areas where the calculated loads are applied. n = 4 is the number of working pedestals (in our case, the nodes for attaching the cables to the plate), taken according to table 11; α, β are tilt angles of the cable, degrees, taken according to table 12. Table 1 shows the calculated values of the loads applied to the surfaces indicated by positions 1 ... 6 in Figure 3 of the floating pontoon for two calculated cases: at a wind speed of 10 m/s (1st case) and at a wind speed of 46 m/s (2nd case).
The floating reinforced concrete pontoon is reinforced with nets and individual rods. The meshes are installed in the top slab with a thickness of 60 mm and in the longitudinal and transverse walls with a thickness of 70 mm. The meshes are made of reinforcing bars 8А500С and have a mesh size of 200200 mm. Additional reinforcing bars 10А500С, 12А500С, 16А500С, 8А500С are installed in the ribs of the pontoons.
For the manufacture of the pontoon, concrete B45 was used. Depending on the concrete class, the thickness of the protective layer of reinforcement for structural elements of the pontoon can be taken as 45 mm, 40 mm or 25 mm. Structurally, the pontoon is a ribbed reinforced concrete slab (рисунок 3). When carrying out a computer calculation [19,20], a reinforced concrete pontoon was modeled using the LIRA-SAPR software package. The geometry of the design scheme included plate elements of the upper flange of the ribbed pontoon slab, its longitudinal and transverse ribs. Additional reinforcing bars of the pontoon slab ribs were modeled with bar elements, similar to the modeling of hidden frames.
The pontoon was fastened in the places of its anchoring in the lower corner zones of the pontoon slab. The strength and deformation characteristics of the materials of the reinforced concrete slab of the pontoon were set.
The loads were set following the loading table. The results of the static calculation are shown in Figure 4. The strength calculation showed the insufficient reinforcement of the pontoon slab to ensure its bearing capacity at a wind speed of 46 m / s.