Structural cracks are a common problem for both industrial and civil buildings. Along with the rapid development of China’s economy, the building structure is developing in the direction of larger span, higher height and narrower size cross-section, and these factors will face more serious stress concentration problems, which lead to structural cracks occurring from time to time.

For industrial buildings, crack control is an important control objective in construction and later maintenance. The presence of cracks adversely affects the overall quality of the project and poses a potential hazard to the safe operation of the plant in the future. The control of cracks is directly related to the structural selection, and the design, material, maintenance and survey should be strictly proven and controlled in the construction process in order to minimize the appearance of cracks.

1, industrial building structure design selection analysis

Industrial building structure design selection specifically includes a variety of factors, such as structural span, structural column height, cross-sectional area of the structure, uneven settlement control range, etc. Therefore, the design institute should select the structural force system according to the function of the building, under the condition that the basic parameters of the structure and the external load size are determined, such as selected frame structure, brick and mortar structure, truss structure, etc. After the structural system is determined After the structural system is determined, the location of structural beams, columns and auxiliary structures should be determined according to the design scheme, and the cross-sectional size, reinforcement type and concrete grade should be calculated at the same time.

For industrial buildings with large height, prestressed concrete should be selected as beam structure as far as possible, and the top structure of scaffolding, scaffolding cover and other overlay materials should be selected as far as possible with lighter mass and greater stiffness, which can reduce the vertical load on the beam and thus reduce the occurrence of cracks.

In addition, in the scaffolding selection, full scaffolding should be set according to the span of the beam, and the scaffolding should be removed only after the concrete reaches eighty percent of the design strength. Crack control should be prevented in advance, both the construction unit and the design unit should be strictly controlled, and the concrete grade and the grade of the main reinforcement should be increased as much as possible when the project cost allows. The design unit should control the cracks mainly by the strength of the structure when designing, and increase the reinforcement amount of the tensile surface of the structure, so that the bending strength of the structure becomes larger.

If the structure’s own weight is large or other reasons cannot avoid the cracks, expansion joints or construction joints should be set in the structure to allow the structure to deform to a certain extent and reduce the damage to the structure by the internal stress of the structure. Preliminary design includes structural concept design and functional design, which refers to the design and planning of the overall function and appearance concept of the structure before the construction of the building structure, specifically the size, material and strength design of the structure.

The functional design of the structure is the precursor of the structural design, and only when the conceptual design meets the functional needs can the existence of industrial buildings be guaranteed. Therefore, when designing the structure, the service life of the structure, the human environment of the structure, the geographical location of the structure and so on should be considered comprehensively, and the mechanical characteristics of the structure should be combined to design the building with the maximum function and the most innovative concept.

In addition, numerical simulation software should be used to analyze the reliability of the structure when designing the structure. The stress distribution law of the structure should be analyzed according to the design load of the structure, and the horizontal and vertical displacement change law of the structure should be depicted, and then the possible cracks of the structure should be calculated according to the data, and the location and size of the maximum cracks should be derived.

Industrial buildings should be designed to meet both the ultimate bearing capacity and normal use bearing capacity requirements, while the crack width control criteria in the design of concrete structures are closely related to the construction environment and functional services. For low-grade concrete, cracks will be extremely easy to appear when the internal force of the rod reaches thirty percent of the design strength, and the allowable range is 0.05mm to 0.1mm, and this crack width has great influence on the load bearing capacity of the structure.

2、Strength and crack analysis of industrial building structure

The reasons for the appearance of cracks in industrial building structures can be roughly divided into three kinds.
The first one, cracks caused by external loads when the structural strength is insufficient.
The second kind, stress concentration caused by excessive internal forces caused by the temperature of the structure, causing cracks to occur.
The third kind, the structure produces cracks under earthquake vibration. For the first case, the structural material strength should be improved, the steel selection and concrete grade selection should be recalculated, and the structural cross-sectional tension should be calculated according to the The location of possible diagonal cracks as well as the size are calculated by the formula of pressure.

At the same time, when calculating the external load, an enlargement factor should be selected for the calculation of external load, and the factor is usually chosen to be 1.3, 1.5, etc. to avoid the load becoming large in special cases and causing the structure to produce unexpected cracks. For the second, the concrete should be judged according to the weather temperature when pouring the construction time, and if the outside temperature exceeds 30 degrees, the concrete should not be poured. Because, the temperature is too high, the process of pouring concrete, the temperature generated can not be well released, resulting in the expansion of concrete, thus creating voids and affecting the quality of the finished concrete structure.

For the third case, the load taking coefficient should be increased according to the ground survey data. In areas where earthquakes are frequent, the load taking value should be determined according to the seismic protection level. In order to increase the strength and stability of the structure, reinforcement can be carried out without changing the structural form, such as setting auxiliary columns in the structural beams, assuming steel trusses in the upper part of the crossbeams, etc.

In addition, when cracks appear in the structure, fixed scaffolding can be set at the location of the cracks to re-pour the concrete.

3、Concrete material control

The hydrothermal properties of concrete affect the size of heat released when concrete is poured. When the hydrothermal coefficient is high, the performance of the member formed by pouring is unstable. When the shrinkage of cement is high, cracks will appear in the formed concrete, causing loss of prestress on the reinforcement.

In addition, the chloride ion content of concrete should be controlled in order to prevent the corrosion of reinforcement by chloride ions. In the configuration of reinforcement, it should be strictly in accordance with the specification “Concrete Structure Design Code”, and should meet the reinforcement rate and reinforcement grade requirements of structural tensile reinforcement. For members with different stress states, the position of the structural reinforcement in the structure should be adjusted to make the reinforcement bear tensile stress as much as possible to reduce the possibility of cracks.

When cracks appear, the surface cracks should be filled and coated. The coating is mainly for the case where the surface cracks are small and submerged, and the concrete cannot enter the deep part of the cracks, and the control effect on the cracks is general. Filling method is to use other materials to fill the cracks, and the material can enter the concrete inside and integrate with the concrete better, this method is suitable for the case of larger and deeper crack width. Another reason for cracks in concrete is that the concrete is not maintained according to the requirements after it is poured, or the maintenance time is short.

Theoretically, concrete maintenance should be covered with materials such as grass fir and wet cotton cloth on the structure, and then the materials are maintained by sprinkling water, but the construction site often sprinkles water directly on the members. This practice can not form a water film on the members, and the maintenance effect is extremely poor. Therefore, in the summer high-temperature season, the members should be maintained by covering the members with shade and water-absorbing materials.

4、Conclusion

The industrial building structure should determine the reasonable structural form according to the service function of the structure, and in order to prevent cracks from appearing, the force state and deformation possibility of each member should be calculated, and the reasonableness of the project cost should be ensured at the same time.

The article analyzes the problems that should be paid attention to in the selection of industrial buildings, takes measures such as setting expansion joints and settlement joints, and then analyzes the problems that occur according to the concrete design and construction maintenance, and draws conclusions that have positive significance for the selection of industrial building structure design and the treatment of concrete cracks.