How to prevent steel structure workshop foundation sinking
Release time:2020-07-19
How to prevent the steel structure workshop from moving down basically should start from the foundation bearing platform. When the steel structure workshop is above 25kg per square meter, the foundation bearing platform must be one meter high, one meter wide and one meter deep. The steel structure factory building from each square meter to 35kg above must be made of ground ring beam and 1.2m foundation. In fact, it must also be based on the condition of the pavement itself.
When the construction of the steel structure workshop has been completed and the early capital investment has not been well done, then how can we prevent the steel structure workshop from moving down basically? Simply speaking, it is the structural reinforcement. All the steel frame structure poles are framed with profiled steel or round steel frames to produce a mesh structure. This method can avoid the basic downward moving effect of steel structure workshop.
In recent years, steel structure workshop is widely used in coal engineering because of its light weight, superior seismic performance, convenient structure and layout, faster production, processing and installation. This paper only discusses the light steel structure of portal steel frame and single-sided industrial workshop of steel bent structure with profiled plate or color steel sandwich panel (hereinafter referred to as "steel structure workshop"). For the steel structure workshop with large crane tonnage, because the upper structure weight is too light, the axial force at the bottom of the column is small, and the bending moment is relatively large, which leads to the excessive eccentricity, which makes the basic design scheme difficult.
1. Basic bearing capacity characteristics of steel structure workshop
The steel structure workshop is generally independent and designed according to the axial force.
For the portal steel structure workshop with low height width ratio and no crane, the connection between the column base and the basic is generally designed according to the stranding design scheme. The top of the basic wall is only subjected to the longitudinal working pressure caused by the upper structure and the leveling force caused by the wind load. The additional axial force bending moment of the basic bottom edge caused by level wind load is small, and the relativity of the basic design scheme is simple.
For the portal steel frame steel structure workshop and steel bent structure industrial workshop with high height width ratio and containing stacking chain crane, especially when the crane tonnage is large (single span two 20t cranes or larger), so as to reasonably improve the structure's anti lateral bending stiffness to operate the vertical and horizontal offset, the general design scheme of column foot is transverse rigid connection and vertical hinge connection. The vertical level load of the workshop is transferred to the top of the basic wall according to the support between columns. In the horizontal direction, due to the light weight of steel frame structure, long natural vibration time and low relativity of horizontal seismic disaster effect, the horizontal level load with operation effect is generally calculated by the load wind formula of crane level braking system, and the axial force of two poles can be different. Based on the basic theory of ductility and stability, the formula is suitable for cross inclined bars with the same length and cross-section.
1) If the cross section of the two bars is the same and does not break at the joint point, then:
Where
2) When the other pole is stressed, the other pole is finally broken at the joint point, but overlapped with the plate reinforcement at the connection point, then:
3) If the cross section of the two bars is the same and does not break at the joint point, then:
4) When the other pole is tensioned, the supporting rod is finally broken at the joint point, but overlapped with the plate reinforcement at the connection point, then:
See Table 1 for comparison of length index measured by new standard and old standard.
It can be seen from the table that the old standards are sometimes more traditional and sometimes less secure.
In some cases, it is not necessary to adopt new standards and new standards in the design of components, which is not conducive to the development of new standards and standards.
2 basic provisions of basic design scheme
The axial force of the basic bottom edge is uneven due to the relatively large axial force load effect, which will lead to a large skew and even endanger the normal application of industrial buildings, especially industrial buildings with cranes. Therefore, the following requirements should be considered for the working pressure of subgrade soil at the basic bottom of powerhouse:
1) For the column foundation without crane load, when the wind load effect is recorded, zero in-situ stress zone is allowed to exist in the subgrade soil at the basic bottom, but the ratio between the length of non-zero in-situ stress zone and the basic length length L '/ L ≥ 0175 must be considered. In addition, the tensile strength of the tensile side of the basic bottom plate under the effect of basic weight and upper soil weight must be calculated by formula.
2) For the column foundation bearing the general crane load effect, there is no zero in-situ stress zone in the subgrade soil of the basic bottom, i.e. Pmin ≥ 0. If considering this standard, it is necessary to specify the eccentricity of the base material e ≤ B / 6.
3 general mode of basic design scheme
According to the above basic bearing capacity characteristics and design scheme, when the crane tonnage is large, the eccentricity usually becomes the operation standard of the basic bottom edge size, and the bearing capacity can not be operated, The large eccentricity will lead to the excessive specification of the basic bottom edge (sometimes the perimeter is more than 6m), which is not economic development, and can not be accepted in engineering projects. Based on the analysis of some practical projects, the author thinks that such problems can be solved in the following ways during the design process:
3.1 selection of axial force
When the eccentricity of the basic bottom is small (E ≤ 015m in general), this method is more reasonable. Its basic principle is to pre add a bending moment in the opposite direction to reduce the axial force effect. However, because the level wind load and crane load of the industrial plant are both double effects, the design scheme should choose the bad composition of the positive and negative directions, and calculate and operate in various aspects. At present, the steel structure building design program flow "STS" has not been able to calculate the axial force basically. The design scheme staff can select two groups which are not easy to form, and rely on "Lizheng" and other auxiliary program processes to calculate the axial force in many aspects.
The axial force can reduce the basic specifications, but for cranes with large tonnage and A6 ~ A8 in operation, this method should be used with caution.
3.2 lifting basic extra net weight
When the basic bottom eccentricity (015m1) increases the basic buried depth, the soil weight at the upper end is relatively increased, and the eccentricity of the bottom material is relatively reduced. At this time, the basic design scheme can be regarded as an independent foundation with concrete structure short column. The cross-section specification of short column is generally decided by the foot size of steel beam, and the reinforcement drawing is determined by calculation. However, in addition to increasing the depth of the foundation, the additional bending moment of the basic bottom caused by the horizontal shear stress of the column base will be relatively increased, and the eccentricity of the base material will also be expanded to a certain extent. Therefore, the above two elements should be considered comprehensively in the design scheme, and the effective foundation depth should be selected after trial calculation.
2) The intensifying wall is selected under the outer bent structure of the industrial plant: the non clay shale brick can be selected for the wall surface, and the net weight can be transferred to the basic structure according to the foundation beam of the tension beam. The wall thickness can be 370mm, and the height width ratio is from the top of foundation beam wall to the lowest balcony. In order to raise the height of the wall, the balcony at the bottom floor can be moderately raised according to the situation. The foundation beam can be prefabricated, and can also be cast-in-place slab with basic short column. The cast-in-place slab foundation beam is beneficial to adjust the uneven foundation settlement adjacent to the foundation.
In the architectural engineering design, the practical effect of the integration of the two methods is stronger.
3.3 selection of pile foundation
When the eccentricity of the basic bottom edge is very large (E > 111m) and the cushion layer is buried deep, which can not be treated by the above method; or when the crane tonnage of industrial workshop is large, the long-term large-scale surcharge of pavement exceeds 60kn / m2, and the bottom material soil is medium and high expansive soil, it is necessary to consider the additional harm of surcharge on the basic. The type of pile foundation can make the soil condition and local construction condition comprehensive and clear.
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