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How to prevent the foundation of steel structure buildings from sinking?

   

    How to prevent the foundation of the steel structure from sinking from the foundation platform, when the steel structure plant reaches 25KG per square meter, the foundation cap needs to reach 1m in height, 1m in width and 1m in depth, and the ash in each square meter is more than 35KG. The steel structure workshop needs to be a ring beam and a 1.2 meter foundation, and it also needs to be based on the ground itself.


     When the steel structure workshop has been completed and no prior investment is made, how can we prevent the steel structure workshop from sinking? Simply put, it is reinforced, and all the steel structure pillars are encased in I-beam or channel steel. , forming a mesh. This method can prevent the foundation sinking of a steel structure factory.

    In recent years, steel structure workshops have been widely used in coal engineering because of their light weight, superior seismic performance, flexible structure arrangement, and fast processing and installation. This article only discusses the single-storey factory buildings (hereinafter referred to as “steel structure buildings”) of light-gauge steel structures and steel girder structures with a gabled steel frame or a sandwich panel for envelope structures. For steel plant buildings with large tonnage of cranes, due to the light weight of the upper structure, the axial force at the bottom of the column is relatively small, and the bending moment is relatively large, resulting in a large eccentricity of the foundation, which brings some difficulties to the basic design.

1 Stress characteristics of steel structure workshop foundation


     Steel plant foundations are usually based on a separate basis and designed to be eccentrically compressed.


     In the case of steel portal plants with low height and without a crane, the connection between the column foot and the foundation is usually articulated. The base top surface is only subjected to the vertical pressure generated by the superstructure and the horizontal force generated by the wind load. The horizontal bottom surface generated by the horizontal wind load has a smaller eccentric bending moment, and the basic design is relatively simple.


   

For high-height gantries with steel girders and steel girder structures with bridge cranes, especially when the tonnage of cranes is large (two single-towers and 20t cranes or more), in order to effectively improve the structure The lateral stiffness is controlled to control the lateral displacement. The column foot is usually designed to be laterally rigid and longitudinally articulated. The vertical horizontal load of the factory building is transmitted to the base top surface through the inter-column support. In the horizontal direction, because the steel structure has light weight, the structure has a long period of natural vibration, and the horizontal seismic effect is relatively small. The lateral horizontal load that controls the control is usually the horizontal braking load plus wind formula of the crane. The axial force of the two rods may not be equal. The formula is based on elastic stability theory.


Applicable to two cross diagonals with the same length and the same cross section.


1) Cross the other rod under pressure, the two rods are of the same cross-section and are not interrupted at the intersection, then:

l:Truss node center spacing(m)

lo:Calculated length of bending buckling(m)

N:Calculate the internal force of the rod(N)

No:Intersecting another piece of internal force(N)

2) Intersecting the other rod under pressure, this other rod is interrupted at the crossover but overlapped with the gusset plate, then:

3) Intersecting the other rod under tension, the two rods are of the same cross-section and are not interrupted at the crossover point,then:

4) Intersecting the other lever is interrupted. This lever is interrupted at the intersection but is overlapped with the gusset plate. Then:


 


The comparison of the calculated length factors of the new and old norms is shown in Table 1.


As can be seen from the table, the old code is sometimes conservative and sometimes less secure.


In the application of the new specification, the author discovered that the new steel rules have some new provisions and calculation methods for the axial force members; the old rules are sometimes conservative and sometimes not very safe. Therefore, in the design work, everyone must To keep up with the times and constantly learn new norms, we can make a good design that is economical and safe.


2 Basic requirements for basic design


    The counterforce at the bottom of the foundation is not evenly distributed due to relatively large eccentric loads, which may result in a large tilt of the foundation and may even affect the normal use of the factory buildings, especially those with cranes. Therefore, the foundation soil underneath the industrial plant foundation is subject to the following pressures:


1) For the column foundation without crane load, when the wind load is taken into account, the zero stress region of the foundation foundation soil is allowed to exist, but the ratio of the length of the non-zero stress region to the base length must be satisfied L'/L ≥ 0175, at the same time It is also necessary to check the bending strength of the tensioned side of the foundation slab under the weight of the foundation and the weight of the upper earth.


2) For the column foundation subjected to normal crane loads, the existence of a zero stress zone in the foundation foundation soil is not allowed, ie, pmin ≥ 0. If this condition is satisfied, the base eccentricity must be e ≤ b/6.


 


3 General methods for basic design


    According to the above-mentioned basis of the force characteristics and design requirements, for the column foot just connected to the crane single-storey steel plant side column, when the crane tonnage is large, if the conventional basis design, eccentricity often becomes the basis of the bottom size Under the control conditions, the bearing capacity of the foundation does not play a role in control, and the larger eccentricity will cause the base floor size to be too large (sometimes more than 6m in length), which is uneconomical and unacceptable in the project. After analyzing and comparing some specific projects, the author believes that such problems can be solved in the design process through the following methods:


3.1 Using Eccentricity


 This method is effective when the base surface eccentricity is small (typically e ≤ 015m). The principle is equivalent to pre-apply a reverse bending moment in the direction of the larger bending distance to reduce the eccentricity. However, due to the two-way effect of horizontal wind load and crane load on the factory building, the unfavorable combination of positive and negative directions should be selected for verification and control. The current steel structure design program “STS” cannot yet verify the eccentricity basis. The designer can select several groups of unfavorable combinations and check them with other auxiliary procedures such as “justification”.


Eccentricity can usually reduce the basic size, but for cranes with larger tonnages and cranes with working levels A6-A8, this method should be used with caution.


3.2 Increase the basic additional weight


    This method is effective when the base surface eccentricity (015m < e ≤ 112m). Adding basic additional weights can be achieved in two ways:


1) Increase the burial depth of the foundation: When the foundation burial depth increases, the soil weight in the upper part of the foundation increases accordingly, and the base eccentricity decreases accordingly. In this case, the foundation can be designed as a separate foundation with a reinforced concrete short column. The cross-sectional size of the short column is usually determined by the size of the steel column foot floor, and its reinforcement is determined by calculation. However, at the same time of increasing the foundation burial depth, the additional bending moment caused by the horizontal shear force of the column foot will increase correspondingly, and the base eccentricity may also increase. Therefore, the above two factors should be comprehensively considered in the design. After a trial and comparison, a reasonable foundation depth should be selected.


2) The weight-increasing wall is used in the lower part of the external protective structure of the plant: The wall can be made of non-clay sintered bricks, and its weight is transmitted to the foundation through the ground beam under the wall. The wall thickness can be 370mm, height from the top of the floor beam to the bottom sill. In order to increase the wall height, the bottom ledge can be properly raised according to the situation. The ground beam can be prefabricated or cast in place with the base short column. The cast-in-place beam is conducive to adjust the uneven settlement of the adjacent foundation.


In engineering design, the combination of the above two approaches works better.


 


3.3 Using pile foundation


    When the foundation bottom eccentricity is relatively large (e>112m) and the depth of the bearing layer is deep, the above method cannot be used to solve the problem; or the tonnage of the plant crane is large, the surface long-term large-area surcharge exceeds 60kN/m2, and the foundation soil is medium. For high compressive soils, pile foundations should be used when the additional impact of the piles on the foundation must be considered. The type of pile foundation can be comprehensively determined according to the ground conditions of the foundation and local construction conditions.

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