The reaction force on the bottom surface of the foundation is unevenly distributed due to the relatively excessive eccentric load, which may cause the foundation to incline greatly and even affect the normal use of the plant, especially the plant with cranes.
Second. Stress characteristics of steel structure factory foundation
The foundation of the steel structure workshop is usually a separate foundation, which is designed according to eccentric compression.
For the portal-type steel frame factory buildings with low height and no crane, the connection between the column foot and the foundation is usually designed according to articulation. The top surface of the foundation is only subjected to the vertical pressure generated by the superstructure and the horizontal force generated by the wind load. The additional eccentric bending moment of the bottom of the foundation caused by horizontal wind load is small, and the foundation design is relatively simple.
In the application of the new code, the author found that the new steel rule has some new rules and calculation methods for the axial force members; the old code is sometimes conservative and sometimes not safe, so everyone in the design work must be Only by advancing with the times and constantly learning new norms can we make good designs that are both economical and safe.
Third. General methods of basic design
It can be done, but it cannot be used flexibly in actual combat. Today, the eccentricity is often the controlling condition for the size of the bottom of the foundation, and the foundation bearing capacity cannot control it. A larger eccentricity will cause the size of the bottom of the foundation to be too large (sometimes the side length) (More than 6m), which is very uneconomical and unacceptable in the project. After analyzing and comparing some specific projects, the author believes that such problems can be solved by the following methods in the design process:
a. Adopt eccentric foundation
The principle is equivalent to pre-adding a reverse bending moment in the direction of the larger bending distance to reduce the eccentric effect. However, since the horizontal wind load and crane load on the plant are bidirectional, the unfavorable combination of positive and negative directions should be selected for verification and control during design. The current steel structure design program "STS" is still unable to check the eccentric foundation. The designer can select several groups of unfavorable combinations and check them with the help of "Lizheng" and other auxiliary programs.
The eccentric foundation can usually reduce the size of the foundation, but this method should be used with caution for cranes with large tonnage and cranes with working class A6 ～ A8.
b. Increase the additional weight of the foundation
The additional weight of the foundation can be achieved in two ways:
1) Application in actual combat The increase of the x20 foundation burial depth will increase the soil weight above the foundation and decrease the eccentricity of the base accordingly. At this time, the foundation can be designed as a separate foundation with reinforced concrete short columns. The cross-sectional size of the short columns is usually determined by the size of the bottom plate of the steel column foot, and its reinforcement is determined by calculation. However, while increasing the depth of the foundation, the additional bending moment of the bottom of the foundation caused by the horizontal shear of the column foot will also increase accordingly, and the eccentricity of the base may also increase. Therefore, the above two factors should be considered in the design. After a trial calculation, a reasonable foundation depth should be selected.
2) The lower part of the external protection structure of the factory building adopts a weighted wall: the wall body can be made of non-clay sintered brick, and its weight is transmitted to the foundation through the ground beam under the wall. The thickness of the wall can be 370mm, and the height is from the top surface of the ground beam to the bottom window sill. In order to increase the height of the wall, the bottom window sill can be raised appropriately according to the situation. The ground beams can be prefabricated or cast-in-place with short foundation columns. The cast-in-place ground beams are useful for adjusting the uneven settlement of adjacent foundations.
In engineering design, the combined use of the above two methods is better.
c. Adopt pile foundation
When the eccentricity of the bottom of the foundation is large (e> 112m) and the depth of the bearing layer is deep, which cannot be solved by the above method; or the crane tonnage of the plant is large, the ground long-term large area stacking exceeds 60kN / m2, and the base soil is medium 1. Highly compressible soil, when the additional impact of pile loading on the foundation must be considered, pile foundation should be used. The type of pile foundation can be comprehensively determined according to the soil quality of the foundation and local construction conditions.
Qingdao Yili Steel Structure professionally undertakes steel structure design and production, budget, dismantling and rendering. Factory building, warehouse, steel structure agricultural building. For more questions, please consult the customer service of steel structure for consultation.