How to prevent the base of steel structure from sinking
How to prevent the steel structure workshop foundation sinking need to start from the foundation pile caps, when the steel structure workshop of 25 kg per square metre above foundation pile caps is needed to reach 1 m high and 1 meter wide and 1 meter deep, to more than 35 kg per square meter of steel structure workshop to do ring beam and 1.2 m, the basis of the specific need according to oneself circumstance on the ground.
When the steel structure factory building has been finished, and put the input, so how can we prevent the steel structure workshop foundation sinking, simple is reinforced, will all the steel columns with circle beams or channels, forming a mesh.This method can prevent the building of steel structure from subsiding.
In recent years, steel structure plant has been widely used in coal engineering for its light weight, excellent seismic performance, flexible structure layout and fast processing and installation speed.In this paper, we only discuss the structure of steel structure and steel frame structure of steel frame structure with the steel structure and steel frame structure.For larger tonnage crane steel structure plant, due to the upper structure weight light, the column bottom small axial force, bending moment is opposite bigger, foundation caused by eccentricity is too big, bring some difficulties to the foundation design.
The mechanical characteristics of the foundation of the steel structure
The foundation of steel structure plant is usually based on a separate foundation and is designed by eccentric compression.
For a rigid frame steel plant with a low height and no crane, the connection of the foot and base is usually articulated.The base surface is only the horizontal force generated by the vertical pressure and wind load generated by the superstructure.The base surface of horizontal wind load is relatively simple and the base design is relatively simple.
To high altitude with bridge crane and gantry frame steel structure factory building and plant steel bent structure, especially when larger tonnage crane (20 t crane single span two or more), in order to effectively improve the resistance to lateral stiffness of the structure to control the lateral displacement, column feet usually transverse just receive, longitudinal hinge design.
The vertical horizontal load of the plant is passed through the column support to the foundation roof.And in the transverse, because of light weight steel structures, structural natural vibration cycle is long, horizontal seismic effect is relatively small, the control action of transverse horizontal load are usually for the level of the crane brake load applied formula of wind two pole axial force can not equal.The formula is based on the elastic stability theory, which is applicable to the same cross diagonal rod with the same length and the same cross section.
The counterforce on the base surface is distributed unevenly due to the relatively large eccentric load, which may cause a large tilt in the base and even affect the normal use of the workshop, especially the crane workshop.Therefore, the pressure of foundation soil under the base of industrial plants must meet the following requirements:
1) for the crane load of column foundation, when included in the wind load, allow the bottom of foundation with zero stress of foundation soil area exists, but must satisfy the zero stress section length and the ratio of the base length L 'p/L, 0175, at the same time, you have to check the tension side of the foundation slab of the foundation under the action of gravity and the upper soil heavy bending strength.
2) for general crane loads under the base of a column, are not allowed to bottom of foundation with zero stress of foundation soil area exists, namely pmin acuity 0, if meet this condition, must demand basal eccentricity e b / 6 or less.
General methods of basic design
Based on the above basis of mechanical characteristics and design requirements, the column foot just pick a side column crane monolayer steel structure workshop, when the crane capacity is larger, if according to the conventional single base design, eccentricity is often the control conditions of foundation bed size, foundation bearing capacity can't afford to control function, large eccentricity will cause the bottom of foundation size is too large (sometimes reach more than 6 m length), is not very economic, engineering can't accept it.After analyzing some specific projects, the author thinks that such problems can be solved through the following methods:
3.1 adopt an eccentric basis
This method is effective when the base surface eccentricity is smaller (generally e is less than 015m).The principle is equivalent to preadding a reverse bending moment in the direction of the larger bending distance to reduce the eccentricity.However, due to the horizontal wind load and the load of crane, both the negative combination of positive and negative directions should be selected and controlled respectively.The current steel structure design procedure "STS" is not able to check the eccentric foundation. The designer can select several groups of unfavorable combinations and check them with other auxiliary procedures such as "li zheng".
The eccentricity base usually reduces the base size, but it should be used with caution for a crane with a large tonnage and a crane with a working grade of A6 ~ A8.
3.2 add additional weight to the base
This method is effective when the base surface eccentricity (015m < e = 112m).Adding additional weight to the base can be achieved in two ways:
1) the foundation is buried deep: the foundation buried depth increases the soil weight in the foundation and the base eccentricity decreases accordingly.At this time, the foundation can be designed as a single basis with reinforced concrete short columns. The section size of the short column is usually determined by the size of the steel column baseboard, and its reinforcement is determined by calculation.However, at the same time the base depth is increased, the additional bending moment of the base surface is also increased by the horizontal shear of the column foot, and the eccentricity of the base may also increase.Therefore, the above two factors should be taken into account in the design. After the comparison, the reasonable foundation shall be selected.
2) the lower part of the outer wall of the workshop adopts an aggravating wall: the wall can be made of non-clay sintering bricks, and the weight can be transferred to the foundation by the earth beam under the wall.Wall thickness can be used 370mm, height from the top of the roof to the bottom sill.In order to increase the wall, the bottom window sill can be appropriately raised according to the situation.The ground beam can be prefabricated, and the base can also be casted. The cast-in-place beam can be used to adjust the uneven settlement of the adjacent foundation.
In engineering design, the above two methods combined use effect is better.