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General methods for steel structure frame building foundation design

Based on the above basis of mechanical characteristics and design requirements, the steel column foot just pick a side steel column crane monolayer prefab steel warehouse/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. Through the analysis and comparison of some specific projects, the author believes that such problems can be solved through the following methods in the design process:

(1)Using eccentric foundation

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 frame design program “STS” cannot yet verify the basis of eccentricity. 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.

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(2)Increase the steel structure building foundation  additional weight

1.Increase the burial depth of the steel structure frame building foundation: When the foundation burial depth increases, the soil weight in the upper part of the foundation increases accordingly, and the base eccentricity decreases correspondingly. In this case, the foundation can be designed as a separate foundation with a reinforced concrete short column. The section 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 lower wall of the external protective structure of the plant adopts a weight-increasing wall: The wall can use non-clay sintered bricks, and the 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 it can be cast in place with the foundation 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)Using pile foundation

When the base surface 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 long-term large-area surcharge of the ground 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 based on the ground conditions of the foundation and local construction conditions.