Steel structure column base node construction

As a modern high-end building structure, the light steel structure system is favored by designers and builders due to its rich and varied structural modeling effects, and is widely used in multi-storey buildings. However, in actual construction, there are several quality problems and potential safety hazards that are more likely to occur at the various parts of the column foot and the support node. How to correctly construct the steel structure column foot node?

1 Analysis of column foot joints

1.1 Existing problems

In the construction of column foot joints with a large number of projects, a large building height, and a large wind load, it often occurs that the concrete in the shear key pit is not easy to pour and compact, and the secondary grouting materials for the reserved space under the column bottom plate use ordinary fine stone concrete, anchors, etc. Bolt position deviation exceeds the limit, bolt holes are reamed by gas cutting, backing plate and base plate are missing welding, fillet weld fillet size is insufficient, etc., resulting in a large difference between the formed column base and the design state, endangering the safety of the upper structure.

1.2 Reasons

(1) The “Code” stipulates that the anchor bolts at the column foot cannot be used to bear the horizontal reaction force at the bottom of the column foot. This horizontal reaction force is borne by the friction between the bottom plate and the concrete foundation or by setting a shear key. The shear key is vertically welded to the bottom of the column bottom plate with channel steel, etc., resulting in a pit on the surface of the foundation. A channel steel is inserted into the pit on the top surface of the foundation. It is impossible to remove the sundries in the pit, and it is also impossible to check whether the pit inserted into the channel steel is grouted. It is even more impossible to check whether it is dense. The secondary grouting material for the reserved gap under the column base plate is filled with ordinary fine stone concrete, and the non-shrinking fine stone concrete configured without expansion cement is used for filling. The concrete shrinks during the hardening process, and a gap is formed between the base plate of the column base and the foundation concrete. Both will lead to the reduction of friction between the base plate of the column base and the foundation concrete or the ineffectiveness of the shear key setting, and the anchor bolts will be damaged due to insufficient horizontal load resistance capacity under earthquake action.

(2) The section size of the short column of reinforced concrete foundation is similar to the size of the bottom plate of the column foot. The steel bars in the short column of reinforced concrete foundation are densely arranged, and the positioning control of the concrete structural reinforcement is not done well during construction. Vehicle; the foundation anchor bolt positioning construction adopts the formwork positioning method, and the anchor bolts of the same column foot are positioned through wooden formwork or steel formwork. Due to the lack of reliable fixation, the anchor bolts are easy to move, and the overall axis deviation of the anchor bolts is opposite to that of the same column foot anchor bolts. The position deviation is very large; during the secondary grouting construction of the reserved space under the column base plate, wooden square logs are used to push into the 50mm gap, and the operation procedure is not carried out correctly, so it is difficult to ensure the density of the secondary grouting layer, especially the base plate of the column foot When the size is large, there will be a cavity in the middle of the bottom plate, part of the axial force is borne by the adjusting nut, the contact surface between the bottom plate of the column foot and the concrete surface of the foundation is reduced, and the friction between the bottom plate and the concrete is reduced; a gap is reserved under the bottom plate of the column foot, and in the main structure Corresponding measures were not taken in time during the installation process. Under the combined load in the construction stage, the anchor bolts at the column foot may be bent; when the gap adjustment between the anchor bolt holes and the anchor bolts on the column bottom plate cannot make the anchor bolts free manholes, the gas cutting method should be used to expand the manhole. Holes, gas cutting and reaming are very irregular, which weakens the effective section of the column foot plate and affects the shear resistance; the design drawing clearly stipulates that the column foot plate and the column bottom plate are welded after the column foot is installed, and welding is missing during construction. The soles of the feet may not be in contact with each other and cannot participate in shear resistance: on-site manual arc welding has poor welding conditions and the weld seam is not full. Both lead to a large difference between the formed support structure and the design state, and the bearing capacity cannot meet the design requirements.

1.3 Prevention suggestions

(1) The shear key generally adopts I-shaped steel, channel steel, and angle steel. Shear welds can avoid making pits on the foundation. The grouting gap under the column bottom plate is to ensure that the shear key does not contact the foundation surface, and a certain gap is left so that the low elevation of the column has an adjustment allowance. The material for the secondary grouting of the reserved space under the column bottom plate must be non-shrinkage mortar, which is prepared by special expansion cement, or dry hard expansion fine stone concrete with a higher strength than the foundation concrete.

(2) During the construction process, the positioning control of the concrete structural steel bars should be done well. When the concrete structure construction meets the installation elevation of the pre-embedded anchor bolts, the size of the concrete structural steel bars should be adjusted and fixed to avoid conflicts with the pre-embedded anchor bolts. The position of the embedded column foot anchor must be accurate. The secondary grouting construction method of the reserved space under the column bottom plate must be the twisting grouting method or the grouting method, and the compactness must be ensured. After the steel column is installed, the column foot should be wedged tightly with steel wedges in time as construction protection. After the wedge-shaped pads are removed, fill the gap with grout. Necessary measures should be taken during pouring to prevent leakage or non-compacting. When the anchor bolt at the column foot cannot pass through the bolt hole freely, the anchor bolt manhole cannot be hit with a hammer. Reaming by gas cutting is strictly prohibited. After the column is installed in place, the backing plate under the nut is fully welded to the bottom plate of the steel column to ensure that all anchor bolts participate in shear resistance. The nut and the backing plate should be tightened and welded firmly. Check the quality assurance certificate and the state of use of welding consumables and flux. Check flux drying records. Welders must hold a welder qualification certificate issued by the labor department, and qualified items must be able to cover all welding positions on site.

(3) The position deviation of the anchor bolt causes the anchor bolt to be unable to freely pass through the bolt hole, and the deviation cannot be repaired, so the hole of the base plate of the column foot or the hole of the shoe beam cover plate must be enlarged. When the position deviation of the anchor bolt hole is greater than 0.2d (d is the diameter of the anchor bolt) and smaller than the diameter of the anchor bolt hole, the method is to expand the original hole to the position of the new hole first, and then add a supplementary plate on the outside of the connected plate. The thickness of the plate to be connected shall not be less than that of the plate to be connected, and the length and width shall not be less than 2 times the diameter of the anchor bolt hole. After the anchor bolt is tightened, the patch plate and the surrounding plate shall be welded with foot welds. The size of the fillet weld shall not Less than 15mm. When the position deviation of the anchor bolt hole is less than 0.2d, the hole can be directly expanded to the design position. It is recommended to use a reamer for trimming. In order to prevent iron filings from falling into the seam of the plate when repairing the hole, all the surrounding anchor bolts should be tightened before reaming.