What are the common types of column bases for steel structure building ?

This article mainly introduces some common forms, designs, and practices for column bases of steel structure

Classification of Steel Column Base

1. According to the type of support, it can be divided into rigidly connected column bases and hinged column bases.

2. According to the structural form, it can be divided into: embedded column base, external column base, inserted column base, and exposed column base.

The Type and Mechanical Properties of Column Base

(1) The exposed column base connects the bottom of the column to a relatively rigid base plate by welding, which is fixed with anchor bolts embedded in the concrete foundation. This form has been in use since the early stages of steel structure development until today. Traditionally, column bases are classified and used as either hinged or rigidly connected. However, the former is difficult to achieve complete articulation, or although it can transmit a certain amount of bending moment, it cannot meet completely indirect rigidity conditions. From a mechanical perspective, it is more appropriate to consider it as a semi rigid connection. The principle mode of transmitting bending moment at this type of column base is shown in Figure 1. On the column base, axial force, bending moment, and shear force are applied. The tension and pressure are transmitted through the contact between the column base plate and the concrete foundation. The shear force is resisted by the friction between the column base plate and the foundation contact surface or transmitted by anchor bolts.

P1 The principle of force transmission for exposed column bases

In most cases, the plastic bending moment at the bottom of a column determined by the yield of anchor bolts is relatively small compared to the fully plastic bending moment at the cross-section of the exposed column base and column root. The mechanical behavior of the column base under bending moment is mainly determined by the performance of the anchor bolts. The load deformation relationship is shown in Figure 2. If the anchor bolt can fully develop plastic deformation after yielding under tension, the restoring force characteristics of the exposed column base under repeated loads are shown in Figure 3. However, in actual engineering design, the column base often breaks at the threaded part before the weakened section of the anchor bolt yields, or there is a high possibility of the anchor bolt being pulled out due to insufficient anchoring. In this case, it is difficult to see sufficient plastic deformation of the column base.

P3 Resilience of exposed column bases under repeated loads

(2)Outsourced column base

The outsourced column base is wrapped in reinforced concrete within a range of 2.5-3 times the height of the column cross-section (maximum cross-sectional size) at the bottom of the steel column (Figure 4). This form of column base is designed as a fixed column base, and if designed correctly, it can ensure the degree of fixation and bearing capacity of the column base. The mechanical behavior of this column base is mainly determined by the mechanical behavior of the concrete wrapped with steel bars.

P4 Outsourced column base

(3) Embedded column base

The embedded column base embeds the bottom of the steel column with a length equivalent to about twice the cross-sectional height into the foundation concrete. The surrounding area is reinforced with reinforced concrete (Figure 5). From the perspective of construction convenience, this type of column base has more processes compared to other forms of column bases, resulting in an unfavorable aspect of the construction period. However, the structural concept is very clear as it can easily meet the requirements of forming plastic hinges at the bottom of the steel column. Therefore, as long as the design and construction are correct, the restoring force characteristics of the column base can display a stable spindle shaped relationship. The main design considerations are to ensure the depth of the column base and the thickness of the surrounding concrete for the embedded part of the steel column.

P 5 Embedded Column Base

(P 6) shows the restoring force characteristics of the column base when the burial depth of the column changes according to 1D, 2D, and 3D (D: column section height). The restoring force characteristics change with the burial depth of the column base, but to the extent that the burial depth reaches twice the height of the column section, it can be considered that it basically becomes a spindle shape. In order to consider the bonding with the surrounding concrete, there is also a design method of welding bolts on the embedded part of the column, but the bolts only work when there is a significant separation between the embedded part of the steel column and the surrounding concrete. Within the expected range of design load, such a large separation will not occur, so it can be considered that there is basically no such effect.

P6  Restoring force characteristics of embedded column feet

The concrete around the embedded column base is subjected to bending moment and shear force acting on the bottom of the column, resulting in significant pressure (see P 7). Therefore, it is necessary to ensure the thickness of the outer layer to prevent punching and shear failure. For the central column, there is generally no problem, but for the edge column and corner column, the outer thickness must be determined based on calculations. If the necessary concrete thickness cannot be guaranteed due to issues such as adjacent buildings or red lines, steel reinforcement should be used for necessary reinforcement.

2、 Basic points of column base design and construction

To ensure the seismic safety of column base nodes, it is necessary to fully understand the mechanical characteristics of the column base and carry out design and construction correctly.

For design, first of all, it is necessary to clearly specify the performance of the column base under the service limit state. Secondly, in the ultimate state of bearing capacity, it is necessary to consider the performance of the steel column and column base. That is to say, under the limit state of use, the column base designed as a hinged connection should not generate bending moments, and to a certain extent, the column base considered as a fixed connection should be able to rotate, which belongs to rigid connections and can bear bending moments. In addition, general exposed column bases, anchor bolts, column base plates, etc. all undergo elastic deformation. A complete rigid connection cannot be achieved. Furthermore, in the ultimate state of bearing capacity, plastic hinges will occur at the column base, which requires a greater plastic rotational capacity. The detailed design of the column base must be carried out in accordance with the requirements of the design policy.

Therefore, for the use of limit state design, the rotational stiffness of the column base should be appropriately evaluated, the stiffness of the column base should be considered in structural analysis to obtain the design internal force, and the internal force acting on the column base should be accurately grasped. Proper design for axial force, bending short, and shear force is a basic requirement.

The design points of various forms of column bases are described separately below.

(1) Exposed column base

It is quite difficult to achieve complete hinge or rigid connection for exposed column bases. However, if the following conditions are met, the column base can basically be assumed to be designed as hinged: the column base plate has great stiffness, which can suppress local deformation and select an appropriate anchor bolt length, so that the threaded part of the anchor bolt does not break before plastic deformation develops within the full length of the anchor bolt; The adhesive part of the anchor bolt in the concrete foundation can prevent punching and shear damage. However, in this situation, the tensile bearing force of the anchor bolt must be ensured to be greater than the column base tension caused by horizontal force (without considering the effect of live load). The smaller value of 1/2 of the tensile yield bearing capacity of the column. It is best to set shear keys under the column base plate to ensure resistance to horizontal forces.

When the exposed column base is designed as a bending resistant column base, the tension generated by the bending moment is borne by the anchor bolt, and the contact surface between the column base plate and the foundation concrete bears the pressure. In the preliminary design stage, it is generally assumed that the column base plate is a rigid connected component, and the anchor bolt is used as the tensile reinforcement, and the area of the foundation plate is used as the cross-section of the reinforced concrete column. Based on this calculation result, the anchor bolt cross-section is selected. In this case, when the cross-section of the column is large to a certain extent, it is often difficult to choose sufficiently thick anchor bolts to ensure the necessary bearing capacity. So it is difficult to design a column base with a bending bearing capacity greater than the bending bearing capacity of the column section. If a certain degree of high-strength anchor bolts can be used, this problem should be solved. However, once high-strength anchor bolts are used, it may cause design and construction problems to prevent the punching and shear failure of the anchored foundation concrete, and the problem of excessive pressure between the column base plate and the foundation concrete below will also arise. Therefore, the design bearing capacity of exposed column bases is mostly determined not by the bearing capacity of the column but by the bearing capacity of the column base. In this case, the bending bearing capacity of the column base must reach at least 1/2 of the bending bearing capacity of the column itself. In addition, to ensure plastic deformation capacity, the design and construction requirements of each part of the column base must also have the same guarantee conditions as the articulated column base mentioned above. Furthermore, in this case, it is necessary to correctly evaluate the rotational stiffness and restoring force characteristics of the column base and reflect them in the design of the frame.

After considering the above points, the specific content of various projects related to the design of exposed column bases is as follows (refer to P 8 and P 9).

P 9 Force transmission principle of external buried column base

                                              

                     1.Rotation due to anchor bolt elongation

                     2.The bottom plate basically does not produce bending deformation

                     3.Punch shear failure starting from anchor components cannot occur

                     4.No damage to the concrete above the ground

                     5.Resist shear force with shear keys

a) Column base plate

To ensure that the column base plate can maintain elasticity even in the ultimate bearing capacity state, and to determine the plate thickness or provide appropriate reinforcement for this purpose. As a result, the designed bottom plate can be considered as a rigid component, and the evaluation of the stiffness of the column base can be done without considering the influence of bottom plate deformation. However, the welding between the base plate and the column adopts a fully welded connection. Under the base plate, welding can resist the shear force acting on it.

b) Anchor bolt

B-1. It is required that the plastic deformation of the column base is generated by the anchor bolt under the ultimate bearing capacity state.

Anchor bolts are made of steel materials such as Q235B and Q355B that can ensure yield strength ratio and plastic deformation ability.

The threaded part of the anchor bolt is processed and divided at both ends of the anchor bolt to ensure that the threaded part does not break before the bolt rod reaches full section yield. The length of the bolt rod is more than 25 times its diameter. Apply anti rust coating or wrapping to the bolt rod section. When fixing the bolt base plate, double nuts are used to prevent rotation. The embedded end in the foundation concrete is equipped with anchoring components to prevent the anchor bolt from pulling out.

B-2. Under the limit state of bearing capacity, it is required that the plastic deformation of the column base is generated by the column

There are no restrictions on the strength, yield ratio, and plastic deformation capacity of anchor bolt materials. However, the selection of the cross-sectional area of the anchor bolt should ensure that the bending moment in the column and the stress in the threaded part of the anchor bolt caused by the tensile force at the limit state are lower than the yield strength of the anchor bolt rod.

Install necessary length threaded parts at both ends of the anchor bolt. The length setting of the anchor bolt rod should consider preventing the foundation concrete from being damaged by punching and shearing starting from the anchor bolt fixing point. When fixing the column base plate, double nuts are used to prevent rotation. The embedded end in the foundation concrete should be equipped with anchor fasteners to prevent the anchor bolt from pulling out.

C Foundation concrete

When the short column part on the foundation concrete is subjected to pressure from the column bottom plate, the common part should not be damaged. Therefore, there should be sufficient size amplification relative to the column base plate. Additional steel bars should be provided on the outer side of the anchor bolt. In addition, when bearing the ultimate limit state, it is necessary to ensure the necessary depth and width to prevent concrete punching shear failure starting from the anchor bolt fixing point.

d. Foundation grouting

Ensure sufficient spacing between the top surface of the concrete foundation and the bottom plate of the column to fill with mortar. The mortar adopts strength non shrinkage mortar.

a) Ensure the plane position and protruding length size of the anchor bolts embedded in the foundation concrete b) Rust prevention treatment of the anchor bolts c) Bonding of the anchor bolts d) Sealing (grouting) of the column bottom plate and the concrete foundation e) Tightening and rotation prevention of the anchor bolts

As mentioned above, there are many issues that need to be addressed in the design and construction of exposed column bases, and any of them cannot achieve the required performance. Therefore, it is necessary to fully understand this. Especially the difficulty in purchasing anchor bolt products that meet the above requirements in the market is a major issue.

On the other hand, there are already exposed column foot construction methods that take into account the above points and understand their mechanical properties through structural experiments. Determine and apply standardized design details in other countries, implement a responsible construction system, and ensure construction quality.

(2) Outsourced column base

The outsourced column base (see P 4) can meet the design requirements of rigid connection at the column bottom as long as the correct design is carried out on the reinforced concrete of the outsourced part. That is, to ensure a height equivalent to 2.5 times the height of the column cross-section. The shear force acting on the top column cross-section of the outer concrete is treated as a concentrated load acting on the cantilever type outer concrete. Design this so that the stress borne by the steel column is transmitted to the wrapped reinforced concrete. In this case, the anchor bolts at the bottom of the column can only bear the internal force during installation. In order to ensure the stress transfer from the steel column to the reinforced concrete at the top position, reinforced steel bars are installed at the top, and longitudinal bars at the four corners of the outer concrete are reinforced with hoops at the top, ensuring the necessary protective layer thickness (refer to P 10).

(3) Buried branch feet

The embedded column base (refer to P 5) should ensure a burial depth equivalent to more than twice the height of the column section. The steel column surrounding the embedded part should be appropriately equipped with steel bars. Through these measures, the column base can be designed with sufficient strength and plastic deformation can occur at the lower end of the column. That is to say, it is relatively easy to design column bases that allow steel columns to exert plastic deformation capacity. However, the embedded column base requires the steel column embedded in the foundation concrete to transmit the steel column reaction force to the surrounding concrete through pressure, ensuring the burial depth of the column and the thickness of the concrete around the column is the key point of the design (refer to P 7). If the burial depth is not enough or the thickness of the surrounding concrete is too small, it will cause punching shear failure of the concrete, and the column base may be damaged before the plastic deformation of the column fully develops (refer to P 11).

The concrete around the column transmits force through mutual compression with the column body, causing local out of plane deformation of the column panel. It is a good method to add a stiffening partition board (refer to Figure 7) or fill the column with concrete in a steel column slightly lower than the top of the foundation concrete.

By designing the embedded column base reasonably, plastic hinges can be formed under the ultimate bearing state of the column. Therefore, the column itself must have sufficient plastic deformation capacity. Therefore, steel that may cause brittle failure at its limit, such as cold formed square steel pipes, should not be used as much as possible in this situation.

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