Embedded Column Base Research Articles

Experimental investigation on seismic performance of embedded column base for circular CFST

Embedded column bases for concrete-filled steel tubes play an essential role in the structural safety of high-rise and super-high-rise buildings. However, the working mechanism and design methods of embedded column bases remain unclear. This paper presents an experimental investigation of five embedded column base specimens subjected to axial compression load and cyclic lateral load, to thoroughly reveal the seismic performance of such column bases with varied parameters. The failure modes, hysteretic curves, energy dissipation capacity, stiffness degradation, and strain developments were discussed. The test results revealed that increasing the embedded depth ratio and utilizing anti-punching shear reinforcements led to an improvement in the seismic performance of the embedded column base. Moreover, the axial load ratio only influenced the seismic performance of the column when the embedded depth was sufficient. Finally, a calculation method was proposed to determine the ultimate strength of the embedded column base. The method considered the resistances resulting from the horizontal interaction between the steel tube and foundation concrete, vertical interaction between the endplate and foundation concrete, and axial deformation of anchor bolts. The predicted results were found to agree well with the test results.

Experimental study and design method of embedded concrete-filled rectangular steel tube column base under cyclic load

Embedded column base has been widely used in concrete-filled steel tube (CFST) structures due to its high rigidity and good seismic performance. However, the strength analysis of the embedded CFST column base was not yet clear since there was little experimental research on it. In this paper, the experimental investigations of six embedded concrete-filled rectangular steel tube (CFRST) column bases under constant axial load and cyclic load were carried out to study the effects of embedded depth, improvement measures and axial compression ratios on the seismic performance of the column bases. The experimental results indicated that setting stiffeners, increasing the embedded depth or axial compression ratios could improve the seismic performance of the column base. In this experiment, the critical embedded depths of the columns with or without stiffeners were 0.7 and 1.5 times the column width B respectively. When the embedded depth was 0.7B, the contribution of anchor bolts to the moment resistance capacity of the column bases could reach up to 14.2%. Based on the analysis of the force transmission mechanism between the column and the concrete foundation in the embedded CFRST column base, a design method considering the contribution of different components to the moment resistance capacity of the CFRST column bases is proposed. The prediction results are in good agreement with the tested results and collected data, within ± 15% error. The parameter analysis based on the design method has revealed the influence patterns of various parameters on the moment resistance capacity of the column bases.

Seismic Performance of Embedded Column Base Connections with Attached Reinforcement: Tests and Strength Models

Embedded column base (ECB) connections used in mid- to high-rise steel moment frames derive moment resistance through bearing of the embedded column and base plate against the concrete footing. Five large-scale tests on ECB connections are presented; these feature cantilever columns subjected to axial compression and cyclic lateral loading. The tests feature reinforcement details including (1) horizontal reinforcement bars attached to the column (either directly welded to the flanges or in the form of U-hairpins wrapped around the column); and (2) vertical reinforcement in the form of stirrups in the footing. These tests complement previous experiments that are nominally identical but without the additional reinforcement. The tests indicate that while the horizontal reinforcement enhances moment strength due to resisting forces in the horizontal direction, it also produces a tension field that decreases the restraint to the rotation of the embedded base plate, ultimately reducing overall moment strength. The addition of vertical reinforcement in the form of stirrups mitigates this issue to an extent. A strength model considering these effects is proposed and shown to predict strength with good accuracy across a range of configurations, encompassing the different configurations of horizontal and vertical reinforcement. Limitations of the approach are discussed.

Experimental investigation on seismic performance of an enhanced embedded base for CFST columns

To reduce the embedded depth and improve the seismic performance of the embedded column base for concrete-filled steel tubes, an enhanced embedded column base is proposed. The column base is strengthened by a pair of strengthening steel beams and anchor bolts. Four embedded column bases were tested under axial compression load and cyclic lateral load, and the test parameters included the type of column base, height of strengthening beam, and axial load ratio. The hysteresis curves, skeleton curves, stiffness degradations, energy dissipation capacities, and strain developments were analyzed based on the test results. The test results indicated that the enhanced embedded column base exhibited high strength and a high energy dissipation capacity compared with the traditional embedded column base. An embedded depth ratio of 0.7 enabled the enhanced embedded column base to achieve the ultimate strength of the CFST column. Finally, a calculation method for the ultimate strength of the enhanced embedded column base was proposed. This method considers the resistance resulting from the interaction between the steel tube and concrete, the interaction between the strengthening steel beam and concrete, and the deformation of anchor bolts.

Experimental investigation on punching shear behavior of the embedded column base for CFSTs

In this paper, the punching shear behavior of the embedded column base for the concrete-filled steel tube is experimentally studied. Push-out tests were firstly conducted to obtain the bond strength between the steel tube and the foundation concrete. Punching tests were then performed on ten specimens to reveal the working mechanism of the embedded column base with shear studs. The test results indicated that shear studs changed the load path, enlarging the punching section of the foundation and improving the punching shear resistance of the column base. It was also found that the diameter of the shear studs and the distance between the shear studs could affect the punching shear resistance significantly. Finally, a method was proposed to determine the punching shear resistance of the embedded column base for the concrete-filled steel tube, by adding up the contributions from the different components of the system, including the bond force and shear resistance of studs above the punching section, and the resistance of foundation. The predicted results showed good agreement with the experimental results, with errors not greater than ± 15%.

Development and Experimental Validation of Dissipative Embedded Column Base Connections for Enhanced Seismic Performance of Steel Moment-Resisting Frames

This paper proposes a novel embedded column base (ECB) connection that defies the current paradigm in capacity-designed steel moment-resisting frames (MRFs) where column bases have been traditionally considered as nondissipative. In the proposed concept, a dissipative zone is introduced as part of the embedded portion of the steel column. This zone features a reduced cross section and is decoupled from the concrete footing with a debonding material layer. The surrounding concrete effectively restrains the embedded section against nonlinear geometric instabilities, thereby retaining simplicity in the design process. Large-scale experiments suggest that, contrary to its conventional counterpart, the proposed dissipative ECB connection exhibits a stable hysteretic response until large lateral drift demands (i.e., 7% rad). It is also demonstrated that the dissipative ECB connection is resilient to local buckling-induced axial shortening, such that it lowers the repairability needs of slender wide-flange steel columns after earthquakes. Column twisting is also minimized throughout the loading history. The results indicate that both the elastic stiffness and flexural yield strength of the proposed ECB connection can be analytically derived for potential use in engineering design. It is found that a simple nondegrading bilinear model suffices to describe the hysteretic response of the proposed ECBs for performance-based assessment of steel MRFs.

Seismic performance of steel columns interacting with embedded column bases while exhibiting inelastic deformations

Observations from past earthquakes indicate that embedded column bases (ECBs) in steel moment resisting frames (MRFs) exhibited inelastic deformations while they were designed to be non-dissipative. Consequently, the amount of inelastic damage concentrated in the respective fixed-end steel MRF columns was somewhat eased. This nonlinear interaction is examined by means of mixed-dimension continuum finite element (CFE) parametric simulations validated to available full-scale experimental data. The examined parameters are the ECB’s peak flexural strength, its plastic deformation capacity, and its elastic stiffness. The influence of the loading conditions along with the steel column geometric characteristics are also investigated. The simulation results indicate that, when ECBs exhibit partial inelastic deformation through controlled cracking, they feature enhanced plastic deformation capacities, which are, on average, 30–50% greater than those in commonly used non-dissipative bases. Moreover, residual axial shortening of first story steel MRF columns is reduced by up to 50% at a reference lateral drift of 4% rads. These benefits are more pronounced for steel columns with slender, but seismically compact cross sections (i.e., h/tw>20), which are prone to local and/or member instabilities at modest lateral drift demands (e.g., 2% rads). The extent of the above effects is controlled by the peak flexural strength ratio between the steel column and the ECB, the plastic rotation of the ECB and the loading history (cyclic versus monotonic). Limitations are discussed.

Seismic design of non-dissipative embedded column base connections

Embedded Column Base (ECB) connections in seismically resistant steel moment frames are commonly designed to be stronger than the connected column, to protect them from inelastic actions. This relies on estimation of demands induced by the column and the strength capacity of the connections themselves. However, recent research indicates that prevalent approaches may be unconservative for both demand and capacity estimation, with the consequence of unintended damage/failure of the connection. Motivated by the above, this research (1) characterizes the seismic demands induced in ECB connections, and (2) evaluates various strength models for these connections, in support of improved design approaches. This is done through a virtual test program that uses validated continuum finite element simulations of interactive column-ECB subassemblies. The results suggest that the current design approaches for ECB connections are unconservative, i.e., they underestimate demands while also overestimating the ECB resistance, with the possibility of unintentional nonlinear behavior within the embedded portion of the ECB connection. This is undesirable, because the ECB connections are not usually detailed to provide plastic deformation capacity. A method is proposed to provide improved estimates of the anticipated flexural demands of non-dissipative ECB connections, along with recommendations regarding the strength models. To accomplish this, the method incorporates local cross-sectional slenderness, gravity-induced axial load demand in conjunction with load combinations imposed by current seismic design standards. The sensitivity to the steel column material grade, the imposed loading history, and axial load demands is studied, and limitations of the approach are outlined.

Seismic Stability of Wide-Flange Steel Columns Interacting with Embedded Column Base Connections

AbstractThe rotational flexibility of column base connections is commonly assumed to be detrimental to the seismic response of steel moment-resisting frames (MRFs) because it increases story drift ...

EXPERIMENTAL INVESTIGATION ON CONCRETE FILLED STEEL TUBULAR COLUMN TO FOUNDATION CONNECTIONS SUBJECTED TO COMBINED AXIAL AND LATERAL CYCLIC LOADING

Abstract This study focuses on performance of CFT column to foundation connections subjected to combined axial and lateral cyclic loading. Experimental investigations were carried out on five specimens with exposed column base and embedded column base connections. The parameters considered were number of stiffeners and embedment depth of CFT column. The force - displacement response of all the specimens were compared. Providing increased number of stiffeners, increase the strength of connection. An additional mesh provided on the embedded connection was found to be good in improving the ductility of the connection which is a feasible solution for composite structures in earthquake prone regions.

A hysteretic model for the rotational response of embedded column base connections

Embedded Column Base (ECB) connections are commonly used in mid- and high-rise steel moment frames, to connect the steel column to the concrete footing. Although recent research has shown these connections to be highly ductile, they are typically designed to be stronger than the adjoining column, resulting in significant cost. To enable assessment of strong-column-weak-base systems that leverage the inherent ductility of these connections, an approach is presented to simulate their hysteretic and dissipative response. The proposed approach simulates ECB connections as an arrangement of two springs in parallel, to reflect moment contributions due to horizontal and vertical bearing stresses. This is informed by recent work that provides physical insight into the internal force transfer within these connections. The springs’ response is defined by the pinched Ibarra-Medina Krawinkler (IMK) hysteretic model, which is able to capture both in-cycle and cyclic degradation in strength and stiffness. The model is shown to reproduce the response of ECB connections with reasonable accuracy. Guidelines to calibrate model parameters are presented; these include physics-based estimation of selected parameters such as strength and stiffness, accompanied by empirical calibration of ancillary parameters associated with cyclic deterioration. Limitations are discussed.

Rotational Stiffness of Deeply Embedded Column–Base Connections

AbstractDeeply embedded column–base (DECB) connections comprise a steel column embedded into a concrete footing, and are common in mid-rise to high-rise steel moment resisting frames. These are oft...

Analysis and design of demountable embedded steel column base connections

This paper describes the finite element model for predicting the fundamental performance of embedded steel column base connections under monotonic and cyclic loading. Geometric and material nonlinearities were included in the proposed finite element model. Bauschinger and pinching effects were considered in the simulation of embedded column base connections under cyclic loading. The degradation of steel yield strength and accumulation of plastic damage can be well simulated. The accuracy of the finite element model is examined by comparing the predicted results with independent experimental dataset. It is demonstrated that the finite element model accurately predicts the behaviour and failure models of the embedded steel column base connections. The finite element model is extended to carry out evaluations and parametric studies. The investigated parameters include column embedded length, concrete strength, axial load and base plate thickness. Moreover, analytical models for predicting the initial stiffness and bending moment strength of the embedded column base connection were developed. The comparison between results from analytical models and those from experiments and finite element analysis proved the developed analytical model was accurate and conservative for design purposes.

Seismic Performance of Embedded Column Base Connections Subjected to Axial and Lateral Loads

AbstractEmbedded column base (ECB) connections are used in mid- to high-rise steel moment frames to transfer base moments and forces into the footing. These connections, which feature a steel colum...

Embedded column base connections subjected to seismic loads: Strength model

Seismically designed mid- to high-rise steel moment resisting frames in the United States are commonly constructed with the lowest portion of the first story column embedded into a concrete footing to transfer large moments and forces. Despite their prevalence, these Embedded Column Base (ECB) connections have not been experimentally examined until recently, such that strength estimation methods for other types of connections are usually adapted for their design. Based on recent experiments, current approaches for designing ECB connections are examined, and a new strength characterization method is presented. Mechanisms for internal force transfer within the connection are postulated; these include horizontal bearing of the column flange against the concrete, panel zone shear, and vertical bearing of the embedded base plate against the concrete. The proposed approach has two components: (1) a process for moment distribution between the horizontal and vertical bearing mechanisms, and (2) idealized stress distributions for estimation of associated limit states. The proposed method is able to characterize the experimentally observed specimen strengths with good accuracy, such that the average test-predicted ratio is 1.01, with a coefficient of variation 0.06. No significant bias in accuracy of the method is observed with respect to test variables. Limitations of the study are discussed; these arise from its empirical aspects which may not be generally extrapolated to configurations significantly different from the experiments upon which the method is based.

Ultimate tensile strength of embedded I-sections: a comparison of experimental and numerical results

Exposed baseplates together with anchor bolts are the customary method of connection of steel structures to the concrete footings. Post-Kobe studies revealed that the embedded column bases respond better to the earthquake uplift forces. The embedded column bases also, offer higher freedom in achieving the required strength, rigidity and ductility. The paper presents the results of the pullout failure of three embedded IPE140 sections, tested under different conditions. The numerical models are then, generated in Abaqus 6.10-1 software. It is concluded that, the steel profiles could be directly anchored in concrete without using anchor bolts as practiced in the exposed conventional column bases. Such embedded column bases can develop the required resistance against pullout forces at lower constructional costs.

Hysteretic Behavior and Strength Capacity of Shallowly Embedded Steel Column Bases

A series of shallowly embedded steel column base consisting of an exposed column base and a covering reinforced concrete floor slab were tested under horizontal cyclic loading to very large deformation. By adjustments to the floor slab thickness, shape, and reinforcing bars in the slab, the initial stiffness, maximum strength, and dissipated energy of the shallowly embedded column base increase significantly with respect to those of the exposed column base. It is found to be practical to strengthen the shallowly embedded column base so that it would behave like a fully embedded column base. Punching shear failure in the floor slab around the column due to the uplift of the base plate occurs when the shallowly embedded column base fails. Based on the plastic theory, a mechanical model that considers the contributions of the anchor bolts and the bearing and punching shear of the floor slab is proposed to evaluate the maximum strength. The evaluated results have good agreement with the test results, with errors not greater than 20%.

EFFECT OF ROTATIONAL SPRING OF ELASTIC CANTILEIVER COLUMN ON MECHANISM OF DAMAGE CONTROL FOR TWO STORY BRACED FRAME WITH RESTRAINT BRACES

In our previous paper, it is shown that story drift concentration is restraint by columns of two braced frames with pinned and fixed column bases. But embedded column bases are nearly fixed and exposed column bases are nearly pinned in the real structures. These column bases is designed as rotational springs. In this paper, the equations of the drift concentration factor for two story braced frames with columns base of rotational springs have been developed from the equilibrium conditions of the frames subjected to lateral force. This paper clarifies the relationship between drift concentration and ratio of column flexural stiffness and column rotational stiffness.

Embedded steel column bases: II. Design model proposal

A design model for an embedded steel column base is proposed in this part of the paper. The proposed model introduces the calculation for the moment, shear and vertical resistance by the use of plastic stress distribution analysis. The design model is based on the experimental results and numerical modelling described in Part I of this paper (Pertold J, Xiao R, Wald F. Embedded column base Part I. Experiment and numerical simulation. Journal of Construction Steel Research). The proposed model reduces the length of embedded column needed to transfer loading forces. Shear and vertical resistance calculation methods have also been improved. Compared with existing experimental results it indicates that the design model is safe and accurate.