Column Base Connections Research Articles

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.

Effect of Column Base Behavior on Seismic Performance of Multi-Story Steel Moment Resisting Frames

Column base is one of the most important elements of steel structures. Exposed column base is commonly used in low-to-medium-rise steel moment resisting frames because of better constructability and low cost. To study the effect of exposed column base behavior on the seismic behavior of low-to-medium-rise steel moment resisting frames, a four-story, four-bay steel moment frame is studied by the nonlinear time history analysis. In the numerical analysis, two types of column base connections (rigid and semi-rigid) are considered. The width–thickness ratio of column and stiffness ratio of column base to column are chosen as the analysis parameters. The characteristics of structural responses, hysteresis loops, and the distribution of plastic energy dissipation are compared. It indicates that the collapse margin ratio is significantly increased when the exposed column base behavior is considered for the moment resisting frames with large width–thickness ratio. Moreover, if the column base connection is allowed to rotate and transfer a portion of the moment, the demand of plastic deformation capacity of steel columns is reduced, then subsequently strength deterioration caused by the local buckling at the bottom of column could be avoided. Also, the whole structure has a better ductility, the ability of plastic deformation and energy absorbance of the moment resisting frame under earthquake are therefore enhanced. The structure with the semi-rigid column base connection has larger potential to avoid the structural collapse caused by the local buckling of first-story columns.

Effect of soil reinforcement on rocking isolation potential of high-rise bridge foundations

Based on recent studies, providing suitable conditions for a soil-structure system to behave non-elastically in response to the forced vibrations resulting from seismic activity is potentially advantageous. High-rise structures with rigid column-base connections often withstand considerable moments which can create plastic hinges at it. The Inclusion of rocking motions in the foundation in structural analysis is a new approach in seismic designs that can reduce the column-base moments and transfer the plastic hinge(s) to the soil. Creating soil slippage on failure surfaces, which in design concept is termed by “rocking isolation”, can act as a “fuse” for protecting the superstructure against damages. Although favorable in terms of limiting the inertial forces applied to the superstructure, rocking motions can cause undesirable foundation settlement in structures with low safety factor against static vertical loads (F.S.v). Since the soil region yielded as a result of rocking motions lies in the shallow depths of the foundation, “shallow soil improvement” can be considered as an option to ensure that F.S.v is sufficiently large and to avoid unpredictable risks regarding residual rotation and increasing settlement. The geogrid and geocell were used as reinforcement elements at different depth ratios. Based on the results, using geocell at depth ratios less than 0.25 would reduce settlements effectively.

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.

Seismic Demands in Column Base Connections of Steel Moment Frames

Methods for the seismic design of base connections in steel moment frames are well-developed and routinely utilized by practicing engineers. However, design loads for these connections are not verified by rigorous analysis. This knowledge gap is addressed through nonlinear time history simulations using design-level seismic excitation that interrogate demands in column base connections in 2-, 4-, 8-, and 12-story steel moment frames, featuring base connections that reflect current U.S. practice. The results indicate that: (1) for exposed base plate connections, lower bound (rather than peak) estimates of axial compression are suitable for design because higher axial forces increase connection strength by delaying base plate uplift; (2) even when designed as pinned (as in low-rise frames), base connections carry significant moment, which can be estimated only through accurate representation of base flexibility; and (3) the failure of embedded base connections is controlled by moment, which may be estimated either through overstrength or capacity-based calculations.

Capacity of exposed column base connections subjected to uniaxial and biaxial bending moments

ABSTRACT Current guidelines do not address the design of column base connections under combined biaxial bending moment and axial load. An analytical study is conducted to develop a biaxial moment-axial load interaction curve for design of exposed, unstiffened column base plates. First, a 3D finite element analysis model is developed for these connections. The response of the model is validated through a series of existing experimental data. Next, the model is used to investigate the response of a few connections subjected to various biaxial moments and axial loads. Based on simulation results, a simple and reasonably accurate interaction equation is proposed. Since the uniaxial capacities of the connection are involved in the interaction curve, an analytical method for predicting the uniaxial capacities, is also presented. This analytical method, which is based on formation of plastic mechanisms, is able to predict the strength of flexible base plates with high accuracy. The behavior of exposed base plates with I-shaped columns under minor-axis moments is also studied. Findings of this study assists practitioners with a reliable and convenient column base connection design.

Evaluation of existing provisions for design of “pinned” column base-plate connections

Low-rise metal buildings are used in all geographic locations, including high seismicity regions. In the design of low-rise metal building systems, column base connections are commonly modeled as pinned supports with no rotational stiffness or moment capacity. However, past studies have indicated that base connections, which are designed as pinned supports, exhibit a non-negligible level of rotational stiffness and strength. Neglecting the rotational stiffness of the base connection may result in a significant overestimation of the lateral displacement of the frames. This additional displacement is addressed by increasing the flexural stiffness of the frame members thereby increasing the cost of low-rise metal buildings. The moment capacity may similarly be exploited for strength design. However, there is a lack of design guidelines to support the use of rotational stiffness or moment capacity at the pinned column bases. This study evaluates the applicability of existing code-formulations to pinned colum base-plate connections. Both the rotational stiffness and the moment capacity from past experiments are compared with those calculated based on the provisions of the European and American design codes. It was found that the Eurocodes do not accurately estimate the rotational stiffness of the connections. A modification to the Eurocodes is recommended to capture the characteristics of pinned connections. It was also observed that the moment capacity of the connections was conservatively estimated by the existing codes.

Dynamic performance of a superelastic column-base connection

Dynamic performance of a superelastic column-base connection

Analysis and design of demountable circular CFST column-base connections

In current engineering practice, circular concrete-filled steel tubular (CFST) columns have been used as effective structural components due to their significant structural and economic benefits. To apply these structural components into steelconcrete composite moment resisting frames, increasing number of research into the column-base connections of circular CFST columns have been found. However, most of the previous research focused on the strength, rigidity and seismic resisting performance of the circular CFST column-base connections. The present paper attempts to investigate the demountability of bolted circular CFST column-base connections using the finite element method. The developed finite element models take into account the effects of material and geometric nonlinearities; the accuracy of proposed models is validated through comparison against independent experimental results. The mechanical performance of CFST column-base connections with both permanent and demountable design details are compared with the developed finite element models. Parametric studies are further carried out to examine the effects of design parameters on the behaviour of demountable circular CFST column-base connections. Moreover, the initial stiffness and moment capacity of such demountable connections are compared with the existing codes of practice. The comparison results indicate that an improved prediction method of the initial stiffness for these connections should be developed.

Effect of grout properties on shear strength of column base connections: FEA and analytical approach

Concrete grout is used in most column base connections to facilitate the construction process and to ensure that full contact is achieved between the steel plate and the concrete pedestal. However, insignificant attention has been given to its use and performance while there is a lack of clear understanding towards its contribution to the shear strength of column base connections. A comprehensive finite element (FE) study is presented herein investigating the shear capacity of the column base connection on the grout thickness and strength. 3D FE models incorporate important behavioural aspects including the surface interaction and multi-axial constitutive models of the assemblages. The results of the investigation indicated that the introduction of grout improves the behaviour and strength of the column base connections significantly by developing a different load path system consisted of the grout strut, the friction between the base plate and grout, and the tension in the anchor rod due to second order effects. It is found that the current design codes of practice do not consider the positive influence of grout and lead to very conservative shear strengths. Furthermore, the paper proposes a mathematical equation to account for the lateral displacement which is overlooked in the current international regulations.

Stiffness-based approach for Belleville springs use in friction sliding structural connections

Sliding hinge joints (SHJs) used in beam-to-column connections of moment frames have a moment-rotational behaviour that depends on asymmetric friction connection (AFC) sliding behaviour. The AFC is also applied to column base connections and friction sliding braces. In the AFC, the slotted sliding plate is clamped between one ideally fixed surface and one partially floating surface. In current practice, the AFC bolts are fully tensioned at installation (i.e. yielded) to provide the clamping force. The AFC bolts are subjected to moment, shear, and axial force (MVP) interaction during joint sliding that is expected to occur only in severe earthquake shaking. The AFC bolt tension as well as SHJ elastic strength are reduced after a few sliding cycles. In this paper, the reasons for the AFC bolt tension loss are discussed, and solutions to prevent this bolt tension loss, including the optimum use of Belleville springs (BeSs) and installing the bolts within the elastic range, are proposed. This paper analytically shows that these solutions can generate significantly improved retention of AFC bolt tension, improved AFC sliding behaviour, higher displacement capacity to accommodate prying effects, and better AFC self-centering characteristics. Examples of AFC bolt installation within elastic range and tension loss with and without BeSs are provided. Similar models are developed for symmetric friction connections (SFCs) and compares differences in behaviour.

Stability and stiffness analysis of a steel frame with an oblique beam using method of least work

For the first time, this paper investigates the stability and stiffness analysis of a single column in a specific case with the connected oblique beam. In this case, the modification factors are analytically derived such that the stiffness of oblique beam is included in the calculation of the well- known parameters GT and GB. The effective length factor for such column can be obtained by these modified GT and GB. It is noted that the effective length factor of the column in the mentioned specific case is assessed for the first time in this research. In the following, a single span- one story steel frame is investigated to determine the lateral stiffness of the frame. In this frame, the applied lateral load and the structural frame are not on the same plane. Accordingly, it is also focused on the investigation of the effects of hinge existed in the beam as well as the changing of column base connection from fixed to hinged forms. The structure is considered as a 3- dimensional steel frame for the analysis upon the least work principle. All effective factors are taken into account including axial and shear loads as well as bending and torsional moments. At the end of the analysis, a relation is obtained through response surface method. The lateral stiffness can be calculated by the derived relation based on the specifications of the steel frame such as geometrical properties of the employed sections, specification of the used material and deviation angle of the beam.

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.

Influence of standard recommendations for the calculation of the column-base connection by socket according to ABNT NBR 9062:2006

Abstract This work studies the column-base connection by external socket in precast structures. A parametric study of the geometric characteristics of the external socket base with smooth interface is presented. In this parametric study, the consumption of concrete and steel are analyzed. The column cross section, the embedded length of the column in the socket base and the thickness of the wall of the socket base were the variables considered in this study. It was observed that with the increase of the embedded length, the minimum cross section of the main horizontal reinforcement reduces. With this modification, the walls of the socket base that are perpendicular to the direction of the applied loads presented a reduction of their stiffness. Besides the parametric study, this paper presents a verification model. This part of the study shows the possibility to generate abacuses that simplifies the project of the socket base foundation. Moreover, a comparative analysis becomes easier to be accomplished

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.

Hysteretic Model for Exposed Column–Base Connections

AbstractA hysteretic model formulation is presented for simulation of the cyclic moment-rotation response of exposed column–base (ECB) connections, which are typically used to connect a steel column to a concrete footing in steel moment-resisting frames. The research is motivated by the potential for using these connections as dissipative elements within seismic design. The physical processes responsible for various forms of hysteretic response are outlined; these include: (1) seating and gapping between the base plate and footing leading to pinched hysteresis, and (2) a recentering effect of axial compressive load resulting in flag-shaped hysteresis. The proposed ECB model utilizes: (1) a trilinear backbone curve; (2) hysteretic rules for pinching, unloading, recentering, and reloading; and (3) modes of deterioration for four quantities, including strength and stiffness. The model has 16 parameters, of which 4 are classified as core parameters (meaning they can be determined through physics-based models)...

Shear behavior of exposed column base connections

Column base connections are critical components in steel structures because they transfer axial forces, shear forces and moments to the foundation. Exposed column bases are quite commonly used in low- to mediumrise buildings. To investigate shear transfer in exposed column base plates, four large scale specimens were subjected to a combination of axial load (compression or tension) and lateral shear deformations. The main parameters examined experimentally include the number of anchor rod, arrangement of anchor rod, type of lateral loading, and axial force ratio. It is observed that the shear resisting mechanism of exposed column base changed as the axial force changed. When the axial force is in compression, the resisting mechanism is rotation type, and the shear force will be resisted by friction force between base plate and mortar layer. The specimens could sustain inelastic deformation with minimal strength deterioration up to column rotation angle of 3%. The moment resistance and energy dissipation will be increased as the number of anchor rods increased. Moreover, moment resistance could be further increased if the anchor rods were arranged in details. When the axial force is in tension, the resisting mechanism is slip type, and the shear force will be resisted by the anchor rods. And the shear resistance was reduced significantly when the axial force was changed from compression to tension. The test results indicated that the current design approach could estimate the moment resistance within reasonable acceptance, but overestimate the shear resistance of exposed column base.

Connections for Timber–Concrete Hybrid Building: Experimental and Numerical Model Results

AbstractIn seismically active regions, the use of timber–reinforced concrete (RC) hybrid construction can overcome the structural limitations that occur with timber construction. This paper investigates three connections used in the timber frame–RC core hybrid system: (1) RC wall-timber beam connection, (2) timber column-base connection, and (3) timber beam-timber column connection. A series of monotonic and reverse-cyclic loading tests were conducted on three replicate connections. Salient features of each connection (i.e., stiffness, ductility, and energy dissipation) were computed from the monotonic loading test results and were used to set the backbone curve of the analytical Pinching4 hysteretic model. Furthermore, the Pinching4 hysteretic models were calibrated with the reverse-cyclic loading test results. Utility of the Pinching4 hysteretic models was shown with performance prediction of a portal frame test. The portal frame results showed good agreement between the finite element model using the c...