Cyclic Loading of CFST Column Connections

By Egbert Padilla

Cyclic loading of CFST (Concrete-Filled Steel Tube) column connections is a critical aspect in structural engineering, particularly in seismic design. CFST columns are widely used due to their high strength, ductility, and resistance to lateral loads, making them suitable for withstanding earthquakes and other dynamic forces.

When subjected to cyclic loading, CFST column connections undergo repeated stress and strain, which can lead to various failure modes such as concrete crushing, steel yielding, and bond slip. Understanding the behavior of these connections under cyclic loading is essential for ensuring structural integrity and safety.

Research in this area focuses on investigating the performance of different types of CFST column connections, including welded, bolted, and hybrid connections, under cyclic loading conditions. Experimental studies, numerical simulations, and analytical models are used to evaluate factors such as connection geometry, material properties, and loading patterns.

The findings from these studies contribute to the development of design guidelines and codes that enhance the resilience of structures against seismic events. Engineers aim to optimize CFST column connections to minimize damage and improve the overall seismic performance of buildings and bridges.

In summary, research on the cyclic loading of CFST column connections plays a crucial role in advancing seismic design practices, ensuring the safety and reliability of structures in earthquake-prone regions.

• Language: English
• Publisher: Egbert Padilla
• Release date: Apr 18, 2024
• ISBN: 9798224973248

Book preview

CHAPTER 1 INTRODUCTION

GENERAL

Steel - concrete composite construction is being popular around the world nowadays, since they integrate the advantages of both steel  and  concrete materials. The most common types of composite sections are  the Steel Reinforced Concrete column (SRC), which is cast by encasing a structural steel section in concrete and the Concrete-Filled Steel Tubular (CFST) column. Concrete filled steel tubular columns have many advantages and better structural properties, such as high strength under compression loading, enhanced ductile behaviour and good capacity in terms of energy absorption. Due to their large compressive stiffness and axial-load carrying capacity, CFSTs are also used as piers, columns, and caissons for deep foundations (Faschan 1992; Viest 1992). With the improvement in ductile behaviour, these composite columns perform well under seismic loading. CFST columns are suitable  for  construction  of  tall  buildings  (Matsumoto  et al. 2014) and structures in seismic vulnerable zones. However, due to lack  of understanding of their structural behaviour and lack of reliable design details, the usage of CFST columns is limited.

The favourable performance of any structure depends fully on the behaviour of the connections. In composite structures, connections are the  most critical zone as several structural members will get affected due to the fracture of a single connection. In 1990, Northridge and Kobe earthquakes, it was reported that many failures were found in the connections.

Hyogo-ken Nambu earthquake (1995) revealed that the behaviour  of column-foundation connection was at risk. After 1995 Hyogo-ken Nambu earthquake, statistical analysis was carried out by Midorikawa  (1997) over  600 moment resisting frame buildings. These buildings were  divided  into three categories as a) the building constructed based on the design code provision before 1980 b) the buildings constructed based on the design codes which were not identified and c) the buildings constructed based on the design code provision after 1995. A comparison was made between the buildings  with damaged column-foundation connections and the buildings with damage on all other structural elements like beams, columns, bolted connections and welded connections. From Figure 1.1, it was clearly noted that, the column- base connection failure was the common damage in all the  building  categories, however the latest design code gradually reduced the damages in column bases from 16.7% to 5.1%.

(Source: Midorikawa 1997)

Figure 1.1 Analysis of Building damages after 1995

Considering all the typical structural connections, column- foundation connection is the most critical part. Even  though  composite column to foundation connections possess enhanced structural performance, their usage is very less due to the lack of knowledge and experience about  their behaviour under various loading scenarios and also due to lack of codal provisions.

Experimental investigations have been carried out on foundation connections using CFST columns, to understand the performance and to provide design details for the connection with good ductility and energy dissipating capacity. For example, Marson et al. (2004) introduced a new foundation concept to achieve the full fixity of the connection. Four large  scale CFST  column-foundation  connection  specimens  with  different  D/t  (D - Diameter and t- thickness of the CFST column) ratios were subjected to both axial load and cyclic bending moment. The diameters of CFST columns were 324 and 406 mm, with a D/t ratio ranging from 34 to 64. All the specimens were embedded in a special foundation exhibited  good ductility  and energy dissipation capacity.

Lehman et al. (2012) evaluated the seismic performance of embedded CFST column-base connections. Cui & Hao (2015) aimed to study the shear behaviour of exposed column bases. Pertold et al. (2000a) carried  out both experimental and numerical investigations on the behaviour of embedded steel column bases. Hitaka et al. (2003) studied the behaviour of connections with different types of base plates. Hsu & Lin (2006) investigated the seismic behaviour of square CFST column foundation connections with different depths of embedment. These studies suggested that the exposed  CFST column-foundation connection could cause excessive deformation demand on tube wall and the base plate connection. These connection failures mainly due to excessive anchor rod elongation, base plate failure and concrete

crushing. In the embedded CFST column-foundation connection, concrete  core surrounded in the embedded portion of the CFST column alleviate the deformation demand, which may improve the overall performance of the connections.

These facts underline that, there is a need for studies on CFST column to foundation connection details that are realistic, provide  better ductile performance, to avoid the possibility of site welding, so that  the  quality of construction and speed can be improved. In the present study, experiments were conducted to investigate the performance of two types of column-foundation connections, between the circular CFST column and the reinforced concrete foundation under combined axial and lateral cyclic displacement controlled load. The types of connections considered include

(i) Exposed column-foundation connection, (ii) Embedded column-foundation connection. The parameters varied include (i) number of stiffeners (four numbers provided parallel and perpendicular to the direction of loading and eight numbers around the CFST column with equal spacing) (ii) depth of embedment (iii) a layer of mesh around the CFST column.

The experimental and numerical investigations established that all the embedded connection subassemblies achieved a ductile behaviour with larger displacements with very small detectable distress on the foundation concrete. Performance of exposed column-foundation connection was compared with the embedded column-foundation connection and their failure mechanism was discussed. A simple finite element model was developed to represent the behaviour of both types of column-foundation connections. The numerical results of the models using ABAQUS software were found to be in good agreement with the actual column-foundation connection behaviour. These finite element models could be used for dynamic simulation of the moment resisting frame with the CFST column-foundation connection.

BACKGROUND - CONCRETE FILLED STEEL TUBULAR COLUMNS

Over past several decades, steel - concrete composite  materials  have been used in the construction industry. CFST column has turned  out to  be one of the most successful composite column especially in heavy construction. It has been widely used because of the full utilization of the infilled concrete and the outer steel tube.

Initially, hollow steel tubes are used as columns, designed to carry the floor loads up to four storeys. In high-rise construction, the hollow steel sections are fabricated or rolled to be raised in front, to support the constructional load from the higher floors. Due to ease of erection and  handling the steel members lead to the investigation on the use of composite columns in the construction industry. The CFST column enhances the  strength, ductility, stiffness with large energy dissipation capacity. Numerous studies were carried out by various researchers (Furlong (1967), Neogi et al. (1969), Knowles & Park (1970), Tomi et al. (1977)) on CFST columns to improve its rigidity, and enhance the design efficiency. CFST columns are  used in heavy constructions since they provide excellent earthquake resistant properties. CFST columns are commercially available in square, rectangular and circular cross sections as illustrated in Figure 1.2. The other cross sections like polygonal, oval and round edged square and rectangular sections are used for aesthetic appearance in construction. The confinement effect and local buckling resistance of circular CFSTs are more than the square  and rectangular CFSTs. But the square and rectangular cross sections are used frequently due to its easier fabrication with beam-column connections.

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Figure 1.2 Various cross sections of CFST column

Advantages of CFST Structure

CFST structural system has numerous distinct advantages in both, structural and constructional performance. The main advantages of CFST structural systems are as follows:

›  The strength of concrete infill is enhanced due to the confinement effect of the outer steel tube. Spalling of concrete is prevented by steel tube and therefore the strength degradation will not be severe.

›  The characteristic failure and the local buckling of the thin steel tube are delayed, and the strength degradation after the occurrence of local buckling is moderated, due to the restraining effect of the in-filled concrete.

›  Drying shrinkage and creep of the concrete in CFST structural system is very small compared to ordinary reinforced cement concrete system.

›  The infilled concrete provides greater contribution to resist the axial compression.

›  The CFST structure provides maximum stiffness, since the outer steel tube lies away from the centroid and gives outstanding contribution to the section