Bending Performance of Rectangular Concrete Filled Steel Tube Members with Different Shear Connector Configurations

Literature Overview

This paper by Yang Yuehua, Jiang Lei (Chang'an University), and Liu Yongjian (Chang'an University and Taizhou Coastal Expressway Command Department, 2017) investigates the bending performance of rectangular concrete filled steel tube (CFST) members with different shear connector configurations. Three specimens were tested: a flat specimen without shear connectors, a rib specimen with conventional transverse ribs, and a PBL (perfobond light) specimen with PBL shear connectors. Pure bending tests were conducted to study the load-displacement curves, neutral axis movement, strain distribution, failure modes, and bending capacity. The research was supported by the National Natural Science Foundation of China and Ministry of Transport construction science and technology projects.

Core Technical Content

Specimen Configuration and Test Setup

Three rectangular CFST beam specimens were designed and tested:

Specimen	Shear Connector Type	Description
Flat	None	Steel tube filled with concrete, no additional connectors
Rib	Conventional transverse ribs	Steel ribs welded to the inner surface of the steel tube
PBL	Perfobond light connectors	Circular holes in transverse steel plates welded to the tube

All specimens were designed with the same steel consumption to ensure a fair comparison of the effectiveness of different shear connector types. The pure bending test setup applied equal and opposite moments at the midspan through a four-point bending arrangement with appropriate loading spans.

Load-Displacement Behavior

The load-displacement curves of the three specimens reveal distinct behaviors:

Stage	Flat Specimen	Rib Specimen	PBL Specimen
Elastic stage	Linear response	Linear response	Linear response
Yield stage	Steel tube yields, concrete cracks	Similar to flat	Similar to flat
Post-yield stage	Slip between steel and concrete	Localized concrete cracking	Composite action maintained
Ultimate stage	Reduced capacity due to slip	Localized failure	High composite capacity

Key Findings

The study reveals several important findings regarding the effectiveness of shear connectors:

Slip behavior - In the flat specimen (no connectors), significant slip developed between the concrete core and the steel tube walls. This slip reduces the composite action and leads to a reduced effective bending capacity. The rib and PBL specimens effectively prevent slip, maintaining full composite action throughout the loading.
Concrete cracking - The rib specimen exhibited severe localized concrete cracking under concentrated transverse loads from the ribs. This localized cracking can initiate premature failure and reduce the ductility of the member. The PBL specimen, with its more uniform load distribution through the perforated plates, did not exhibit severe localized cracking.
Composite action - The PBL specimen demonstrated the best overall composite behavior, with no concrete slip and no severe localized cracking. The perfobond light connectors provide effective shear transfer while distributing the loads more uniformly over the concrete surface.
Bending capacity enhancement - The concrete infill significantly improves the bending capacity of the rib and PBL specimens compared to the equivalent empty steel tube. However, for the flat specimen, the capacity improvement is minimal due to the loss of composite action from slip.

Neutral Axis Movement and Strain Distribution

The study also examined the movement of the neutral axis and the strain distribution in the concrete and steel:

In all specimens, the neutral axis moved upward with increasing load, indicating the contribution of the concrete compression zone to the overall flexural capacity.
The strain distribution in the concrete was more uniform in the PBL specimen compared to the rib specimen, where localized high strains developed near the rib locations.
The steel tube strains followed the expected linear distribution in the elastic stage, with nonlinear redistribution in the plastic stage.

Engineering Practice Implications

Shear Connector Selection for CFST Beams

The study provides clear guidance for the selection of shear connectors in rectangular CFST beam design:

PBL connectors are preferred - The perfobond light connectors provide the best combination of shear transfer effectiveness, load distribution uniformity, and composite action maintenance. They are recommended for applications where full composite action is required.
Conventional ribs are acceptable but with limitations - Transverse ribs can provide adequate shear transfer but may cause localized concrete cracking, particularly under high loads. The rib spacing and geometry should be optimized to minimize stress concentrations.
No connectors is not recommended for composite action - The flat specimen demonstrates that without shear connectors, the composite action between steel and concrete is not achieved, and the bending capacity is not significantly improved over the empty steel tube.

Welding Quality Requirements

From a welding engineering perspective, the fabrication of CFST members with shear connectors involves several critical welding operations:

Welding Operation	Quality Requirement	Inspection Method
PBL plate to tube wall	Full-penetration fillet welds, no undercut	Visual, UT, MT
Rib to tube wall	Adequate weld size, no porosity	Visual, PT
Tube longitudinal seam	Full fusion, NDE inspected	RT, UT, ET
Tube circumferential joints	Full fusion, stress-relieved	RT, UT
Connector plate holes	Clean, deburred, no defects	Visual, dimensional

The welding quality of the shear connectors is critical to the overall performance of the CFST member. Poor weld quality can lead to:

Premature weld failure under shear loads
Reduced effective shear transfer capacity
Stress concentrations that initiate cracks in the steel tube or concrete
Reduced fatigue life under cyclic loading

Concrete Placement and Curing

The quality of the concrete infill is also critical to the performance of CFST members:

Complete filling - The concrete must completely fill the steel tube interior without voids or honeycombing. Voids reduce the effective concrete area and create stress concentrations.
Adequate compaction - Proper compaction ensures good concrete-steel bond and eliminates trapped air. Vibrators or rodding should be used as appropriate for the tube geometry.
Proper curing - Adequate curing ensures the concrete achieves its design strength and durability. Curing methods must be compatible with the confined conditions inside the steel tube.
Concrete mix design - The concrete mix should be designed for pumpability (if pumping is required) while maintaining adequate strength and durability. The aggregate size should be compatible with the tube dimensions and shear connector spacing.

Study Insights and Critical Reflection

This study provides valuable experimental data on the bending performance of rectangular CFST members with different shear connector configurations. The findings clearly demonstrate that shear connectors are essential for achieving full composite action and maximizing the bending capacity of CFST members.

The superiority of PBL connectors over conventional ribs is attributed to the more uniform load distribution provided by the perforated plate design. The circular holes in the PBL plate allow the concrete to flow through and create mechanical interlock, while the plate itself provides a large contact area for friction and adhesive bond. This results in a more gradual load transfer between the steel and concrete, reducing stress concentrations and localized cracking.

From a practical engineering perspective, the PBL connector system offers several advantages:

Simplified fabrication - The PBL plates can be prefabricated and welded to the tube in a controlled shop environment.
Reduced on-site complexity - The PBL connectors are installed before concrete placement, eliminating the need for in-situ welding of shear studs.
Improved quality control - The welding of PBL plates can be inspected and qualified before concrete placement.
Design flexibility - The PBL connector spacing and plate dimensions can be adjusted to achieve the required shear transfer capacity.

However, the PBL connector system also has some limitations:

The hole diameter and spacing must be carefully designed to ensure adequate concrete flow and mechanical interlock.
The plate thickness must be sufficient to resist shear and bending without excessive deformation.
The welding of the PBL plates to the tube walls requires skilled welders and proper welding procedures, particularly for high-strength steel tubes.

For steel pipe suppliers and fabricators, this study emphasizes the importance of providing high-quality steel tubes with accurate dimensions, smooth surfaces, and adequate mechanical properties. The steel tube serves as both a structural member and a formwork for the concrete, and its quality directly affects the performance of the composite member. Surface defects, dimensional variations, and inadequate mechanical properties can all compromise the shear connector weld quality and the overall composite action.

The study also highlights the importance of integrated design, where the steel tube, shear connectors, and concrete are designed as a unified system. The interaction between the different components must be carefully considered, and the design should account for the construction sequence, quality control measures, and potential degradation mechanisms during service. Engineers should adopt a systems-based approach to CFST design, recognizing that the performance of the composite member depends on the quality and compatibility of all its constituent parts.