Jerome F. Hajjar - Department of Civil and Environmental Engineering

Lai and Varma 2018

Experimental Study and Results

This study proposes stress-strain relationship and works to create design equations for rectangular CFT members which can be used in a compilation of studies on CFT members in order to fill the gaps in this database for high-strength rectangular CFT columns. This knowledge can then be used to propose a new design approach for CFT short columns, and establish a strength reduction factor to be used with these design equations. Many studies and tests have been performed on CFT columns, and these tests agree that the results depend on concrete compressive strength, steel yield stress, tube width-to-thickness ratio, and member length. According to the AISC 360-16. In order to test the behaviour of high-strength rectangular CFT columns, 124 tests were performed where materials were divided into three types depending on material strength. The first type was 40 CFT columns with high-strength steel and conventional-strength concrete (HS-CC), the second type consisted of 41 CFT columns with conventional-strength steel and high-strength concrete (CS-HC), and the third consists of 43 specimens with high-strength steel and high-strength concrete (HS-HC).

Finite Element Analysis

The concrete damaged plasticity material model is used, where a user inputs dilation angle, eccentricity, biaxial to uniaxial compressive strength ratio, and ratio of compressive to tensile meridians of the yield surface. The model was used to predict the strength of the columns tested in the experimental database, however there is still a lack of high-strength CFT columns with slender sections, thus more analyses were performed. Stress-strain relationships were developed, which were made to include behavior for both tensile and compressive directions. For steel and concrete fibers in tension, bilinear curves were assumed, however the compressive fibers were far more complicated. It was found that the stress-strain relationship for concrete in compression is a two-branch curve, which focuses on improving the ductility due to confinement while keeping peak stress constant (f’c). Type CS-HC columns exhibited the steel tube reaching its strength first due to yielding or local buckling, however after buckline, the CFT column and concrete infill reached their strengths at the same time because of concrete crushing. Type HS-HC columns exhibited the CFT column and steel tube reaching their strengths first due to yielding of the steel tube, when a low tube width-to-thickness ratio was used.When the steel tube becomes thinner, local steel buckling and concrete crushing occured at the same time.

Reference

Lai, Z.,Varma, A. (2018) “High-Strength Rectangular CFT Members: Database, Modeling, and Design of Short Columns.” Journal of Structural Engineering, 144(5), May. [[doi:10.1061/(ASCE)ST.1943-541X.0002026[]]