Date of Award

June 2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Eric M. Lui

Keywords

Cyclic performance, High strength steel, Mechanical properties, Post-fire behavior, Q690 steel, Residual stresses

Subject Categories

Engineering

Abstract

Behavior of High Strength Steels (HSS) after exposure to high temperature has become an important research topic in recent years. A number of studies have demonstrated that different grades of HSS can exhibit noticeable differences in their mechanical properties under and after fire exposure, and different cooling methods may have an effect on the post-fire mechanical properties of HSS. In this research, the post-fire mechanical properties of Q690 steel and the post-fire residual stress distributions of Q690 welded I-sections heated to various temperatures using different heating rates and cooled using two cooling methods are determined experimentally and simple empirical equations to represent these measured data are proposed. Furthermore, the cyclic behavior of welded I-shaped columns made from Q690 steel after fire exposure is investigated.

In the first phase of the study, the post-fire mechanical properties of Q690 steel, which is a typical HSS with 690MPa nominal yield strength, are determined experimentally and discussed. The major variables considered are the level of temperature exposure and cooling methods used. The temperature used in the experimental work ranges from room temperature to 900°C, and two cooling methods – natural air and quenching water – are used to study whether they have an effect on the post-fire mechanical properties of HSS. In addition, the effect of the use of different heating methods, consideration of repeated heating and cooling, and various loading conditions are also studied. The test results show that while the post-fire elastic modulus is not too sensitive to the exposed temperature level and the manner of cooling, it decreases about 10% when a higher initial heating rate, repeated heating and cooling, or a load is applied to the specimen. The post-fire yield strength tends to decrease with the exposed temperature level when the temperature reaches 400°C if the air cooling method is used and 500°C if the water quenching method is used. Further reduction in yield strength occurs when the specimen is subjected to a higher initial heating rate, repeated heating and cooling, or an applied load. The post-fire tensile strength does not show significant variations if air cooling is used but for specimens heated to a temperature above 700°C and rapidly cooled by submersion in water, noticeably higher post-fire tensile strength is observed as a result of the formation of martensite. Martensite formation also reduces the ductility (as measured by the fracture strain) of steel heated above 700°C and cooled suddenly.

In the second phase of the work, the residual stresses of Q690 welded I-sections after fire exposure are determined using the sectioning method. Like phase one, temperature and cooling method are the two main parameters that are studied. Furthermore, the effect of section dimensions will be considered. The results show that when the exposed temperature is below 300°C, the influence is not very important. However, when the exposed temperature exceeds 300°C, the magnitudes of the maximum residual stresses start to decrease. Once the temperature reaches 700°C, the maximum residual stress magnitudes are less than 5% of the nominal steel yield stress. The heating rate does not seem to affect the residual stress results. However, for specimens heated to a temperature at or above 700°C and suddenly cooled by water quenching, noticeable residual stresses are generated on the edges of the flanges and at the web-flange junctions. The residual stress magnitudes on the flange edges are -0.13Fy for 700°C and -0.24Fy for 900°C, while the magnitudes at the web-flange junctions are +0.29Fy for 700°C and +0.21Fy for 900°C (where Fy is the nominal yield stress of Q690 steel and +/- represents tension or compression).

In the last phase of this research, a Finite Element Model (FEM) is developed, calibrated and verified against the test results of cyclic behavior of Q690 welded I-shaped columns reported by other researchers. Using this FEM, the loss in energy dissipation under cyclic loads after fire exposure is investigated. The analysis results show that energy dissipation tends to decrease when the level of temperature exposure increases.

Finally, to facilitate design, empirical equations for the post-fire mechanical properties of Q690 steel, and the post-fire residual stress patterns of Q690 welded I-sections are developed and proposed. An equation to describe the capacity loss of Q690 welded I-shaped columns under cyclic loads after fire exposure is also proposed.

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