Research team member, Matthew Smith, and Engineer at global consulting firm, Entuitive, successfully defended his masters thesis: Towards a Performance-Based Fire Design Framework for Composite Steel Deck Construction in Canada . His thesis and defense were of the highest quality that there were no corrections necessary. Matt was subsequently nominated for the university’s senate medal (he has also just been awarded the SFPE National Capital Region Chapter Scholarship in Fire Safety Engineering in Canada for his work. A big thank you to his examiners whom I would say have a combined experience of over 60 years practicing steel construction. Also a big thank you to project sponsors: CISC and Entuitive, as well as to reps from FM Global and the National Research Council, NIST for their previous feedback and contributions.
Our new paper, Creep of Prestressing Steels in fire, is now in press in the journal Fire and Materials (you can access it here; an open access version is to come). It is a great piece about prestressing steels used in Post-tensioned structures. We show how high- temperature creep is effected by the chemical composition of the steel. While it may seem obvious to some, the effect this can have for prestressing steel in high temperatures seen in real fires is astounding.
Consider several different prestressing steels, all behaving the same in ambient service temperatures, but when stressed and heated (700 MPa and 427C) as if they were in a fire, one fails after 5 hours, were the others fail below 1.5 hours. All likely due to the chemical composition from the production process of the steel (see the above figure). These tests are described in the paper (that and about 80 others). The same trends shows the same effect in simple strength tests at high temperature.
The paper also describes the use of digital image correlation to measure the strain at high high temperature. An innovated technique, but leading to important insights about true areas and strains as the steel fails in high temperature. A compiled video of a creep test is provided below which illustrates how quickly tertiary (third stage) creep takes effect before failure.
The above video shows the prestressing steel coated in a black and white speckle texture pattern. Image correlation is a powerful technique for measuring high temperature strains. Its a very simple process described below
We did a very ad hoc style image correlation test using a steel stub column in the undergrad labs. Though not really an accurate way to adhere image correlation paint though, however; it was a good illustration of observing yielding and buckling effects on the column as the test progressed. Images in this video were taken at 1 second intervals, and the video represents 100s of image put together. Images were taken using a Canon 5d Mark 3.
One of the great things of testing materials with image correlation technologies at ambient and high temperatures is developing educational movies about engineering. Last summer Tom Parker and myself compiled thousands of images from my digital image correlation experiments into these videos. I’ve posted one example above which is to help teach the principles behind necking of steel for students. Ill post back later with an expanded blog entry with more videos but for now enjoy!