Conventional ways of strengthening steel beams include adding additional steel plates by either welding or bolting. With advances in technology, the introduction of Carbon Fiber Reinforced Polymers (CFRPs) has offered an alternative approach which not only minimizes the addition of dead load to the structure but also enables the use of Ultra-High Modulus (UHM) CFRP materials that possess an elastic modulus greater than that of steel. In essence, such an approach enables an increase in load transfer in steel beams. Additionally, FRP materials have the benefits of high strength to weight ratio, corrosion resistance, and the ability to be adhesively bonded. Nevertheless, previous research highlighted the premature debonding failure of splice plates when used with UHM CFRP laminates.
Preceding studies have shown that the strip panels used are especially advantageous when strengthening requires the use of UHM CFRP splice plates to maintain laminate continuity. Nonetheless, debonding failure is still eminent. To address this, University of Kentucky researchers: Dr. Abheetha Peiris and Professor Issam Harik proposed a new method where they investigated the performance of CFRP strip panels with finger joints linking two panels. The original research article is currently published in the journal, Engineering Structures.
In their approach, the panels were fabricated using narrow UHM CFRP strips mounted on a fabric mesh, which was designed to maintain the necessary clear spacing between individual strips. Flexural tests were then carried out under 4-point bending on steel wide flange beams. A 50 mm wide UHM CFRP laminate strengthened beam without laminate splicing was compared with the novel strip panel type splice strengthened girders. Moreover, two different strip widths of 5 mm and 10 mm were tested and all beams, strengthened with the same CFRP area, were evaluated against an un-strengthened control steel beam.
The authors reported that the steel beam strengthened with the 50 mm wide UHM CFRP laminate failed in rupture. To be specific, it was seen that the failure load for the 50 mm UHM CFRP strengthened beam was 39% larger than for the control beam. Two beams strengthened with 5mm & 10mm UHM CFRP strip panels failed in debonding. Notably, the failure load for the 10mm UHM CFRP strip panel beam was 27% larger than control beams, while the failure load for the 5mm UHM CFRP strip panel beam was reported to be 26% larger than the control beam. Overall, steel yielding was not detected before failure in all UHM strengthened beams.
In summary, the study by Dr. Peiris and Professor Harik demonstrated the viability of UHM CFRP strip panels as an alternative method for strengthening steel beams. Remarkably, a finite element analysis of the tested beams was carried out and the results matched well with the experimental results. It is noteworthy to mention that while the final observed failure was debonding for the UHM CFRP strip panel strengthened beams, the experimental results indicated that the UHM CFRP strip panels can increase the load at which yielding initiates. In a statement to Advances in Engineering, Professor Issam Harik who is Raymond-Blythe Professor of Civil Engineering explained that attaching strip panels provides a modular construction method that requires lesser labor and overall equipment costs when compared with the conventional laminate strengthening of long-span beams.
A. Peiris, I. Harik. Steel beam strengthening with UHM CFRP strip panels. Engineering Structures: volume 226 (2021) 111395.