PSI - Issue 64

Urs Meier et al. / Procedia Structural Integrity 64 (2024) 29–39 Meier/Winistörfer / Structural Integrity Procedia 00 (2019) 000 – 000

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The post-tensioning of the external CFRP stirrups of beam #1, on the other hand, resulted in an increase in the maximum load of 51% compared to the not post-strengthened beam #2 and an increase of 47% compared to beam #3. The external CFRP stirrups deliberately had only a small overall cross-section to ensure that the post-strengthened beams also failed in shear before the longitudinal reinforcement began to yield. The small overall cross-section was decisive for the rapid increase in stress in the external CFRP stirrups from the point at which the steel stirrups reached their yield point and for the failure of the CFRP stirrups in all post-strengthened beams. The post-tensioning caused a compressive stress of only 0.93 MPa in the concrete and had only a minor influence on the crack widths. While the vertical deformations of the disc areas of the post-tensioned beams were slightly lower than those of the non-post-tensioned beam #2 at the same load level, the post-strengthened beams showed significantly larger vertical deformations of the disc area, as well as significantly larger crack widths and average strains in the steel stirrups compared to beam #2 at the highest load levels. The CFRP stirrups took on additional loads at a time when the yield point of the steel stirrups had already been exceeded. The strains in the concrete were strongly influenced by the different load-bearing behavior of the post-strengthened and non-post-strengthened beams. The principal compressive and principal tensile strains of the post-strengthened beams measured at the respective highest load levels are many times greater than those of beam #2. The reorientation of the principal compressive stress directions is also more pronounced in the case of the post-tensioned beams than in the case of beams #2 and #3. The pre-damage of beam #4 resulted in a significantly softer behavior in the lower and medium load range. However, as the load continued to increase, the deformations were almost identical to those of beams #1 and #5. The maximum load of the beam was only slightly lower than that of the non-pre-damaged beam #1. At the highest comparable load level, the crack widths of beam #4 were even noticeably smaller than those of beam #1. Beam #4 reached 96% of the maximum load of beam #1. Before reaching the failure load, however, the pre-damage of beam #5 led to a pronounced widening of the concrete in one of the compression zones, which occurred only once during the test series. This expansion led to the failure of the beam, as the nearest CFRP stirrup failed when it reached its breaking strain. Apart from the numerous crack branches in the concrete compression zone in question and the cracks resulting from the pre-damage, the crack patterns of the beam #5 and #1 beams were almost identical. The crack widths of both beams were also of the same order of magnitude. The maximum load reached was 92% of the maximum load of beam #1. The suitability of external post-tensioned shear reinforcement made of CFRP is impressively demonstrated by means of large-scale experiments carried out by Stenger (2001) and described above. The external CFRP stirrups, which were clearly undersized in the experiments, already had a significant influence on the maximum load achieved, on the deformation capacity of the shear beams and on the crack widths under maximum load. The appropriate post tensioning of an external CFRP shear strengthening thus results in a more ductile system behavior and a more pronounced indication of failure. This is not self-evident in view of the brittle CFRP tapes, which behave purely elastically without plastic deformation capacity up to tensile failure. 2.2. A pioneering application The Leuenhof, formerly the headquarters of Bank Leu , stands as an iconic commercial edifice along Zurich’s renowned Bahnhofstrasse. Erected between 1913 and 1916, it was originally designed to house the bank, boasting significant historical value with most of its components designated as heritage sites. During the pioneering era of iron reinforced concrete construction, structures often featured low shear reinforcement ratios. The compressive strength of the rammed concrete varied widely. The reinforcing bars had a smooth surface. The material-efficient slabs were engineered to sustain a service load of 200 kg/m 2 . The structure required according to Galmarini et al. (2022) extensive rehabilitation. Initial static assessments revealed inadequate shear reinforcement within the concrete beams, particularly concerning the increased service load mandated by contemporary standards for office spaces. Before 1916, the permissible shear stress in concrete stood at 0.45 MPa, with stirrups - commonly used today for shear reinforcement - largely absent, except for a sparse provision of 7 mm stirrups to secure the longitudinal main reinforcing bars. To optimize the existing structure and minimize necessary strengthening efforts, an extensive testing regimen was devised. Service load assessments and two full-scale destructive tests of reinforced concrete floor beams extracted