Seismic in-plane behavior of URM walls upgraded with composites

Existing unreinforced masonry (URM) buildings, many of which have historical and cultural importance, constitute a significant portion of existing buildings around the world. Recent earthquakes have shown the vulnerability of such URM buildings. This thesis investigates the in-plane seismic behavior of URM walls retrofitted using composites. The thesis includes an extensive dynamic and static cyclic tests followed with development of an analytical model. For the dynamic tests, five half-scale single wythe URM walls were built using either strong or weak mortar and half-scale hollow clay brick units. These five walls were dynamically tested as reference specimens. Then, these reference specimens were retrofitted on single side only using composites and retested. As consequence a total of eleven specimens were tested on the earthquake simulator at ETHZ. For the static cyclic tests, five half-scale single wythe URM walls were built using weak mortar and half-scale hollow clay brick units. Of them, three specimens were tested as reference specimens. Then, two specimens of these three reference specimens were retrofitted using composites and tested again. The third reference specimen was retrofitted using post-tensioning and tested; then, the post-tension forces were released and the specimen was retrofitted using composites and retested. Finally, two virgin specimens were retrofitted directly after construction and tested. As consequence a total of nine specimens were tested in the Structural Laboratory at EPFL. For analytical models, an innovative shear model is developed. In addition, a simple flexural model is developed. For shear analysis, masonry, epoxy, and composites in a URM wall retrofitted using composites (URM-FRP) were idealized as different layers with isotropic homogenous elastic materials. Then, using principles of theory of elasticity the governing differential equation of the system is formulated. A double Fourier sine series was used as the solution for the differential equations. The solution can be used to model the linear shear behavior of URM-FRP. To take into consideration material nonlinearity, step-by-step stiffness degradation has been implemented in a computer program. For flexural analysis, a simple model using linear elastic approach with the well-known assumptions of Navier-Bernoulli and Whitney's equivalent stress block is developed. The experimental work shows that the retrofitting technique improved the lateral resistance of the URM walls by a factor ranged from 1.3 to 5.9 depending on the applied normal force, the reinforcement ratio, and mode of failure. However, improvement in lateral drift was less significant. Moreover, no uneven response was observed during tests due to single sided retrofitting. Several phenomena and relationships have been correctly determined by the model. These phenomena and relationships are originally observed in the literature during tests on reinforced concrete beams that were retrofitted using composites. This includes the relationship between strains in FRP and reinforcement ratio as well as the interaction between masonry lateral resistance and FRP contribution to the lateral resistance of URM-FRP. In addition, effects of epoxy ductility and allowable shear stresses as well as masonry ductility and allowable shear stresses have been studied. Such development is of interest to the structural engineering community and material producers. Regarding flexural analysis, the simple model leads to unconservative designs. Correlation analysis of the test data show that the ratio between the experimental lateral resistance to the estimated flexural lateral resistance is proportional to reinforcement mechanical ratio times the square of the effective moment/shear ratio up to a certain limit. Within the limits of experimental testing, a correlation factor is proposed.

Assessment of Shear Strength for Existing Bridges with Low Amounts of Shear Reinforcement

Miguel Fernández Ruiz, Aurelio Muttoni, Michael Markus Rupf

Design of concrete girder bridges has significantly evolved during the last decades. This has been particularly relevant with respect to shear design, motivated by changes in actions and design models. As a consequence, assessing the shear strength of existing bridges leads in many cases to unsatisfactory safety levels. Furthermore, many existing bridges do not comply with current code regulations with respect to minimum amounts of shear reinforcement. This situation can lead to expensive retrofitting and strengthening of a significant number of existing bridges. The assessment of the shear strength of these bridges has thus become a significant task for structural engineers. Design codes are not always appropriate for assessing the strength of existing bridges. They propose safe models providing sufficient accuracy for design of a wide number of structures. However, these models do not account for some particularities of prestresses bridges and neglect a number of shear-transfer actions that can be relevant for their strength. This is typically the case of shear carried by the inclination of the compression chord, the increase of the stress in the tendons or the effective strength of concrete in the web of cracked prestressed girders. Accounting for more realistic approaches for these parameters may significantly increase the estimated strength of a structure with respect to code provisions, avoiding in many cases unnecessary retrofitting or strengthening. In this paper, the results of a tests campaign carried out at Ecole Polytechnique Fédérale de Lausanne on 10 prestressed concrete girders (10 meters long, 0.78 m high) are outlined. The specimens were provided with very low amounts of shear reinforcement and some of them present defective stirrup anchorage to simulate realistic conditions of existing structures. The experimental results are discussed with reference to the stress field method where the role of the different shear-transfer actions is investigated and compared to current code provisions.

fib Symposium Tel-Aviv 20132013

Zürich Stadtspital Triemli Personalhäuser – Resource assessment of structural elements

Maléna Bastien Masse, Julie Rachel Devènes, Corentin Jean Dominique Fivet, Célia Marine Küpfer

The Triemli Personalhäuser are three equal 15-story buildings located on the Zürich Triemli Stadtspital campus and erected between 1964 and 1969. Cast-in-place reinforced concrete (RC) slabs and walls form the building cores and their surrounding corridors. The rooms are arranged around these cores. The load-bearing intermediate walls, made of prefabricated masonry, support thin precast slabs used as permanent forms. A RC layer is cast over these precast slabs and connects the room slabs with the cast-in-place slabs of the corridors and cores. The self-supporting facade consists of precast RC panels. The City of Zürich plans the deconstruction of these three buildings, thus making available a large amount of RC elements composing the structure and the facades of these buildings. Little-known and rarely implemented, the reuse of concrete elements from obsolete buildings in new projects is a sustainable approach that promotes a circular economy. When reusing, the components of obsolete buildings are carefully dismantled without being crushed. They are then cleaned, possibly repaired or trimmed, and reused without many transformations in a new project, maintaining their shapes, technologies, and mechanical properties. In addition to maintaining the embodied energy and history of the reused components, reuse allows the construction industry to reduce demolition waste, greenhouse gas emissions, and material consumption. This report is a preliminary resource assessment and aims at inventorying and assessing all structural elements of the Triemli Personalhäuser, focusing on their potential value for reuse. Both precast and cast-in-place RC elements are included. They are part of the load-bearing structure or are self-supporting such as the precast facade elements. The proposed methodology allows identifying all properties needed to evaluate the potential for reuse of an element: geometry, material properties, current condition, aesthetics, accessibility, resistance, future durability and environmental impacts. After reviewing available reports and drawings on the buildings, onsite visits are carried out to complete the information and visually inspect the structural elements. During the inspection, the elements are assessed with regards to their suitability for reuse and their condition is classified into a five-grade scale. The investigations are completed with destructive and non-destructive testing of the material properties. Together, Buildings A, B and C are made up of approximately 7 000 m3 of materials constituting their load-bearing system, with approximately 2 500 m3 of precast concrete, 3 400 m3 of cast-in-place concrete and 1 100 m3 of masonry. Of this total, approximately 4% of the volume is dropped from the analysis due to the bad condition of the elements, namely the balcony slabs, the roof slab, and the external stairs. The other elements are in a good or acceptable condition and ae inventoried and analysed in detail. The inventoried elements are divided into 5 categories: (1) facade elements; (2) slab elements; (3) wall elements; (4) column elements; and (5) staircases. Each of these categories are subdivided into a certain number of element types for which a complete factsheet is prepared, including pictures, drawings and useful information on their condition. The volume and weight of each element types are given, as well as their share of the total material volume. The embodied global warming potential (in kgCO2eq) for fabrication and demolition of the elements is also calculated. The results of the investigation on material properties confirm sufficient compressive strength for all elements. The carbonation depths measured on the cores are lower than the cover thickness of the reinforcement. Thus, the concrete is not carbonated in the reinforcement areas and the risk of corrosion is kept low, insuring a good durability of the elements. This document should serve as a base for designing and planning future reuse applications for the concrete elements extracted when deconstructing the Triemli Personalhäuser. The information presented here will help the planners to prioritize the reuse strategy on the elements in the best conditions, with the largest volume share and thus with the largest embodied global warming potential.

EPFL2022

Assessment of Shear Strength for Existing Bridges with Low Amounts of Shear Reinforcement

Miguel Fernández Ruiz, Aurelio Muttoni, Michael Markus Rupf

fib Symposium Tel-Aviv 20132013

Zürich Stadtspital Triemli Personalhäuser – Resource assessment of structural elements

Maléna Bastien Masse, Julie Rachel Devènes, Corentin Jean Dominique Fivet, Célia Marine Küpfer

EPFL2022