Development of a displacement-based design approach for modern mixed RC-URM wall structures

The recent re-assessment of the seismic hazard in Europe led for many regions of low to moderate seismicity to an increase in the seismic demand. As a consequence, several modern unreinforced masonry (URM) buildings, constructed with reinforced concrete (RC) slabs that provide an efficient rigid diaphragm action, no longer satisfy the seismic design check and have been retrofitted by adding or replacing URM walls with RC walls. Of late, also several new construction projects have been conceived directly as buildings with both RC and URM walls. Despite the widespread use of such construction technique, very little is known about the seismic behaviour of mixed RC-URM wall structures and codes do not provide adequate support to designers. The aim of the paper is therefore to propose a displacement-based design methodology for the design of mixed RC-URM edifices and the retrofit of URM buildings by replacing or adding selected URM walls with RC ones. The article describes also two tools developed for estimating important quantities relevant for the displacement-based design of structures with both RC and URM walls. The tools are (i) a mechanical model based on the shear-flexure interaction between URM and RC walls and (ii) an elastic model for estimating the contribution of the RC slabs to the overturning moment capacity of the system. In the last part of the article the proposed design method is verified through non-linear dynamic analyses of several case studies. These results show that the proposed design approach has the ability of controlling the displacement profile of the designed structures, avoiding concentration of deformations in one single storey, a typical feature of URM wall structures.

Seismic Behaviour and Design of Mixed RC-URM Wall Structures

Alessandro Paparo

In several countries of central Europe, many modern residential buildings are braced by reinforced concrete (RC) and unreinforced masonry (URM) walls coupled with RC slabs. Such mixed constructions, in fact, behave better under seismic loading than buildings with URM walls alone. Similarly, the insertion of RC walls is a technique used to retrofit existing modern URM constructions that feature RC slabs, since both strength and displacement capacities of the repaired structure increase with respect to the un-retrofitted configuration. Although modern mixed RC-URM wall buildings are rather common, very little is known about their seismic behaviour and codes do not provide adequate design and assessment guidelines. Hence, the thesis focuses on four objectives: (1) to provide experimental data on the seismic behaviour of mixed RC-URM wall structures; (2) to formulate recommendations for setting up numerical models for structures with both RC and URM walls; (3) to develop a mechanical model that represents the interaction between RC and URM walls; (4) to propose a displacement-based seismic approach for the design of new mixed RC-URM wall structures and the retrofitting of URM wall buildings by adding RC walls or replacing URM walls with RC ones. An experimental investigation of two mixed RC-URM wall substructures leads to new findings with respect to the actual distribution of the reaction forces between RC and URM walls and gives insight into the displacement profile of mixed RC-URM wall structures. The results are used to validate two numerical strategies and recommendations for setting up models for structures with both RC and URM walls are formulated. A simple mechanical model, which takes into account the most important parameters influencing the seismic behaviour of mixed RC-URM wall structures, is proposed and validated. The model is based on the shear-flexure interaction as URM and RC walls display dominant shear and flexural deformations, respectively. In the last part of the thesis a displacement-based design approach for mixed RC-URM wall structures is developed. The design method is verified through inelastic time-history analyses (ITHA) of three-to-five-storey case study buildings. Comparison between the design values and the results from ITHA suggests that the design methodology controls the horizontal deflection of the structures, being almost linear over their height, and avoids concentrations of deformations in the bottom storey, a typical feature of URM wall structures. On the other hand, for the three-storey configurations, it was observed that the approach overestimates the maximum horizontal displacement.

EPFL2015

Performance-based approach for the retrofit of URM wall structures by RC walls

Katrin Beyer, Alessandro Paparo

In several countries of moderated seismicity, with the re-evaluation of the seismic hazard most unreinforced masonry (URM) buildings with reinforced concrete slabs failed to satisfy the seismic design check. A possible seismically retrofit solution consists of adding RC walls to the existing structure or replacing selected critical URM walls with RC ones. Experimental and numerical studies have shown that this retrofit technique can be very effective since it modifies the global deformed shape of the structure, leading to an increase in the system’s displacement capacity. The paper proposes a Displacement-Based Design methodology for the retrofit of URM structures by replacing selected URM walls by RC walls. The methodology follows the Direct Displacement-Based Design (DBD) approach by Priestley et al. (2007) and is based in particular on the DBD procedure for frame-wall buildings (Sullivan et al., 2005 and 2006). The design procedure consists of three main phases: (i) A preliminary design check of the URM building by means of the DBD approach. (ii) If the structure does not fulfil the design check and exhibits at the same time a dominant shear behaviour, replacing the critical URM wall or walls with RC ones leads to an improved system’s behaviour. (iii) In the final phase, the DBD design of the mixed RC-URM wall structure, in which both the URM and the RC walls are taken into account, is carried out. The design methodology is then investigated through non-linear dynamic analyses of one case study.

2014

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.