Geodesic Lines on Free-Form Surfaces - Optimized Grids for Timber Rib Shells

In addition to glue-laminated timber, screw-laminated timber has increasingly been applied to rib shells. The ribs are made from laminated timber boards, which are joined together with the aid of pin-like fasteners such as screws or nails. The current research is meant to optimize these grids on free-form surfaces with regard to the bending stress due to initial curvature of the boards. This can be done by arranging the ribs according to geodesic lines which allows the application of straight boards, subjected only to bending about their week axis and to torsion. The determination of such geodesic grids on free-form surfaces, currently enjoying great popularity in contemporary architecture, is quite complex. In order to satisfy this demand and to improve automation of the production process, the software GEOS has been developed in close collaboration with the Laboratory of Timber Construction (EPFL/IBOIS) and the chair of Geometry (EPFL/GEOM). This software calculates grids of geodesic lines on free-form surfaces and provides all geometrical data necessary for computer-controlled sawing. Free-form surfaces are treated as Bézier surfaces. In order to proved the reliability of the assumptions the program is based upon, a timber rib shell prototype (length/width/height: 8000 mm x 3000 mm x 2060 mm) was build. Thin timber laths (12/60 mm) were used. All laths were sawn and predrilled by means of computer-controlled sawing. The prototype could thus be built in relatively short time. In general, it can be stated that even after assembly of the fourth layer the predrilled holes at the intersections fitted precisely about 85% of the time. This confirms the reliability of the calculation and shows that the demanded precision is kept even for significant curvatures. For a better understanding of the load-bearing behavior of the structure load tests were carried out. The comparison of the measured deformations with the calculated deformations enabled us to evaluate the calculation model, based on a framed load-bearing system. For small charges, good accordance between calculation and reality could be noticed. The software GEOS provides an important tool for the design and the realization of timber rib shells. It contributes to clarifying current uncertainties in the design planning process and will highly improve the confidence of engineers and architects in conceiving and realizing this type of challenging lightweight spatial structures.

Structural implications of ultra-high performance fibre-reinforced concrete in bridge design

Ana Spasojevic

The present research represents a theoretical and experimental contribution to the understanding of the structural behaviour of elements made of ultra-high performance fibre-reinforced concrete (UHPFRC). UHPFRC is investigated as an advanced cementitious material offering particular potential in innovative bridge design. The optimised material composition results in high compressive strength and non-negligible tensile strength and ductility, provided by multi-microcracking. This allows significant tensile forces to be sustained by elements in bending even without the use of ordinary reinforcement. Thanks also to the material's resistance to environmental degradation, very thin structural elements can be constructed. This research focuses primarily on the bending behaviour and design of thin UHPFRC beams and slabs. Punching-shear is also investigated as a possible failure mode of thin UHPFRC slabs. One of the main differences between other concretes and UHPFRC is that the latter requires mechanical models capable of taking tensile behaviour into account for rational structural application. Analytical and numerical models are developed in this study to simulate the non-linear bending response of UHPFRC beams and slabs. This permits the assessment of element behaviour at service states and prediction of failure loads. Theoretical research on both bending and punching-shear failure is supported by experimental research on beams and slabs made of BSI® UHPFRC with 2.5 % volume of 20-mm long steel fibres. The analytical model for beams in bending takes both material multi-microcracking and macrocrack propagation with tensile softening into consideration. Multi-microcracking is modelled as a pseudo-plastic tensile behaviour, while the macrocrack is simulated based on the assumptions of the fictitious crack model. The results are in good agreement with experimental data and simulations obtained from a developed finite element model. Using theoretical results and experimental data it is demonstrated that pre-peak behaviour and bending strength are mainly governed by multi-microcracking. The propagation of the macrocrack provides only a minor additional contribution to bending strength, but, in the case of thin beams, plays an important role in providing ductility in bending. Theoretical results demonstrate that, due to the presence of pronounced pseudo-plastic deformations, size effect on bending strength is much less significant for UHPFRC than for other quasi-brittle materials, which corresponds to experimental observations. It is however shown that, even if the pseudo-plastic phase is less pronounced, thin elements develop behaviour similar to that of elements with high pseudo-plastic tensile deformations, owing to the low stress decrease in tensile softening. Nonetheless, in the absence of pseudo-plastic tensile deformations, the behaviour of thick elements approaches that of typical quasi-brittle materials, with a more pronounced size effect. In the case of thin statically indeterminate beams and slabs, it is shown that a high level of tensile ductility can allow sufficient internal force redistribution to occur, leading to a significant increase in load-bearing capacity. Moreover, high rotations can be sustained after cracking while almost constant bending strength is maintained, resulting in a plastic-like behaviour. It is demonstrated that the theory of plasticity can thus be applied: a formulation is proposed to predict the resistant plastic moment, enabling easy estimation of the bending failure load for thin elements. The analysis results show good agreement with test results for slabs of different thicknesses. However, due to the remarkable size effect on ductility in bending, the rotation capacity of UHPFRC elements thicker than approximately 100 mm is limited, and the theory of plasticity does not apply. Experimental and theoretical research on the punching-shear failure of thin UHPFRC slabs demonstrates the influence of structural parameters on achieved shear resistance. A proposal is made for considering fibre contribution in shear resistance as a structure-dependent parameter, relating the critical shear crack opening to slab rotation. This approach results in more accurate predictions for thin elements with larger deformations as compared to current code predictions that overestimate resistances for such elements. With a view to the structural application of UHPFRC in bridge design, the concept of ribbed deck slab is studied. Based on the theoretical and experimental results it is demonstrated that thin UHPFRC slabs (40-60 mm) without ordinary reinforcement can be effectively used in this concept: sufficient bending and punching-shear resistances to locally applied traffic loads can be ensured. With prestressed ribs, UHPFRC ribbed slabs attain high load-bearing capacity, while structural dead weight is significantly decreased. This concept could open up new vistas in the design of new structures and offers effective possibilities for the structural repair or widening of existing bridges.

EPFL2008

Poinçonnement des planchers-dalles avec colonnes superposées fortement sollicitées

Roberto Guidotti

Flat slabs are commonly used in buildings due to their easiness of construction and economy. In order to keep these advantages, columns are usually not continuous through the slabs in multi-storey buildings. In these cases, the slabs are subjected to large compressive stresses at the support area of the columns, which can exceed the uniaxial compressive strength of the concrete of the slab. This critical zone is in addition subjected to large shear forces and bending moments due to the loads applied on the slab. This leads to a series of potential failure modes: crushing of the concrete of the slab between columns, flexural failures or punching shear failures. Most research has previously focused on the influence of bending of the slab on the column strength. However, no works have provided in-depth investigation of the strength of the slab when large column loads are applied. In this research, an extensive experimental program has shown that the stresses applied at the support area of the columns can be significantly larger than the uniaxial compressive strength of concrete. The test results have clearly shown that no special confinement or load transfer devices are required between columns for most cases (moderate column loads). In addition, two phenomena have been observed. The first one is a reduction on the flexural strength as column loads are applied. The second corresponds to a significant increase on the punching shear strength and deformation capacity with column loading. Existing theoretical approaches for flat slab behaviour and strength are shown not to be directly applicable for slabs subjected to large column loading. In this research, the principles of two general theories (the theory of plasticity and the critical shear crack theory) are thus used to investigate such cases. The theory of plasticity allows calculating a plastic failure envelope accounting for bending and column loading, whereas the critical shear crack theory, which in this work as been further investigated theoretically, is used to derive a failure criterion accounting for punching shear failure in presence of column loading. The results for both theories are finally presented in terms of a single interaction diagram between column loading and slab loading (bending and shear of the slab). The theoretical approaches require however the help of rather refined numerical tools for estimating the strength of a flat slab. In order to use the theoretical approaches for design, a simplified approach has been developed, allowing to calculate the strength as well as the deformation capacity of flat slabs. These tools were implemented in a design method for slab-column joints in multi-storey building. This design approach allows to derive simplified interaction diagrams that can be compared with the loading history of the structural element analysed.

EPFL2010

Timberfabric: Applying Textile Principles on a Building Scale

Markus Hudert, Yves Weinand

Practical and material orientated academic research has become increasingly important for architectural practice, due to several factors. First, it contributes to contemporary concepts in architecture and improves their implementation. Today’s architects are looking for a deeper understanding of technical and technological questions related to architecture: technology, construction methods and structural considerations are no longer seen as merely bothersome necessities, as was often the case in the past. The importance of such aspects and the potential of including them as active stimuli in the architectural design process are largely recognised. It is the limitations in time and capacities that more often than not confound the realisation of such ambitions. Academic research can fill this gap and provide architectural practices with the necessary resources. Second, research has a duty to address one of the biggest architectural challenges of our time, namely how to achieve sustainable building. Society’s burgeoning awareness of the urgent need to use renewable materials for building construction is an undeniable reality and has become an important parameter for architectural production. As a result, timber constructions experience a new popularity and the importance of research on timber has increased. The potential of this field becomes evident with some of its latest developments and innovations. Cross- laminated timber panels open up new dimensions for massive timber construction and prefabrication in context with the digital chain. Technologies such as wood welding and the densification of wood create new possibilities not only for architecture but also for furniture and product design. Timber as a building material is therefore capable of satisfying both the demands of contemporary architecture as well as the requirements of sustainable building.