Structural steel

Summary

Structural steel is a category of steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a of a specific cross section. Structural steel shapes, sizes, chemical composition, mechanical properties such as strengths, storage practices, etc., are regulated by standards in most industrialized countries. Most structural steel shapes, such as -beams, have high second moments of area, which means they are very stiff in respect to their cross-sectional area and thus can support a high load without excessive sagging. Common structural shapes The shapes available are described in many published standards worldwide, and a number of specialist and proprietary cross sections are also available. *-beam (-shaped cross-section – in Britain these include Universal Beams (UB) and Universal Columns (UC); in Europe it includes the IPE, HE, HL, HD and other sections; in the US it includes Wide Flange (WF

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https://en.wikipedia.org/wiki/Structural_steel

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Embrittlement induced by the synergistic effects of radiation damage and helium in structural steels

Kun Wang

Ferritic/martensitic steels are candidate materials for fusion reactor structural components, liquid metal containers of spallation neutron sources, and accelerator driven systems, with good radiation resistance and thermo-mechanical properties. However, embrittlement resulting from the combined effects of radiation induced displacement damage (measured in dpa = displacement per atom) and transmutation products, especially helium gas, is one of the key issues. Four different steels were selected for mechanical and microstructural studies to understand the mechanisms of embrittlement induced by the combined effects of displacement damage and helium after irradiation in SINQ, the Swiss spallation source. The irradiations conditions were in the range: 10.7 ¿ 20.4 dpa with 850-1750 appm He at 160-300 °C. The evolution of the mechanical properties after irradiation was investigated by tensile and hardness tests. Radiation-induced defect clusters and helium bubbles were quantified by transmission electron microscopy (TEM). Emphasis was put on the deformation mechanisms under the different observed fracture mode, (ductile, quasi-cleavage and intergranular), whose occurrence depends on the irradiation conditions (dpa, He content and irradiation temperature). The tensile stress-strain curves and the scanning electron microscopy images of fracture surfaces showed distinct fracture mechanisms under different irradiation and test conditions. The tensile tests showed a yielding stress increase and loss of ductility of irradiated specimens. Hardness was measured on the specimens before tensile testing. The hardness results demonstrated an increasing trend with irradiation dose and helium content. TEM observations were done for all irradiated fractured specimens. Small defect clusters were observed in the 12.3 dpa specimen, but large defect clusters with loop-shape were very few. In the specimens of 17.2, 17.7 and 20.4 dpa, many large dislocation loops were detected besides small clusters. In addition, helium bubbles were observed in all specimens. The average size of defect clusters increased from 4.2 nm to 11.8 nm with dose increasing from 12.3 dpa to 20.4 dpa, whereas the number density did not change significantly. Meanwhile, the average size of visible helium bubbles increased from 1.03 nm to 1.93 nm. The microstructures in deformed area of irradiated specimens were observed and defect free channels with {110} and {112} slip planes were found in some specimens, indicating plastic flow localization. The average width of the channels is about 100 nm. Regarding the brittle samples, the TEM-lamella were extracted directly below intergranular fracture surfaces or cleavage surfaces by focused ion beam. Strikingly, deformation twinning was observed as the main feature in three irradiated specimens at high dose. Only twins with {112} planes were observed in all of these samples. The average thickness of twins is about 34 nm. Twins started from a fracture surface became gradually thinner with distance away from the fracture surface and stopped in the matrix finally. Features such as twin-precipitates interaction, twin-grain boundary and/or lath boundary interaction were observed. Twinning bands were seen to be arrested by grain boundaries or large precipitates, but could penetrate martensitic lath boundaries. Unlike the case of defect free channels, inside twins small defect-clusters, dislocation loops and dense small helium bubbles were observed.

EPFL2015

On the compressive and bond strength of reinforced concrete as structural properties

Francesco Rafael Alberto Moccia

Traditionally, the concrete strength is measured on cubes or cylinders having normalized dimensions, suitable vibration and curing conditions and their strength is assessed in laboratory under fast loading rates. However, the in-situ strength of structural elements can considerably differ from that of a small and homogenous control specimen due to a number of issues. Notably, phenomena taking place during the consolidation process of fresh concrete may affect the compressive resistance of tall members as well as the bond strength of reinforcing bars located in top layers. During concrete consolidation, water migrates in the direction of the free surface while concrete settles downwards, phenomena referred as concrete bleeding and plastic settlement respectively. Under these circumstances, a decrease in the concrete properties near the upper surface is observed as well as a development of cracks and voids surrounding horizontal reinforcing bars, causing potential disturbances on the compressive stresses and affecting the mechanical engagement between bars and concrete. In addition, the response of structural concrete may differ from that of material samples due to non-uniform stress states, the material brittleness, the cracking induced by imposed strains, the rheological response of concrete and the presence of embedded disturbances. As a result, the strength measured in material samples needs to be corrected with strength reduction factors to ensure suitable structural analysis. In this thesis, an in-depth investigation is performed on the different phenomena affecting the compressive and bond strength of structural members. These aspects are assessed by means of several testing programmes instrumented with refined measurements techniques such as tomography and Digital Image Correlation. An extensive experimental programme comprising 76 column and prism tests was carried out to evaluate the influence of casting position, loading direction and bar disturbances on the compressive resistance of structural elements. The detailed measurements performed at the fresh and hardened state resulted in the proposal of consistent design rules accounting for the investigated phenomena. Focus was also given on the influence of material brittleness and the implications of internal stress redistributions on the structural response of reinforced concrete columns and compression zones of members in bending. The pertinence of the investigations were validated based on more than 400 column tests collected from the literature. The implications of casting conditions on pull-out and spalling failures were also assessed by means of 137 pull-out tests on reinforcing bars presenting variable diameter, concrete cover, casting height and embedded length. The investigations resulted in the proposal for a physically-consistent approach, evaluating the pull-out resistance as function of casting conditions and reinforcement characteristics. Finally, the phenomenon of cover spalling was examined with respect to the action of a radial inner pressure, as originated by bond engagement or associated to the volumetric expansion of corroded reinforcement. The mechanisms inducing spalling were analysed by means of a comprehensive experimental programme comprising 56 specimens instrumented with Digital Image Correlation. A mechanical model is eventually proposed to evaluate bond-related cases of cover spalling.

EPFL2021

Designing new concrete members and assessing the strength of existing elements subjected to in-plane stress state conditions is often conducted by means of Stress Fields (SF) and Strut-and-Tie Models (STM). These methods are usually accounting for identical load-carrying mechanisms for design and assessment, even if the goals for these two tasks are not the same. When designing new elements, the aim is to obtain solutions that are in equilibrium with external actions, with simple reinforcement layouts while ensuring satisfactory behaviour at serviceability limit state. When assessing the ultimate strength of existing elements, the goal is to avoid an unnecessary strengthening and limit the amount of retrofitting. The required complexity of assessment models depends on the strength requirements, and need to be gradually refined if the results from current models prove to be insufficient. This refinement process ultimately leads to increasingly more exact solutions that eventually correspond to the largest possible strength according to the limit analysis. This thesis presents various strategies which can be employed to develop SF and STM suitable for the design and assessment of structural concrete members. The idea of a gradual model refinement (both for design and assessment) is introduced through practical examples, while potential challenges related to each solution are indicated and discussed. Moreover, the accuracy and the generality of exact solutions obtained using Elastic-Plastic Stress Fields (EPSF) are investigated. To do this, ultimate loads estimated with EPSF are compared to test results found in the literature. To facilitate further studies by other researchers, they are available online. The analysis of structural members with insufficient anchorage and indirectly supported concrete elements with EPSF are presented and discussed. Furthermore, this thesis focuses on a sensitivity analysis of the EPSF, to investigate the stability of the results as a function the size, shape and orientation of the finite elements. The influence of the number of iteration steps on the accuracy of EPSF models is evaluated, and clear recommendations are provided. Stress fields based on exact solutions of the theory of plasticity simulate the physical behaviour of structural concrete members more accurately than current code provisions. On this basis, a procedure for tailoring partial safety factors for steel and concrete is presented and discussed. Reduced PSF could potentially be used when assessing the strength of existing reinforced and prestressed concrete elements, which would (could?) lead to significant cost reductions in the field of structural maintenance. To better understand the mechanical origins of concrete compression softening (important for an accurate application of stress fields), a mechanical model for estimating the effective concrete compressive strength is developed and discussed. Concrete cover spalling, concrete crushing and crack sliding are taken into account. Special attention is given to the dowel action of the reinforcement and its effects on the surrounding concrete matrix. The model is validated using experimental results found in literature. Finally, the pertinence of existing semi-empirical approaches for determining the effective concrete compressive strength is evaluated and discussed.

EPFL2017