Internal Stiffening Research Articles

Mechanical performance of concrete canvas reinforced with insert-bar structures under quasi-static and impact loading

Concrete canvas (CC) is a promising composite based on a combination of three-dimensional fabric and cement-based powder, with the comprehensive advantages of easy storage and transport, convenient construction, rapid strength development, and low dust pollution. In civil and military engineering, interest and exploration work within the field of CC have been ongoing for many years. While external measures exist to enhance the mechanical properties of CC, substantial internal measures are still lacking. In this paper, an internal stiffening strategy was proposed by coupling warp-knitted spacer structure with a series of insert-bar structures, attempting to improve the mechanical performance of CC. The reinforcing effect of flexible bar stiffeners were investigated through quasi-static bursting test and impact test. Damage morphology analysis was then performed to reveal the progressive failure mechanism. The obtained results demonstrated that flexible bar stiffeners with low modulus were highly beneficial reinforcements for CC due to their favorable synergy and great stress dissipation capacity. The load-bearing capacity and toughness of CC were increased with the insertion height and groups of bar combination. The vertical combination could further enhance the penetration resistance by altering stress transfer.

Exploratory Thermo-Mechanical Assessment of the Bottom Cap Region of the EU DEMO Water-Cooled Lead Lithium Central Outboard Blanket Segment

The Water-Cooled Lead Lithium (WCLL) Breeding Blanket (BB) is one of the two BB concept candidates to be selected as the driver blanket for the EU DEMO fusion reactor. In this regard, the development of a sound architecture of the WCLL Central Outboard Blanket (COB) Segment, ensuring the fulfilment of the thermal and structural design requirements, is one of the main goals of the EUROfusion consortium. To this purpose, an exploratory research campaign has been launched to preliminarily investigate the thermo-mechanical performances of the Bottom Cap (BC) region of the WCLL COB segment because of its peculiarities making its design different from the other regions. The assessment has been carried out considering the nominal BB operating conditions, the Normal Operation (NO) scenario, as well as a steady-state scenario derived from the in-box LOCA accident, the Over-Pressurization (OP) scenario. Starting from the reference geometric layout of the WCLL COB BC region, a first set of analyses has been launched in order to evaluate its structural performances under a previously calculated thermal field and to select potential geometric improvements. Then, the analysis of a complete BC region was conducted from both the thermal and structural standpoints, evaluating its structural behaviour in compliance with the RCC-MRx code. Finally, after some iterations and geometric updates, a promising geometric layout of the BC region has been obtained even though some criticalities still persist in the internal Stiffening Plates and First Wall. However, the obtained results clearly showed that the proposed layout is worthy to be further assessed to achieve a robust enough configuration. The work has been performed following a theoretical-numerical approach based on the Finite Element Method (FEM) and adopting the quoted Ansys commercial FEM code.

Comprehensive study of compressive behavior of CFST columns with confinements and stiffeners

This paper presents a performance study of circular-shaped short steel-concrete-filled columns (CFST) with external confinement and internal stiffening under axial loading conditions. Thirty specimens of different types of HST and CFST columns were fabricated and tested. The variables studied included concrete type, inner wall design with/without vertical strips, and shear studs. The failure pattern, deflection, axial strain, hoop strain, types of lateral confinement and their combined effects, and the effect of longitudinal strips were closely examined. The results showed that using external confinement, such as spiral or ring-type steel rods, and internal stiffening, such as vertical strips and shear studs, improved the axial load-carrying capacity and stiffness of the CFST columns. The theoretical and empirical findings of the axial load-carrying ability of CFST columns were found to be in excellent agreement with experimental results. Reliability analysis was carried out, and its results signified that the CFST column specimens with studs and stiffeners were highly reliable.

Full-Scale Testing and Analytical Modeling of Rebar Cages Reinforced with Mechanical U-Bolt Connectors

Rebar cages are the skeletons of reinforced concrete structures. These temporary structures consist of longitudinal and transverse reinforcing bars connected by tie-wires. Given the relatively low strength of tie-wire connections, replacing tie-wires with mechanical connectors such as U-bolts can improve the stability and strength of rebar cages. This paper aims to understand the behavior of large prefabricated rebar cages reinforced with mechanical U-bolt connectors through experimental and analytical investigations. Twenty-six full-scale experimental tests are performed on five different underground pile-shaft rebar cages with tie-wire and U-bolt connectors to determine their behavior during different site handling conditions. The data obtained from the experiments are used to develop and calibrate detailed finite element models that can predict the complex response behavior of rebar cages reinforced with U-bolt connectors. The results show that U-bolt connectors can effectively ensure rebar cage integrity even under extreme loading conditions. Additionally, it is concluded that the presence of U-bolt connectors allows for the elimination of internal stiffening elements, which are common for large rebar cages, adding to the simplicity and efficiency of the construction process. The results of this study can be used as a basis for establishing analysis, design, fabrication, and handling guidelines for rebar cages.

Thermo-Mechanical Analysis and Design Update of the Top Cap Region of the DEMO Water-Cooled Lithium Lead Central Outboard Blanket Segment

Within the framework of the EUROfusion research and development activities, the Water-Cooled Lithium Lead (WCLL) Breeding Blanket (BB) is one of the two candidates to be chosen as the driver blanket for the European DEMO nuclear fusion reactor. Hence, an intense research work is currently ongoing throughout the EU to develop a robust conceptual design able to fulfil the design requirements selected at the end of the DEMO pre-conceptual design phase. In this work, the thermo-mechanical analysis and the design update of the top cap (TC) region of the DEMO WCLL Central Out-board Blanket (COB) segment is presented. The scope of the work is to find a design solution of the WCLL COB TC region able to fulfil the design requirements, prescribed by the reference RCC-MRx code, under the selected nominal and accidental steady state loading scenarios. The activity herein presented moved from the WCLL COB reference design, purposely modified in compliance with the adopted thermal and mechanical requirements in order to attain a robust TC region geometric layout. In the end, a geometric configuration called “TC region-mod++” was determined, foreseeing a TC able to safely withstand both nominal and accidental loads. Nevertheless, some criticalities still hold in the internal stiffening plates and, therefore, further and finer analysis are necessary to fully match the goal. In any case, it was also found that the proposed approach for the design update is promising and worthy to be further pursued. The work was performed following a theoretical–numerical approach based on the finite element method (FEM) and adopting the quoted Ansys commercial FEM code.

Experimental study and application of manufactured sand self-compacting concrete in concrete-filled-steel-tube arch bridge: A case study

Utilizing manufactured sand (MS) with high content of stone dust, mix proportion tests were carried to investigate the influence factors of preparing self-compacting concrete (SCC) with good performance. Results showed that, with the stone dust content no more than 9% and the addition of expansion agent not exceed 10%, the prepared MS-SCC has no slump loss within 4 h for the fresh status and have compressive strength over 55 MPa for the harden status. To further verify the workability of the prepared MS-SCC for the “filling down and pumping up” concrete pouring method, field full-scale filling simulation test was carried out. Results showed that the prepared MS-SCC performed well in construction workability, which could pass through the headed studs and internal stiffening plates in the semi-open tube segment smoothly. All the above results have been successfully applied on both the Zongxi River Bridge and the Xianghuoyan Bridge, which are two similar concrete-filled-steel-tube (CFST)arch bridge located adjacent in mountainous areas with poor engineering conditions. Therefore, although the research results of this paper have certain particularity, they are still valuable to form application guidelines of MS-SCC with high stone dust content in the mountainous areas of southwest China.

Repowering Steel Tubular Wind Turbine Towers Enhancing them by Internal Stiffening Rings

This paper presents a robust repowering approach to the structural response of tubular steel wind turbine towers enhanced by internal stiffening rings. First, a structural response simulation model was validated by comparison with the existing experimental data. This was then followed with a mesh density sensitivity analysis to obtain the optimum element size. When the outdated wind turbine system needs to be upgraded, the wall thickness, the mid-section width-to-thickness ratio and the spacing of the stiffening rings of wind turbine tower were considered as the critical design variables for repowering. The efficiency repowering range of these design variables of wind turbine towers of various heights between 50 and 250 m can be provided through the numerical analysis. Finally, the results of efficiency repowering range of design variables can be used to propose a new optimum design of the wind turbine system when repowering a wind farm.

Effect of partial replacement of coarse aggregate by polystyrene balls on the shear behaviour of deep beams with web openings

Thirteen specimens were experimentally tested under single midspan concentrated loads to study the shear behavior of lightweight concrete (LWC) and normal weight concrete (NWC) deep beams with web openings. In this research, the term LWC refers to the concrete obtained by partially replacing aggregate by polystyrene foam balls not the concrete containing lightweight aggregate. This resulted in a weight reduction of LWC beams in this research by approximately 30% compared to NWC compartments. The studied variables were the dimensions and location of openings, transverse reinforcement ratio, and shear span to depth ratio (a/d). It was found that the overall shear behavior and failure mode for LWC deep beams are comparable to those of the NWC specimens. This is very promising and encouraging to build lighter deep beams of similar structural behaviour as that of NWC deep beams. Dimensions of the openings have a significant effect on the behaviour of failure and shear strength of LWC and NWC deep beams. It was found that increasing the depth of the opening from 20 % to 40 % of the beam depth led to a reduction in the ultimate load by up to 46.4 %. Finite element modelling of the test beams was carried out to verify numerical results versus experimental work and both were very well correlated. In addition, a parametric study was conducted to assess the effect of internal stiffening around openings in deep beams. The maximum enhancement in the shear capacity was approximately 30 % for beams, internally strengthened by additional reinforcement on the perimeter of openings compared to the beams without any reinforcement around the openings. Strut-and-Tie model (STM) was carried out as a rational approach to predict the shear behaviour of studied beams. It was found that STM underestimates the shear of the studied beams compared to experimental results for different tested beams but the agreement between both of them was acceptable. It is recommended that the depth of opening should not exceed 20 % of the depth of the deep beam and if the depth of opening is more than that or lies in the shear span it is highly recommended to strengthen the opening internally by additional reinforcement around its perimeter.

Behavior of Rectangular-Sectional Steel Tubular Columns Filled with High-Strength Steel Fiber Reinforced Concrete Under Axial Compression.

This paper studies the effect of high-strength steel fiber reinforced concrete (SFRC) on the axial compression behavior of rectangular-sectional SFRC-filled steel tube columns. The purpose is to improve the integrated bearing capacity of these composite columns. Nine rectangular-sectional SFRC-filled steel tube columns and one normal concrete-filled steel tube column were designed and tested under axial loading to failure. The compressive strength of concrete, the volume fraction of steel fiber, the type of internal longitudinal stiffener and the spacing of circular holes in perfobond rib were considered as the main parameters. The failure modes, axial load-deformation curves, energy dissipation capacity, axial bearing capacity, and ductility index are presented. The results identified that steel fiber delayed the local buckling of steel tube and increased the ductility and energy dissipation capacity of the columns when the volume fraction of steel fiber was not less than 0.8%. The longitudinal internal stiffening ribs and their type changed the failure modes of the local buckling of steel tube, and perfobond ribs increased the ductility and energy dissipation capacity to some degree. The compressive strength of SFRC failed to change the failure modes, but had a significant impact on the energy dissipation capacity, bearing capacity, and ductility. The predictive formulas for the bearing capacity and ductility index of rectangular-sectional SFRC-filled steel tube columns are proposed to be used in engineering practice.

Achieving targeted levels of reliability for low-rise seismically isolated structures

By interpreting the seismic responses of two seismically isolated low-rise case-study buildings probabilistically using FEMA P695 methodology, the study indicates for the design of seismically isolated structures, providing isolator capacities equal to risk-targeted maximum considered earthquake (MCER) demand does not achieve targeted levels of reliability specified in ASCE 7–16. To do so, isolation system capacities beyond average MCER demand are required. The minimum required capacities for using three types of enhanced isolation system (isolator without displacement restraint, isolator combined with external hard-stopping mechanism, and isolator with internal stiffening behavior at large horizontal displacement) are calculated with nonlinear response history analysis following probabilistic framework for different design risk categories numerically. The results indicate that isolator displacement capacities ranging from 1.5 to 2.60 times the average MCER demand and isolation system shear capacities ranging from 1.5 to 5 times the average MCER demand are required, depending on seismic risk categories and isolation system types. The use of an isolator with internal stiffening behavior is an efficient option to provide the required capacities for achieving reliability goals, especially for high risk-category design.

Structural performance of short concrete-filled steel tube columns with external and internal stiffening under axial compression

External confinement and internal stiffening have been widely adopted for strengthening concrete-filled steel tube (CFST) columns. This paper presents an attempt to study the possibility of combining the external confinement using reinforcing rings and the internal stiffening using reinforcing bars for strengthening CFST columns. In order to investigate the structural performance of the proposed combination method, a total of 18 specimens were tested under axial compression. These specimens were strengthened using three different methods: confinement in the form of external reinforcing rings (ERRs), stiffening in the form of internal reinforcing bars (IRBs), and a combination of both methods. For all tested specimens, the thickness of steel tube, outer diameter, and tube length were 4.15 mm, 114.3 mm, and 260 mm, respectively. CFST specimens were filled using self-compacting concrete (SCC) of normal compressive strength. The main parameters considered in the test procedure are the spacing between the ERRs, the number of IRBs, and a combination of the ERRs spacing and the number of IRBs. The experimental results show that the proposed combination method is more effective to improve the structural performance of short CFST columns compared to using only ERRs or IRBs. Further improvement is achieved when the spacing between the ERRs decreases and the number of IRBs increases. The proposed combination method increases the compressive load-carrying capacity up to 27.2% and 57.4% compared to ERRs and IRBs, respectively. It deserves noting that the proposed combination method, ERRs, and IRBs improve the compressive load-carrying capacity up to 120.3%, 93.5%, and 40%, respectively, compared to the control specimen. In addition, specimens strengthened by the proposed combination method show an excellent ductility behavior.

Pressure Losses in Solar Chimney Power Plant

This technical brief develops a theoretical model of all the pressure losses in the solar chimney power plant (SCPP, also called solar updraft power plant) and analyzes the pressure losses for different chimney internal stiffening appurtenance (SA) structures, different roof heights, and different collector support parameters. Results show that the exit dynamic pressure drop (EDPD) accounts for the majority of the total pressure loss (TPL), while other losses constitute only small proportions of the TPL, and the collector inlet loss is negligible. Pressure losses are strongly related to the mass flow rate, while reasonable mass flow rates excluding too low flow rates have little influence on the pressure loss ratios (PLRs, defined as the ratios of the pressure losses to the TPL) and the total effective pressure loss coefficient (TEPLC). Designing of the SA structure in view of reducing the drag, for example, using the ring stiffeners without wire spoked instead of the spoked bracing wheels (SBWs), reducing the width of the chimney internal rims of SAs, or reducing the number of SAs results in large reduction of the SA PLR and the TPL. Lower roof leading to higher velocity inside the collector, larger supports, or shorter intersupport distance leads to the increase in the support PLR. This technical brief lays a solid foundation for optimization of SCPPs in future.

Preparation of random poly(butylene alkylate-co-terephthalate)s with different methylene group contents: crystallization and degradation kinetics

Random copolyesters having 1,4-butanediol units were synthesized from a transesterification process between homopolymers constituted by aliphatic dicarboxylates (i.e. succinate, adipate or sebacate) and the aromatic therephthalate derivative, as verified by NMR spectroscopy. Biodegradability of resulting copolyesters was studied via enzymatic hydrolysis using Pseudomonas cepacia lipase at pH = 7.2 and 37 °C. Kinetics of degradation showed that in all cases the degradation rate decreased after 19 days of exposure. The observed glass transition temperatures, T g, of the random copolyesters showed a non-linear dependence on composition, a feature that was explained in terms of the internal stiffening effect of butylene terephthalate units. Copolymers with higher aliphatic (i.e. 50 and 70 mol-%) and methylene (i.e. adipate and sebacate units) contents showed double melting peaks in DSC thermograms. These copolyesters resulted in two different crystalline rich phases after melt-crystallization and subsequent cooling. The ratio between these phases logically depended on the predominant aliphatic or aromatic dicarboxylate content. The copolymers initially crystallized via the aromatic units through a heterogeneous nucleation and a spherulitic growth. The presence of aliphatic dicarboxylate units hindered the beginning of the crystallization process, but the overall growth kinetic constant was similar for all samples. The secondary nucleation constants were determined and showed higher values for samples with higher adipate and sebacate contents.

Structural bionic design for a machine tool column based on leaf veins

Some biological structures, such as leaf veins, bamboo and animal bones, have excellent mechanical properties after millions of years evolution. By studying the distribution characteristics of biological structures, the performances of mechanical components can be improved using structural bionic design method. In this paper, the internal stiffening ribs of a machine tool column are rearranged based on the structure of leaf veins, and a bionic column is designed. Static and modal analysis of the conventional and bionic column is conducted to investigate the static and dynamic performances by finite element method. Then, static loading experiment and modal test are carried out for further verification. The simulation results have good agreement with the experiment results. Compared with the conventional column, the maximum deformation of the bionic column in experiment is reduced by 24.69%, and the first six-order natural frequencies of bionic column are increased by 48.39%, 12.98%, 10.70%, 5.11%, 3.07%, and 8.44%, respectively.

Analysis of a preliminary configuration for a floating wind turbine

There are many theoretical analyses and experimental studies of the hydrodynamics for the tension leg platform (TLP) of a floating wind turbine. However, there has been little research on the arrangement of the TLP\'s internal structure. In this study, a TLP model and a 5-MW wind turbine model as proposed by the Minstitute of Technology and the National Renewable Energy Laboratory have been adopted, respectively, to comprehensively analyze wind effects and wave and current combinations. The external additional coupling loads on the TLP and the effects of the loads on variables of the internal structure have been calculated. The study investigates preliminary layout parameters-namely, the thickness of the tension leg body, the contact mode of the top tower on the tension leg, the internal stiffening arrangement, and the formation of the spoke structure-and conducts sensitivity analyses of the TLP internal structure. Stress is found to be at a maximum at the top of the tension leg structure and the maximum stress has low sensitivity to the load application point. Different methods of reducing maximum stress have been researched and analyzed, and the effectiveness of these methods is analyzed. Filling of the spoke structure with concrete is discussed. Since the TLP structure for offshore wind power is still under early exploration, arrangements and the configuration of the internal structure, exploration and improvements are ongoing. With regard to its research and analysis process, this paper aims to guide future applications of tension leg structures for floating wind turbine.

Investigation of stiffening scheme effectiveness towards buckling stability enhancement in tubular steel wind turbine towers

Current climate conditions along with advances in technology make further design and verification methods for structural strength and reliability of wind turbine towers imperative. Along with the growing interest for green energy, the wind energy sector has been developed tremendously the past decades. To this end, the improvement of wind turbine towers in terms of structural detailing and performance result in more efficient, durable and robust structures that facilitate their wider application, thus leading to energy harvesting increase. The wind tower industry is set to expand to greater heights than before and tapered steel towers with a circular cross-section are widely used as more capable of carrying heavier loads. The present study focuses on the improvement of the structural response of steel wind turbine towers, by means of internal stiffening. A thorough investigation of the contribution of stiffening rings to the overall structural behavior of the tower is being carried out. These stiffening rings are placed along the tower height to reduce local buckling phenomena, thus increasing the buckling strength of steel wind energy towers and leading the structure to a behavior closer to the one provided by the beam theory. Additionally to ring stiffeners, vertical stiffening schemes are studied to eliminate the presence of short wavelength buckles due to bending. For the purposes of this research, finite element analysis is applied in order to describe and predict in an accurate way the structural response of a model tower stiffened by internal stiffeners. Moreover, a parametric study is being performed in order to investigate the effect of the stiffeners' number to the functionality of the aforementioned stiffening systems and the improved structural behavior of the overall wind converter.

Modification of a liquid hydrogen tank for integrated refrigeration and storage

The modification and outfitting of a 125,000-liter liquid hydrogen tank was performed to provide integrated refrigeration and storage capability. These functions include zero boil-off, liquefaction, and densification and therefore require provisions for sub-atmospheric tank pressures within the vacuum-jacketed, multilayer insulated tank. The primary structural modification was to add stiffening rings inside the inner vessel. The internal stiffening rings were designed, built, and installed per the ASME Boiler and Pressure Vessel Code, Section VIII, to prevent collapse in the case of vacuum jacket failure in combination with sub-atmospheric pressure within the tank. For the integrated refrigeration loop, a modular, skeleton-type heat exchanger, with refrigerant temperature instrumentation, was constructed using the stiffening rings as supports. To support the system thermal performance testing, three custom temperature rakes were designed and installed along the 21-meter length of the tank, once again using rings as supports. The temperature rakes included a total of 20 silicon diode temperature sensors mounted both vertically and radially to map the bulk liquid temperature within the tank. The tank modifications were successful and the system is now operational for the research and development of integrated refrigeration technology.

Effect of internal stiffening rings and wall thickness on the structural response of steel wind turbine towers

In this paper, the structural response of steel tubular wind turbine towers with various design configurations is analysed using FEM modelling. Towers of various heights between 50 and 250m are considered and investigated with three different design options as follows: (i) thick walled tower with internal horizontal stiffening rings, (ii) thick walled tower without stiffening rings and (iii) thin walled tower with stiffening rings. Based on this analysis, weight reduction ratios are examined in relation to the horizontal sway and von Mises stress increase ratios in order to identify a more efficient design approach between reducing the wall thickness and adopting internal stiffeners. All studied design solutions satisfy the strength and serviceability requirements as specified by the design codes of practice. In the final part of paper, the dynamic characteristics of these three types of towers have been examined to obtain the natural frequencies and mode shapes. The studied model ignored the mass of nacelle-rotor system and the wind turbines, namely, only the isolated tower was included. Furthermore, the recommendations to avoid resonance for each height case are proposed.

Ground vehicle blunt impact damage formation to composite aircraft structures

Composite aircraft structures such as fuselage and wings are subject to impact from many sources. Of particular interest are impacts which can impart severe damage that is difficult to visually detect, or is even not visually detectable. Such damage modes include delamination, separation/delamination of stringers and stiffeners from skin, and fracture of internal stiffening components such as stringers and frames. Extensive damage can be produced by blunt impact sources involving wide-area contact, which reduces the propensity to leave visible signs that damage has formed, especially with high-strength composite outer skin. Ground service equipment (GSE) is a realistic and oft-occurring source of blunt impact damage due to: (i) the interactions between GSE and aircraft typically involving a soft rubber bumper material, (ii) the close proximity with which GSE operate around aircraft, and (iii) the high mass of GSE involves very large impacting energies despite their slow speeds. Blunt impact damage can produce both local and non-local damage since, as the contact area increases, higher energy levels are involved for blunt impact damage to occur. Therefore, inspections to find damage from GSE impact events require that locations relatively far away from the impact sites must also be inspected.

Use of superplastic deformation and diffusion welding to form a tee designed with a complex set of internal stiffening elements

A method is described for making a tee designed with a complex set of internal stiffening elements. The article describes the main stages of the modern process of pneumothermal forming in the superplastic regime with the use of diffusion welding. The production of a tee by superplastic pneumothermal forming after diffusion welding is modeled and the calculated results are presented.