This study focuses on the response of flat unsymmetric laminates to an inplane compressive loading that for symmetric laminates are of sufficient magnitude to cause bifurcation buckling, postbuckling, and secondary buckling behavior. In particular, the purpose of this study is to investigate whether or not the concept of bifurcation buckling is applicable to unsymmetric laminates. Past work by other researchers has suggested that such a concept is applicable for certain boundary conditions. The study also has as an objective the determination of the response of flat unsymmetric laminates if bifurcation buckling does not occur. The finite-element program ABAQUS is used to obtain results, and a portion of the study is devoted to becoming familiar with the way ABAQUS handles such highly geometrically nonlinear problems, particularly for composite materials and particularly when instabilities and dynamic behavior are involved. Familiarity with the problem, in general, and with the use of ABAQUS, in particular, is partially gained by considering semi-infinite unsymmetrically laminated cross- and angle-ply plates, a one-dimensional problem that can be solve in closed form and with ABAQUS by making the appropriate approximations for the infinite geometry. In this portion of the study it is found that semi-infinite cross-ply laminates with clamped boundary conditions and semi-infinite angle-ply plates with simple-support boundary conditions remain flat under a compressive load until the load magnitude reaches a certain level, at which time the out-of-plane deflection become indeterminate, essentially an eigenvalue problem as encountered with classic bifurcation buckling analyses. Obviously, a linear analysis of such problems would not reveal this behavior and, in fact, there are other revealed significant differences between the predictions of linear and nonlinear analyses. Transversely-loaded and inplane-loaded finite isotropic plates are studied by way of semi-closed form Rayleigh-Ritz-based solutions and ABAQUS in a step to approaching the problem with unsymmetric laminates. A method to investigate the unloading behavior of postbuckled finite isotropic plates is developed that reveal multiple plate configurations in the postbuckled region of the response, and this method is then extended to the study of finite inplane-loaded unsymmetric laminates. To that end, two specific laminates, a symmetric and an unsymmetric cross-ply laminates, and a variety of boundary conditions are used to study the response of inplane-loaded unsymmetric laminates. The symmetric laminate is included to provide a familiar baseline case and a means of comparison. Plates with all four edges clamped and a variety of inplane boundary conditions are studied. Of course the symmetric cross-ply laminate exhibits bifurcation behavior, and when the tangential displacement on the loaded edges and the normal displacement on the unloaded edges are restrained, secondary buckling behavior occurs. For the unsymmetric cross-ply laminate, bifurcation buckling behavior does not occur unless the tangential displacement on the loaded edges and the normal displacement on the unloaded edges are restrained, or the tangential displacement on the loaded edges and the normal displacement on the unloaded edges are free. If either of these conditions are not satisfied, the unsymmetric cross-ply laminate exhibits what could be termed 'near-bifurcation' behavior. In all cases rather complex behavior occurs for high levels of inplane load, including asymmetric postbuckling and secondary buckling behavior. For clamped loaded edges and simply-supported unloaded edges, bifurcation buckling behavior does not occur unless the tangential displacement on the loaded edges and the normal displacement on the unloaded edges are restrained. For this case, rather unusual asymmetric bifurcation and associated limit point behavior occur, as well as secondary buckling. This is a very interesting boundary condition case and is studied further for other unsymmetric cross-ply laminates, including the use of a Rayleigh-Ritz-based solution in attempt to quantify the problem parameters responsible for the asymmetric response. The overall results of the study have led to an increased understanding of the role of laminate asymmetry and boundary conditions on the potential for bifurcation behavior, on the response of the laminate for loads beyond that level. / Ph. D.

Viscoelastic materials have been used widely in different applications, such as constructing tires, artificial joints, shoe heels, and soles. A study on the different characteristics of viscoelastic materials has always been a matter of interest in order to improve their properties for various applications. In the automotive industry, rubber, as a viscoelastic material, has been used in several subsystems, such as vehicle interior, suspension, steering joints, and tires. The tire and terrain's contact characteristics are among the essential factors for assessing the performance of the tire and the vehicle in general. Friction and tread wear are two of these contact characteristics. Considering the tire's functionality, for most applications, it is desired to have higher friction to have better traction and a lower wear rate to minimize the material loss of the tread. The friction coefficient and the rubber's wear rate depend on various parameters such as rubber material properties, terrain characteristics, temperature (tread and the environment), and the load. To obtain the wear rate and friction of a viscoelastic material, three approaches have been used for this study: Experimental, Analytical, and Numerical. The results obtained using these approaches have been compared and validated. Several test setups have been designed and implemented to study the wear and friction of the rubber experimentally. Also, a new linear friction tester has been designed and manufactured by the author to achieve this project's objectives. The new test setup has several advantages over existing test setups in this field, such as covering a higher range of velocities while maintaining high precision. The designed Linear Friction tester and the modified dynamic friction tester at the CenTiRe laboratory at Virginia Tech were used to measure the rubber's friction and wear for different testing conditions such as different normal loads, different velocities, and various surfaces such asphalt and sandpaper. The data collected by the experiment will later be used for the validation of the developed models. In order to obtain the wear rate of the rubber using the analytical approach, the real contact area and friction of the rubber were calculated using Persson's model. The simulation has created the surface to obtain the friction coefficient and the real contact area. After obtaining the friction coefficient and the real contact area, the rubber's wear rate was calculated using a novel approach by combining the Persson Powdery Rubber Wear model with the Crack Propagation model. The results from the improved model compare well with the results from the original model. For the last step of this project, a Finite Element approach was used for modeling a tread block and round rubber sample. A new semi-empirical model for wear was developed by improving the Archard wear model. The novel approach was implemented to Abaqus by using the Umeshmotion subroutine and adaptive mesh motion (ALE) and subroutine UFric and UFric_Coef in two categories: The Node base method and the Ribbon base method. For finite element modeling, the visco-hyper elastic material model has been used to define the rubber's material properties. / Doctor of Philosophy / Viscoelastic materials have been used widely in different applications, such as constructing tires, artificial joints, shoe heels, and soles. Therefore, studying the different characteristics of viscoelastic materials has always been a matter of interest in improving their properties for various applications. In the automotive industry, rubber, as a viscoelastic material, has been used in several subsystems, such as vehicle interior, suspension, steering joints, and tires. The tire and terrain's contact characteristics are among the essential factors for assessing the performance of the tire and the vehicle in general. Friction and tread wear are two of these contact characteristics. Considering the tire's functionality, for most applications, it is desired to have higher friction to have better traction and a lower wear rate to minimize the material loss of the tread. This study used different approaches such as experimental, analytical, and numerical methods to predict the friction and wear of the rubber sample in contact with different surfaces. For the experimental parts, the author designed and manufactured a linear friction test setup. For the numerical parts, a new semi-empirical model was created to predict rubber samples' wear accurately.

Vatteninträngning är en viktig faktor som måste tas i beaktande vid konstruktion av komponenter som avses användas i krävande miljöer. Testning av vatteninträngning kan ske genom nedsänkning i en öppen vattenbehållare till önskat djup, men även genom ett simuleratvattendjup i en vattenfylld behållare som trycksätts. I följande arbete testas ett grundkoncept för konstruktion av en trycktank för IP-testnig av klasserna IPx7 samt IPx8, vilketinnebär att den utsätts för förhållandevis låga tryck. Grundkonceptet testas för att identifiera dess brister och förbättringar föreslås. Alternativa lösningar för konstruktionen testassedan och jämförs med grundkonceptet. Analysen av konstruktionerna bygger i huvudsakpå finita emementanalyser, men har även inslag av mekanikberäkningar. Resultatet visaratt en trycktank med rätblocksform kan vara en möjlig konstruktion om hörnförstyvningaranvänds, samt dess infästnigspunkter placeras in från trycktankens kanter. En cylindriskform på trycktank är dock att föredra framför en i formen av ett rätblock. Det visar ävenatt halvsfäriska ändar på trycktanken är att föredra jämfört med plana. Resultatet visaräven att en trycktank tillverkad i låglegerat stål är att föredra ur konstruktionssynpunkt,jämfört med en tillverkad i polymera material. Fortsatta arbeten krävs dock för analys avcylindriska trycktankar i stål med sfäriska ändar. / Water ingress is an important factor that should be considered when designing componentsintended for use in harsh environments. Water ingress can be tested by immersion to thedesired depth in an open water container, an alternative to this method is simulatingthe water depth in a pressurized water-filled container. In this study, a basic concept forthe construction of a pressure tank for IP testing of class IPx7 and IPx8 is examined,which means that it is exposed to relatively low pressures. The basic concept is examinedto identify its shortcomings and improvements are suggested. Alternative solutions forthe construction of a pressure tank are then investigated and compared with the basicconcept. The analysis of the constructions is mainly based on finite element analyses, butalso has elements of classical mechanics equations. This study shows that a pressure tankwith a shape like a cuboid could possibly be constructed if corner braces are used and itsattachment points are placed away from the edges of the pressure tank. This study showsthat a pressure tank in a cylindrical shape is preferable to one in a cuboid shape and thathemispherical pressure tank heads are preferable to flat ones. The study also shows thata pressure tank made from low-alloy steel is preferable from a construction standpoint,compared to one made of polymeric materials. However, further studies are required forthe analysis of cylindrical steel pressure tanks with hemispherical heads.

Recently planar magnetic technologies have been widely used in power electronics, due to good cooling and ease of fabrication. High frequency operation of magnetic components is a key to achieve high power density and miniaturization. However, at high frequencies, skin and proximity effect losses in the planar windings become significant, and parasitics cannot be ignored. This piece of work deals with the modeling and design of planar integrated magnetic component for power electronics applications. First, one-dimensional eddy current analysis in some simple winding strategies is discussed. Two factors are defined in order to quantify the skin and proximity effect contributions as a function of frequency. For complicated structures, 2D and 3D finite element analysis (FEA) is adopted and the accuracy of the simulation results is evaluated against exact analytical solutions. Then, a planar litz structure is presented. Some definitions and guidelines are established, which form the basis to design a planar litz conductor. It can be constructed by dividing the wide planar conductor into multiple lengthwise strands and weaving these strands in much the same manner as one would use to construct a conventional round litz wire. Each strand is subjected to the magnetic field everywhere in the winding window, thereby equalizing the flux linkage. 3D FEA is utilized to investigate the impact of the parameters on the litz performance. The experimental results verify that the planar litz structure can reduce the AC resistance of the planar windings in a specific frequency range. After that, some important issues related to the planar boost inductor design are described, including core selection, winding configuration, losses estimation, and thermal modeling. Two complete design examples targeting at volume optimization and winding parasitic capacitance minimization are provided, respectively. This work demonstrates that planar litz conductors are very promising for high frequency planar magnetic components. The optimization of a planar inductor involves a tradeoff between volumetric efficiency and low value of winding capacitance. Throughout, 2D and 3D FEA was indispensable for thermal & electromagnetic modeling. / Master of Science

This dissertation focuses on studying membrane air dehumidification for a membrane moisture exchanger in a membrane heat pump system. The study has two parts: an optimization of membrane moisture exchanger for air dehumidification in the macroscale, and diffusion of water vapor in polymer nanocomposites membrane for humid air dehumidification in the nanoscale. In the first part of the research, the mass transport of water vapor molecules through hydrophilic silica nanochannel chains in hydrophobic polyurethane matrix was studied by simulations and experiments for different membrane moisture exchanger design configurations. The mass transport across the polymer nanocomposite membrane occurs with the diffusion of moist air water vapor molecules in the membrane moisture exchanger in a membrane heat pump air conditioning system for air dehumidification purposes. The hydrophobic polyurethane matrix containing the hydrophilic silica nanochannel chains membrane is responsible for transporting water vapor molecules from the feed side to the permeate side of the membrane without allowing air molecules to pass through.In the second part of the research, diffusion analysis of the polymer nanocomposite membrane were performed in the nanoscale for the polymer nanocomposite membrane. The diffusion phenomena through the polymer, the polymer nanocomposite without modifying the silica surfaces, and the polymer nanocomposite with two different silica modified surfaces were studied in order to obtain the highest water vapor removal through the membrane. Different membrane moisture exchanger configurations for optimal water vapor removal were compared to get the desired membrane moisture exchanger design using the finite element method (FEM) with the COMSOL Multiphysics software package. The prediction of mass transport through different membrane configurations can be done by obtaining the mass flux value for each configuration. An experimental setup of one membrane moisture exchanger design was introduced to verify the simulation results. Also, for different membrane structures, permeability was measured according to the ASTM E-96 method. The prediction of water vapor diffusion through the polymer nanocomposite was studied by molecular dynamics simulation with the MAPS 4.3 and LAMMPS software packages. As a new nanocomposite material used in air dehumidification application, water vapor diffusivity through Silica-Polyurethane nanocomposite membranes was measured by the random movement of water vapor molecules through the formed nanochannels in the nanocomposite. For the diffusivity value, the Einstein's relationship was employed for the movement of each single water vapor molecule during the simulation time for all suggested membranes. The results of the proposed research will contribute to enhancing the energy efficiency of air conditioning systems by choosing the membrane moisture exchanger configuration which maximizes water vapor removal while, at the same time, enhancing the silica surfaces with the desired surface modifier that will maximize diffusion through the membrane itself.

This thesis intends to investigate the enhancements made to the pneumatic system in the University of Central Florida shock tube, with a focus on expanding control over final temperature and pressure conditions for low-pressure, high-temperature environments. The project explores key upgrades, including a higher-pressure actuator to increase breaking pressure and an optimized cage design that allows the cage face to withstand a higher force, improving overall durability. Additionally, it tests the limits of the new actuator to ensure reliability under extreme conditions. These advancements enable the collection of accurate data from consistent shocks, which is essential for rocket plume speciation and high-temperature gas dynamics in hypersonic flight.

The objective of this research is to determine how the sandwich's physical characteristics have an impact on the mechanical properties, determine under what conditions the specimens will be lighter and mechanically stronger, and determine if the use of an aluminum honeycomb sandwich as a construction material is feasible. The research has aimed at the use of aluminum sandwiches as light and strong material. The study of the structural layers' damage resistance and tolerance demonstrated that the top and bottom layers play a crucial role. The thesis presents three test results from aluminum honeycomb sandwich compression horizontal, compressive vertical, and bending tests. Also, each group was displayed mechanically and simulated in Abaqus. The study determines the mechanical properties such as maximum elastic stress-strain, ultimate stress-strain, fracture point, density, poison ration, young modulus, and maximum deflection was determined. The energy absorbed by the FEA, such modulus of elasticity, resilience, and toughness, the crack propagation, the test's view shows aluminum honeycomb behaved like a brittle material with both compression test. And the maximum deflection, crack propagation, shear forces, bending moment, and images illustrated that the layers play a crucial role in the 3-point bend test.

Detta examensarbete har utförts åt Ålö, som är en tillverkare av frontlastare och redskap till dessa. Ett av deras redskap, vars syfte är snöröjning, är utrustat med hydrauliskt justerbara sidoklaffar som kan fällas bakåt och nyttjas som plogblad. De fyller dock inte denna uppgift tillfredsställande då snö tenderar falla tillbaka bakom dem. Det bildas även en triangulär spalt mellan klaffarna och underlaget vid plogning, detta lämnar kvar en sträng av snö, utöver det som faller ner bakom klaffarna. Målet i detta projekt var att utveckla en plogklaff som löser tidigare nämnda problem genom att ersätta den högra sidoklaffen till skopan. Utvecklingen har initierats med en studie av rapporter, existerande plogar, kontakt med brukare och fastställande av de begränsningar som råder. Detta har sedan utgjort grunden för en rad koncept, av vilka de mest lovande sammanfogades till en prototyp. Kraftberäkningar utifrån det lastfall som uppstår vid plogning har sedan använts för att bl.a. dimensionera skruvförband och skapa randvillkoren för en finita element-analys. Finita element-analysen har sedan nyttjats för att göra en uppskattning av antalet cykler med momentan maximal belastning plogklaffen klarar innan utmattningsbrott sker. Kraftberäkningarna visade att hydraulcylinderns tryckbegränsare öppnar redan innan maximal belastning uppnås. Finita element-analys och kraftberäkningar till skruvförband förutsätter likväl att maximal belastning kan erhållas momentant i syfte att ge konservativa resultat. Under denna förutsättning skiljer belastningen i skruvförbanden endast ca 7-10 % mellan finita element-analys och handberäkningar. Samtliga beräkningar är inom spannen för skruvförbandens hållfasthet. Utmattningsberäkningarna visade att antalet cykler plogklaffen klarar vid maximal belastning uppgår till ca 346 000. Detta avser områden av intresse och inkluderar således ej delar vilka redan utvärderats i originalklaffen. Svetsförband, som i regel är dimensionerande i situationer som dessa, har ej sett någon utmattningsmässig utvärdering utan kommer istället behandlas vid tester av prototypen. Prototypen uppfyller de utsatta målen och tillverkningsunderlag i form av 3D-CAD modeller och 2D-ritningar (inklusive svetsritningar) har levererats. En fysisk prototyp är även beställd och kommer tillverkas av Vännäs Verkstads AB, planerat leveransdatum är 2017-06-08. Den kommer utvärderas under vintern 2017/2018. / This thesis work has been carried out for Ålö, a manufacturer of front loaders and implements for these. One of their implements, whose purpose is snow clearing, is equipped with hydraulically adjustable flaps that can be folded backwards and used as plows. However, they do not fill this task satisfactorily as snow tends to fall down behind them. A triangular gap between the flaps and the ground is also formed during plowing. This leaves a string of snow behind, aside from the snowmass already falling down behind the flaps. The goal for this project was to develop a plow that solves the aforementioned problems by replacing the right side flap on the bucket. The development was initiated with a study of reports, existing plows, contact with users and determining existing constraints. This has since provided the basis for a series of concepts, of which the most promising ones were joined to a prototype. Force calculations based on the load scenario that occurs during plowing have been used to find appropriate fasteners and create boundary conditions for a finite element analysis. The finite element analysis has then been used to estimate the number of cycles with maximum load the plow can be subjected to before it succumbs to fatigue failure. Force calculations showed that the pressure limiter for the hydraulic cylinder opens even before maximum load is reached. Despite this, finite element analysis and force calculations for fasteners were still based on the condition that maximum load is achieved momentarily to produce convervative results. Under this condition, the load on fasteners differs only ca 7-10 % between finite element analysis and calculations made by hand. All calculations are within the span for the fasteners proof strength. Fatigue calculations showed that the number of cycles the plow can handle with maximum load amounts to approximately 346 000. This refers to areas of interest and thus does not include parts already evaluated in the original flap. Welds, which usually dictate minimum strength in situations like these, have not seen any fatigue evaluations, but will instead be addressed while testing the prototype. The prototype meets the set goals and production documentation in the form of 3D-CAD models and 2D drawings (including weldment drawings) have been delivered. A physical prototype is also ordered and will be manufactured by Vännäs Verkstads AB, scheduled delivery date is 2017-06-08. It will be evaluated during the winter of 2017/2018.

The interest in using finite element analysis (FEA) as a product development tool is present within IKEAof Sweden (IoS). To gain additional support from within IoS a project was issued to identify the meritsand demerits of FEA. This was done by applying FEA to optimise an existing product, namely theEktorp sofa frame. Three important questions are addressed in this report: • How can the finite element method efficiently and reliably be applied to improve an existingproduct? • What types of improvements can be anticipated from using the finite element method andhow can these be achieved? • What products are suitable to be developed with the use of FEA? As the sofa frame mainly consists of wood a literature study is conducted to answer several questionsregarding the use of wood in conjunction with FEA. Real life tests are performed to verify acquiredmaterial data. The original design is analysed with FEA to act as reference for newer concepts and also to identifyareas of improvement. Additional design tools are also applied to complement the FEA. Concepts are generated to improve a number of factors such as weight, number of components andcost. These concepts are analysed with FEA to be compared to the original design and incrementallychanged until meeting set requirements. A final concept where the cost, weight and number of components are reduced is thoroughlydescribed and presented as a CAD model. The final result is the Ektorp sofa frame with its weightreduced by 10.9%, number of components reduced by 18.2% and material cost reduced by 13.4%. The conclusion from this process is that FEA can efficiently and reliably be applied to improve anexisting product. This, however, requires thorough knowledge in FEA. Strength, weight and cost arethe most obvious factors that can benefit from the use of FEA while almost all design changes canbenefit indirectly as well. Almost any product can benefit from the use of FEA; however, severalfactors decide the efficiency and reliability of these analyses, such as analysis experience and productcomplexity. / Det finns ett intresse för att använda finita element analyser (FEA) som ett produktutvecklingsverktygpå IKEA of Sweden (IoS). För att få ett ökat stöd inom IoS skapades ett projekt där för och nackdelarnamed FEA kan identifieras. Detta projekt baserades på att optimera en existerande produkt, nämligenEktorp soffans ram. Tre viktiga frågor behandlas i och med detta projekt: • Hur kan finita element metoden appliceras på ett effektivt och tillförlitligt sätt för att optimeraen existerande produkt? • Vilka typer av förbättringar kan man förvänta sig genom att använda finita element metodenoch hur uppnås dessa? • Vilka produkter lämpar sig att utvecklas med hjälp av FEA? Eftersom soffans ram till större delen består av trä inleds en litteraturstudie för att besvara frågor somberör användandet av trä i samband med FEA. Verkliga tester utförs för att verifiera insamladematerialdata. Originalkonstruktionen analyseras med FEA för att skapa en referens till kommande koncept och ävenför att identifiera möjliga förbättringsområden. Flera designverktyg används för att komplettera FEA. Koncept genereras för att förbättra en rad faktorer såsom vikten, antalet komponenter och kostnaden.Dessa koncept analyseras med FEA och jämförs mot originalkonstruktionen för att förbättrasinkrementellt tills det att fastställda krav uppfylls. Ett slutgiltigt koncept där kostnad, vikt och antalet komponenter är minskat beskrivs i detalj ochpresenteras som en CAD modell. Det slutgiltiga resultatet är Ektorp-soffans ram där vikten ärreducerad med 10,9%, antalet komponenter reducerade med 18,2% och materialkostnadernareducerade med 13,4%. Slutsatsen från projektet är att FEA kan appliceras på ett effektivt och tillförlitligt sätt för att förbättraen existerande produkt. Detta kräver dock omfattande kunskap inom FEA. Hållfasthet, vikt ochkostnad är de mest uppenbara parametrar som kan tjäna på användandet av FEA medan nästan allakonstruktionsändringar kan dra nytta av FEA indirekt. Nästan alla produkter kan dra nytta av FEA, detfinns dock en rad faktorer som avgör hur pass effektiva och tillförlitliga analyserna blir. Exempel pådessa är användarerfarenhet och produktkomplexitet.

Kalmar Cargotec is a company offering solutions for container and heavy industrial material handling. One of their products, EC-truck (empty container), lifts objects as heavy as 11 tonnes and sometimes as high as 16 meters vertically up in the air. Lifting very heavy objects high up in the air can cause the pulse to rise if unwanted movement is caused due to strong winds. This thesis was raised by Kalmar Cargotec, to study if the swinging effect of the elevated load could be reduced by increasing the torsional resistance of the truck chassis framework. The project work will be conducted at the engineering consultancy firm Citec AB in Karlstad, whom together with Kalmar Cargotec, offered the opportunity for this thesis. Concepts were generated for the chassis framework to find a new and improved design. The conceptual designs were compared both numerically, through the finite element analysis based program Mechanical, and analytically. The three best performing concepts were studied further with more variables accounted for where one was picked to be implemented in a large scale computational model. It was discovered that it is possible to reduce the swinging effect of the mast by replacing the current U-profiled beams in the chassis framework with hollow rectangular cross-sectional beams. / Kalmar Cargotec är ett företag som erbjuder lösningar till kontainer- och materialhantering i tungviktsindustrin. En av deras produkter, EC-trucken (empty container), lyfter föremål så tunga som 11 ton och ibland s˚a högt som 16 meter vertikalt upp i luften. Att lyfta så tunga föremål, så högt upp i luften kan väcka oro ifall föremålet börjar förflytta sig på grund av starka vindar. Den här masteruppsatsen presenterades av Kalmar Cargotec för att studera om svängningseffekten av det upphöjda föremålet kan reduceras genom att öka vridmotståndet i truckens chassi-ramverket. Projektet kommer att utföras på ingenjörsinriktade konsultfirman Citec AB i Karlstad, som genom Kalmar Cargotec, erbjöd möligheten för att utföra detta examensarbete. Koncept genererades på chassi-ramverket med syfte att hitta nya och förbättrade lösningar. De konceptuella utformningarna jämfördes både numeriskt, genom det finita element-baserade programmet Mechanical, och analytiskt. De tre koncept som presterade bäst valdes för vidare studier med fler variabler inräknade, där en till slut valdes för att implementeras i en större beräkningsmodell. Det visade sig vara möjligt att reducera den svängande effekten av stativet med lasten genom att byta ut de befintliga U-profil balkarna i chassi-ramverket mot balkar med rektangulärt ihåliga tvärsnitt.