SCIENCE EDUCATION > Engineering

Structural Engineering

This collection introduces students to fundamental concepts and protocols for material characterization, with specific emphasis on common construction materials such as steel, wood, and concrete.

  • Structural Engineering

    11:22
    Material Constants

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    In contrast to the production of cars or toasters, where millions of identical copies are made and extensive prototype testing is possible, each civil engineering structure is unique and very expensive to reproduce (Fig.1). Therefore, civil engineers must extensively rely on analytical modeling to design their structures. These models are simplified abstractions of reality and are used to check that the performance criteria, particularly those related to strength and stiffness, are not violated. In order to accomplish this task, engineers require two components: (a) a set of theories that account for how structures respond to loads, i.e., how forces and deformations are related, and (b) a series of constants that differentiate within those theories how materials (e.g. steel and concrete) differ in their response. Figure 1: World Trade Center (NYC) transportation hub. Most engineering design today uses linear elastic principles to calculate forces and deformations in structures. In the theory of elasticity, several material constants are needed to describe the relationship between stress and strain. Stress is defined as the force per unit area while strain is defined as the change in dimension when subjected to a force divided by the original magnitude of that dimension. The two most common of

  • Structural Engineering

    13:25
    Stress-Strain Characteristics of Steels

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    The importance of materials to human development is clearly captured by the early classifications of world history into periods such as the Stone Age, Iron Age, and the Bronze Age. The introduction of the Siemens and Bessemer processes to produce steels in the mid-1800s is arguably the single most important development in launching the Industrial Revolution that transformed much of Europe and the USA in the second half of the 19th century from agrarian societies into the urban and mechanized societies of today. Steel, in its almost infinite variations, is all around us, from our kitchen appliances to cars, to lifelines such as electrical transmission networks and water distribution systems. In this experiment we will look at the stress-strain behavior of two types of steel that bound the range usually seen in civil engineering applications - from a very mild, hot rolled steel to a hard, cold rolled one.

  • Structural Engineering

    14:52
    Stress-Strain Characteristics of Aluminum

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    Aluminum is one of the most abundant materials in our lives, as it is omnipresent in everything from soda cans to airplane components. Its widespread use is relatively recent (1900AD), primarily because aluminum does not occur in its free state, but rather in combination with oxygen and other elements, often in the form of Al2O3. Aluminum was originally obtained from bauxite mineral deposits in tropical countries, and its refinement requires very high-energy consumption. The high cost of producing quality aluminum is another reason why it is a very widely recycled material. Aluminum, especially when alloyed with one or more of several common elements, has been increasingly used in architectural, transportation, chemical, and electrical applications. Today, aluminum is surpassed only by steel in its use as a structural material. Aluminum is available, like most other metals, as flat-rolled products, extrusions, forgings, and castings. Aluminum offers superior strength-to-weight ratio, corrosion resistance, ease of fabrication, non-magnetic properties, high thermal and electrical conductivity, as well as ease of alloying.

  • Structural Engineering

    07:31
    Charpy Impact Test of Cold Formed and Hot Rolled Steels Under Diverse Temperature Conditions

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    One of the more insidious types of failures that can occur in structures are brittle fractures,which are mostly due to either poor quality materials or poor material selection. Brittle fractures tend to occur suddenly and without much material inelasticity; think of a bone fracture, for example. These failures often occur in situations where there is little ability for the material to develop shear stresses due to three-dimensional loading conditions, where local strain concentrations are high, and where a logical and direct force path was not provided by the designer. Examples of this type of failure were observed in the aftermath of the 1994 Northridge earthquake in multi-story steel structures. In these buildings, a number of the key welds fractured without displaying any ductile behavior. Fractures tend to occur near connections, or at interfaces between pieces of base materials, as welding tends to introduce local discontinuities in both, materials and geometry, as well as three-dimensional stresses due to cooling. When specifying materials for a structure that will see very low operating temperatures (i.e., the Alaska pipeline) many cycles of loading (a bridge on an interstate highway), or where welding is used extensively, it is necessary to have a simple test that characterizes the material

  • Structural Engineering

    08:00
    Rockwell Hardness Test and the Effect of Treatment on Steel

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    Hardness testing is one of the most universally valuable mechanical tests available to engineers, as it is both simple and relatively inexpensive for the wealth of information and data it produces. Hardness testing, generally in the form of a surface penetration test, is both quicker and less destructive than tensile testing. Hardness provides a linear relationship with tensile strength over a wide range of strengths for many materials, such as steel. Hardness tests are empirical, rather than derived from theory, as the results conflate effects from many different materials properties (Young's modulus, yield strength, etc.). Hardness is a characteristic of a material used to describe how much plastic deformation (yield) that a material will undergo when a known force is applied). One can characterize hardness in three manners: scratch, indentation, and rebound hardness. A common early example of a hardness (scratch) test is the Mohs scale (1820), derived for minerals, and in which talc has a value of 1 and diamond a value of 10. In indentation testing using the Rockwell approach, small indenters are used with different loads. The most common are the Rockwell Hardness B (HRB), which uses a 1/16 in. hardened steel ball indenter along with a 100 kg weight, and the Rockwell Hardness C (HRC), which u

  • Structural Engineering

    11:13
    Buckling of Steel Columns

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    In the design of civil works, it is important to deliver structures that are not only safe under unexpected loads, but also provide excellent performance under everyday loads at a reasonable economic cost. The latter is often tied to minimum use of materials, ease of fabrication, and rapid construction in the field. Structures made of steel members can be very economical because of the great strength of the material and the extensive prefabrication of their members and connections, which help maximize the speed of construction on site. Generally, the skeleton of a steel structure will be very slender as compared to a reinforced concrete one. While its behavior in tension is governed primarily by the strength of the material, steel in compression is governed by another failure mode common to all materials- buckling. This behavior is easily demonstrated by pressing down on a slender wooden ruler, which under a compressive load will suddenly move sideways and lose load carrying capacity. This phenomenon will occur in any slender member of a structure. In this lab, we will measure the buckling capacity of a series of slender aluminum columns to illustrate this failure mode, which over time has led to many catastrophic failures including that of the Quebec River Bridge, which was erected in 1918.

  • Structural Engineering

    12:02
    Dynamics of Structures

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    It is rare nowadays that a whole year goes by without a major earthquake event wreaking havoc somewhere around the world. In some cases, like the 2005 Banda Ache earthquake in Indonesia, the damage involved large geographic areas and casualties in the six figures. In general, the number and intensity of earthquakes is not increasing, however, the vulnerability of the built environment is rising. With increasing unregulated urbanization around seismically active areas, such as the Circum-Pacific "belt of fire," sea rising in low-laying coastal area, and increasing concentrations of both energy production/distribution and digital/telecommunication network critical nodes in vulnerable areas, it is clear that earthquake-resistant design is key to future community resilience. Designing structures to resist earthquake damage has progressed greatly in the last 50 years, primarily through work in Japan following the 1964 Niigata Earthquake, and in the United States following the 1971 San Fernando Valley Earthquake. The work has advanced along three parallel tracks: (a) experimental work aimed at developing improved construction techniques to minimize damage and loss of life; (b) analytical studies based on advanced geometrical and non-linear material models; and, (c) synthesis of the results in

  • Structural Engineering

    09:57
    Fatigue of Metals

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    The importance of studying metal fatigue in civil infrastructure projects was brought into the spotlight by the collapse of the Silver Bridge in Point Pleasant, West Virginia in 1967. The eyebar-chain suspension bridge over the Ohio River collapsed during evening rush hour, killing 46 people as a result of the failure of a single eyebar with a small 0.1-inch defect. The defect reached a critical length after repeated loading conditions and failed in a brittle fashion causing the collapse. This event garnered attention in the bridge engineering community and highlighted the importance of testing and monitoring fatigue in metals. Under normal service conditions, a material can be subjected to numerous applications of service (or everyday) loads. These loads are typically at most 30%-40% of the ultimate strength of the structure. However, after the accrual of repeated loadings, at magnitudes substantially below the ultimate strength, a material can experience what is termed fatigue failure. Fatigue failure can occur suddenly and without significant prior deformation and is linked with crack growth and rapid propagation. Fatigue is a complex process, with many factors affecting fatigue resistance (Table 1). This complexity underscores the integral need for routine and thorough inspection of structures subje

  • Structural Engineering

    08:43
    Tension Tests of Polymers

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    Polymeric materials are widely used in civil structures, with uses ranging from very soft sealants to more rigid pipes in water and wastewater systems. The most basic definition of a polymer is a molecular structure with repeating subunits. The term polymer comes from Greek, where "poly" means many, and "-mer" means basic unit. Monomers, or single mers, are the specific repeating units. With polymers, the structure, including the length of the carbon backbone and the varying flexibility, will dictate the properties of the polymer. Polymers are classified into 3 subcategories: plastics, elastomers, and rigid rod polymers. Plastics are further subdivided into thermosets, which do not soften on heating, and thermoplastics, which do soften when heated and harden on cooling. Additionally, thermoplastics are mostly linear or branched polymers with little to no cross-linking, whereas thermosets exhibit 3D structure and have extensive cross-linking. Elastomers, or rubbers, are long, coiled chains and can be stretched to twice the original length, but will contract back to the original size when released, whereas rigid rod polymers do not stretch and are strong, crystalline structures. In this laboratory, we will look at several different polymeric materials, including high density polyethyle

  • Structural Engineering

    08:41
    Tension Test of Fiber-Reinforced Polymeric Materials

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    Fiber-reinforced polymeric materials (FRP) are composite materials that are formed by longitudinal fibers embedded in a polymeric resin, thereby creating a polymer matrix with aligned fibers along one or more directions. In its simplest form, the fibers in FRP materials are aligned in an orderly, parallel fashion, thus imparting orthotropic material characteristics, meaning that the material will behave differently in the two directions. Parallel to the fibers, the material will be very strong and/or stiff, whereas perpendicular to the fibers will be very weak, as the strength can only be attributed to the resin instead of the whole matrix. An example of this unidirectional configuration is the commercially available FRP reinforcing bars, which mimic the conventional steel bars used in reinforced concrete construction. FRP materials are used both as stand-alone structures such as pedestrian bridges and staircases, and also as materials to strengthen and repair existing structures. The thin, long plates are often epoxied to existing concrete structures to add strength. In this case, the FRP bars act as external reinforcement. The FRP bars and plates are lighter and more corrosion resistant, so they are finding applications in bridge decks and parking garages, where de-icing slats lead to rapid deteriora

  • Structural Engineering

    10:09
    Aggregates for Concrete and Asphaltic Mixes

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    Concrete and asphalt are by far the most common construction materials used today. Concrete is a composite material consisting of cement, water, air, coarse aggregate, and fine aggregates. Fine aggregates are typically sands and coarse aggregates are natural or crushed rocks. Chemical admixtures to modify certain specific properties are also commonly used (i.e., superplasticizers to make the concrete fluid during casting). Asphaltic mixes consist primarily of asphalts, coarse aggregates, and fine aggregates, in addition to a number of emulsifiers and other additives used to improve viscosity during placement. In both concrete and asphaltic mixes, aggregates make up a very significant portion of the mix volume, as economy requires that the amount of cement and asphalt be minimized. Two types of aggregates are commonly recognized: coarse aggregates, defined as particles larger than about 4.75mm (rocks), and fine aggregates, consisting of smaller particles (sands). Other important characteristics of aggregates are that they be rigid, durable, and chemically inert with respect to the concrete mortar or asphalt. Aggregates are intended to be fillers, but they are not intended to play a key role in the behavior of either material. However, the stiffness and strength of the aggregates needs to be highe

  • Structural Engineering

    11:22
    Tests on Fresh Concrete

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    Concrete is one of the most common construction materials and consists of two phases: the mortar phase, comprised of concrete, water and air, and the aggregate phase, comprised of coarse and fine aggregates. There are two key considerations when designing a concrete mix. First, the concrete must be workable and easy to cast in the forms in its fresh condition, even when the forms are packed with steel reinforcement. In this condition, it is the rheology of concrete that is important. Second, the mix must produce a hardened concrete of specified strength at 28 days (or similar specified time) that is durable and provides good serviceability. In this laboratory exercise, a method of concrete mixture proportioning, named the trial batch method, will be explored. The concrete produced will be used in conducting typical tests to determine the principal characteristics of fresh concrete, including slump, flowability, air content, and density. The trial batch method is a simple, empirical approach to mixture design. The objectives of this experiment are fourfold: (1) to use the trial batch mix method to determine optimum proportions of aggregates, cement, and water for concrete to meet specified slump requirements, (2) to learn concrete mixing practice in a laboratory environment, (3) to observe the

  • Structural Engineering

    08:07
    Compression Tests on Hardened Concrete

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    There are two distinct stages in a construction project involving concrete. The first stage involves batching, transporting, and casting fresh concrete. At this stage, the material is viscous, and the workability and finishability are the key performance criteria. The second stage occurs when the hydration process begins shortly after the concrete is placed in the form, and the concrete will set and begin to harden. This process is very complex, and not all of its phases are well understood and characterized. Nevertheless, the concrete should achieve its intended design strength and stiffness at about 14 to 28 days after casting. At this point, a series of tests will be conducted on concrete cylinders cast at the time of placement to determine the concrete's compressive and tensile strengths, as well as on occasion, its stiffness. The objectives of this experiment are threefold: (1) to conduct compressive cylinder tests to determine the 7-, 14-, and 28-day strength of concrete, (2) to determine the modulus of elasticity at 28 days, and (3) to demonstrate the use of a simple non-destructive test to determine in situ concrete strength.

  • Structural Engineering

    10:08
    Tests of Hardened Concrete in Tension

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    In a previous laboratory focused on concrete in compression, we observed that concrete can withstand very large stresses under uniaxial compressive forces. However, the failures observed were not compressive failures but failures along shear planes where maximum tensile forces occur. Thus, it is important to understand the behavior of concrete in tension and particularly its maximum strength as that will govern both its ultimate and service behavior. From the ultimate standpoint, combinations of tension and shear stresses will lead to cracking and immediate and catastrophic failure. For that reason, concrete is seldom if ever used in an unreinforced condition in structural applications; most concrete members will be reinforced with steel so that these cracks can be stopped and the crack widths limited. The latter is important from the serviceability standpoint because controlling crack widths and distribution is the key to durability, as this will impede deicing salts and similar chemicals from penetrating and corroding the reinforcing steel. The objectives of this experiment are threefold: (1) to conduct tensile split cylinder tests to determine concrete tensile strength, (2) to conduct beam tests to determine concrete tensile strength, and (3) to demonstrate the influence of steel reinforcement

  • Structural Engineering

    11:07
    Tests on Wood

    Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

    Wood is a ubiquitous material that has been used in construction from the earliest times. Wood is a renewable, sustainable material with great aesthetic value. Today, there are probably more buildings constructed with wood than any other structural material. Many of these buildings are singlefamily residences, but many larger apartment buildings, as well as commercial and industrial buildings, also use wood framing. The widespread use of wood in construction has appeal from both an economic and aesthetic basis. The ability to construct wood buildings with a minimal amount of equipment has kept the cost of woodframe buildings competitive with other types of construction. On the other hand, where architectural considerations are important, the beauty and warmth of exposed wood is difficult to match with other materials. The objectives of this experiment are to conduct tensile and compressive tests on three types of wood to investigate their stress-strain behavior, and to conduct a four-point bending test on a wood beam to ascertain its flexural performance. In a four-point bending test, a simply-supported beam is loaded with two equal point loads at its third points, resulting in a central portion with constant moment and zero shear. This is an important test because wood structural elements ar

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