Unsupported Spans Research Articles

A Review on Relative Linear and Nonlinear Strains Performances of Laminated Composite Shell Roofs

This paper comprehensively reviews research articles on linear and nonlinear performances of laminated shells. The study on composite surfaces gained popularity since the last decade. The composite shell problems for complex forms are difficult to solve analytically. Hence, the numerical solutions became popular. The invent of fast computers popularized the numerical approaches further among practicing engineers. Thin shell surfaces are widely popular to cover long unsupported spans in civil engineering; however, these are vulnerable to instability especially under transverse overloading. The stiffened surfaces addressed such problems. The detailed review on bare and stiffened surfaces highlights gaps in literature which can help the researchers to find out scopes of future research problems.

Experimental out-of-plane damage limits of historical stone masonry walls

In stone masonry buildings, failure under strong ground motions usually occurs in the form of out-of-plane overturning of the walls before reaching their in-plane strength, as the walls have long unsupported spans perpendicular to their plane and the lack of slabs ensuring a diaphragm effect. Post-earthquake damage observations reveal this specific weakness of the historic masonry walls. Experimental determination of the out-of-plane damage limits of these walls, emerges as a need for use, especially during numerical evaluations. This article presents the results of static and dynamic testing of double- and three-leaf large-scale U-shaped masonry wall specimens. For this purpose, nondestructive material characterization was conducted on a reference historical masonry structure. Large-scale wall specimens were designed considering these findings and material tests performed on mortar samples. Quasi-static cyclic tests and ambient vibration tests for both initial and damaged conditions were performed on these specimens to capture the damage evolution and the corresponding drift limits. Damage limits are suggested to be used in performance-based evaluations of other territorial structures.

Propagation, proppant transport and the evolution of transport properties of hydraulic fractures

Hydraulic fracturing is a widely used method for well stimulation to enhance hydrocarbon recovery. Permeability, or fluid conductivity, of the hydraulic fracture is a key parameter to determine the fluid production rate, and is principally conditioned by fracture geometry and the distribution of the encased proppant. A numerical model is developed to describe proppant transport within a propagating blade-shaped fracture towards defining the fracture conductivity and reservoir production after fracture closure. Fracture propagation is formulated based on the PKN-formalism coupled with advective transport of an equivalent slurry representing a proppant-laden fluid. Empirical constitutive relations are incorporated to define rheology of the slurry, proppant transport with bulk slurry flow, proppant gravitational settling, and finally the transition from Poiseuille (fracture) flow to Darcy (proppant pack) flow. At the maximum extent of the fluid-driven fracture, as driving pressure is released, a fracture closure model is employed to follow the evolution of fracture conductivity with the decreasing fluid pressure. This model is capable of accommodating the mechanical response of the proppant pack, fracture closure of potentially contacting rough surfaces, proppant embedment into fracture walls, and most importantly flexural displacement of the unsupported spans of the fracture. Results show that reduced fluid viscosity increases the length of the resulting fracture, while rapid leak-off decreases it, with both characteristics minimizing fracture width over converse conditions. Proppant density and size do not significantly influence fracture propagation. Proppant settling ensues throughout fracture advance, and is accelerated by a lower viscosity fluid or greater proppant density or size, resulting in accumulation of a proppant bed at the fracture base. ‘Screen-out’ of proppant at the fracture tip can occur where the fracture aperture is only several times the diameter of the individual proppant particles. After fracture closure, proppant packs comprising larger particles exhibit higher conductivity. More importantly, high-conductivity flow channels are necessarily formed around proppant banks due to the flexural displacement of the fracture walls, which offer preferential flow pathways and significantly influence the distribution of fluid transport. Higher compacting stresses are observed around the edge of proppant banks, resulting in greater depths of proppant embedment into the fracture walls and/or an increased potential for proppant crushing.

Engineered 3D-printed artificial axons

Myelination is critical for transduction of neuronal signals, neuron survival and normal function of the nervous system. Myelin disorders account for many debilitating neurological diseases such as multiple sclerosis and leukodystrophies. The lack of experimental models and tools to observe and manipulate this process in vitro has constrained progress in understanding and promoting myelination, and ultimately developing effective remyelination therapies. To address this problem, we developed synthetic mimics of neuronal axons, representing key geometric, mechanical, and surface chemistry components of biological axons. These artificial axons exhibit low mechanical stiffness approaching that of a human axon, over unsupported spans that facilitate engagement and wrapping by glial cells, to enable study of myelination in environments reflecting mechanical cues that neurons present in vivo. Our 3D printing approach provides the capacity to vary independently the complex features of the artificial axons that can reflect specific states of development, disease, or injury. Here, we demonstrate that oligodendrocytes’ production and wrapping of myelin depend on artificial axon stiffness, diameter, and ligand coating. This biofidelic platform provides direct visualization and quantification of myelin formation and myelinating cells’ response to both physical cues and pharmacological agents.

Megalithic stone beam bridges of ancient China reach the limits of strength and challenge size effect in granite

Abstract In all ancient monuments, stone beams and architraves have unsupported spans that seldom reach 7 m, while ordinary spans are usually much less. These structural elements were and still are believed to be prone to failure, so that several relieving systems (arches, chambers, gaps) were adopted through history to prevent collapse. The perception that stone beams could not exceed a certain span is coherent with the so-called size-effect theory of rock and concrete, which predicts that large elements are proportionally weaker than small ones. While the rest of the world started using architectural design to avoid these problems, in the Fujian region of China (near Xiamen) from the XI to the XII century megalithic stone beam bridges with spans of up to 21 m were being built. These bridges have resisted over the centuries. A spectacular example of these bridges, tending to disprove the size-effect theory and challenging all previous ancient constructions, is the Jiangdong bridge, of which only a part survives, but which should be restored, preserved, and declared human heritage monument.

New Methods Developed to Manage Subsea Vibration-Induced Fatigue

Technology Update The vibration assessment of subsea systems has largely been limited to vortex-induced vibration of riser systems and unsupported pipeline spans (i.e., environmental loading) caused by flow past the outside of a riser or conductor. But now, piping vibration caused by process excitation is becoming an issue on manifolds and jumpers, in part associated with increasing production rates. Though the likelihood of a failure is low, the consequences can be high, resulting in an unacceptable level of risk. As well as piping integrity issues, additional vibration-related problems have also been experienced with valves and instrumentation. Process Excitation Mechanisms There are a number of different sources of excitation and these generally depend on the type of process fluid within the system. The following three mechanisms are the most common. Flow-Induced Turbulence. This is caused by broad band, low-frequency energy, generated by “single phase” turbulent flow through valves, expansions and bends—essentially anything that disturbs the flow. This can lead to excitation of the low-frequency modes of the piping system by energy transfer from fluid momentum to the pipe, resulting in low-frequency vibration (up to possibly 50 Hz). This is widespread in most processes and depends heavily on the velocity and density of the fluid. Multiphase Flow. This encompasses slug, bubble, annular, and churn flow. The frequency range of the resulting excitation is heavily dependent on void fraction and flow regime. Flow-Induced Pulsation. This is linked to gas flow through a flexible riser or jumper, sometimes known as the “singing riser.” This is where gas across the internal corrugated geometry within the flexible piping causes vortex generation. This can cause a pressure fluctuation, or pulsation, with a frequency and amplitude that are dependent on the gas velocity. This pulsation can drive the flexible piping at either end at very high frequencies—in some situations as high as 1,000 Hz—causing fatigue damage to accrue very quickly. The phenomenon is typically only experienced on dry gas systems, so it can be an issue on gas export, gas injection, and gas lift systems. The Hidden Threat Pipe design and construction can present a number of unique challenges in subsea developments, given their remoteness and complexity to access. A vibration issue may occur subsea without any obvious sign topsides.

Horizontal Skyscraper: Innovative Structural Design of Shenzhen VANKE Center

The Shenzhen VANKE Center included both the required ground clearance to the first floor of 10–15 m as well as unsupported spans in excess of 50 m. The concept of building on cable-stayed bridges was finally selected. In this design, a system consisting of inclined prestressed cables and rigid composite floor systems is used to transfer the gravity loads from the superstructure to the tubes, walls, and columns. The simulation of the construction sequence of the cable prestressing and the structural behaviors are revealed. The innovative structural concept and design methods could be used as a reference. Long-span buildings are usually associated with specialpurpose structures (stadiums, airports, etc.), and the type of structural systems that consequently have evolved can be viewed as an extension of the concepts used in the construction of domes and arches. Until now, the conventional design of typical long-span office and mixed-use buildings has involved the use of transfer structures to support the main superstructure, such as in the incorporation of megatrusses, in which the height can exceed that of a single story. These transfer truss systems also provide additional lateral stiffness to the overall structure. The challenges posed by the architectural design of the Shenzhen VANKE Center included both the required ground clearance to the first floor of 10–15 m as well as that required for unsupported spans in excess of 50 m. Following a comparison of several structural schemes, the concept of building on cable-stayed bridges was finally selected. Moreover, after considering the architectural specifications and the requirements of the surrounding space, it was evident that anchoring cables could not be utilized in the proposed cable-stayed building scheme. Therefore, in this design, a system consisting of inclined prestressed cables and rigid composite floor systems was used to transfer the gravity loads from the superstructure to the tubes, walls, and columns. Four innovative concepts were introduced in the realization of the project: (1) techniques for preventing slack in the cables; (2) precise adjustment of prestressing forces in the cables; (3) self-balanced structural design and construction; and (4) control of lateral displacements during construction of the structure. The details of these concepts as well as the construction sequence of the cable prestressing and critical aspects of the structural system are presented in this paper.

Normal Fault Rupture Interaction with Strip Foundations

Observations after earthquakes where surface fault ruptures crossed engineering facilities reveal that some structures survived the rupture almost unscathed. In some cases, the rupture path appears to divert, avoiding the structure. Such observations point to an interaction between the propagating rupture, the soil, and the foundation. This paper (i) develops a two-step nonlinear finite-element methodology to study rupture propagation and its interaction with strip foundations; (ii) provides validation through successful Class A predictions of centrifuge model tests; and (iii) conducts a parameter study on the interaction of strip foundations with normal fault ruptures. It is shown that a heavily loaded foundation can substantially divert the rupture path, which may avoid outcropping underneath the foundation. The latter undergoes rigid body rotation, often detaching from the soil. Its distress arises mainly from the ensuing loss of support that takes place either at the edges or around its center. The average pressure q on the foundation largely dictates the width of such unsupported spans. Increasing q decreases the unsupported width, reducing foundation distress. The role of q is dual: (1) it compresses the soil, flattening fault-induced surface anomalies; and (2) it changes the stress field underneath the foundation, facilitating rupture diversion. However, even if the rupture is diverted, the foundation may undergo significant stressing, depending on its position relative to the fault outcrop.

Design and Operational Considerations for Unsupported Offshore Pipeline Spans

Summary Design and operational considerations have been examined in determining the significance of unsupported offshore pipeline spans that may develop during pipeline installation or field operation. Allowable unsupported span lengths determined during design are generally based on strict code compliance and a design foundation encompassing the worst possible environmental and operational loads. During operation, however, unsupported spans develop beyond the allowable limits, perhaps as a result of various unforeseen local conditions. Applying original design criteria is likely to result in cost-prohibitive repair predictions, while lack of action may result in loss of production. Without the design code requirements of an existing or a proposed pipeline system being violated, realistically safe design can be approached through evaluation of the sensitivity of the key design parameters. Priorities for remedial repair of the unsupported spans can be established for the pipelines in service while operating constraints, budget, and schedule are recognized. On the basis of periodic surveys, properly formatted survey data, and reanalysis of the design parameters, pipeline span repair cost estimates and schedules can be appreciably reduced. Similar principles can be applied to a pipeline in planning and design stages to ensure a safe operating design and to eliminate an uneconomically conservative design. These design and operational considerations are illustrated through a case study.

Full-scale tornado-missile impact tests

Full-scale poles, pipes, and rods, representing postulated tornado-borne missiles, were rocket-propelled into reinforced concrete panels with thicknesses typical of walls and roofs in the auxiliary buildings of nuclear power plants. Data from the 18 tests can be used directly for structural design or for validating design and analysis techniques. The test panels, constructed with 3000-psi design strength concrete and minimum allowable reinforcement, were 12, 18, and 24 in. thick with 15 × 15-ft unsupported spans. The results show that a 1500-lb utility pole, 1-in rod, and 3-in. pipe are ineffective for producing significant local and structural damage even under the improbably severe tornado-missile impact conditions represented by the tests. The front several feet of the poles disintegrated upon impact without damaging the face of the panels. Although 12-in. pipes produced craters in the face of the panels, impact tests with these missiles showed that 18-in. thick walls are adequate for preventing backface scabbing (secondary missiles) in the highest tornado-intensity region of the US, while 12-in. thick walls are adequate in other regions. Data on penetration depths and scabbing thresholds for 12-in. pipe impacts could be fitted reasonably well only with a modified NDRC design formula. Examination of structural response was aided by electronic records of reaction forces, missile deceleration, panel velocity, and strains during impact. Contrary to the predictions of conventional structural design methods, which do not account for missile deformation, no overall permanent deflections of the panels were produced by any of the missiles.