The Millennium Tower in San Francisco, which was the tallest building in western United States, recorded excessive settlements shortly after construction. There were two possible reasons for the abnormal behavior, inadequate foundation design to support excessively heavy building load, and changes in subsurface conditions due to dewatering at the nearby construction site. This research aims to identify the most reasonable causes of the settlement of Millennium Tower. In this study, a centrifuge model was constructed to simulate the settlement of Millennium Tower and to predict the settlement trend in the future. A loading device using airbag was used to apply incremental load in-flight. Dewatering was also simulated during consolidation. The centrifuge modeling was performed based on the field subsurface data obtained from geotechnical reports. Ground materials were prepared by mixing different kinds of soil, and the foundation system was modeled in detail based on the information of foundation design. The model was tested under a centrifugal acceleration of 120-g. An airbag loading system was used to simulate the multi-stage construction sequence. After simulating the construction sequence, long-term consolidation settlement over a period of 20 years was conducted. The groundwater level was lowered to study the change in settlement caused by dewatering at the nearby construction site. Eight laser transducers measured the settlements of building and each ground layer, and the pore water pressure transducers measured the pore water pressure in the clay layers at four different depths. The average settlement obtained from the centrifuge model test showed good agreement with the field measurements. The centrifuge model testing showed that the temporary change in groundwater level did not affect the long-term settlement.

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The South African National Standard for the structural use of cold-formed steel (SANS 10162-2) provides capacity prediction models for screwed connections. Screwed connections are designed against shear failure of the screw(s), section tear-out, net section failure and tilt-andbearing failure. Previous studies (Rogers & Hancock, 1997) showed that the capacity is typically determined by the tilt-and-bearing type failure mode. The aim of this document is to report on the reliability of single screwed connections in cold-formed steel against this critical failure mode. Predicted nominal capacities depend on the ultimate tensile strength of the steel, the thickness of the connected plates and the diameter of the screw. Design capacities are obtained by multiplying the nominal capacities by a capacity reduction factor of 0.5, according to SANS 10162-2. Reliability is assessed by means of FORM analyses, taking uncertainty in the prediction model and variability of input parameters into account. Laboratory testing of 222 single screwed connections allowed to statistically describe the model factor, i.e. the ratio of actual tested- over unbiased predicted capacity. For each connection, the steel strength, plate thickness and screw diameter were measured, with the measured values used to predict capacity. This implies that the model factor accounts for uncertainty in the prediction model and experimental setup, while the variability of input parameters is separately accounted for through appropriate statistical modelling. Variability in the input parameters was described using appropriate statistical distributions from expert literature (Holicky, 2009:199; JCSS, 2000). For steel strength, the mean value and standard deviation were obtained from tensile tests, while mean values and standard deviations of the plate thickness and screw diameter were obtained from the above mentioned measurements. The experimental work and numerical analysis resulted in a model factor with a mean just exceeding unity and a small standard deviation. This suggests that the design code under consideration is able to accurately predict the nominal capacity of screwed connections. The FORM analysis resulted in computed reliability indexes significantly higher than the corresponding target values which suggest conservative and reliable design formulations. Die eksperimentele werk en numeriese analise het gelei tot 'n model faktor met 'n gemiddeld hoër as een en 'n klein standaardafwyking. Dit dui daarop aan dat die ontwerp-kode onder oorweging in staat is om die nominale kapasitiet van skroef verbindings akkuraat te voorspel. Die betroubaarheid analise het gelei tot betroubaarheidsindekse aansienlik hoër as die ooreenstemmende teiken waardes wat daarop dui dat die ontwerp formulerings betroubaar en hoogs konserwatief is. / AFRIKAANSE OPSOMMING: Die Suid-Afrikaanse Nasionale Standaard vir die strukturele gebruik van koud gevormde staal (SANS 10162-2) bied voorspellingsmodelle vir die kapasitiet van skroef verbindings. Skroef verbindings word ontwerp teen skroef faling, staal profiel faling, die uitskeer van skroewe en ook faling weens skroef kanteling. Vorige studies (Rogers & Hancock, 1997) het getoon dat die kapasiteit gewoonlik bepaal word deur die skroef-kantel falingsmodus. Die doel van hierdie navorsing is om verslag te doen oor die betroubaarheid van tipiese enkel skroef verbindings in koud gevormde staal strukture teen hierdie kritiese falingsmodus. Voorspelde nominale kapasiteite hang af van die treksterkte van die staal, die dikte van die verbonde profiele en die diameter van die skroef. Volgens die SANS 10162-2 word die ontwerp kapasiteit verkry deur die nominale kapasiteit met 'n kapasiteitsverminderingsfaktor van 0.5 te vermenigvuldig. Betroubaarheid word ontleed deur middel van ʼn eerste orde betroubaarheidsmetode analise, met die in ag neming van onsekerheid in die voorspellingsmodel en wisselvalligheid van die parameters. Laboratoriumtoetse van 222 enkel skroef verbindings het ʼn statistiese beskrywing van die model faktor toegelaat. Die model faktor is bereken as die verhouding tussen die getoetste kapasitiet en die voorspelde kapasitiet. Die staal sterkte, profiel dikte en skroef diameter is gemeet vir elke verbinding met die gemete waardes wat gebruik is om die kapasiteit te voorspel. Dit beteken dat die model faktor slegs onsekerhede in die voorspellingsmodel en van die eksperimentele opstelling in ag neem, terwyl die wisselvalligheid van die parameters afsonderlik in ag geneem word deur toepaslike statistiese modellering. Variasie in die parameters is beskryf met gepaste statistiese verdelings voorgestel deur verskeie literatuur (Holicky, 2009:199; JCSS, 2000). Aangaande die staal sterkte, is die gemiddelde waardes en standaardafwykings verkry deur standaard trek toetse terwyl die gemiddelde waardes en standaardafwykings van die plaat dikte en skroef diameter verkry is deur die bogenoemde metings.

The field of structural design optimization is one with great breadth and depth in many engineering applications. From the perspective of a designer, three distinct numerical methodologies may be employed. These include size, shape, and topology optimization, in which the ordering typically (but not always) corresponds to the order of increasing complexity and computational expense. This, of course, depends on the particular problem of interest and the selected numerical methods. The primary focus of this research employs density-based topology optimization with the goal of improving structural resistance to failure. Beginning with brittle fracture, two topology optimization based formulations are proposed in which low weight designs are achieved with substantially increased fracture resistance. In contrast to the majority of the current relevant literature which favors stress constraints with linear elastic physics, we explicitly simulate brittle fracture using the phase field method during the topology optimization procedure. In the second formulation, a direct comparison is made against results obtained using conventional stress-constrained topology optimization and the improved performance is numerically demonstrated. Multiple enhancements are proposed including a numerical efficiency gain based on the Schur-complement during the analytical sensitivity analysis and a new function which provides additional path information to the optimizer, making the gradient-based optimization problem more tractable in the presence of brittle fracture physics. Subsequently, design for ductile failure and buckling resistance is addressed and a numerically efficient topology optimization formulation is proposed which may provide significant design improvements when ductile materials are used and extreme loading situations are anticipated. The proposed scheme is examined regarding its impact on both the peak load carrying capacity of the structure and the amount of external work required to achieve this peak load, past which the structure may no longer be able to support any increase in the external force. The optimized structures are also subjected to a post-optimization verification step in which a large deformation phase field fracture model is used to numerically compare the performance of each design. Significant gains in structural strength and toughness are demonstrated using the proposed framework. Additionally, the failure behavior of 3D-printed polymer composites is investigated, both numerically and experimentally. A large deformation phase field fracture model is derived under the assumption of plane-stress for numerical efficiency. Experimental results are compared to numerical simulations for a composite system consisting of three stiff circular inclusions embedded into a soft matrix. In particular, we examine how geometric parameters, such as the distances between inclusions and the length of initial notches affect the failure pattern in the soft composites. It is shown that the mechanical performance of the system (e.g. strength and toughness) can be tuned through selection of the inclusion positions which offers useful insight for material design. Finally, a size optimization technique for a cardiovascular stent is proposed with application to a balloon expandable prosthetic heart valve intended for the pediatric population born with Congenital Heart Disease (CHD). Multiple open heart surgical procedures are typically required in order to replace the original diseased valve and subsequent prosthetic valves with those of larger diameter as the patient grows. Most expandable prosthetic heart valves currently in development to resolve this issue do not incorporate a corresponding expandable conduit that is typically required in a neonate without a sufficiently long Right Ventricular Outflow Tract (RVOT). Within the context of a particular design, a numerical methodology is proposed for designing a metallic stent incorporated into the conduit between layers of polymeric glue. A multiobjective optimization problem is solved, not only to resist the retractive forces of the glue layers, but also to ensure the durability of the stent both during expansion and while subject to the anticipated high cycle fatigue loading. It is demonstrated that the surrogate-based optimization strategy is effective for understanding the trade-offs between each performance metric and ultimately efficiently arriving at a single optimized design candidate. Finally, it is shown that the desired expandability of the device from 12mm to 16mm inner diameter is achievable, effectively eliminating at least one open heart surgical procedure for certain children born with CHD.

The need to protect marine structures from the high-intensity impulsive loads created by underwater explosions has stimulated renewed interest in the mechanical response of sandwich structures. The objective of this combined numerical and experimental study is to analyze the dynamic response of composite sandwich structures and develop material-structure-property relations and design criteria for improving the blast-resistance of marine structures. Configurations analyzed include polymer foam core structures with planar geometries. A novel experimental facility to generate high-intensity underwater impulsive loads and carry out in-situ measurements of dynamic deformations in marine structures is developed. Experiments are supported by fully dynamic finite-element simulations which account for the effects of fluid-structure interaction, and the constitutive and damage response of E-glass/polyester composites and PVC foams. Results indicate that the core-density has a significant influence on dynamic deformations and failure modes. Polymeric foams experience considerable rate-effects and exhibit extensive shear cracking and collapse under high-magnitude multi-axial underwater impulsive loads. In structures with identical masses, low-density foam cores consistently outperform high-density foam cores, undergoing lesser deflections and transmitting smaller impulses. Calculations reveal a significant difference between the response of air-backed and water-backed structures. Water-backed structures undergo much greater damage and consequently need to absorb a much larger amount of energy than air-backed structures. The impulses transmitted through water-backed structures have significant implications for structural design. The thickness of the facesheets is varied under the conditions of constant material properties and core dimensions. The results reveal an optimal thickness of the facesheets which maximizes energy absorption in the core and minimizes the overall deflection of the structure.