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In order for a human-powered helicopter (HPH) to fly, lifting the weight of its human pilot-engine and the weight of its own structure, the rotary wings need to be extremely large and exceptionally lightweight. Through centuries of dreaming and decades of modern attempts, no design so far has been able to obtain the combination of an adequately large rotor size, sufficiently lightweight structure, and an inherently stable aircraft. This thesis describes a concept of a wire-braced semi-rigid elevated rotor system for a proposed HPH. Then, using scale models and quantitative analysis, tests a series of supporting hypotheses in order to prove that such a large rotor system could be sufficiently lightweight, maintain its geometry to overcome coning and twisting, avoid interplanar interference, produce sufficient lift, yield inherent aircraft stabilty, and demonstrate that the drag penalty induced by external bracing wires would be more than offset by the benefits of wire bracing.

Ductile braced frames are often used to resist lateral earthquake loads in steel buildings; however the presence of a brace element can sometimes interfere with architectural features. One common type of ductile braced frame system sometimes used to accommodate architectural features is the eccentrically braced frame (EBF). In order to dissipate seismic forces, EBF beam regions (called links) must sustain large inelastic deformations. EBF links with column connections must transmit large moments and shear forces to facilitate link rotation. Experiments have shown that welded link-to-column connections tend to fracture in the link flange prior to large link rotations. This study investigated methods for improving EBF link-to-column connection performance, and proposed an alternative ductile braced frame system for accommodating architectural features. Several EBF links with reduced web and flange sections were analytically investigated using validated finite element models in ABAQUS. Results indicated that putting holes in the link web reduced stress and strain values in the link flanges at the connection, but increased the plastic strain and stress triaxiality in the web at the edges of holes. Removing area from the link flanges had little effect on connection stresses and strains. Thus, the reduced web section and reduced flange section methods are not a promising solution to the EBF link-to-column connection problem. The alternative braced frame system proposed in the dissertation used ductile beam splices and buckling-restrained braces in eccentric configurations (BRBF-Es) to accommodate architectural features. Design considerations for the BRBF-Es were determined and dynamic BRBF-E performance was compared with EBF performance. BRBF-E system and component performance was determined using multiple finite element methods. Inter-story drifts and residual drifts for the BRBF-Es were similar to those for EBFs. Results indicated that BRBF-Es are a viable alternative to the EBF, and may result in better design economy than EBFs. With the BRBF-E, damage was isolated within the brace, and in the EBF, damage was isolated within the link, indicating simpler repairs with the BRBF-E. Shop welding of BRBF-E members may replace the multiple field welds required in EBF construction.

It is possible to reduce residual and maximum drifts in buildings by adding “elastic stories” that engage gravity columns in seismic response. An elastic story is a story wherein the buckling restrained brace frame (BRBF) size is increased to prevent yielding when an earthquake occurs. Buildings ranging from 4–16 stories were designed with various elastic story brace sizes and locations to determine the optimal combination to best reduce drifts. Gravity column stiffnesses were also varied in elastic story buildings to determine the effects on drifts. Computer models were used to analyze these buildings under a suite of earthquakes. Adding elastic stories reduce residual drifts 34% to 65% in 4- to 16-story BRBF buildings. General recommendations are made to achieve optimal reductions in drifts. For buildings with six or more stories, drifts were generally reduced most when an elastic story was added to every 4th story starting at level 1 (the bottom story). The most effective size for the braces in the elastic story appears to be three times the original brace size. For buildings with less than six stories, adding a three times elastic story to the bottom level was observed to reduce drifts the most. Further research is also recommended to confirm the optimal location and size of elastic stories for buildings with differing number of stories. Increasing gravity column stiffnesses in buildings with elastic stories helps to further reduce drifts, however it may not be economical. Residual drifts were observed to decrease significantly more than maximum drifts when elastic stories were added to buildings. Maximum drifts generally decreased at some levels, but also increased at others when elastic stories were used.

This paper provides an assessment of the seismic performance of a code-designed buckling restrained braced frame building using the performance-based analysis procedures prescribed in ASCE 41-06. The building was designed based on the standards of the ASCE 7-05 for a typical office building located in San Francisco, CA. Nonlinear modeling parameters and acceptance criteria for buckling restrained brace components were developed to match ASCE 41-06 design standards for structural steel components, since buckling restrained braces are not currently included in ASCE 41-06. The building was evaluated using linear static, linear dynamic, nonlinear static and nonlinear dynamic analysis procedures. This study showed that the linear procedures produced more conservative results, with the building performing within the intended Life Safety limit, while the nonlinear procedures predicted that the building performed closer to the Immediate Occupancy limit for the 2/3 maximum considered earthquake hazard. These results apply to the full maximum considered earthquake hazard as well, under which the building performed within the Collapse Prevention limit in the linear analysis results and within the Life Safety limit in the nonlinear analysis results. The results of this paper will provide data for the engineering profession on the behavior of buckling restrained braced frames as well as performance based engineering as it continues to evolve.

In mid- to high-rise structures, dual systems (DS) enable a structural designer to satisfy the stringent drift limitations of current codes without compromising ductility. Currently, ASCE 7-05 permits a variety of structural systems to be used in combination as a dual system yet the design requirements are limited to the following statement: Moment frames must be capable of resisting 25% of the seismic forces while the moment frames and braced frames or shear walls must be capable of resisting the entire seismic forces in proportion to their relative rigidities. This thesis assesses the significance of the 25% design requirement for the secondary moment frames (SMF) in dual systems with consideration of current structural engineering practice. Three 20-story buckling-restrained braced frame (BRBF) dual system structures were designed with varying relative strengths between the braced and special moment frame systems. The SMF system wa designed for 15%, 25%, and 40% of seismic demands and the BRBF system design has been adjusted accordingly based on its relative stiffness with respect to the moment frame. These structures were examined with nonlinear static and nonlinear dynamic procedures with guidance from ASCE 41-06. The drift, displacement and ductility demands, and the base shear distribution results of this study show similar responses of the three prototype structures. These results indicate a secondary moment frame designed to less than 25% of seismic demands may be adequate for consideration as a dual system regardless of the 25% rule.

The buildings that are designed according to the building codes generally perform well at severe performance objectives (like life safety) under high earthquake hazard levels. However, the building performance at low performance objectives (like immediate occupancy) under low earthquake hazards is uncertain. The motivation of this research is to modify the design and detailing rules to make the traditional systems perform better at multi-level hazards. This research introduces two new structural steel systems: hybrid Buckling Restrained Braced Frames (BRBF) and hybrid steel Moment Frames (MF). The "hybrid" term for the BRBF system comes from the use of different steel material including carbon steel (A36), high-performance steel (HPS) and low yield point (LYP) steel. The hybridity of the moment frames is related to the sequence in the plastification of the system which is provided by using weaker and stronger girder sections. Alternative moment frame connections incorporating the use of LYP steel plates are also investigated. The hybrid BRBF approach was evaluated on seventeen regular (standard) frames with different story heights, seismic design categories and building plans. By varying the steel areas and materials in the BRB cores, three hybrid BRBFs were developed for each regular (standard) frame and their behavior was compared against each other through pushover and incremental dynamic analyses. The benefits of the hybridity were presented using different damage measures such as story accelerations, interstory drifts, and residual displacements. Collapse performance evaluation was also provided. The performance of hybrid moment frames was investigated on a design space including forty-two moment frame archetypes. Two different hybrid combinations were implemented in the designs with different column sections and different strong column-weak beam (SC/WB) ratios. The efficiency of the hybrid moment frame in which only the girder sizes were changed to control the plastification was compared with regular moment frame designs with higher SC/WB ratios. As side studies, the effect of shallow and deep column sections and SC/WB ratios on the moment frame behavior were also investigated.   In order to provide adequate ductility in the reduced capacity bays with special detailing, alternative hybrid moment frame connections adapting the use of low strength steel were also studied. / PhD

The purpose of this study is to examine the multidisciplinary design optimization (MDO) of a strut-braced wing (SBW) aircraft compared to similarly designed cantilever wing aircraft. In this study, four different configurations are examined: cantilever wing aircraft, fuselage mounted engine SBW, wing mounted engine SBW, and wingtip mounted engine SBW. The cantilever wing design is used as a baseline for comparison. Two mission profiles were used. The first called for a 7380 nmi range with a 305 passenger load based on a typical Boeing 777 mission. The second profile was supplied by Lockheed Martin Aeronautical Systems (LMAS) and has a 7500 nmi range with a 325 passenger load. Both profiles have a 0.85 cruise Mach number and a 500 nmi reserve range. Several significant refinements and improvements have been made to the previously developed MDO code for this study. Improvements included using ADIFOR (Automatic Differentiation for FORTRAN) to explicitly compute gradients in the design code. Another major change to the MDO code is the improvement of the optimization architecture to allow for a more robust optimization process. During the Virginia Tech SBW study, Lockheed Martin Aeronautical Systems (LMAS) was tasked by NASA Langley to evaluate the results of previous SBW studies. During this time, the original weight equations which were obtained from NASA Langley's Flight Optimization System (FLOPS) was replaced by LMAS proprietary equations. A detailed study on the impact of the equations from LMAS on the four designs was done, comparing them to the designs that used the FLOPS equations. Results showed that there was little difference in the designs obtained using the new equations. An investigation of the effect of the design constraints on the different configurations was performed. It was found that in all the design configurations, the aircraft range proved to be the most crucial constraint in the design. However, results showed that all three SBW designs were less sensitive to constraints than the cantilever wing aircraft. Finally, a double-deck fuselage concept was considered. A double deck fuselage configuration would result in a greater wing/strut intersection angle which would, in turn, reduce interference drag at that section. Due to the lack of available data on double deck fuselage aircraft, a detailed study of passenger and cargo layout was done. Optimized design showed that there was a small improvement in takeoff gross weight and fuel weight over the single-deck fuselage SBW results when compared with a similarly designed cantilever wing aircraft. / Master of Science

The objective of this study is to use Multidisciplinary Design Optimization (MDO) to investigate the use of truss-braced wing concepts in concert with other advanced technologies to obtain a significant improvement in the performance of transonic transport aircraft. The truss topology introduces several opportunities. A higher aspect ratio and decreased wing thickness can be achieved without an increase in wing weight relative to a cantilever wing. The reduction in thickness allows the wing sweep to be reduced without incurring a transonic wave drag penalty. The reduced wing sweep allows a larger percentage of the wing area to achieve natural laminar flow. Additionally, tip-mounted engines can be used to reduce the induced drag. The MDO approach helps the designer achieve the best technology integration by making optimum trades between competing physical effects in the design space. To perform this study, a suite of approximate analysis tools was assembled into a complete, conceptual-level MDO code. A typical mission profile of the Boeing 777-200IGW was chosen as the design mission profile. This transport carries 305 passengers in mixed class seating at a cruise Mach number of 0.85 over a range of 7,380 nmi. Several single-strut configurations were optimized for minimum takeoff gross weight, using eighteen design variables and seven constraints. The best single-strut configuration shows a 15% savings in takeoff gross weight, 29% savings in fuel weight, 28% increase in L/D, and a 41% increase in seat-miles per gallon relative to a comparable cantilever wing configuration. In addition to the MDO work, we have proposed some innovative, unconventional arch-braced and ellipse-braced concepts. A plastic solid model of one of the novel configurations was created using the I-DEAS solid modeling software and rapid prototyping hardware. / Master of Science