Damping capacity of a model steel structure

The damping capacities of seven model steel structures, each consisting of a heavy steel platform supported on four columns, have been determined from forced vibration tests. The vibrations were produced by an eccentric-mass vibration generator, and the amplitudes ranged from small displacements up to slightly greater than yield displacement. The tests were terminated at the higher amplitudes once fatigue cracks formed in any of the joint welds in the columns. For vibration amplitudes up to a critical amplitude slightly less than yield displacement, the damping factors of the structures were constant, independent of amplitude, and ranged between .15 and .25~ for different structures. At vibration amplitudes greater than the critical amplitude, the damping factors of the structures became functions of amplitude. For example, the damping factor of one test structure increased from .15 to 1.~ as the displacement amplitude increased from 1.1 to 1.4 inches. The last structure tested revealed that a few cycles of vibration at amplitudes greater than the critical amplitude would increase these values slightly. Finally, the results summarized above are compared with the results of: {i) experimental work conducted by Lazan; {ii) reversed loading tests conducted on cantilever beams of similar construction to those tested in the work described in this report; and (iii) experimental work conducted by Hanson

The dynamic behavior of steel frame and truss buildings

This report describes experimental and analytical studies of the dynamic behavior of the East Building of the University of California\u27s new Medical Center at San Francisco, California. It also describes experimental tests conducted on the East Building\u27s mechanical service tower and on an adjacent elevator tower. Experimental tests on the East Building were conducted in the summers of 1964 and 1965. Vibrations were produced by eccentric-masstype vibration generators. Frequency responses, time responses, and mode shapes were observed. In the summer 1964 tests, the first mode east-west had 2% of critical damping and the first mode north-south had 1.8%; the values of damping for the higher modes lay in the range 0.4% to 0.9% of critical In the summer 1965 tests, the damping capacity measurements were mostly in the range 5 to 10% of critical. The cause of this increase in damping capacity was investigated in the fall 1965 tests. The fall 1965 tests showed that although the East Building was designed as free-standing, it was dynamically coupled to its adjacent buildings The relatively large damping capacity of this coupled system has been attributed to the Coulomb damping provided by non-structural connections between buildings An analytical model of the East Building was formulated to provide behavior corresponding to the summer 1964 test results. A standard openframe- type model was found capable of representing the dynamic behavior of the real structure. The analytical model was subjected to the El Centro earthquake ground acceleration record in a digital computer analysis. With 5% critical damping in the first few modes, no members of the frame yielded, but when the intensity of the earthquake was increased by a factor of 1.3, some members did yield

Extension of Bethe's diffraction model to conical Geometry: application to near field optics

The generality of the Bethe's two dipole model for light diffraction through a subwavelength aperture in a conducting plane is studied in the radiation zone for coated conical fiber tips as those used in near field scanning optical microscopy. In order to describe the angular radiated power of the tip theoretically, we present a simple, analytical model for small apertures (radius < 40 nm) based on a multipole expansion. Our model is able to reproduce the available experimental results. It proves relatively insensitive to cone angle and aperture radius and contains, as a first approximation, the empirical two-dipole model proposed earlier

Diffraction of light by a planar aperture in a metallic screen

We present a complete derivation of the formula of Smythe [Phys.Rev.72, 1066 (1947)] giving the electromagnetic field diffracted by an aperture created in a perfectly conducting plane surface. The reasoning, valid for any excitating field and any hole shape, makes use only of the free scalar Green function for the Helmoltz equation without any reference to a Green dyadic formalism. We compare our proof with the one previously given by Jackson and connect our reasoning to the general Huygens Fresnel theorem.Comment: J. Math. Phys. 47, 072901 (2006

Theory of imaging a photonic crystal with transmission near-field optical microscopy

While near-field scanning optical microscopy (NSOM) can provide optical images with resolution much better than the diffraction limit, analysis and interpretation of these images is often difficult. We present a theory of imaging with transmission NSOM that includes the effects of tip field, tip/sample coupling, light propagation through the sample and light collection. We apply this theory to analyze experimental NSOM images of a nanochannel glass (NCG) array obtained in transmission mode. The NCG is a triangular array of dielectric rods in a dielectric glass matrix with a two-dimensional photonic band structure. We determine the modes for the NCG photonic crystal and simulate the observed data. The calculations show large contrast at low numerical aperture (NA) of the collection optics and detailed structure at high NA consistent with the observed images. We present calculations as a function of NA to identify how the NCG photonic modes contribute to and determine the spatial structure in these images. Calculations are presented as a function of tip/sample position, sample index contrast and geometry, and aperture size to identify the factors that determine image formation with transmission NSOM in this experiment.Comment: 28 pages of ReVTex, 14 ps figures, submitted to Phys. Rev.

Evaluation of thermal evaporation conditions used in coating aluminum on near-field fiber-optic probes

This is the published version, also available here: http://dx.doi.org/10.1063/1.1148836.The effects that the thermal evaporation conditions have on the roughness of aluminum-coated near-field fiber-optic probes were investigated using the high-resolution capabilities of atomic force microscopy. The coating conditions studied include the effects of background gas composition, base vacuum pressure, and aluminum evaporation rate. The effects of aging on the aluminum-coated tips were also evaluated. The results from topography measurements of the resulting aluminumfilm indicated that the most dramatic improvements in the tip coatings can be achieved using high aluminum evaporation rates at base vacuum pressures below 10−5 Torr. These results agree with other studies on thin aluminumfilms and reflect a decrease in oxide formation. For demanding applications of near-field microscopy requiring maximal resolution, the results presented here indicate that it may also be necessary to reduce oxygen and/or water from the vacuum chamber prior to coating

Probing single molecule orientations in model lipid membranes with near-field scanning optical microscopy

This is the published version, also available here: http://dx.doi.org/10.1063/1.481367.Single molecule near-field fluorescence measurements are utilized to characterize the molecular level structure in Langmuir–Blodgett monolayers of L-α-dipalmitoylphosphatidylcholine (DPPC).Monolayers incorporating 3×10−4 mol % of the fluorescent lipid analog N-(6-tetramethylrhodaminethiocarbamoyl)-1,2-dihexadecanoyl-sn- glycero-3-phosphoethanolamine, triethylammonium salt (TRITC–DHPE) are transferred onto a freshly cleaved mica surface at low (π=8 mN/m) and high (π=30 mN/m)surfacepressures. The near-field fluorescence images exhibit shapes in the single molecule images that are indicative of the lipid analog probe orientation within the films. Modeling the fluorescence patterns yields the single molecule tilt angle distribution in the monolayers which indicates that the majority of the molecules are aligned with their absorption dipole moment pointed approximately normal to the membrane plane. Histograms of the data indicate that the average orientation of the absorption dipole moment is 2.2° (σ=4.8°) in monolayers transferred at π=8 mN/m and 2.4° (σ=5.0°) for monolayers transferred at π=30 mN/m. There is no statistical difference in the mean tilt angle or distribution for the two monolayer conditions studied. The insensitivity of tilt angle to filmsurfacepressure may arise from small chromophore doped domains of trapped liquid-expanded lipid phase remaining at high surfacepressure. There is no evidence in the near-field fluorescence images for probe molecules oriented with their dipole moment aligned parallel with the membrane plane. We do, however, find a small but significant population of probe molecules (∼13%) with tilt angles greater than 16°. Comparison of the simultaneously collected near-field fluorescence and force images suggests that these large angle orientations are not the result of significant defects in the films. Instead, this small population may represent a secondary insertion geometry for the probe molecule into the lipidmonolayer

Multipole Expansion of Bremsstrahlung in Intermediate Energy Heavy Ion Collisions

Using a multipole expansion of the radiated field generated by a classical electric current, we present a way to interprete the bremsstrahlung spectra of low energy heavy ion collisions. We perform the calculation explicitely for the system ^{12}C+ ^{12}C at 84AMeV and compare the result with the experimental data of E. Grosse et al. Using simple model assumptions for the electromagnetic source current we are able to describe the measured data in terms of coherent photon emission. In this context, the information contained in the measured data is discussed.Comment: LaTex, 4 Figure

Diffraction by a small aperture in conical geometry: Application to metal coated tips used in near-field scanning optical microscopy

Light diffraction through a subwavelength aperture located at the apex of a metallic screen with conical geometry is investigated theoretically. A method based on a multipole field expansion is developed to solve Maxwell's equations analytically using boundary conditions adapted both for the conical geometry and for the finite conductivity of a real metal. The topological properties of the diffracted field are discussed in detail and compared to those of the field diffracted through a small aperture in a flat screen, i. e. the Bethe problem. The model is applied to coated, conically tapered optical fiber tips that are used in Near-Field Scanning Optical Microscopy. It is demonstrated that such tips behave over a large portion of space like a simple combination of two effective dipoles located in the apex plane (an electric dipole and a magnetic dipole parallel to the incident fields at the apex) whose exact expressions are determined. However, the large "backward" emission in the P plane - a salient experimental fact that remained unexplained so far - is recovered in our analysis which goes beyond the two-dipole approximation.Comment: 21 pages, 6 figures, published in PRE in 200

Characterization of a hollow core fibre-coupled near field terahertz probe

We describe the design and performance of a freely positionable THz near field probe based on a hollow core photonic crystal fibre-coupled photoconducting dipole antenna with an integrated sub-wavelength aperture. Experimental studies of the spatial resolution are compared with detailed finite element electromagnetic simulations and imaging artefacts that are a particular feature of this type of device are discussed. We illustrate the potential applications with descriptions of time domain near field studies of surface waves on a metamaterial and multimode propagation in a parallel plate waveguide