Delta Frame Research Articles

Innovative Design and Construction of Chesapeake and Delaware Canal Bridge

An outstanding accomplishment of bridge design, construction, and management, the Chesapeake and Delaware (C&D) Canal Bridge demonstrates that precast, segmental, cable-stayed bridges can be an economical and aesthetically pleasing solution in an area of the country where steel bridges are predominant. The C&D Canal Bridge is the first major concrete segmental cable-stayed bridge structure to be completed in the Northeast. The $58 million C&D Canal Bridge is a precast concrete, segmental structure 1417 m (4,650 ft) in length. Twin parallel box girders were designed to provide six lanes of travel with a total bridge width of 38.7 m (127 ft), with each precast box girder containing three 3.65-m (12-ft) lanes and two 3-m (10-ft) shoulders. Innovative construction techniques, economy, and aesthetics were the primary focus of the construction and design solutions used for the 229-m (750-ft) cable-stayed main-span crossing. The bridge was completed on schedule, with no claims or suits, at a cost slightly under the original bid. A quality-based selection process for the bridge designer, a prequalification process for the selection of the contractor, a disputes review panel, and informal partnering during construction all contributed greatly to a very successful project. The use of precast concrete elements provided an efficient and time-saving technique for the construction of this bridge. In addition, precast concrete box girders, box piers, and delta frames were designed to anchor the cable stays and make the parallel trapezoidal box girders monolithic throughout the main span.

Vertical profiles and horizontal scales of bubble clouds in the surf zone as measured with a distributed array of upward-looking sonars

A collaborative, multi-institute experiment was recently performed in the vicinity of the Scripps Pier, whose goal was the study of bubble-field generation, transport, and distribution, as influenced by surf zone conditions. The collaboration produced an ensemble of instrumentation at the site for measuring bubbles, ambient noise, temperature and salinity, currents, and surface waves. This paper emphasizes data gathered by APL-UW using a set of four upward-looking transducers (frequency 240 kHz), which simultaneously measured vertical profiles of acoustic volume scattering from bubbles from four locations. The primary locus of measurement activity was defined by NRL-SSC’s triangular frame (Delta Frame) and the four transducer locations for the APL-UW system bracketed this region. The transport of bubbles via rip currents emerged as a key feature surf zone bubble environment. Images of volumetric backscattering strength versus time and depth reveal episodic events (by way of increased scattering level) which last O(100) s and can occur in intervals of O(10) min. Time lags for increased scattering at the four locations were consistent with a seaward flow of rip currents equal to ∼0.25 m/s. Other features of the data that complement the single-depth measurements made from the Delta Frame will be discussed. [Work supported by ONR.]

Tomographic reconstruction of evolving bubble fields in the Scripps Pier bubble experiment

The Delta Frame, designed and operated by NRL-SSC, was a major component of the 1997 Scripps Pier shallow water bubbles experiment. The frame supported two sources and eight receivers. Acoustic travel time and attenuation were measured at eight frequencies between 39 and 244 kHz over the resulting 16 intersecting ray paths. Paths ranged in length from 2.5 to 8.6 m. For a given transmission sequence, the measurements at a particular frequency can be combined using tomography to yield a quantitative cross-sectional image of the ensonified region. By generating images for successive sequences, the temporal evolution of the medium can be visualized. A tomography algorithm [D. Rouseff and F. S. Henyey, J. Acoust. Soc. Am. 101, 3032 (1997)] is applied to the Delta Frame data. The images are produced for time periods during the experiment where the bubble field was probed by other independent measurements. Tomography results are compared to the other measurements. [Work supported by ONR.]

Observations of attenuation during the Scripps Pier bubble experiment

An experiment that measured the effects of bubbles just offshore from active surf was performed in the spring of 1997 off the pier at the Scripps Institution of Oceanography. A region from a few tens of meters to 1000 m from the beach was instrumented by several researchers. An emphasis was placed on a region approximately 400 m2 in area and about 300-m offshore where most of the instruments were clustered. Located in this area was a triangular-shaped frame called the Delta Frame, which had sources at two vertices and eight hydrophones along the perimeter (each side was 9.4 m long). Eight frequencies, from 39 to 244 kHz, were propagated sequentially from each source and received on each hydrophone. All 16 transmissions occurred within 12 ms and were repeated at a 1-s interval during each of ten 88-min data runs. Bubbles produced in the surf zone were carried out to the experimental region by rip currents and had dramatic effects on all the instruments. This paper discusses their effects on attenuation as measured at the Delta Frame. [Work supported by ONR.]

Tomographic reconstruction of shallow-water bubble fields observed in the Scripps Pier bubble experiment

An experiment was performed in the Spring of 1997 off the pier at the Scripps Institute of Oceanography. The objective was to measure the near-shore properties of bubble fields. A major component of the experiment was the Delta frame designed at NRL-SSC. The triangular frame, 9.4 m on each side, supported two sources and eight hydrophones. Measurements of acoustic travel time and attenuation were made at eight frequencies between 39 and 244 kHz. Details of the measurement apparatus will be presented elsewhere at this meeting [J. W. Caruthers et al., ‘‘Observations of attenuation during the Scripps Pier bubble experiment’’]. In this paper, a tomography algorithm [D. Rouseff and F. S. Henyey, J. Acoust. Soc. Am. 101, 3032 (1997)] is applied to a sampling of the data to produce cross-sectional mappings of attenuation. The bubble concentrations are shown to be inhomogeneous and carried across the frame by rip currents. Possible improvements to the tomography algorithm for imaging strongly attenuating structures are discussed. [Work supported by ONR.]

The Chesapeake and Delaware Canal Bridge Design-Construction Highlights

This award-winning $58 million, 4650 ft (1417 m) long structure is the first major all-precast, prestressed concrete segmental cable-stayed bridge built in the northeast United States. Situated on the new state relief route (SR-1) near St. Georges, Delaware, the bridge will be a big step forward in alleviating traffic congestion on this important transportation corridor. This article will concentrate on the 1650 ft (503 m) cable-stayed portion of the the project which has a main span of 750 ft (229 m). A major feature of the design is the use of precast delta frames, trapezoidal box girder segments, box pier segments and piles - all precast components were barged 180 miles (290 km) to the bridge site. The innovative design features and new construction techniques contributed to making this bridge into a very efficient and economical structure. The author presents the conceptual design and structural design features of the bridge while discussing the production, transportation, and erection aspects of the precast concrete segments.

Spliced Segmental Prestressed Concrete 1-Beams for Shelby Creek Bridge

The Shelby Creek Bridge is a five-span, high-level highway bridge nearly 1000 ft (305 m) long over a narrow valley in mountainous eastern Kentucky. Equal length prestressed concrete I-beams were spliced together in a unique design that eliminated costly falsework and provided full-length continuity over end spans of 162 ft 3 in. (49.5 m) and three equal interior spans of 218 ft 6 in. (66.6 m). Stay-in-place, precast, prestressed concrete deck panels eliminated most on-site forming and helped to speed up construction of the deck. The segmental prestressed concrete I-beam design was bid against two alternate steel designs and was $417,000 less than the continuous composite welded plate girder alternate and $2 million less than the originally designed steel delta frame girded bridge.

An Evaluation of the Bending Stiffness of Various Tibia1 Fixation Methods

The relative stiffness of human tibial fixation by plaster casts, intramedullary rods, external fixators, and dynamic compression plates was compared to the stiffness of the intact tibia. Four-point bending was performed on each specimen, first intact, and then after sequential instrumentation, thus allowing each specimen to serve as its own control. The mean stiffnesses obtained for the bone-device complexes, as a percentage of the intact tibial stiffness, were 113 +/- 9% for the delta frame fixator, 57 +/- 14% for the dynamic compression plate, 28 +/- 2% for the unilateral external fixator (UEF) with stainless steel sidebar, 18 +/- 1% for the UEF with aluminum sidebar, 7 +/- 2% for the flexible intramedullary (IM) rods, and 6 +/- 2% for the cast. Even relatively flexible unilateral external fixators are much stiffer than cast or IM rod fixation, methods that usually lead to satisfactory union by secondary healing. If secondary union with callus formation is desirable, modifications of the external fixator may be necessary to provide more flexible fixation. This study may be one of the first attempts to quantitate the stiffness of tibial fixation in a standard cast and compare it with other devices.

Stability of ten configurations of the Hoffmann external-fixation frame.

The rigidity, load to yield, and load to failure of ten configurations of the Hoffmann external fixator were investigated using a model of wooden pylons with a simulated fracture that consisted of either a reduced transverse cut or a ten-millimeter gap. The axial compressive, torsional, anterior-posterior bending, and medial-lateral bending characteristics of four forms of the single half-frame (half-pinned), four double half-frame, and two full-frame (transfixion-pinned) configurations were examined. Of the single half-frame configurations, a system with a second stacked connecting-rod proved to be superior; however, the system yielded at a mean axial compressive load of only 199 newtons and failed totally at 355 newtons. The delta frame (two rods connecting or triangulating two half-frames set at an angle of 45 degrees to one another) was as rigid as the quadrilateral full frame in axial compression; however, it exhibited low loads to yield and to failure, with means around 200 and 350 newtons, respectively. The use of only two pins in each pin-cluster did not significantly affect the performance of the delta frame. The two full-frame systems performed poorly in torsion and particularly poorly in anterior-posterior bending. The loads that caused a one-millimeter movement within the fracture gap in axial compression were notably low: for the stacked half-frames the load did not exceed a mean value of 174 newtons; for the double half-frame, 190 newtons; and for the quadrilateral frame, 412 newtons. We concluded that no frame had a good over-all performance with regard to rigidity.(ABSTRACT TRUNCATED AT 250 WORDS)