Section properties for cellular decks subjected to negative bending

Cellular decks are formed by attaching cold-formed “hat-shaped” deck sections on top of cold-formed steel sheets. The attachment is typically made using resistance spot welds spaced at a specific interval. The void left underneath the deck flutes and above the steel sheet provides a convenient means for the distribution of wiring and data cables throughout building systems. The section properties of cellular decks subjected to positive bending can be determined using the provisions of Chapter B of the 2001 AISI Specification (AISI, 2001). However, the provisions of Chapter B do not apply to cellular decks subjected to negative bending unless a specific weld spacing requirement is met. This requirement, set by Section D1.2 Spacing of Connections in Compression Elements (AISI, 2001), limits weld spacing so as to completely prevent column-like buckling between welds and provide adequate resistance to horizontal shear forces. Using section D1.2 limits weld spacing to a range of 1 in. to 2 in. for most cellular decks. It is standard industry practice to space cellular deck welds at 4 in. to 8 in. on center, exceeding the limits of Section D1.2. If the spacing limits of Section D1.2 are exceeded, the 2001 AISI Specification requires that the steel sheet be neglected when determining the section properties of cellular deck in negative bending. This is done because column-like buckling is likely to occur in the sheet when it is subjected to compression forces. Although the 2001 AISI Specification has provisions in place to account for the effects of local buckling, it has no provisions in place to account for the post column-like buckling strength of the steel sheet. However, a procedure for determining the post-buckling strength of cellular decks was developed by Luttrell and Balaji (1992), and is based on the results of 82 negative bending tests performed on six cellular deck profiles. The procedure developed by Luttrell and Balaji (1992) utilizes a dimensional reduction factor, ρm, which is used to determine the effective width of the steel sheet when column-like buckling is an issue. The factors having the greatest influence on ρm include steel sheet thickness, steel sheet yield strength, weld spacing, and the depth of the deck. Although the method correlated well with the 82 bending tests performed, a ballot containing his method was not passed by AISI. The principal reason for its rejection was 2 that the reduction factor, ρm, was dimensional, which violates an AISI directive that all equations be non-dimensional so they apply to both US Standard and SI units. The primary objective of this research was to modify the method developed by Luttrell and Balaji such that the dimensional reduction factor is non-dimensional. Using Luttrell\u27s method, section properties for 49 of the 82 cellular decks tested in negative bending were determined. Section properties were not determined for the remaining 33 ECP266 and EPC3 cellular decks due to a lack of information with regard to the deck dimensions. However, a dimensionless reduction factor was developed based on the section properties of the EP-type cellular deck. The equation used to predict the reduction factor was optimized so as to reduce the error between observed and theoretical bending strength to a minimum

Strength of arc-spot welds made in single and multiple steel sheets

The objective of this research was to establish a relationship between arc spot weld shear strength and the arc time used to form the weld. Lap shear tests were performed on both 3/4 in. and 5/8 in. nominal diameter welds. Each weld was formed in one-, two-, or four-layers of sheet steel ranging from 22 gauge (0.028 in.) to 16 gauge (.057 in.). Three distinct time series were tested for each unique weld size, thickness of sheet steel and layer configuration. The first of these series were the full-time welds. The two remaining series, 2/3-time and 1/3-time welds, had arc times equal to 2/3 and 1/3 of the average full-time weld arc time, respectively. Both weld shear strength tests and weld sectioning were performed for each series of weld. Strength tests were performed on a minimum of three specimens from every weld series. If the strength of any specimen deviated by over ten percent from the mean strength, an additional specimen was tested, helping to better understand the true behavior of the weld. Comparisons were made between the strengths of full-time, 2/3- time and 1/3-time welds. Comparisons were also made between the observed strength of each weld and the strengths calculated using the 2001 AISI Specification. Each sectioning test involved measuring and documenting the visual diameter, average diameter and effective diameter of the weld. Weld penetrations were also documented as sufficient or insufficient and any porosity was noted. A single sectioning test was performed for each full-time series, while three were performed for every 2/3- time and 1/3-time series. The data taken from the strength tests and the sectioning samples proved that welds formed using reduced arc times were considerably smaller and weaker than fulltime welds. The tests also proved that proper penetration is not dependent on the arc time, but is instead a function of the welding current and sheet steel thickness

A new search for interstellar C3

A new, very sensitive search for interstellar triatomic carbon has resulted in upper limits for a few diffuse clouds of order 1010 cm – 2, or about 10 – 11 with respect to hydrogen. These limits are consistent with recent cold diffuse cloud chemistry models, but may be in conflict with shocked cloud models such as those invoked to explain CH + abundances. Our results may also be argue against linear carbon-chain molecules as carriers of the diffuse interstellar bands

Predicting peculiar interstellar extinction from gaseous abundances

Molecular and atomic abundances are examined for 19 lines of sight through dense clouds, each with a peculiar selective extinction curve. The interstellar clouds in the present study appear to fall into two distinct categories: CN-rich, with relatively small amounts of neutral iron, or CN-poor, with large amounts of neutral iron. Lines of sight, having a CN/(Fe i) abundance ratio about two (~ 0.3 dex) or greater, are found to have a shallow (2.57 ± 0.55 mag) 2175 Å feature relative to the underlying extinction, while the strength of the bump is 3.60 ± 0.36 for the other dense clouds in the present study. The difference in the strength of the extinction bump between these two ensembles is 1.03 ± 0.23. Several atomic abundances are examined as potential indicators of peculiar extinction. Mn i abundances in particular are sought at 10 times greater sensitivity than previous studies because of a possible empirical connection between a small (Mn ii)/(Fe ii) abundance ratio and a weak 2175 Å bump reported in the literature. Unfortunately, the abundances of the neutral atoms do not appear to scale with the abundance of CN, reducing the effectiveness of Mn i as a diagnostic tool. Nevertheless, the Mn i upper limits in the present study support Mn being preferentially depleted. Fe i is underabundant relative to K i by 0.7 (dex) in the large (CN)/(Fe i) compared to the small (CN)/(Fe i) lines of sight. In addition, the data suggest that potassium is substantially depleted in both types of dense cloud

Predicting peculiar interstellar extinction from gaseous abundances

Molecular and atomic abundances are examined for 19 lines of sight through dense clouds, each with a peculiar selective extinction curve. The interstellar clouds in the present study appear to fall into two distinct categories: CN-rich, with relatively small amounts of neutral iron, or CN-poor, with large amounts of neutral iron. Lines of sight, having a CN/(Fe i) abundance ratio about two (~ 0.3 dex) or greater, are found to have a shallow (2.57 ± 0.55 mag) 2175 Å feature relative to the underlying extinction, while the strength of the bump is 3.60 ± 0.36 for the other dense clouds in the present study. The difference in the strength of the extinction bump between these two ensembles is 1.03 ± 0.23. Several atomic abundances are examined as potential indicators of peculiar extinction. Mn i abundances in particular are sought at 10 times greater sensitivity than previous studies because of a possible empirical connection between a small (Mn ii)/(Fe ii) abundance ratio and a weak 2175 Å bump reported in the literature. Unfortunately, the abundances of the neutral atoms do not appear to scale with the abundance of CN, reducing the effectiveness of Mn i as a diagnostic tool. Nevertheless, the Mn i upper limits in the present study support Mn being preferentially depleted. Fe i is underabundant relative to K i by 0.7 (dex) in the large (CN)/(Fe i) compared to the small (CN)/(Fe i) lines of sight. In addition, the data suggest that potassium is substantially depleted in both types of dense cloud

Evidence for over-dispersion in the distribution of clinical malaria episodes in children.

BACKGROUND: It may be assumed that patterns of clinical malaria in children of similar age under the same level of exposure would follow a Poisson distribution with no over-dispersion. Longitudinal studies that have been conducted over many years suggest that some children may experience more episodes of clinical malaria than would be expected. The aim of this study was to identify this group of children and investigate possible causes for this increased susceptibility. METHODOLOGY AND PRINCIPAL FINDINGS: Using Poisson regression, we chose a group of children whom we designated as 'more susceptible' to malaria from 373 children under 10 years of age who were followed up for between 3 to 5 years from 1998-2003. About 21% of the children were categorized as 'more susceptible' and although they contributed only 23% of the person-time of follow-up, they experienced 55% of total clinical malaria episodes. Children that were parasite negative at all cross-sectional survey were less likely to belong to this group [AOR = 0.09, (95% CI: 0.14-0.61), p = 0.001]. CONCLUSIONS AND SIGNIFICANCE: The pattern of clinical malaria episodes follows a negative binomial distribution. Use of lack of a clinical malaria episode in a certain time period as endpoints for intervention or immunological studies may not adequately distinguish groups who are more or less immune. It may be useful in such studies, in addition to the usual endpoint of the time to first episode, to include end points which take into account the total number of clinical episodes experienced per child

OH and HO2 chemistry in the North Atlantic free troposphere

Interactions between atmospheric hydrogen oxides and aircraft nitrogen oxides determine the impact of aircraft exhaust on atmospheric chemistry. To study these interactions, the Subsonic Assessment: Ozone and Nitrogen Oxide Experiment (SONEX) assembled the most complete measurement complement to date for studying HO(x) (OH and HO2) chemistry in the free troposphere. Observed and modeled HO(x) agree on average to within experimental uncertainties (±40%). However, significant discrepancies occur as a function of NO and at solar zenith angles >70°. Some discrepancies appear to be removed by model adjustments to HO(x)-NO(x) chemistry, particularly by reducing HO2NO2 (PNA) and by including heterogeneous reactions on aerosols and cirrus clouds

Spatial prediction of malaria prevalence in an endemic area of Bangladesh

<p>Abstract</p> <p>Background</p> <p>Malaria is a major public health burden in Southeastern Bangladesh, particularly in the Chittagong Hill Tracts region. Malaria is endemic in 13 districts of Bangladesh and the highest prevalence occurs in Khagrachari (15.47%).</p> <p>Methods</p> <p>A risk map was developed and geographic risk factors identified using a Bayesian approach. The Bayesian geostatistical model was developed from previously identified individual and environmental covariates (p < 0.2; age, different forest types, elevation and economic status) for malaria prevalence using WinBUGS 1.4. Spatial correlation was estimated within a Bayesian framework based on a geostatistical model. The infection status (positives and negatives) was modeled using a Bernoulli distribution. Maps of the posterior distributions of predicted prevalence were developed in geographic information system (GIS).</p> <p>Results</p> <p>Predicted high prevalence areas were located along the north-eastern areas, and central part of the study area. Low to moderate prevalence areas were predicted in the southwestern, southeastern and central regions. Individual age and nearness to fragmented forest were associated with malaria prevalence after adjusting the spatial auto-correlation.</p> <p>Conclusion</p> <p>A Bayesian analytical approach using multiple enabling technologies (geographic information systems, global positioning systems, and remote sensing) provide a strategy to characterize spatial heterogeneity in malaria risk at a fine scale. Even in the most hyper endemic region of Bangladesh there is substantial spatial heterogeneity in risk. Areas that are predicted to be at high risk, based on the environment but that have not been reached by surveys are identified.</p

A case study of transport of tropical marine boundary layer and lower tropospheric air masses to the northern midlatitude upper troposphere

Low‐ozone (ppbv) air masses were observed in the upper troposphere in northern midlatitudes over the eastern United States and the North Atlantic Ocean on several occasions in October 1997 during the NASA Subsonic Assessment, Ozone and Nitrogen Oxide Experiment (SONEX) mission. Three cases of low‐ozone air masses were shown to have originated in the tropical Pacific marine boundary layer or lower troposphere and advected poleward along a warm conveyor belt during a synoptic‐scale disturbance. The tropopause was elevated in the region with the low‐ozone air mass. Stratospheric intrusions accompanied the disturbances. On the basis of storm track and stratospheric intrusion climatologies, such events appear to be more frequent from September through March than the rest of the year