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Full-scale study of wind loads on roof tiles and felt underlay and comparisons with design data
Robertson, A.P.,Hoxey, R.P.,Rideout, N.M.,Freathy, P. Techno-Press 2007 Wind and Structures, An International Journal (WAS Vol.10 No.6
Wind pressure data have been collected on the tiled roof of a full-scale test house at Silsoe in the UK. The tiled roof was of conventional UK construction with a batten-space and bitumen-felt underlay beneath the interlocking concrete tiles. Pressures were monitored on the outer surface of selected tiles, at several locations within the batten-space, and beneath the underlay. Data were collected both with and without ventilator tiles installed on the roof. Little information appears to exist on the share of wind load between tiles and underlays which creates uncertainty in the design of both components. The present study has found that for the critical design case of maximum uplifts it would be appropriate to assign 85% of the net roof load to the tiles and 15% to the underlay when an internal pressure coefficient of -0.3 is used, and to assign 60% to the tiles and 50% to the underlay when an internal pressure coefficient of +0.2 is assumed (an element of design conservatism is inherent in the apparent 110% net loading indicated by the latter pair of percentage values). These findings indicate that compared with loads implied by BS 6399-2, UK design loads for underlay are currently conservative by 25% whilst tile loads are unconservative by around 20% in ridge and general regions and by around 45% in edge regions on average over roof slopes of $15^{\circ}-60^{\circ}$.
Wind-induced fatigue loading of tubular steel lighting columns
Robertson, A.P.,Hoxey, R.P.,Short, J.L.,Burgess, L.R.,Smith, B.W.,Ko, R.H.Y. Techno-Press 2001 Wind and Structures, An International Journal (WAS Vol.4 No.2
Two 12 m high tubular steel lighting columns have been instrumented to determine the wind-induced fatigue loading experienced by such columns. Each column supported a single luminaire mounted on a 0.5 m long bracket. One column was planted in soil, and the other bolted through a welded baseplate to a substantial concrete base. The columns were strain gauged just above the shoulder weld which connected the main shaft to the larger base tube. Forced vibration tests were undertaken to determine the natural frequencies and damping of the columns. Extensive recordings were made of response to winds with speeds from 4 m/s to 17 m/s. Selected records were analysed to obtain stress cycle counts and fatigue lives. Mean drag coefficients were also derived from the strain data to investigate experimentally the effect of Reynolds Number.
Robertson, Abigail,Mohamed, Tamer M.A.,El Maadawi, Zeinab,Stafford, Nicholas,Bui, Thuy,Lim, Dae-Sik,Cartwright, Elizabeth J.,Oceandy, Delvac Elsevier 2017 Stem cell research Vol.20 No.-
<▼1><P>Adult fibroblasts can be reprogrammed into induced pluripotent stem cells (iPSC) for use in various applications. However, there are challenges in iPSC generation including low reprogramming efficiency, yield, cell survival and viability. Since the Hippo signalling pathway is a key pathway involved in regulating cell proliferation and survival, we here test whether modification of the Hippo pathway will enhance the efficiency of iPSC generation and improve their survival.</P><P>The Hippo pathway was modified by genetic ablation of the mammalian sterile-20 like kinase 1 (Mst1), a major component of the pathway. Using adult skin fibroblasts isolated from Mst1 knockout mice (Mst1<SUP>−/−</SUP>) as a source of iPSC we found that genetic ablation of Mst1 leads to significantly increased reprogramming efficiency by 43.8%. Moreover, Mst1<SUP>−/−</SUP> iPSC displayed increase proliferation by 12% as well as an increase in cell viability by 20% when treated with a chemical hypoxic inducer. Mechanistically, we found higher activity of YAP, the main downstream effector of the Hippo pathway, in iPSC lacking Mst1.</P><P>In conclusion, our data suggests that Mst1 can be targeted to improve the efficiency of adult somatic cell reprogramming as well as to enhance iPSC proliferation and survival.</P></▼1><▼2><P><B>Highlights</B></P><P>•<P>Genetic deletion of Mst1 increases the efficiency of cell reprogramming.</P>•<P>iPSC lacking Mst1 displays higher proliferation rate than WT iPSC.</P>•<P>In response to chemical hypoxia Mst1<SUP>−/−</SUP> iPSC demonstrates higher survival.</P></P></▼2>
Robertson, Alex W.,Lin, Yung-Chang,Wang, Shanshan,Sawada, Hidetaka,Allen, Christopher S.,Chen, Qu,Lee, Sungwoo,Lee, Gun-Do,Lee, Joohee,Han, Seungwu,Yoon, Euijoon,Kirkland, Angus I.,Kim, Heeyeon,Suenag American Chemical Society 2016 ACS NANO Vol.10 No.11
<P>Dopants in two-dimensional dichalcogenides have a significant role in affecting electronic, mechanical, and interfacial properties. Controllable doping is desired for the intentional modification of such properties to enhance performance; however, unwanted defects and impurity dopants also have a detrimental impact, as often found for chemical vapor deposition (CVD) grown films. The reliable identification, and subsequent characterization, of dopants is therefore of significant importance. Here, we show that Cr and V impurity atoms are found in CVD grown MoS2 monolayer 2D crystals as single atom substitutional dopants in place of Mo. We attribute these impurities to trace elements present in the MoO3 CVD precursor. Simultaneous annular dark field scanning transmission electron microscopy (ADF-STEM) and electron energy loss spectroscopy (EELS) is used to map the location of metal atom substitutions of Cr and V in MoS2 monolayers with single atom precision. The Cr and V are stable under electron irradiation at 60 to 80 kV, when incorporated into line defects, and when heated to elevated temperatures. The combined ADF-STEM and EELS differentiates these Cr and V dopants from other similar contrast defect structures, such as 2S self-interstitials at the Mo site, preventing misidentification. Density functional theory calculations reveal that the presence of Cr or V causes changes to the density of states, indicating doping of the MoS2 material. These transferred impurities could help explain the presence of trapped charges in CVD prepared MoS2.</P>
Stability and Dynamics of the Tetravacancy in Graphene
Robertson, Alex W.,Lee, Gun-Do,He, Kuang,Yoon, Euijoon,Kirkland, Angus I.,Warner, Jamie H. American Chemical Society 2014 NANO LETTERS Vol.14 No.3
<P>The relative prevalence of various configurations of the tetravacancy defect in monolayer graphene has been examined using aberration corrected transmission electron microscopy (TEM). It was found that the two most common structures are extended linear defect structures, with the 3-fold symmetric Y-tetravacancy seldom imaged, in spite of this being a low energy state. Using density functional theory and tight-binding molecular dynamics calculations, we have determined that our TEM observations support a dynamic model of the tetravacancy under electron irradiation, with Stone–Wales bond rotations providing a mechanism for defect relaxation into lowest energy configurations. The most prevalent tetravacancy structures, while not necessarily having the lowest formation energy, are found to have a local energy minimum in the overall energy landscape for tetravacancies, explaining their relatively high occurrence.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2014/nalefd.2014.14.issue-3/nl500119p/production/images/medium/nl-2014-00119p_0010.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl500119p'>ACS Electronic Supporting Info</A></P>
Atomic Structure and Dynamics of Epitaxial Platinum Bilayers on Graphene
Robertson, Alex W.,Lee, Gun-Do,Lee, Sungwoo,Buntin, Parker,Drexler, Matthew,Abdelhafiz, Ali A.,Yoon, Euijoon,Warner, Jamie H.,Alamgir, Faisal M. American Chemical Society 2019 ACS NANO Vol.13 No.10
<P>Platinum atomic layers grown on graphene were investigated by atomic resolution transmission electron microscopy (TEM). These TEM images reveal the epitaxial relationship between the atomically thin platinum layers and graphene, with two optimal epitaxies observed. The energetics of these epitaxies influences the grain structure of the platinum film, facilitating grain growth <I>via</I> in-plane rotation and assimilation of neighbor grains, rather than grain coarsening from the movement of grain boundaries. This growth process was enabled due to the availability of several possible low-energy intermediate states for the rotating grains, the Pt-Gr epitaxies, which are minima in surface energy, and coincident site lattice grain boundaries, which are minima in grain boundary energy. Density functional theory calculations reveal a complex interplay of considerations for minimizing the platinum grain energy, with free platinum edges also having an effect on the relative energetics. We thus find that the platinum atomic layer grains undergo significant reorientation to minimize interface energy (<I>via</I> epitaxy), grain boundary energy (<I>via</I> low-energy orientations), and free edge energy. These results will be important for the design of two-dimensional graphene-supported platinum catalysts and obtaining large-area uniform platinum atomic layer films and also provide fundamental experimental insight into the growth of heteroepitaxial thin films.</P> [FIG OMISSION]</BR>
Partial Dislocations in Graphene and Their Atomic Level Migration Dynamics
Robertson, Alex W.,Lee, Gun-Do,He, Kuang,Fan, Ye,Allen, Christopher S.,Lee, Sungwoo,Kim, Heeyeon,Yoon, Euijoon,Zheng, Haimei,Kirkland, Angus I.,Warner, Jamie H. American Chemical Society 2015 NANO LETTERS Vol.15 No.9
<P>We demonstrate the formation of partial dislocations in graphene at elevated temperatures of ≥500 °C with single atom resolution aberration corrected transmission electron microscopy. The partial dislocations spatially redistribute strain in the lattice, providing an energetically more favorable configuration to the perfect dislocation. Low-energy migration paths mediated by partial dislocation formation have been observed, providing insights into the atomistic dynamics of graphene during annealing. These results are important for understanding the high temperature plasticity of graphene and partial dislocation behavior in related crystal systems, such as diamond cubic materials.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2015/nalefd.2015.15.issue-9/acs.nanolett.5b02080/production/images/medium/nl-2015-02080e_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl5b02080'>ACS Electronic Supporting Info</A></P>