Browsing by Author "Gao, Tian"
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- Direct Strength Method for the Flexural Design of Through-Fastened Metal Building Roof and Wall Systems under Wind Uplift or SuctionGao, Tian (Virginia Tech, 2012-08-03)The design of metal building roof and wall systems under uplift and suction wind loading is complicated because the laterally unbraced purlin and girt's free flange is compressed, and the cross-section rotates due to the shear flow. The objective of this thesis is to introduce a Direct Strength Method (DSM) prediction approach for simple span purlins and girts with one flange through-fastened under uplift or suction loading. This prediction method is also applicable for the case when rigid board insulation is placed between the metal panel and through-fastened flange. The prediction method is validated with a database of 62 simple span tests. To evaluate the prediction for the case when rigid board is used, 50 full-scale tests with rigid board insulation are conducted by the author of this thesis. In the experimental study panel failure, connection failure and member (purlin and girt) failure are observed, and they all limit the system's capacity. Another important contribution of this thesis is that it builds the foundation for future study of a general, mechanics-based limit state design approach for metal building roof and wall systems that can accommodate uplift and gravity loads, simple and continuous spans, and through-fastened and standing seam roofs.
- Experimental Evaluation of a Vehicular Access Door Subjected to Hurricane Force Wind PressuresGao, Tian; Moen, Cristopher D. (Virginia Polytechnic Institute and State University, 2009-11-01)This report presents findings on the behavior of a typical rolling sheet vehicular access door under a hurricane force wind pressure. The objectives are to quantify the structural behavior of a rolling sheet vehicular access door and the attached frame under both positive pressure (pushing the door into the building) and negative pressure (suction pulling the door away from the building), including the direct measurement of the catenary forces in the wind locks with strain gauges. The results will be used to optimize existing design methods for a rolling sheet vehicular access door and the supporting door frame.
- Flexural Strength of Exterior Metal Building Wall Assemblies with Rigid InsulationGao, Tian (Virginia Polytechnic Institute and State University, 2011-08-01)The goal of this research study is to observe and quantify the influence of rigid board insulation on through-fastened girt capacity. Rotational restraint tests are performed to study local rotational stiffness at the flange-insulation contact point. Vacuum box tests on through-fastened wall systems are conducted to explore the effect of insulation thickness on the R-factor. The results will be used to support new code language in AISI S100-07 D6.1 that accommodate capacity prediction of metal building wall systems with rigid board insulation.
- Mode-I Fracture in Bonded Wood: Studies of Adhesive Thermal Stability, and of the Effects of Wood Surface DeactivationGao, Tian (Virginia Tech, 2010-02-01)This work included two separate studies; the common theme in each was the use of mode-I fracture testing to evaluate wood adhesion. In the first study, mode-I fracture testing was used to compare the thermal stability of polyurethane (PUR) and resorcinol-formaldehyde (RF) wood adhesives. Bonded specimens for both adhesives were subjected to prolonged thermal exposure, and fracture testing was subsequently conducted after re-equilibration to standard test conditions. It was found that both PUR and RF suffered a significant fracture energy loss after heat treatment, and that RF was more thermally stable than PUR, as expected. However, both adhesives suffered significant thermal degradation, and fracture testing did not distinguish the RF system as being clearly superior to PUR. Dynamic mechanical analysis (DMA) was also used to analyze and compare the thermal softening of PUR and RF in terms of the decline in storage modulus. DMA results indicated that PUR specimens suffered greater stiffness loss due to simple thermal softening. Because fracture testing indicated that both adhesives suffered significant degradation, the DMA results suggested that the generally superior fire resistance of RF adhesives is born from greater high temperature stiffness; whereas the more compliant PUR suffers greater immediate softening during thermal exposure. In other words, both systems suffer from thermal degradation, but the more highly cross-linked RF system suffers less thermal softening and therefore maintains a greater load carrying capacity during fire exposure. In the second study, mode-I fracture testing was used to test the effects of wood surface thermal deactivation (surface energy reduction) on the adhesion between southern pine wood (Pinus spp.) and polyethylene (PE). Pine specimens were progressively surface deactivated by 185°C heat treatments for periods of 5, 15, and 60 minutes. Control and deactivated pine laminae were subsequently hotpressed/bonded using PE film as the adhesive. Mode-I fracture testing was conducted under the assumption of linear elasticity, however load/displacement test curves suffered from a severe degree of nonlinearity believed to be caused by PE bridging behind the advancing crack tip. Instead of applying a nonlinear data analysis, a standard linear elastic analysis was conducted and deemed acceptable for comparative purposes within this study. Under dry conditions (unweathered specimens), 5 and 15 minute thermal treatments resulted in progressively worse adhesion (lower fracture energies) when compared to control surfaces; but the 60 minute heat treatment improved adhesion relative to 5 and 15 minute treatments, and showed a trend of improving adhesion as surface deactivation became more extreme. Simulated-weather resistance was also studied and it was determined that the highest degree of surface deactivation slightly improved weather durability in comparison to control surfaces. Overall, the findings here were similar to those in a previously published work- thermal deactivation of wood surfaces shows promise as a method to improve adhesion between wood and nonpolar polyolefins.
- Rice Chalky Grain 5 regulates natural variation for grain quality under heat stressChandran, Anil Kumar Nalini; Sandhu, Jaspreet; Irvin, Larissa; Paul, Puneet; Dhatt, Balpreet K.; Hussain, Waseem; Gao, Tian; Staswick, Paul; Yu, Hongfeng; Morota, Gota; Walia, Harkamal (Frontiers, 2022-10)Heat stress occurring during rice (Oryza sativa) grain development reduces grain quality, which often manifests as increased grain chalkiness. Although the impact of heat stress on grain yield is well-studied, the genetic basis of rice grain quality under heat stress is less explored as quantifying grain quality is less tractable than grain yield. To address this, we used an image-based colorimetric assay (Red, R; and Green, G) for genome-wide association analysis to identify genetic loci underlying the phenotypic variation in rice grains exposed to heat stress. We found the R to G pixel ratio (RG) derived from mature grain images to be effective in distinguishing chalky grains from translucent grains derived from control (28/24 degrees C) and heat stressed (36/32 degrees C) plants. Our analysis yielded a novel gene, rice Chalky Grain 5 (OsCG5) that regulates natural variation for grain chalkiness under heat stress. OsCG5 encodes a grain-specific, expressed protein of unknown function. Accessions with lower transcript abundance of OsCG5 exhibit higher chalkiness, which correlates with higher RG values under stress. These findings are supported by increased chalkiness of OsCG5 knock-out (KO) mutants relative to wildtype (WT) under heat stress. Grains from plants overexpressing OsCG5 are less chalky than KOs but comparable to WT under heat stress. Compared to WT and OE, KO mutants exhibit greater heat sensitivity for grain size and weight relative to controls. Collectively, these results show that the natural variation at OsCG5 may contribute towards rice grain quality under heat stress.
- Test Videos- Flexural Strength Experiments on Exterior Metal Building Wall Assemblies with Rigid InsulationGao, Tian (2012-06-12)This research program evaluated the flexural capacity of a metal building wall system with rigid board foam insulation sandwiched between C- and Z-section girts and through-fastened steel panels. Vacuum box tests were conducted to simulate wind suction on the wall system, and distinct failure modes were observed. The metal panel pulled over the screw heads for wall systems without insulation, however when rigid board insulation was added, the insulation acted as a washer and girt failure or screw fracture was observed. Screw bending and fracture were common in the specimens with the thickest rigid board insulation and for locally stocky cross-sections because a concentrated moment could be developed in the fastener. In these cases wall system capacity was decreased by the presence of rigid board insulation. For wall system specimens with locally slender girts, rigid board insulation did not influence girt capacity because girt deformation under load was primarily in the cross-section and not at the through-fastened connections.
- Vehicular Access Doors under Hurricane Force Wind Pressure: Experiments to Study Jamb BehaviorGao, Tian; Moen, Cristopher D. (Virginia Polytechnic Institute and State University, 2012-03-01)In August of 2009, two 10 ft by 10 ft DBCI 5000 vehicular access doors were tested with a simulated hurricane force wind pressure at the DBCI in Douglasville, GA (Gao and Moen 2009). After evaluating the results from this study, it was hypothesized that the door curtain deflection and wind-lock axial force are sensitive to the jamb stiffness: a stiffer jamb limits curtain deflection, but at the same time increases wind-lock axial force. A subsequent analytical and computation study (Janas and Moen 2011) confirmed this hypothesis, leading to the development and validation of a general mechanics-based prediction model that was implemented as a computer program CSBA. In this study, two 10 ft by 10 ft access doors are tested with different wind-lock details and door jambs than those considered in the August 2009 study. Jamb stiffness and jamb deformation are directly measured, providing useful data for validating the CSBA prediction model. The access doors (Janus 3100) were provided by Janus International, and the tests were again conducted at DBCI in Douglasville, GA in November of 2011.