Acinetobacter baumannii is currently posing a serious threat to global health. Lipopolysaccharide (LPS) is a potent virulence factor of pathogenic Gram‐negative bacteria. To explore the antigenic ...properties of A. baumannii LPS, four Kdo‐containing inner core glycans from A. baumannii strain ATCC 17904 were synthesized. A flexible and divergent method based on the use of the orthogonally substituted α‐Kdo‐(2→5)‐Kdo disaccharides was developed. Selective removal of different protecting groups in these key precursors and elongation of sugar chain via α‐stereocontrolled coupling with 5,7‐O‐di‐tert‐butylsilylene or 5‐O‐benzoyl protected Kdo thioglycosides and 2‐azido‐2‐deoxyglucosyl thioglycoside allowed efficient assembly of the target molecules. Glycan microarray analysis of sera from infected patients revealed that the 4,5‐branched Kdo trimer was a potential antigenic epitope, which is attractive for further immunological research to develop carbohydrate vaccines against A. baumannii.
Four inner core oligosaccharides from the lipopolysaccharide (LPS) of A. baumannii strain ATCC 17904 were chemically synthesized (see structures). Antigenic investigation indicated that the 4,5‐branched Kdo trisaccharide, a common inner core structure of A. baumannii LPS, is a potential antigenic epitope that could be used for further immunological research to develop diagnostic tools or vaccines against A. baumannii.
Comprehensive Summary
An efficient synthesis of a complex tetrasaccharide fragment 1 structurally related to rhamnogalacturonan II side chain A has been accomplished through a stepwise glycosylation ...strategy. Challenges involved in the synthesis include the facile construction of the sterically crowded l‐fucopyranose core and the stereoselective formation of two 1,2‐cis‐glycosidic linkages. The 3,4‐disubstituted l‐fucopyranoside structure was successfully constructed through a ‘counterclockwise’ glycosylation sequence, namely, the less reactive axial 4‐OH group of the central fucose unit was glycosylated first, then the 3‐OH. Besides, a 2‐pyridinecarbonyl‐assisted α‐d‐xylosylation was developed to synthesize the α‐d‐xylopyranosidic linkage and a 3,4‐O‐benzoyl‐controlled α‐l‐galactosylation reaction was used for the stereoselective synthesis of the corresponding α‐l‐galactopyranosidic linkage.
An efficient synthesis of a complex tetrasaccharide fragment 1 structurally related to rhamnogalacturonan II side chain A has been accomplished through a stepwise glycosylation strategy. Challenges involved in the synthesis include the facile construction of the sterically crowded l‐fucopyranose core and the stereoselective formation of two 1,2‐cis‐glycosidic linkages. The 3,4‐disubstituted l‐fucopyranoside structure was successfully constructed through a ‘counterclockwise’ glycosylation sequence, namely, the less reactive axial 4‐OH group of the central fucose unit was glycosylated first, then the 3‐OH. Besides, a 2‐pyridinecarbonyl‐assisted α‐d‐xylosylation was developed to synthesize the α‐d‐xylopyranosidic linkage and a 3,4‐O‐benzoyl‐controlled α‐l‐galactosylation reaction was used for the stereoselective synthesis of the corresponding α‐l‐galactopyranosidic linkage.
To investigate the disparity in collapse performance between multi-story composite frames with equal or unequal spans, experimental tests were statically conducted on two 1/3 scaled two-bay ...three-story composite planar frames, featuring span ratios of 1:1 and 4/3:1, respectively, subjected to an internal-column removal scenario. Analysis encompasses collapse resistance response, failure patterns, deformation characteristics, axial force development, and load-carrying mechanisms. The findings indicate that an increase in beam span correlates with a continuous decrease in both the maximum load at the bearing capacity stage and the first fractured load. Specifically, the unequal-span composite frame specimen exhibited its maximum collapse resistance at the flexural mechanism stage, while the equal-span frame reached its maximum collapse resistance at the ultimate failure point due to cooperative action. Subsequently, modeling methods were validated, and extensive investigations into resistance enhancement for unequal-span composite frames using steel braces were conducted. Detailed design methodologies featuring various brace types were proposed based on collaborative stress analysis of unequal-span double-span beams. The accuracy of these methodologies was validated through numerous numerical examples, underscoring their significant research and practical implications in effectively augmenting bearing capacities and mitigating collapse risks in unequal-span frame structures.
•A horizontal loading device that considers boundary constraints for collapse tests of multi-story frames was proposed.•Two one-third scale experimental tests on multi-story composite frames with different span ratios were conducted.•Detailed methods that use steel braces to improve the performance of unequal-span composite frames were proposed.
Controlling the collapse area of a structure is a direct collapse-resistant design requirement. However, existing theoretical models for collapse-resistant design establish the demand–capability ...relationship by considering only the lower-limit capacity of the damaged regions of a structure, and ignore the limit state of the surrounding structures. With the development of chain failure behaviors in a damaged structure, the member mechanism cannot resist accumulated unbalanced loads. Therefore, a higher level collapse-resistant mechanism should be considered in the design to ensure that the structure still has the controllable and expected failure modes. To this end, this paper proposes a collapse-resistant design concept with multilevel collapse-resistant behaviors characterized by a steel frame structural system with force-controlled members (FCMs). The FCM was designed based on the upper- and lower-limit internal force-controlled equations proposed in this study. Under the designed load, following the failure of a key member, unbalanced loads are redistributed throughout the remaining structure via a member mechanism. A regional mechanism is triggered (connection between FCM and surrounding regions is disconnected) to control the failure mode of the structure if a certain region fails. Two lines of defense at the member and regional levels are provided to prevent progressive collapse. To evaluate the efficacy of the proposed method, corresponding full-scale steel frame models were designed and tested using reliable numerical models based on the proposed method. The results indicated that the collapse-resistant behaviors of the structural system at the member and regional levels meet the design requirements.
•A steel frame structure with force-controlled members (FCM) was proposed.•Two lines of defense at the member and regional levels were provided.•The FCM was designed based on the internal force-controlled equations.
•A novel theoretical method was proposed to predict the failure patterns of angle steel-bolt connected steel frames exposed to fire under progressive collapse condition.•The potential failure ...mechanisms of connections and columns were considered.•The collapse-resistant actions under different fire exposure scenarios were investigated using multi-scale models.
Partially restrained connections comprise intricate systems of multiple components, each exhibiting varying degrees of performance degradation under fire conditions. Consequently, the damage patterns within these connections are reconfigured. Moreover, owing to the insulating effect of floor slabs, the thermal and mechanical states of remaining structures differ in hogging and sagging bending moment regions after removing critical members, thereby influencing the potential damage modes and collapse-resistant mechanisms. To elucidate this impact mechanism, a comprehensive analysis of the influence parameters on the failure mechanism and collapse-resistant action of a typical partially restrained connection (angle steel-bolted connection) steel frames exposed to fire was conducted based on multi-scale models. The results revealed that steel frame structures with different connection configurations all enter the compressive arching action (CAA) stage after the heating phase due to the thermal expansion behavior of beams. After the completion of load application, the deformations of all three connection models remain below 0.2 rad. The variation of beam load magnification factor α does not affect the structural failure mode, while a higher column load ratio causes the damage location to shift from the connection to the column under fire conditions and triggers a chain failure reaction. Compared to axial constraint kas, the rotational constraint krs is a critical factor influencing the damage mode. The negative axial force generated by the thermal expansion behavior of the fire-affected region can enhance the flexural action resistance in the unheated region by employing the CAA. Finally, a theoretical method is proposed to predict the failure mechanism of steel frames with angle steel-bolted connections exposed to fire under progressive collapse condition, providing a novel approach for identifying weak components of similar structures and improving anti-collapse capabilities.
Acinetobacter baumannii is currently posing a serious threat to global health. Lipopolysaccharide (LPS) is a potent virulence factor of pathogenic Gram‐negative bacteria. To explore the antigenic ...properties of A. baumannii LPS, four Kdo‐containing inner core glycans from A. baumannii strain ATCC 17904 were synthesized. A flexible and divergent method based on the use of the orthogonally substituted α‐Kdo‐(2→5)‐Kdo disaccharides was developed. Selective removal of different protecting groups in these key precursors and elongation of sugar chain via α‐stereocontrolled coupling with 5,7‐O‐di‐tert‐butylsilylene or 5‐O‐benzoyl protected Kdo thioglycosides and 2‐azido‐2‐deoxyglucosyl thioglycoside allowed efficient assembly of the target molecules. Glycan microarray analysis of sera from infected patients revealed that the 4,5‐branched Kdo trimer was a potential antigenic epitope, which is attractive for further immunological research to develop carbohydrate vaccines against A. baumannii.
Four inner core oligosaccharides from the lipopolysaccharide (LPS) of A. baumannii strain ATCC 17904 were chemically synthesized (see structures). Antigenic investigation indicated that the 4,5‐branched Kdo trisaccharide, a common inner core structure of A. baumannii LPS, is a potential antigenic epitope that could be used for further immunological research to develop diagnostic tools or vaccines against A. baumannii.
•The effects of three unequal-span cases on the collapse performance are analyzed.•The resistant contributions of different mechanisms are quantitatively separated.•Theoretical formulas are proposed ...for the different resistant contributions.
Following the sudden removal of an internal column from a steel-frame composite-floor system, the two-bay beams connected to the failed column play a key role in the internal force redistribution and rebalancing of the remaining structure. Moreover, the span–depth ratios of the adjacent two-bay beams have a significant influence on the collapse-resistant performance of the structure. To date, studies on structural progressive collapse of beams with unequal spans have been limited. In this study, based on the results of static loading tests on composite beam–column assemblies with cover-plate joints under three different span ratios, the finite element modeling method was validated and numerical models of the beam–column assemblies with three unequal spans were established. The influence of the unequal span of two-bay beams on the internal force development and load-resisting mechanisms of the composite beam–column assemblies were analyzed in detail. To gain deeper insight into the load-resisting mechanisms, the contributions of the flexural and catenary mechanism resistances to the total resistance of two-bay beams were quantitatively separated using the energy equilibrium theory, and it was found that the resistant contributions of the catenary mechanism, in general, account for less than a half of the structure’s resistance capacity. Moreover, theoretical formulas for the different resistant contributions of two-bay beams were proposed as recommendations for structural progressive collapse design and practical engineering applications. The analysis of results demonstrated that the span–depth ratios of two-bay beams determined the resistant contribution coefficients for the flexural mechanism, whereas the span ratios of two-bay beams determined the resistant contribution coefficient for the catenary mechanism, whereas the beam depth had little influence thereon.
Steel-braced concentric frames (SBCFs) are a popular form of structure and has been widely employed in seismic regions owing to their high strength and elastic stiffness; however, the effects of key ...parameters on the collapse behavior of SBCFs are unclear due to insufficient investigations, which should be further explored in column removal scenarios. To this end, numerical models were employed to explore the key parameters affecting the collapse behavior of SBCFs in this study. The numerical models of steel frames with or without braces were validated based on previous experimental results. According to the refined model, the effects of brace members of various types and with different slenderness ratios on the collapse behavior were illustrated. The results showed that an X brace type with smaller slenderness ratio is the most appropriate for enhancing robustness against progressive collapse. Thereafter, full-scale numerical models were established to investigate the effect of the column removal position, and the results showed that the frame relied purely on flexural action in a side column loss. Moreover, the effect of the position of horizontal brace layers on the collapse behavior was investigated in the middle and side column removal scenarios. To gain deeper insight into the load-resisting mechanisms, the specific contributions to the total resistance of different mechanisms in different stories, bare steel frames and braces, and tensile and compressive braces were separated. Finally, a strategy for the robustness enhancement of high-rise steel frames was empirically proposed, and it was suggested that the provision of a brace layer for approximately every 10 stories would be effective for the anti-collapse design of high-rise buildings.
•Progressive collapse performance of SBCFs are studied.•Effects of brace members with various types and with different slenderness ratios were analyzed.•The effect of the position of horizontal brace layers on collapse behavior were investigated.•A strategy for robustness enhancement of high-rise steel frames was empirically proposed.
This study presents the investigation on the anti-collapse behavior of multi-story structures at different structural levels with different connection types. A three-story composite sub-frame with a ...weld cover-plated flange (WCPF) connection is quasi-statically tested and subjected to an internal column-removal scenario. The accuracy of the numerical modeling method is verified by quasi-static tests and combined with previous experimental tests on two single-story beam–column assemblies with top-and-seat double web-angle (TSDWA) and WCPF connections; and on a multi-story sub-frame with a TSDWA connection. Based on the modeling method, numerical models considering peripheral frames are established using the hybrid element modeling method, and the effects of different connection types and structural scales on the collapse-resistant performance are analyzed in detail. The results indicate that when the actual boundary restraints are considered, the collapse resistance of multi-story frames can be approximately summed up to derive the total resistance of the single-story beam-column assemblies with TSDWA connections. However, for models with WCPF connections, it is unsafe to apply the collapse resistance of single-story beam-column assemblies to represent that of multi-story planar frames. For a more thorough understanding of the load-resisting mechanisms, the contribution of different resistance mechanisms is quantitatively distinguished by introducing contribution coefficients. Finally, resistance improvement strategies for multi-story planar frames with WCPF and TSDWA connections are proposed. The ultimate load of models with WCPF and TSDWA connections increase by 65% and 76%, respectively, indicating that these strategies are beneficial for the effective exertion of catenary action.
•A three-story sub-frame with weld cover-plated flanges connection was tested.•The model predictions and experimental outcomes were in good agreement.•Diverse frame stiffness connections and structural scales were utilized and compared.•Resistance improvement strategies for multi-story frames are proposed.
