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To investigate the variability of ground motion characteristics (GMC) with the angle of seismic incidence (ASI) and the impact of seismic incident directionality on structural responses, first, a large-scale database of recorded ground motions was used to analyze the causes of GMC variability due to the seismic incident directionality effect (SIDE). Then a single-mass bi-degree-of-freedom system (SM-BDOF-S) with different types of symmetrical sections was selected to explore the influence mechanism of SIDE on the seismic responses. The results illustrated that the GMC has substantial variability with the ASI, which is independent of the earthquake source, propagation distance, and site condition, and exhibits complex random characteristics. Additionally, a classification method for ground motions is proposed based on this GMC variability to establish a criterion for selecting ground motions in seismic analysis considering the SIDE. Moreover, for an SM-BDOF-S, the response spectral plane is proposed to explain the transition behavior of spectral responses that are very similar among different stiffness ratios, but divergent for different types of ground motions. The influence of SIDE on structures is determined by their stiffness and stiffness ratio in the x - and y -directions, as well as the type of ground motion. [ABSTRACT FROM AUTHOR]

Large grandstands provide areas for human activities and leisure pursuits. The degree of vibration comfort for people has an important effect on the functionality of grandstands. In this paper, the changes of the dynamic properties of the stand owing to different human postures are demonstrated. The comfort degree for human vibration in the large stand is also examined by full-scale measurements and questionnaire surveys. In addition, the analysis of the influence of different parameters on acceleration responses of the stand is presented via numerical simulation. Thereafter, it has been observed that ISO10137-2007 and BS6472 are more suitable than GB10070 and AISC-11 for evaluating vibration serviceability owing to crowd jumping loads during football matches in the large grandstand environment. Moreover, it has been observed that females and the staff are more likely to feel uncomfortable during matches, whilst students younger than 30 years old are the least sensitive group to vibrations of large stands. When the frequency of the jumping loads is close to the fundamental frequency of the stand or the synergy rate is high, the resonance is more likely to occur, which should be avoided by designers and users. [ABSTRACT FROM AUTHOR]

Unbalanced snow loads on roofs have been a turbulent and significant problem for the building industry. Gables are one of the most common roof shapes, which are widely adopted as the roof shape for family residences, factories, ware houses, and garages. Series of experiments on an isolated gable-roof with or without scuttles and different roof pitches were done for the purpose of exploring the unbalanced patterns of snow loads. The model buildings made of wood have the same floor space, 2000 × 2000 mm but different roof pitches. A new open air snow–wind combined experimental facility was adopted in the experiments. The facility, which is like an open air wind tunnel with a snowfall simulator, can produce a stable wind field and snowfall environment for a specific duration. Artificial snow was used in the experiments, and particles properties were compared with natural snow previously proposed by the other authors. The experiment results indicated that the unbalanced snow loads were strongly connected with various wind velocities and roof pitches and a scuttle or other salient on the roof would bring an evident influence of patterns of snow distribution on the roofs. [ABSTRACT FROM AUTHOR]

Featured Application: This study provides a simplified method that can be applied to help preliminarily estimate the snow load on a complex roof by combining the snow loads on several simple roofs, which have been provided in the load code or obtained by numerical studies. A considerable number of studies have been carried out for predicting snowdrifts on roofs over the years. However, few studies have focused on snowdrifts on complex long-span roofs, as the complex shape and fine structure pose significant challenges. In this study, to simplify the calculation requirements of snow load on such roofs, work was conducted to decompose the snowdrift on a complex roof into snowdrifts on several simple roofs. First, the snow–wind tunnel test similarity criteria were investigated based on a combined air–snow–wind experimental system. Thereafter, with reference to the validated experimental similarity criteria, a series of snow–wind tunnel tests were performed for snowdrifts on a complex long-span structure under the conditions of different inflow directions. Finally, based on empirical orthogonal function (EOF) analysis, the snowdrifts on the complex roof were decomposed into basic characteristic distribution modes, including snowdrifts caused by the local and overall roof forms. The snow distribution under a specific inflow direction could be derived from the weighted combination of the basic characteristic modes, based on the wind direction coefficients. Therefore, it is possible for the snow load on a complex roof to be estimated preliminarily based on the snow distributions on several simple roofs. [ABSTRACT FROM AUTHOR]

Featured Application: A new approach for investigating snowdrifts around buildings and snow loads on building roofs based on a new facility named the "snow–wind combined experimental facility" is proposed in this paper. First, the method proposed in this paper is convenient and economical, and also the experimental results prove that the method is feasible and reliable. In this paper, only experiments on snowdrift around a cube were chosen for the verification for the sake of generality, but this method could also be adopted for the prediction of snowdrift around complex building environments and snow loads on various shapes of building roofs. Unbalanced, or non-uniform, snow loads caused by snow drifting or sliding in cold regions with heavy snowfalls, can be a serious problem for the building industry. However, the methods for predicting snow distribution still need to be improved. Field observation is the most direct and reliable method to study snow distribution, but because the natural environment is uncontrollable and varies dramatically, sometimes conclusions may be confused under the influence of the many variables in the investigation. This paper proposes a snowing experiment approach using an outdoor snow–wind combined experiment facility for the study of snow distribution. The facility can produce a stable and controllable wind field and snowfall environment. Experiments which focused on snowdrift around a building were conducted during the winter to make an evaluation of the repeatability and reliability of the new approach. Finally, from the analysis of results, it was demonstrated that the experimental facility was stable and that the similarity criterion adopted for the snowing pattern was reliable. Especially, the minimum value of the friction speed ratio was suggested to ensure the test accuracy. [ABSTRACT FROM AUTHOR]

Reciprocal grid structure is a structural system consisting of mutually supporting bars. Its geometry should satisfy not only the demands of architectural intents but also the requirements of the lapping relationship of the bars. Thus, form generation is the key problem in its design. Inspired by the physical building process of a reciprocal grid structure, self‐adaptive form generation method is proposed. In this method, an initial plane grid with the topology of required structural configuration is created first. Next, elements are iteratively lapped to realize a self‐adaptive generation of structural form. Two new parameters, bar lapping factor and rotation angle, are proposed to describe reciprocal grid structures quantitatively and make the form generation process easy. The efficiency and accuracy of the method are illustrated by comparisons with other form generation methods. Numerical examples demonstrate the applicability of the proposed method in the complex and realistic reciprocal grid structures. [ABSTRACT FROM AUTHOR]

This experimental investigation focused on the mechanical and damping properties of raw and alkali treated flax fibers reinforced cementitious composites. The objective of this study was to research the influence of flax fiber for the damping properties of cementitious composites. The mechanical strengths were obtained using compressive and flexural tests and damping properties were measured using a dynamic mechanical analysis. Structural modifications to the treated flax fibers were analyzed using scanning electronic microscopy and mercury intrusion porosimetry. Results showed that cement compressive strength was decreased, and flexural strength was increased by the addition of untreated flax fibers. Alkali treatment improved the mechanical strength of flax reinforced cement. The addition of flax fibers improved cement damping properties, and the damping effect of untreated flax fibers was greater than that of alkali treated fibers. The loss tangent of the cementitious composites reinforced with raw and alkali treated flax fibers at an amount of 1 wt % of cement was improved by 45.83% and 37.5% at 20 Hz, respectively. The damping properties of flax fiber was attributed to friction between elementary fibers and between adjacent fiber cell walls and slipping between the fiber and matrix. Callouses, pectin, and lignocellulose in untreated flax fiber contributed to damping and their removal by alkali treatment was the main reason why untreated flax fibers had a greater damping effect as compared to alkali-modified flax fibers. [ABSTRACT FROM AUTHOR]

Abstract: Free‐form grid structure is a kind of structure whose shape cannot be expressed by analytical functions. Although this kind of structure has been widely used in practice, how to generate the structural shapes that not only satisfy architectural requirements but also have excellent mechanical properties is still a problem. This article focuses on the shape generation of complex grid structures and proposes a novel shape‐generation method for free‐form grid structures. The method integrates numerical inverse hanging method and multiobjective optimization. First, it uses numerical inverse hanging method to generate structural shapes in low bending moment level. Next, it uses dynamic weight multiobjective firefly optimization algorithm to adjust the parameters of the inverse hanging method adaptively so that the structural shape can satisfy architectural requirements. Comparisons with former shape‐generation methods show that the proposed method is more effective. Numerical examples based on real structures illustrate the applicability of the proposed method in practice. [ABSTRACT FROM AUTHOR]

Cross-linkable poly(arylene ether nitrile)s containing side propenyl groups (DPEN) were synthesized through nucleophilic aromatic substitution polymerization from biphenol, 2,2′-diallylbisphenol A (DBA), and 2,6-dichlorobenzonitrile. The structures of resultant copolymers were characterized by Fourier transform infrared and proton nuclear magnetic resonance spectroscopies. The properties of copolymers were investigated by thermal (differential scanning calorimetry and thermogravimetric analysis) analysis, inherent viscosity measurements, and mechanical tests. The results showed that allyl groups rearranged into propenyl groups during the polymerization. The copolymers with increasing content of DBA had high glass transition temperatures ranging from 207°C to 169°C and good thermal and thermo-oxidative stability with the 5 wt% loss temperatures in the range of 511–423°C under nitrogen and 471–402°C in air atmospheres, respectively. They also exhibited good mechanical property with tensile strength of 105–79 MPa. Meanwhile, the cross-linking reaction was studied through heat treatment. The result showed that the properties of DPEN containing side propenyl groups changed obviously after heat treatment, due to the fact that the double bond of PEN can be readily cross-linked to form net-shape polymers at high temperature. [ABSTRACT FROM AUTHOR]

An investigation of the seismic behaviour and a design method for single-layer reticulated shells with semi-rigid joints is described in this paper. A semi-rigid joint system has some bending and rotational rigidity that lying between that of a rigid-joint and a pin-joint. The seismic capacity of Kiewit reticulated shells with bolt-ball joints is analyzed in detail on the basis of the bending rigidity of the bolt-ball joint reported by others. The change rules of natural frequency and seismic internal force coefficient are reviewed by analyzing of large-scale shells with different span, depth-to-span ratio, bending rigidity, roof load, section size of members and seismic wave parameters. The seismic internal force coefficient (under the influence of bending moment) for shells with a semi-rigid joint system is summarized statistically to provide the maximal seismic response envelop. [ABSTRACT FROM AUTHOR]

A new control method is proposed to improve the seismic performance of double-layered Kiewitt Spherical reticulated shells. The approach taken is to replace selected bars in the shell with passive viscoelastic dampers; thus, the grid of the shell doesn't need to be changed. Computational models of the shell with the eight different damper topologies are developed using ANSYS®. The seismic response of the shell is simulated to determine the performance of the various damper topologies. Aparameter study is conducted to determine optimal values for the damping coefficient of dampers. Finally, the dynamic stability and failure modes of the shell are investigated through incremental dynamic analysis. If designed well, failure of the controlled shell is usually the result of a strength failure, rather than a dynamic instability. [ABSTRACT FROM AUTHOR]