Advanced Materials Research Vols. 639-640

Abstract: Research on progressive collapse has advanced greatly in the past forty years. Motivated by heightened research interest around the globe stemming from recent high profile events, the rate of progress has been especially rapid in the past decade. Research in this area has been primarily enabled by massive improvements in computational simulation tools and hardware as well as structural testing at a scale and level of sophistication never seen before. While the research effort shows no signs of slowing down, several agencies have already undertaken large codification efforts in an attempt to synthesize the rapidly growing knowledge base into practical and meaningful guidelines for collapse-resistant design. This keynote paper presents the state-of-the-art in progressive collapse research. The paper sheds light on several topics including: methods for assessment of structural robustness; methodologies for enhancement of system collapse resistance; probabilistic models for progressive collapse risk assessment; and current trends and research needs, which discusses current gaps in our understanding of progressive collapse research and identifies research efforts needed to address them.

Abstract: According to Federal Highway Administration, impact by moving trucks is the 3rd leading cause of bridge failure or collapse in the country. Although current AASHTO LRFD Guide Specifications prescribe designing bridge piers by applying a 400 kips static load at a height of 4ft to improve their impact resistance, recent studies have shown that the dynamic forces because of truck impacts may be significantly higher than that recommended by the AASHTO Guide Specifications. In this paper, we present an extensive investigation on the impact of a three-span steel girder bridge with reinforced concrete piers by trucks running at different speeds through models of bridge and the truck in LS-DYNA, including a correlation between seismic and impact resistance of bridge piers. Results also present a comparison between static load prescribed by AASHTO Guide Specifications and dynamic impacts loads observed during numerical simulations. A performance based approach is proposed to design bridge piers against truck impacts.

Abstract: The authors review investigations which have provided fundamental knowledge on the use of a new generation of composite materials, namely textile-reinforced mortar (TRM) and textile-reinforced concrete (TRC), as strengthening and seismic retrofitting materials of existing concrete and masonry structures, as well as in the prefabrication of new reinforced concrete (RC) structural elements. In the first part of the paper, TRM are investigated as a means to provide confinement in concrete, to increase the deformation capacity of old-type RC columns subjected to simulated seismic loading, to increase the shear and flexural resistance of RC members and to increase the out-of-plane or in-plane strength of unreinforced masonry walls. In all cases, the effectiveness of TRM systems is quantified through comparison with equivalent fiber-reinforced polymer (FRP) ones. It is concluded that TRM jacketing is an extremely promising new technique, which is expected to enjoy the attention of the research community and to be employed in numerous applications in the near future. In the second part, the paper gives a brief overview of the application of TRC in the field of advanced prefabricated systems, with a focus on stay-in-place (or permanent) formwork elements in hybrid construction projects. Along these lines, the paper provides experimental results on the behavior of TRC/RC composite beams and one-way slabs under flexure. The results indicate that the use of prefabricated TRC stay-in-place formwork elements is a promising solution for achieving reduction of the construction time, minimization of labor cost and defect-free finishing of external surfaces.

Abstract: This paper summarizes the recent work by the first author’s research group related to the performance evaluation of existing bridges under vehicle dynamic effects. Based on the data from short-term monitoring of existing bridges, a framework to estimate the extreme structure responses from the live load in a mean recurrence interval is developed in the first part. The Gumbel distribution of the extreme values was derived from an extreme value theory and Monte Carlo Simulation. In the second part, a framework of fatigue damage and reliability assessment for existing bridges is presented to include the effects of the progressively deteriorated road conditions and random dynamic vehicle loads in bridge’s life cycle. The random effects of vehicle speed and type, road profiles, and stress ranges are included. Studies have shown that the vehicle-induced dynamic allowance IM value prescribed by the AASHTO LRFD code may be underestimated under poor road surface conditions (RSCs) of some existing bridges. In addition, multiple dynamic stress ranges induced by vehicles cannot be included in the maximum displacement-based dynamic allowance IM values. In the third part of this paper, the reliability indices of a selected group of prestressed concrete girder bridges are calculated by modeling the IM explicitly as a random variable for different RSCs. Nevertheless, a reliability based dynamic amplification factor on stress ranges (DAFS) for fatigue design is proposed to include the fatigue damages from multiple stress range cycles due to each vehicle passage at varied vehicle speeds under various road conditions in the bridge’s life cycle.

Abstract: Yielding can be emulated in a structural system by adding an adaptive “negative stiffness device” (NSD) and shifting the “yielding” away from the main structural system-leading to the new idea of “apparent weakening” that occurs ensuring structural stability at all displacement amplitudes. This is achieved through an adaptive negative stiffness system (ANSS), a combination of NSD and a viscous damper. By engaging the NSD at an appropriate displacement (apparent yield displacement that is well below the actual yield displacement of the structural system) the composite structure-device assembly behaves like a yielding structure. The combined NSD-structure system presented in this study has a re-centering mechanism thereby avoids permanent deformation in the composite structure-device assembly unless, the main structure itself yields. Essentially, a yielding-structure is “mimicked” without any, or with minimal permanent deformation or yielding in the main structure. As a result, the main structural system suffers less accelerations, less displacements and less base shear, while the ANSS “absorbs” them. This paper presents comprehensive details on development and study of the ANSS/NSD. Through numerical simulations, the effectiveness and the superior performance of the ANSS/NSD as compared to a structural system with supplemental passive dampers is presented. A companion paper presents the NSD and its mechanics in detail.

Abstract: Hybrid simulation is a testing method for examining the seismic response of structures using a hybrid model comprised of both physical and numerical substructures. Because of the unique feature of the method to combine physical testing with numerical simulations, it provides an opportunity to investigate the seismic response of structures in an efficient and economically feasible manner. It is this feature of the method which made it gain widespread use in recent years. This paper presents the theory of the method including an overview of the previous research related to various aspects of the method, an overview of two hybrid simulation applications, and the future directions for transforming the method to its next generation.

Abstract: Engineered wood is increasingly used in large structures in Europe, though little is known of its behavior in cold climate. This paper presents the structural health monitoring (SHM) system of a newly built suspension bridge with a deck of glulam timber as well as a bond stability study regarding cold climate performance of engineered wood. The bridge is located in Skellefteå in northern Sweden, and it connects two parts of the city situated on opposite shores of the Skellefteå river. In this ongoing study of the timber-bridge, a structural health monitoring system is employed to verify structural design and long-term performance. This 130m-span bridge is monitored using GNSS receivers, MEMS accelerometers, laser positioning systems, wireless moisture content sensors, strain gauges and weather stations. Data from the monitoring systems is analyzed regarding accuracy, complexity, costs and reliability for long time use. Engineered wood application in bridges, sports centers and timber buildings are discussed. Bond stability of glulam structures in cold climate is also examined in a range of experiments ranging from small glued wood joints to full size glulam bridge performance over time. From an engineered wood material point of view, the study is relevant to cold regions such as Scandinavia, Canada, Alaska, Russia, and the northern parts of China and Japan etc. The engineered wood constructions in these areas will be exposed to low temperature in a quite long period each year. The goal is to determine how engineered wood behaves when exposed to temperatures between 20 °C to -60 °C.

Abstract: Modern timber structure holds many virtues in the fields of construction, such as energy-saving, green, aesthetics, ect, superior to concrete and steel bridge. Meanwhile，it is the mechanical properties of timber structure that have been attracted much attention compared with other structures. It is not only glue laminated timber(glulam) but also tri-axial grids sandwich panels（TGSP）satisfy high load-bearing capacity and long-span requirements of modern timber bridge. As two essential parts of elements used as columns，decks in modern timber structure, glulam column had more full hysteretic curve with high energy-consuming ability and good seismic performance under reciprocating load was showed in this paper , and excellent compression property and bending property of TGSP were also studied . Finally, an engineering application was introduced. Filling the domestic gaps in bridge construction successfully, the first modern timber bridge in China has remarkable significant and greatly boosts the development of timber work.

Abstract: It is well known that one of the most significant sources of uncertainty and variability in seismic demand prediction arises from ground motion selection. The task of selecting appropriate ground motions can become a formidable given the limited database of earthquake records that satisfy the required site parameters. Moreover, from a practical consideration, it is necessary to limit the number of ground motions used in the evaluation process while at the same time minimizing the dispersion in the demand estimation. This presentation will examine the effectiveness of three ground-motion selection schemes: (i) magnitude scaling; (ii) spectrum matching; and (iii) conditional mean spectra. Findings from comprehensive nonlinear time history simulations indicate that while spectral matching is slightly more effective in reducing dispersion compared to scaling, it may modify ground motion content which can alter the location of peak demands. The conditional mean spectrum is less effective in reducing dispersion in demands when amplitude scaling is used