Simplified design method for energy dissipating devices in retrofitting of seismically isolated bridges

Abstract: Highway bridges have great values in a country because in case of any natural disaster they may serve as lines to save people’s lives. Being vulnerable under significant seismic loads, different methods can be considered to design resistant highway bridges and rehabilitate the existing ones. In this study, base isolation has been considered as one efficient method in this regards which in some cases reduces significantly the seismic load effects on the structure. By reducing the ductility demand on the structure without a notable increase of strength, the structure is designed to remain elastic under seismic loads. The problem associated with the isolated bridges, especially with elastomeric bearings, can be their excessive displacements under service and seismic loads. This can defy the purpose of using elastomeric bearings for small to medium span typical bridges where expansion joints and clearances may result in significant increase of initial and maintenance cost. Thus, supplementing the structure with dampers with some stiffness can serve as a solution which in turn, however, may increase the structure base shear. The main objective of this thesis is to provide a simplified method for the evaluation of optimal parameters for dampers in isolated bridges. Firstly, performing a parametric study, some directions are given for the use of simple isolation devices such as elastomeric bearings to rehabilitate existing bridges with high importance. Parameters like geometry of the bridge, code provisions and the type of soil on which the structure is constructed have been introduced to a typical two span bridge. It is concluded that the stiffness of the substructure, soil type and special provisions in the code can determine the employment of base isolation for retrofitting of bridges. Secondly, based on the elastic response coefficient of isolated bridges, a simplified design method of dampers for seismically isolated regular highway bridges has been presented in this study. By setting objectives for reduction of displacement and base shear variation, the required stiffness and damping of a hysteretic damper can be determined. By modelling a typical two span bridge, numerical analyses have followed to verify the effectiveness of the method. The method has been used to identify equivalent linear parameters and subsequently, nonlinear parameters of hysteretic damper for various designated scenarios of displacement and base shear requirements. Comparison of the results of the nonlinear numerical model without damper and with damper has shown that the method is sufficiently accurate. Finally, an innovative and simple hysteretic steel damper was designed. Five specimens were fabricated from two steel grades and were tested accompanying a real scale elastomeric isolator in the structural laboratory of the Université de Sherbrooke. The test procedure was to characterize the specimens by cyclic displacement controlled tests and subsequently to test them by real-time dynamic substructuring (RTDS) method. The test results were then used to establish a numerical model of the system which went through nonlinear time history analyses under several earthquakes. The outcome of the experimental and numerical showed an acceptable conformity with the simplified method.

L'orientation et la propriété de mémoire de forme des polymères cristallins liquides à chaînes latérales covalents et supramoléculaires

In many studies of the side-chain liquid crystalline polymers (SCLCPs) bearing azobenzene mesogens as pendant groups, obtaining the orientation of azobenzene mesogens at a macroscopic scale as well as its control is important, because it impacts many properties related to the cooperative motion characteristic of liquid crystals and the trans-cis photoisomerization of the azobenzene molecules. Various means can be used to align the mesogens in the polymers, including rubbed surface, mechanical stretching or shearing, and electric or magnetic field. In the case of azobenzene-containing SCLCPs, another method consists in using linearly polarized light (LPL) to induce orientation of azobenzene mesogens perpendicular to the polarization direction of the excitation light, and such photoinduced orientation has been the subject of numerous studies. In the first study realized in this thesis (Chapter 1), we carried out the first systematic investigation on the interplay of the mechanically and optically induced orientation of azobenzene mesogens as well as the effect of thermal annealing in a SCLCP and a diblock copolymer comprising two SCLCPs bearing azobenzene and biphenyl mesogens, respectively. Using a supporting-film approach previously developed by our group, a given polymer film can be first stretched in either the nematic or smectic phase to yield orientation of azobenzene mesogens either parallel or perpendicular to the strain direction, then exposed to unpolarized UV light to erase the mechanically induced orientation upon the trans–cis isomerization, followed by linearly polarized visible light for photoinduced reorientation as a result of the cis–trans backisomerization, and finally heated to different LC phases for thermal annealing. Using infrared dichroism to monitor the change in orientation degree, the results of this study have unveiled complex and different orientational behavior and coupling effects for the homopolymer of poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate} (PAzMA) and the diblock copolymer of PAzMA-block- poly{6-[4-(4-cyanophenyl) phenoxy]hexyl methacrylate} (PAzMA-PBiPh). Most notably for the homopolymer, the stretching-induced orientation exerts no memory effect on the photoinduced reorientation, the direction of which is determined by the polarization of the visible light regardless of the mechanically induced orientation direction in the stretched film. Moreover, subsequent thermal annealing in the nematic phase leads to parallel orientation independently of the initial mechanically or photoinduced orientation direction. By contrast, the diblock copolymer displays a strong orientation memory effect. Regardless of the condition used, either for photoinduced reorientation or thermal annealing in the liquid crystalline phase, only the initial stretching-induced perpendicular orientation of azobenzene mesogens can be recovered. The reported findings provide new insight into the different orientation mechanisms, and help understand the important issue of orientation induction and control in azobenzene-containing SCLCPs. The second study presented in this thesis (Chapter 2) deals with supramolecular side-chain liquid crystalline polymers (S-SCLCPs), in which side-group mesogens are linked to the chain backbone through non-covalent interactions such as hydrogen bonding. Little is known about the mechanically induced orientation of mesogens in S-SCLCPs. In contrast to covalent SCLCPs, free-standing, solution-cast thin films of a S-SCLCP, built up with 4-(4’-heptylphenyl) azophenol (7PAP) H-bonded to poly(4-vinyl pyridine) (P4VP), display excellent stretchability. Taking advantage of this finding, we investigated the stretching-induced orientation and the viscoelastic behavior of this S-SCLCP, and the results revealed major differences between supramolecular and covalent SCLCPs. For covalent SCLCPs, the strong coupling between chain backbone and side-group mesogens means that the two constituents can mutually influence each other; the lack of chain entanglements is a manifestation of this coupling effect, which accounts for the difficulty in obtaining freestanding and mechanically stretchable films. Upon elongation of a covalent SCLCP film cast on a supporting film, the mechanical force acts on the coupled polymer backbone and mesogenic side groups, and the latter orients cooperatively and efficiently (high orientation degree), which, in turn, imposes an anisotropic conformation of the chain backbone (low orientation degree). In the case of the S-SCLCP of P4VP-7PAP, the coupling between the side-group mesogens and the chain backbone is much weakened owing to the dynamic dissociation/association of the H-bonds linking the two constituents. The consequence of this decoupling is readily observable from the viscoelastic behavior. The average molecular weight between entanglements is basically unchanged in both the smectic and isotropic phase, and is similar to non-liquid crystalline samples. As a result, the S-SCLCP can easily form freestanding and stretchable films. Furthermore, the stretching induced orientation behavior of P4VP-7PAP is totally different. Stretching in the smectic phase results in a very low degree of orientation of the side-group mesogens even at a large strain (500%), while the orientation of the main chain backbone develops steadily with increasing the strain, much the same way as amorphous polymers. The results imply that upon stretching, the mechanical force is mostly coupled to the polymer backbone and leads to its orientation, while the main chain orientation exerts little effect on orienting the H-bonded mesogenic side groups. This surprising finding is explained by the likelihood that during stretching in the smectic phase (at relatively higher temperatures) the dynamic dissociation of the H-bonds allow the side-group mesogens to be decoupled from the chain backbone and relax quickly. In the third project (Chapter 3), we investigated the shape memory properties of a S-SCLCP prepared by tethering two azobenzene mesogens, namely, 7PAP and 4-(4'-ethoxyphenyl) azophenol (2OPAP), to P4VP through H-bonding. The results revealed that, despite the dynamic nature of the linking H-bonds, the supramolecular SCLCP behaves similarly to covalent SCLCP by exhibiting a two-stage thermally triggered shape recovery process governed by both the glass transition and the LC-isotropic phase transition. The ability for the supramolecular SCLCP to store part of the strain energy above T[subscript g] in the LC phase enables the triple-shape memory property. Moreover, thanks to the azobenzene mesogens used, which can undergo trans-cis photoisomerization, exposure the supramolecular SCLCP to UV light can also trigger the shape recovery process, thus enabling the remote activation and the spatiotemporal control of the shape memory. By measuring the generated contractile force and its removal upon turning on and off the UV light, respectively, on an elongated film under constant strain, it seems that the optically triggered shape recovery stems from a combination of a photothermal effect and an effect of photoplasticization or of an order-disorder phase transition resulting from the trans-cis photoisomerization of azobenzene mesogens.