Current Issue : July - September Volume : 2014 Issue Number : 3 Articles : 6 Articles
Cracking of newly placed binary Portland cement-slag concrete adjacent to bridge deck expansion dam replacements has been\nobserved on several newly rehabilitated sections of bridge decks. This paper investigates the causes of cracking by assessing the\nconcrete mixtures specified for bridge deck rehabilitation projects, as well as reviewing the structural design of decks and the\nconstruction and curing methods implemented by the contractors. The work consists of (1) a comprehensive literature review of\nthe causes of cracking on bridge decks, (2) a review of previous bridge deck rehabilitation projects that experienced early-age\ncracking along with construction observations of active deck rehabilitation projects, and (3) an experimental evaluation of the two\nmost commonly used bridge deck concrete mixtures. Based on the literature review, the causes of concrete bridge deck cracking\ncan be classified into three categories: concrete material properties, construction practices, and structural design factors. The most\nlikely causes of the observed early-age cracking were found to be inadequate curing and failure to properly eliminate the risk\nof plastic shrinkage cracking. These results underscore the significance of proper moist curing methods for concrete bridge decks,\nincluding repair sections. This document also provides a blueprint for future researchers to investigate early-age cracking of concrete\nstructures....
Surface explosions resulting fromterrorist attacks will produce a hemispherical shock wave in the air,which, upon release, can affect\nfive faces of the building which is in front of it. Given the fact that conventional buildings are usually exposed to such explosions,\nthis study examined the effect of pressure to each face of a building on the responses of the structure and has compared different\nrelevant scenarios. This study, which includes the following two steps, was conducted as a case study on earthquake resistant RC\nbuildings, with the help of UFC guideline and using the software SAP2000. In the first step, responses of loading on each face\nwere separately calculated so that they were compared with the responses from overall loading on all faces. The sensitivity of the\nresponses and their ratio to the variables considered were evaluated in the second step. Accordingly, an outline was formed on the\nexplosion hazards considered for these types of buildings....
Active control devices, such as active mass dampers, are mainly employed for the reduction of wind-induced vibrations in highrise\nbuildings, with the final aim of satisfying vibration serviceability limit state requirements and of meeting appropriate comfort\ncriteria. When such active devices, normally operating under wind loads associated with short return periods, are subjected to\nseismic events, they can experience large amplitude vibrations and exceed stroke limits. This may lead to a reduced performance of\nthe control system that can even worsen the performance of the whole structure. In this paper, a nonlinear control strategy based\non a modified direct velocity feedback algorithm is proposed for handling stroke limits of an active mass driver (AMD) system.\nIn particular, a suitable nonlinear braking term proportional to the relative AMD velocity is included in the control law in order\nto slowdown the device in the proximity of the stroke limits. Experimental and numerical free vibration tests are carried out on a\nscaled-down five-story frame structure equipped with an AMD to demonstrate the effectiveness of the proposed control strategy....
New approach is presented for controlling the structural vibrations. The proposed active control method is based on structural\ndynamics theories in whichmultiactuators and sensors are utilized. Each actuator force is modeled as an equivalent viscous damper\nso that several lower vibration modes are damped critically. This subject is achieved by simple mathematical formulation. The\nproposed method does not depend on the type of dynamic load and it could be applied to control structures with multidegrees\nof freedom. For numerical verification of proposed method, several criterions such as maximum displacement, maximum kinetic\nenergy, maximum drift, and time history of controlled force and displacement are evaluated in two- , five- , and seven-story shear\nbuildings, subjected to the harmonic load, impact force, and the Elcentro base excitation. This study shows that the proposed\nmethod has suitable efficiency for reducing structural vibrations. Moreover, the uncertainty effect of different parameters is\ninvestigated here....
Fire response of concrete structural members is dependent on the thermal, mechanical, and deformation properties of concrete.\nThese properties vary significantly with temperature and also depend on the composition and characteristics of concrete batch\nmix as well as heating rate and other environmental conditions. In this chapter, the key characteristics of concrete are outlined.\nThe various properties that influence fire resistance performance, together with the role of these properties on fire resistance,\nare discussed.The variation of thermal, mechanical, deformation, and spalling properties with temperature for different types of\nconcrete are presented....
This paper presents the structural behaviour of precast lightweight foam concrete sandwich panel (PFLP) under flexure, studied\nexperimentally and theoretically. Four (4) full scale specimens with a double shear steel connector of 6mm diameter and steel\nreinforcement of 9mm diameter were cast and tested. The panel�s structural behavior was studied in the context of its ultimate\nflexure load, crack pattern, load-deflection profile, and efficiency of shear connectors. Results showed that the ultimate flexure\nload obtained from the experiment is influenced by the panel�s compressive strength and thickness. The crack pattern recorded in\neach panel showed the emergence of initial cracks at the midspan which later spread toward the left and right zones of the slab.\nThe theoretical ultimate load for fully composite and noncomposite panels was obtained from the classical equations. All panel\nspecimens were found to behave in a partially composite manner. Panels PLFP-3 and PLFP-4 with higher compressive strength and\ntotal thickness managed to obtain a higher degree of compositeness which is 30 and 32.6 percent, respectively....
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