- ترتیب محصولات: پیش فرض
A non-emulative moment connection for progressive collapse resistance in precast concrete building frames0 تومان
This paper documents the experimental development of a new spandrel-to-column moment connection detail for progressive collapse resistance in precast concrete building frames. This study focuses on a 10-story prototype precast concrete frame building with perimeter special moment frames (SMF) that are subjected to a groundfloor column removal. The experimental subassembly represents a spandrel-to-column connection on the perimeter SMF ear the middle of the building face (i.e. not at the corners). The connection is non-emulative and utilizes unbonded high-strength steel post-tensioning (PT) bars which pass through ducts in the column and are anchored to the spandrels via bearing plates. The proposed design strives for construction simplicity, avoids field welding and/or grouting, and maximizes ductility by allowing the high strength steel bars to act as structural “fuses” when yielding. A full-scale quasi-static pushdown test is performed on two variants of the proposed connection: one with higher moment-rotation capacity and limited ductility, and another with lower capacity and higher ductility. The results show that the connection can reliably achieve its design yield capacity, performs well under service level demands, and can achieve moderate-to-high ductility. The experimental results are then applied to a system-level computational model of the prototype building frame under a column removal scenario. The results of a nonlinear dynamic analysis demonstrate that the system can arrest progressive collapse in the event of a single column loss scenario when either variant of the proposed connection is considered.
An efficient framework for the inelastic performance assessment of structural systems subject to stochastic wind loads0 تومان
The modeling and estimation of the inelastic response of wind excited structures is attracting growing interest with the introduction of performance-based wind engineering. While frameworks based on direct integration have been widely adopted in earthquake engineering for estimating inelastic responses, the significantly longer duration of typical windstorms, as compared to seismic events, makes this approach extremely computationally challenging in the case of wind excited systems. This is especially true in the case of modern performance-based wind ngineering frameworks, which are based on probabilistic metrics estimated through simulation and therefore repeated evaluation of the system. This paper addresses this challenge through the development of a simulation framework based on dynamic shakedown theory. In particular, an efficient path-following algorithm is proposed for estimating not only the shakedown multipliers, but also the plastic strains and deformations associated with occurrence of the state of shakedown. The efficiency with which this information can be estimated for any given wind load time history enables the development of a simulation-based framework, driven by general stochastic wind load models, for the estimation of the system-level inelastic performance of the structure. The validity and practicality of the proposed framework is illustrated on a large-scale case study.
An experimental evaluation of the blast resistance of heterogeneous concrete-based composite bridge decks0 تومان
Because of the current geopolitical situation, research on improving the resistance of the civil and transport infrastructure to blast or impact loads has gained considerable attention. This paper presents the results of fullscale blast experiments designed to characterize the resistance of concrete-based composite bridge decks ubjected to close-in blast loading. Three composite decks with different degrees of heterogeneity were proposed and tested: a slab with basalt fiber meshes in multiple layers along the depth of the specimen, a slab with recycled textile sheets 100 mm in total thickness, and a typical hollow-core prestressed slab. The dependence of the extent of the blast damage on the material characteristics of the composite material was studied. A detailed study of the damage to the specimen caused by the close-in explosion found apparent delamination of all tested composite specimens. The heterogeneity of the layered composite material converts the blast damage due to internal rebounds into layer delamination.
Characterization of activated sludge settling properties with a sludge collapse-acceleration stage0 تومان
The sedimentability of the activated sludge can be affected by the presence of a large variety of coagulants and polymers from a previous physical-chemical process. In this paper, the activated sludge settling process in industrial wastewater treatment plants where the sludge does not settle in a conventional way is studied. The two observed constant hindered settling velocity stages and the instant the intermediate sludge acceleration period occurs are described. A variation of the Richardson and Zaki model is used to characterize the two stages of constant settling velocity. The concentration of suspended solids, where a sudden decrease of hindered settling velocity was observed, is calculated. Finally, a new hypothesis to explain the processes triggering the collapse of the initial homogeneous sludge structure and the existence of an acceleration period is formulated.
Comparison of in-plane and out-of-plane failure modes of masonry arch bridges using discontinuum analysis0 تومان
This research aims to provide a better understanding of the structural behavior of masonry arch bridges using advanced modeling strategies. Two main contributions are achieved in this article; first, triggering mechanisms for the out of plane failure of spandrel walls are established; second, the influence of soil backfill on the behavior and strength of the bridges is presented through a comprehensive parametric study. Here, masonry arch bridges are modeled using a discontinuum approach, composed of discrete blocks, including also a continuum mesh to replicate infill material, adopting a framework of discrete element modeling. The equations of motion for each block are solved by an explicit finite-difference method, using the commercial software 3DEC. The results of the preliminary analyses are ompared with analytical solutions and limit state analysis for validation purposes. Different arch bridge models, representing common geometrical properties in the northwest Iberian Peninsula are analyzed. Transverse effects, damage patterns and collapse mechanisms are discussed under different types of loading. The analysis demonstrated the severe capacity reduction due to spandrel wall failures and the importance of soil backfill in results, only possible by taking advantage of the performed numerical modeling strategy.
Earthquake responses of a base-isolated structure on a multi-layered soft soil foundation by using shaking table tests0 تومان
Soil-structure interaction (SSI) has a significant effect on the earthquake response of a base-isolated structure, particularly on the rotation response of the SSI system and the isolation performance of the isolation layer, as demonstrated by previous shaking table tests (Zhuang et al., 2014). On a softer soil foundation, the SSI should have a greater influence on the seismic response of an isolated structure. To this end, a new shaking table test is conducted to estimate the effect of SSI on the dynamic characteristics of a base-isolated structure on a multilayered soil foundation including a soft clay layer. As expected, the isolation efficiency of the isolation layer is reduced by the SSI effects, especially with increasing peak ground acceleration (PGA) of the input motion. Compared with the test results for an isolated structure on a harder soil foundation, the rotation responses of the pile cap and the isolation layer in this study are stronger. Additionally, the rotation responses of the pile cap are significantly amplified by the isolation layer. This type of amplification effect can become stronger with increasing PGA of the input motion, which differs from the results for previous tests with a base-isolated structure on a harder sand foundation. Meanwhile, when the natural isolation property of the softening soil layer is considered, the seismic responses of a base-isolated structure are reduced by the SSI effects because the natural isolation of the soft soil layer can compensate for the lost isolation ability of the isolation layer.
Effect of concrete creep on dynamic stability behavior of slender concretefilled steel tubular column0 تومان
An analytical procedure for dynamic stability of CFST column accounting for the creep of concrete core is proposed. The long-term effect of creep of concrete core is formulated based on the creep model by the ACI 209 committee and the age-adjusted effective modulus method (AEMM). The equations of boundary frequencies accounting for the effects of concrete creep are derived by the Bolotin’s theory and solved as a quadratic eigenvalue problem. The effectiveness of the proposed method and the characteristics of time-varying distribution of instability regions are numerically surveyed. It is shown that the CFST column becomes dynamically unstable even when the sum of the sustained static load and the amplitude of the dynamic excitation is much lower than the static instability load. It is also found that due to the time effects of concrete creep under the sustained static load, the same excitation, that cannot induce dynamic instability in the early stage of sustained loading, can induce the dynamic instability in a few days later. The critical amplitude and frequency of the dynamic excitation can ecrease by 6% and 3% in 5 days, and 11% and 6% in 100 days.
Experimental study on seismic performance of post-fire reinforced concrete frames0 تومان
To investigate the post-fire seismic behaviour of reinforced concrete (RC) frames with different column-to-beam bending capacity ratios, four specimens were fabricated, which included a strong-beam-weak-column frame and a strong-column-weak-beam frame either under room temperature or after being exposed to fire. The fire test was conducted in a furnace chamber, followed by quasi-static tests under a low-frequency cyclic load. The crack patterns, hysteretic loops, plastic hinges, and failure modes were investigated in the loading process. The influence of two factors, i.e., the fire exposure and the column-to-beam bending capacity ratio, on the mechanical performance, ductility, stiffness degradation, and energy dissipation was compared and analysed. The experimental results indicated that the ultimate bearing capacity, the stiffness, the ductility factor, and the energy dissipation capacity of the RC frames decreased after fire exposure. The bearing capacity of the strong-beamweak-column frame decreased even more seriously. Although the yielding displacements of the post-fire frames increased, their ultimate displacements decreased. In addition, the strong-column-weak-beam frame under room temperature failed in the form of beam-end plastic hinging, while after being exposed to fire, the failure mode changed to shear-bond failure in column.
Large-scale test as the basis of investigating the fire-resistance of underground RC substructures0 تومان
Fire disasters occurring in underground structures generally result in severe damage. In order to investigate the fire performance of typical underground structures, a large-scale fire test on a three-span frame structure was carried out. The tested model was loaded both by vertical and horizontal earth pressures. The applied temperature history, representing the time-dependent fire load, was determined by simulations of fire scenarios in the underground structure with the help of the software FDS. The paper contains test results of the temperature of the air in the furnace, the concrete and the steel bars in the structural model, and the state of deformation of the model. Apparent phenomena, such as cracking and spalling, were observed throughout the whole testing process. The results show that a very dangerous situation of the underground structure exposed to fire may occur during the cooling process. Curling of the top slab was observed during the heating process. More attention in fire-resistance design must be paid to the columns. The fire test results are useful for validation of numerical models and for further fire-resistance research of underground structures.
Perforated shear + reinforcement bar connectors in a timber-concrete composite solution. Analytical and numerical approach0 تومان
This paper presents a study of a novel shear connector in a timber-concrete composite solution, focussing on the determination of an analytical expression that makes it possible to predict its behaviour and a numerical analysis that describes it accurately. The shear connector is composed of a perforated steel plate inserted into a slot within the timber rib and glued, in combination with reinforcing corrugated steel bars affixed to the top of the plate. Previous tests made it possible to establish failure mode in different T composite section plate-rebar configurations. These results determine the effectiveness of the system in terms of force-slip behaviour, with systematic failure in the timber section. A simple predictive model is proposed to determine the ultimate capacity of the joint, taking into account the mechanical properties of timber in relation with the fracture plane and the timber-adhesive interface. This model makes it possible to apply a design process that is able to predict the stiffness of the connection. FEM models were analysed for each configuration in a variable load process equal to that used in the test, according to the standard procedure. A variable friction coefficient in contact definition made it possible to achieve an accurate descriptive model in association with the test procedure.
Punching strength of conventional slab-column specimens0 تومان
This paper presents an improved rational method for predicting the punching strength of conventional reinforced concrete slab-column specimens extending to the nominal line of contraflexure in a flat slab structure. The proposed method of analysis is for square and circular, isotropically reinforced, conventional slab-column specimens, concentrically loaded using square or circular columns. The method is based on an earlier two-phase approach, in which the punching strength was predicted as the lesser of the flexural punching strength and the shear punching strength. The earlier approach had previously been shown to be more reliable than other methods, including the major building code methods, and the proposed method represents a further significant improvement. The improvement in the proposed method is due to the incorporation of slab depth factors for both the flexural and shear modes of punching failure and refinements to the effects of concrete strength, reinforcement percentage and reinforcement yield strength, for the shear mode of punching failure. Comprehensive test data is presented for 217 tests on conventional slab-column specimens reported in the literature in the sixty year period 1956–2016. Analysis of these results by the proposed method resulted in ignificantly improved correlation over that of the authors’ previous two-phase approach. The method is also shown to be significantly more accurate and consistent than the current Eurocode 2 (2004) method, the ACI 318-14 (2014) method and the fib Model Code (2010) method for predicting the punching strength of conventional reinforced concrete slab-column specimens.
Reverse time migration of acoustic waves for imaging based defects detection for concrete and CFST structures0 تومان
Ultrasonic non-destructive testing (NDT) technology has been widely used for defect inspection of concrete structures in civil engineering. However, most of the current data processing methods can only provide qualitative information regarding the existence of concrete inner defects. In this study, an ultrasonic inner defects inspection approach with a high-resolution imaging method which combines travel time tomography (TTT) and reverse time migration (RTM) is proposed for concrete and concrete-filled steel tube (CFST) columns. TTT estimates a reasonable distribution of ultrasonic velocity over the cross-section of the concrete and CFST columns from the first arrival time of the ultrasonic transmission signal. The velocity distribution is used as an input of the initial model forRTM to image the defects inside the concrete and CFST column cross-sections with a high resolution. Numerical experiments demonstrate that the air cavity inside the concrete and CFST columns, and the debonding between the concrete core and the steel tube of the CFST column can be identified clearly, and that the location, size and shape of both defects can be determined accurately. It is concluded that the proposed defect detection approach with a high-resolution imaging method is efficient for the non-destructive inspection of concrete and .CFST structures using ultrasonic waves
آسفالت بتنی (معایب،محاسن،روش های اجرا و استفاده)0 تومان
با توجه به افزایش سطح تولید سیمان در کشور و روند رو به افزایش آن و توجه به اینکه امکان صادرات کامل محصول مازاد سیمان کشور به سهولت مسیر نیست لازم است در زمینه کاربردهای جدید سیمان در کشور بازاندیشی نماییم تا بتوانیم برای سیمان اضافی موجود در بازار در حال و آینده زمینه مصرف مناسبی فراهم آوریم. یکی از زمینه های کاربرد پرمصرف سیمان که در کشور ما از آن استفاده نشده است، کاربرد سیمان در جاده سازی و به عبارت دیگر احداث جاده های بتنی است. با توجه به عملکرد نامناسب آسفالت در بسیاری از جاده های کشور به نظر می رسد مطالعه و بررسی در این زمینه ضروری باشد.
آشنائی با معماری جهان0 تومان
این کلیسای زیارتی در Via Tolosa معروف قرار داشت، مسیری که از Arleo تا(saint sernin) Toulouoe با مقصد نهائی آن در Santiago de Compostela کشیده می شد. این جاده توسط زائرانی که از ایتالیا ، سوئد یا اروپای مرکزی می آمدند استفاده می شد ، همچون “Romieux” که بعد از روم ، به Compostela یا برعکس ، می رفتند. آنها بعد از عبور از آلپ در گذرگاه Montgenvere یااز جاده ای در طول ساحل Ligarian آمده بودند. این مسیر اگر چه به نام Via Aegidiana نیز شناخته می شد. نشانه ای از اهمیت معبد Seint Gilleo (Aegidiuo به لاتین)بوده است . در 1116 ميلادی ، برای ساخت سرداب در کلیسای تحتانی ، یک سری ساختمان سازی عمده آغاز شد. این قسمت با معبد Saint Gilleo ، مکان مهمی برای زائرین بود، اگر چه سر در که ساخت آن بعدها شروع شد، به علت تماشایی ترین مجموع کنده کاری قابل ملاحظه است. Carra Fergusan Omeara ادعا می کند که سنگینی در پیچیدگی ترکیب معماری و کنده کاری آن با دروازه های غربی کلیسای جامع Chartreo و صومعه ی Saint Denis برابری می کند … متاسفانه ، اکثر این تزیینات ، در طول جنگ های مذهبی قرن 16 ناقص شدند. شماری از سنگ تراشان و دستیاران آنها بدون شک این مجموع را بر سردر کنده کاری کرده اند، تا حدودی به این علت که سبک های آنها بسیار متفاوت است. stoddard این پروژه ، از اواخر دهه 1120 زمانبندی می کند، اگر چه دیگر دانشمندان تاریخ آن را از اواخر قرون 12 و اوایل قرن 13 قرار داده اند.
آشنايي با ساختمان و عملكرد نيمه هادي ديود و ترانزيستور0 تومان
نيمه هادي ها عناصري هستند كه از لحاظ هدايت ، ما بين هادي و عايق قرار دارند، و مدار آخر نيمه هاديها ، داراي 4 الكترون ميباشد. ژرمانيم و سيليكون دو عنصري هستند كه خاصيت نيمه هادي ها را دارا ميباشند و به دليل داشتن شرايط فيزيكي خوب ، براي ساخت نيمه هادي ديود ترانزيستور ، آي سي (IC ) و …. مورد استفاده قرار ميگيرد. ژرمانيم داراي عدد اتمي32 ميباشد . اين نيمه هادي ، در سال 1886 توسط ونيكلر كشف شد. اين نيمه هادي ، در سال 1810توسط گيلوساك و تنارد كشف شد. اتمهاي نيمه هادي ژرمانيم و سيليسيم به صورت يك بلور سه بعدي است كه با قرار گرفتن بلورها در كنار يكديگر ، شبكه كريستالي آنها پديد ميآيد . اتم هاي ژرمانيم و سيليسيم به دليل نداشتن چهار الكترون در مدار خارجي خود تمايل به دريافت الكترون دارد تا مدار خود را كامل نمايد. لذا بين اتم هاي نيمه هادي فوق ، پيوند اشتراكي برقرار ميشود. بر اثر انرژي گرمائي محيط اطراف نيمه هادي ، پيوند اشتراكي شكسته شده و الكترون آزاد ميگردد. الكترون فوق و ديگر الكترون هائي كه بر اثر انرژي گرمايي بوجود ميآيد در نيمه هادي وجود دارد و اين الكترون ها به هيچ اتميوابسته نيست. در مقابل حركت الكترون ها ، حركت ديگري به نام جريان در حفره ها كه داراي بار مثبت ميباشند، وجود دارد. اين حفره ها، بر اثر از دست دادن الكترون در پيوند بوجود ميآيد.