نشریه مهندسی عمران مدرس
سال سیزدهم شماره 4 (زمستان 1392)

Eccentrically braced frames (EBF) by covering the advantages of moment-resisting frames (MRF) and concentrically braced frames (CBF) have been used as seismic load resisting systems in buildings for more than two decades. In eccentrically braced frames (EBFs)، the link beams transmit bracing forces through themselves into the columns and other bracings and، in the end، create dominant forces in the bracings. Link beams، similar to ductile fuses، in addition to avoiding bracing buckling، attract earthquake energies. In EBF system، failure and yielding should happen in the link beams، and other members of the structure must remain in elastic behaviour. On the other hand، link beams prevent transmitting of more forces to the other members by yielding، therefore، these link beams are so important. Typically، the link beams، which are relied upon for energy dissipation through inelastic deformation، have had a wide-flange or I-shaped cross-section that requires lateral bracing to prevent lateral torsional buckling. This has limited the use of eccentrically braced frames in bridge piers and towers، as lateral bracing is difficult to provide in those situations. Tubular cross-sections of link beams have substantial torsional stability، making them less susceptible to lateral torsional buckling than I-shaped cross-sections in eccentrically braced frames، and may thus not require lateral bracing. Web of I-shaped cross-sections because of having clamped boundary conditions in its four sides than web of tubular cross-sections that have simply supported boundary conditions in its four sides، having advantage. Web buckling of link beams in eccentrically braced frames cause rapid strength and stiffness degradation، and this significantly impedes the energy dissipation capabilities of the system. Web stiffeners can be used to delay web buckling beyond a certain rotation level. Stiffener spacing of link beam is of important causes for the delay of web buckling. Stiffener spacing is based on the boundary condition of web that is between the flanges and the stiffeners. In this study، link beams with different tubular and I-shaped cross-sections in non-elastic limits are investigated and has been tried by changing link beams stiffeners array cause increasing the distances and decreasing the dimensions of them. For this purpose nonlinear finite element software and AISC-2005، loading protocol are used. The numerical result ofdifferent link beams models indicate that tubular and I-shaped cross-sections if having same geometric profile، I-shaped cross-sections have higher rotation capacity because ofbetter performance of their web، this increasing is approximatelly twice. So study of results indicate that new arraying of stiffeners of tubular link beam inceases the distances of them approximatelly 30% and decreasing the dimensions of them approximatelly 15% anddoesn’t have any bad effect on rotation angle capacity of them. An investigation of the effect of stiffener dimensions indicated that stiffeners dimensions if having minimum area and moment of inertia requirements don’t effect on rotation angle capacity.

Falling avalanche is one of incidents that may happen during cold months of winter in Iran. This event may lead to closure of some main roads of the country. Road closure is a challenging issue، particularly on roads connecting Tehran capital to the northern cities. Detecting avalanche prone locations on roads is usually conducted by data gathering، surveying and investigating aerial photos. Field investigation usually requires collecting regional data including slope stability، geology and number and repetition of road traffic. In this study importance of each parameter was determined By an AHP method. After interviewing experts، importance of each parameter was determined pair wise. Final weights for each parameter were also determined using Expert Choice software. Searching for important contributing parameters and methods for measuring risk of avalanche in mountainous roads in the literature، main parameters of falling avalanche were selected and used in this study. It is obvious that many of recognized parameters should be considered in every effort of measuring danger. That is why such parameters are similarly used in most methods. The most important reference for categorizing the methods of risk measuring was guidelines on preventing falling stones and avalanche published by US National Institute of Highways and the Main Office of Highways. In these guidelines، traffic repetition is taken as an important factor increasing the risk after a falling event happens. This study framed the experts’ opinions and optimized risk analysis with regards to local geotechnical and geologic conditions in Iran. Experts selected the snow depth، hillside slope، vegetation condition on hillside and number and repetition of the road traffic. Importance of parameters were calculated as 44. 1% for traffic number and repetition، 19. 1% for the hillside slope، 17. 2% for vegetation، 15. 6% for snow depth and 4% for traffic rate. These parameters were selected as the factors contributing in avalanche. In continue each parameter was rated in a four level category (very high، high، medium and low). Model of avalanche danger calculates the avalanche risk in every point in a road network and all roads can be evaluated in terms of the danger of potential avalanche. For example the model was applied on Karaj-Chalus highway connecting Alborz to Mazandaran provinces. This highway is an important arterial between the capital and the northern cities. Using the method، 15 dangerous avalanche prone locations were determined and prioritized. They are located at 65، 73 and 60 km from Karaj. Preventive efforts should be taken for maintaining these locations and keeping them safe against potential avalanches

The use of polymeric and flammable building materials has been considerably increased in construction industry of Iran in recent years. Most of these materials are highly flammable and can seriously contribute in growth of a probable fire in buildings. In this research، the fire behavior of some polymeric building materials used as finishes was evaluated with cone calorimeter method at 50 kW/m2. The thermal fire parameters and smoke toxicity produced from the burning specimens were assessed; including time to ignition، flaming duration، peak of heat release rate، total heat release، smoke production rate، average specific extinction area، carbon dioxide and carbon monoxide production. The correlation between the thermal parameters was discussed and it was concluded the total heat release had direct dependence on combustible nature of the material and the surface density and reverse dependence on time to ignition. Meanwhile، the fire toxicity hazard parameters including carbon monoxide production rates and smoke toxicity were evaluated. The smoke toxicity was expressed in terms of the two parameters: fractional effective dose (FED) and toxicity index (LC50). The obtained results showed that the most tested materials had a dangerous behavior in case of fire and can cause flashover in the room. They released considerable amount of heat with high peaks of heat release rate. Especially epoxy، MDF and polycarbonate samples showed a high potential for contribution in fire growth. In addition، the epoxy، PVC and polycarbonate produced the highest amounts of smoke and carbon monoxide. It should be noted that the production amount of carbon monoxide was measured in an open system in which air is circulated with a certain velocity. So in a real fire، these values ​​can be accumulated and increased in the room of fire. Among the tested samples، polycarbonate (PC) and PVC flooring (PVC-F) showed the highest value of FED، however it seems that the assessment of carbon monoxide versus time could provide more important information for assessment of smoke toxicity. The results showed that it is needed to regulate the fire behavior of building finishes and restrict their application in buildings. For this purpose، a fire risk classification was needed، which will be studied in further steps of the research.

In this paper the behavior of framee، the process of plastic hinge formation and energy absorption of frames with two spans and one floor with three types of slab including bubble deck slab، hollow core slab and reinforced slab under three earthquake accelerations have been analyzed and compared. The results show that bubble deck slab and hollow core slab as rigid as normal reinforced slab، although bubble deck slab has higher strength and stiffness compared to other slabs. Partnering slab in analysis make period of slab reduce more over bubble deck slab and hollow core to the comparison of reinforced slab، have more effect on period reduction. Ultimate displacement of frame with reinforced slab reach to failure mechanism is more than two mentioned case، however frame with bubble deck slab reach to failure mechanism under stronger earthquake acceleration and smaller displacement than reinforced slab. Comparison base shear of three discussed case shows that maximum base shear is in bubble deck slab and minimum base shear is in normal reinforced slab. Formation of plastic hinge in frame with bubble deck slab is similar with that in frame with hollow core slab with the difference that plastic hinge in former occurs later at the top end of the middle column and two ends of middle beams. In fact، formation of plastic hinges in this frame requires higher acceleration because of the higher amount of concrete and stiffness. In all samples، plastic hinge first occur in the frame and then yielding lines occur in the tensile region of the slabs. The failure mechanism of slab and steel frame occur at the same time in frame with hollow core slab and reinforced slab; however، this is not the case in the frame with bubble deck slab and even though with occurring of yielding lines، the slab does not fail. The stress distribution due to gravity loads is symmetric across all the slabs; however، the increase rate of stress is different. This difference is particularly notable in seismic behavior of slabs in a way that the formation of plastic hinge and yielding lines in hollow core slab، because of the holes، is totally different with that of in reinforced slab. In comparison with other slabs and due to the formation of plastic hinge، reinforced slab absorb lower energy. Columns، beams and connections play different role in energy dissipation. In all frame، the contribution of connections to dissipate energy is minor and this is because yielding does not occur in connections. Contrary to the frame with reinforced slabs، because of yielding in several places of columns، columns dissipate energy more than beams in the frames with hollow core slabs. It was concluded that hollow core slab and bubble deck slab have maximum and minimum contributions to the energy dissipation، respectively.

Dry pluviation، or the “raining of sand through air”، is a commonly employed technique used to prepare sand samples in the laboratory to some specified initial state. It is important، however، that the techniques are calibrated so that the density of the samples produced is known accurately and precisely. Several methods have been used to prepare sand samples for use in experimental studies. This paper presents the results of a study in which the characteristics of pluviated samples of Stockton Beach silica sand are determined as a function of deposition rate and free-fall height. The experimental arrangement used to generate the data for this study is a simple set with 500 mm of diameters mold. Sand was rained from the hopper، through holes ranging in size from 6mm to 15mm diameter، at spacing’s (S) ranging from 20 to 100mm، with holes arranged in a triangular pattern. In all cases، a single diffuser screen، with an aperture size of 2. 36mm was employed (except where the pluviation rate was so great that sand accumulated on the screen، at which point، the diffuser was removed). The fall heights (free fall distance traveled by the sand from the diffuser to the top of mould) considered were 61mm، 236mm، 481mm، 641mm. A 1 liter sampler mold was used to catch the sand. The dry pluviation technique is an effective method of preparing sand samples to a wide range of target densities. A comprehensive set of data has been obtained and reported to assist in the design of pluviation systems. Data is also provided on the consistency of the achieved densities، and how this varies as a function of the target density. In general، it is found that the higher densities achieved under slower rates of deposition are more repeatable. There is no unique relationship between shutter porosity and the deposition rate، but rather there is a trend for shutters with fewer، larger holes to achieve significantly greater deposition rate than shutters with a greater number of smaller holes، with the same porosity. Clearly، retardation of flow at the margins of the shutter holes is reduced in situations of fewer، larger holes، as the ratio of circumference to area is smaller in these cases. The results show that samples with relative densities from around 20% to 100% are readily achieved by simple combinations of hole size and hole spacing. In general، it is apparent that the dry pluviation techniques is more suited to the preparation of samples of greater density، with many combinations of hole size and hole spacing able to achieve relative densities between about 70 and 90%. Densities between 20 and 60% are achieved less readily، while densities below 20% are the least readily achieved. Previous observations that sand fall heights greater than 0. 5m have no affect on density are confirmed by the results of this study.

In recent years using steel plate shear wall system، because of its advantages in comparison with other earthquake resistant systems، has been a matter of attention. Some of its advantages relative to other systems include abundance advantage، high ductility، good hysteretic behavior and energy absorption capacity، high stiffness and economic advantages. Regarding that in Iran there is high seismicity risk and the need to strengthen old and unsafe urban textures and buildings، using this system as a lateral load resistant system seems appropriate and economical. In the present research strengthening of x-braced steel frames with steel plate shear walls is evaluated. Addition of bracing to unbraced frame spans، substituting braces with thin steel shear wall panels and adding thin steel shear wall panels to unbraced spans which do not have architectural requirements are considered as retrofitting strategies. The focus is on the methods in which retrofitting is only done by adding steel plate shear wall elements to braced frames. Some of these methods have many economic and practical advantages. Others are only proper for some special cases. In this study a number of x-braced steel frames designed by the first edition of Iranian code of practice for seismic resistant design of buildings (Iranian Standard No. 2800) are taken as basic frames which need to be retrofitted. These basic frames are retrofitted by adding steel panels with different methods. Then nonlinear static analysis (pushover analysis) with displacement control pattern has been done on both basic and retrofitted finite element frame models and the capacity curves (diagram of story displacements against base shear) of basic frames and retrofitted frames are compared. Considering the results of the pushover analysis of models in which seismic retrofitting is done by replacing x-bracing earthquake resistant system with steel plate shear walls and the results of other methods of strengthening، it is seen that seismic behavior of retrofitted frames is more desirable in terms of overstrength factor (Ω) and overall ductility of structure (). The failure and fracture mode in most of the medium-rise frames was ductile but in the short-rise frames the fracture was brittle. Thus، replacing the braces in short-rise structure with thin steel shear walls seems irrational and unjustified economically. But it is to be mentioned that strengthening and increasing the moment of inertia of the adjacent columns of steel shear wall panels in structures with brittle fracture mode could result the change from brittle to ductile fracture. The results of this research show that in the case of steel braced frames with regard to some scientific، technical and practical points; replacing concentric steel bracing earthquake resistant system by steel plate shear walls can be used as a suitable method for retrofitting a wide range of existing steel structures in Iran.

Water-storage capacity of reservoir reduces mainly due to sediment laden. Turbidity current has an important role on sediment transfer in reservoir. It is necessary to study sediment interaction and flow in order to predict mechanism of turbidity current. In this paper effects of changes in entrance hydraulic condition of turbidity current on head velocity، layer-average thickness، layer-average velocity، body velocity and turbulent structure have investigated experimentally. The front velocity of the head of turbidity current was determined by video recording and body velocity and turbulence parameters measured by Vecterino. When the initial Froude number decreases the maximum velocity increases in body and head. Positive shear Reynolds stress near bed indicates that major contributor in this region is sweep or ejection while major contributor near interface is inward interaction or outward interaction. Entrainment is dominated at interface. The investigation shows that head velocity depends on inlet Froude number and inlet Reynolds number. Variation of head velocity along channel is exponential. The maximum reduction of head velocity takes place at whereas variation of head velocity at is negligible. Driving forces at are inertial force and gravity force. Driving force decreases after hydraulic jump and only gravity force remains as driving force. Therefore head velocity is constant at. Head velocity increases when inlet Reynolds number increases. Body velocity increases when inlet Froude number decreases، as gravity force increases when inlet Froude number decreases. Effects of inlet Froude as number on body velocity is negligible at the end of channel. Negative value of body velocity at the interface of turbidity current and ambient fluid indicates entrainment phenomenon at this region. When inlet Froude number decreases، vertical component of velocity increases too،then maximum velocity approaches to the bed. Elevation of maximum velocity increases along the channel due to sedimentation of particles and decreases of vertical component of velocity. Body velocity decreases along the channel due to decrease of inertial force. Vertical Reynolds stress decreases when inlet Froude number decreases. Because of increase in particle turbulence dissipates and therefore vertical Reynolds stress decreases. Oscillation of vertical Reynolds stress is due to turbulence at this region. The maximum of vertical Reynolds stress tacks place near bed and interface of turbidity current and ambient fluid and minimum of vertical Reynolds stress tacks place near maximum velocity elevation. Shear Reynolds stress have two maximum values. One is near the bed and the other one is near the interface of turbidity current and ambient fluid. Maximum Reynolds shear stress is positive near bed and negative near interface. Minimum of Reynolds shear stress take place near maximum velocity elevation.

Study on the physics of sediment particle movement at grain scale is essential for better understanding sediment transport phenomenon and estimating the rate of sediment transport in rivers and marine environment. Sediment particles basically transport in two modes of bed and suspended load. Bed load takes place through sliding، rolling and saltation، from which the latter is dominant. Many parameters influence on saltation phenomenon، which their effects are not fully understood. These influencing parameters make the saltation a stochastic phenomenon. In the present article the influence of the affecting parameters on movement of sediment particles at saltation mode of transport under unidirectional steady flow are investigated. A numerical model is developed to simulate the particle motion in bed load saltation with considering the main contributor forces. Then the influencing parameters that effect on the jump length and average velocity of the particles are studied. Among them are the initial condition، the particle position between other particles and the shape of particles. The influence of the velocity profile on the jump length and average velocity of the particles are also studied. In summary، the change in the initial condition including the initial velocity and angle produces less than 10% variation on the particle jump length and velocity. On the other hand the position of the grain between the other particles is considerably influential with 40% change in the jump length and average velocity. The particle shape is most important parameter in term of the influence on the jump length and average velocity; there is a 50% difference between the jump length of spherical particles and flake-shape particles، for average velocity it is about 10%. The result of the study improves our understanding of particle motion at grain scale and ultimately results in the better estimation of sediment transport rate.