STRUCTURE AND STEEL
Year:2010 | Volume:5 | Issue:6|Papers Count:9

Stiffness degradation and increase in vibration amplitude of cracked steel beams, cause to lose the efficiency of structure in service loads. Bonding CFRP laminates on tension flange is one of the rehabilitating methods of these beams. At present, finite element method is used to calculate the value of stiffness and strength of rehabilitated beams. In this paper, a closed form formulation is presented for calculating stiffness of rehabilitated beams. In this method, the stiffness of rehabilitated beam is calculating based on the stiffness of perfect beam, stiffness of cracked beam, CFRP and adhesive properties and dimensions of structure. Cracked section of beam and reinforcing plate are modeled by an equivalent rotational spring. A procedure is proposed for calculating the stiffness of rotational spring. Formulation is studied in two categories of definite and indefinite beams, and implemented in two special cases of fixed and clamped beams. In order to verify proposed formulation, eighteen hypothetical specimens have been modeled by finite element method. Comparison of proposed formulation and F. E. results shows good agreement. Also another formula is suggested for practical applications by simplification of previous formulas. Simplified and accurate formulas results match very well. Using simplified formula, it’s not necessary to know the depth of crack to determine the size of reinforcing CFRP plate.

Seismic design codes are prepared for prevent the life lost, damage control and economic design; to do this, generally combination of the strength and ductility are used. The main different between seismic design code and seismic strength is that the design is preformed in the elastic region while the strength is calculated based on plastic behavior, ductility, and the location of plastic hinges. Based on the research in this paper, the pseudo static qualifies for all responses such as displacement, drift and story shears. However, based on the 2800 standard the dynamic analysis is necessary for regular structures with height more than 50m, therefore it must be investigated that the structures designed using the modal dynamic analysis is able to resist earthquakes scaled to 0.35g. In this paper 15, 20 and 25 story 2D-moment resisting frames which are designed based on design spectrum of 2800 Iranian code are studied. The nonlinear acceleration time history analyses are preformed and the story displacements and drifts are extracted and are compared with code allowable amounts. The story ductility demands are also calculated and compared with the capability of story ductility. The local failure of the structural members is controlled. Finally, the damage level of the structures is investigated.

One of the methods for seismic retrofit of mid-rise buildings is the use of dual systems such as moment resisting frame accompanied with another lateral load carrying system such as steel bracing frame or reinforced concrete shear wall. The selection of shear wall or steel braced frame is one of the most important matters. Therefore, investigation of the behavior in these structures and knowing the effect of those two systems must be considered in coupling with moment resisting frame system. In this study, the effect of shear wall and steel braced frame in the behavior of structure and the dual system interaction is investigated by the analysis of steel moment resisting frame in a 20 story building using ETABS 2000 software. The analytical result show that shear wall have more lateral stiffness than X-braced system and more logical when considered from executive point of view. For controlling the lateral deformation of structures the analytical results show that the increase in cross sectional area of steel braces have a limited effect on the response, but increasing the number of braced bays is more effective.

Two kinds of Post-Northridge proposed beam-to-column connections are End palte and T-stub bolted connections. The aim of this paper is assessment and comparison of cyclic behavior of the mentioned connections due to horizontal variation of column bolts from centerline of beam web by finite element modeling of eight mentioned connections substructures in ANSYS, ver10. The results show that, if the mentioned connections are designed for equal sections of beam and column according to the AISC code, T-stub connection will have more moment resistance capacity and elastic stiffness than End plate connection while they have equal total energy dissipation approximately. Finally, analytical inspection on the mentioned parameter represented, attention to the behavior sensitivity and failure mode variation probability for bolted T-stub connection under cyclic load due to this partameter variation, despite T-stub connection has more moment resistance capacity, it is recommended to design bolted End plate connection rather than T-stub connection in cases that a weak construction is probable.

The beam-to-column connections in steel frames are assumed to be either pinned or rigid. Using pinned or rigid connections simplify the analysis, but they can not present the real behavior of the structure. Because, the experiments have shown that the connections under the loads will have some flexibility. Therefore, it is recommended that the flexibility of the connection should be included in the frame analysis. In this paper, the elastic secant stiffness matrix of a beam-column element with semi rigid connections under axial force, end moments and transverse distributed loads are derived. Second-order elastic and stability analysis of two-dimensional steel frames are carried out, afterward. The proposed stiffness matrix is very general and can be applied to a frame member with any type of connection. Furthermore, it is more accurate and efficient than the corresponding matrices obtained by other authors. In addition, this method uses only one element for each member and presents a single formulation for both tensile and compressive members in the analysis. Load-deflection curve and numerical values obtained by the suggested technique are compared with those found by other investigators. The effect of the connection flexibility on the equilibrium path and the stability load are also discussed.

The use of energy-based method is discussed in this paper, with taking into account the gravity effects, P-D effects, and other equations to prevent the formation of undesirable mechanisms. In this method the seismic design forces are derived from input energy for a selected plastic yield mechanism, and a selected target deformation level. The aim of this method is to study the ability of predicting structural behavior in the ultimate level and to use the maximum structure capacity in order to dissipate seismic input energy. For comparison between response of structures designed with equivalent static method and energy-based method, two 10 and 15-story steel moment frames are designed considering these two methods, for which the results from nonlinear analyses of the frames are compared. The obtained results indicate that the seismic design by energy-based method improves the seismic behavior of structures compared to the typically used equivalent static method.

Friction dampers have low cost and good performance. Therefore they are in a good position on passive control devices. There are performance indexes to optimize the slip load of friction dampers. Also investigators created a simplified method to determine the optimal slip load of Pall friction dampers. Rotational friction damper was introduced at 2000. Experimental and numerical tests were done on this damper. In this paper, steel frames with rotational friction dampers, and variable slip loads, are investigated, by 540 nonlinear dynamic time history analysis. These analyses were done on three frames with different heights and different accelerograms. Also slip load is obtained from the simplified method. These results are achieved, that the different performance indexes of damper, give a nearly response to determine the slip load. Performance of dampers is better by increasing the heights of structure. And the error of simplified method to determine the optimal slip load of rotational friction damper less than 2 percent, in comparison with exact method.