Seismic Rehabilitation of Steel Frame Connections Through Asymmetrically Weakening the Beam

Mohammad Bahirai,Mohsen Gerami 한국강구조학회 2019 International Journal of Steel Structures Vol.19 No.4

Poor seismic performance of steel frame connections were proved after the Northridge earthquake (1994) which led to fundamental change in acceptance criteria of the steel frame connections. New design codes consider the plastic hinge to be located far from the column face to prevent brittle fractures at or near welds. In these cases there is a need to retrofi t the existing steel frame buildings which have been designed according to Pre-Northridge seismic codes. In current study, several experimental tests were conducted to investigate the infl uence of two retrofi t techniques on cyclic behavior of a Pre- Northridge connection. The techniques involved intentionally weakening the beam to trigger the plastic hinge to a predefi ned location in the beam. The retrofi t scenario was implemented through reduction and annealing the beam bottom fl ange. For the Pre-Northridge connection a tear damage mechanism was observed near the welded joint accompanied by a sudden drop in connection strength, while the retrofi tted specimens were able to relocate the plastic hinge away from the column face. In reduced specimens, top and bottom fl anges of the beam showed local buckling near the joint at the end of loading, however in annealing technique low stress demands in near weld region was resulted. It was concluded that area reduction of the beam would make the connection prone to out of plane buckling.

Post Tensioned Tendons for Seismic Retrofitting of Weak Bolted T-stub Connections

Vahid Saberi,Mohsen Gerami,Ali Kheyroddin 한국강구조학회 2017 International Journal of Steel Structures Vol.17 No.3

Current design philosophy for conventional steel moment resisting frames (MRFs) in high seismic regions is that the frames should not collapse for major earthquakes. However, significant structural damage and residual drift due to inelastic deformations in beams and columns may cause loss of building occupancy or operation after major earthquakes. On the other hand, Selecting the optimum technique for rehabilitation of existing structures with weak connections has been a challenge for engineers in the recent years. In this study, the authors proposed using post-tensioned tendons for rehabilitation of bolted Tstub connections with weak bolts or weak T-stub flange as well as a technique for changing pinned connections to moment connections. Six corner connection specimens are made and tested under SAC cyclic loading protocol. The results of this study show that this rehabilitation technique not only modifies the cyclic behavior of weak rigid connections and changes simple bolted connections into moment connections, but also it improves the behavior of the rehabilitated connections in a way that their behavior is more desirable than that of the reference rigid connection designed according to AISC. For example, flexural capacity and rotational stiffness of the retrofitted connections are higher than those of the reference connection by 18 and 26% in average respectively. Besides, rehabilitation using post tensioned tendons add self-centering ability to the retrofitted connection that has a major role in preventing permanent deformations in frame and thus, the possibility of using frictional dampers in self-centering frames for increasing the energy absorption capacity is provided. This rehabilitation technique in bolted connection with weak T-stub flange has better cyclic behavior compared to that of a connection with weak bolts, since the T-stub participates in energy absorption and higher energy absorption is reached.

The Effect of Irregularity of Lateral Stiffness in Estimating the Separation Gap of Adjacent Frames

Mostafa Khatami,Mohsen Gerami,Ali Kheyroddin,Navid Siahpolo 대한토목학회 2020 KSCE JOURNAL OF CIVIL ENGINEERING Vol.24 No.1

Structural pounding can lead to local or total damage to the stories at the collision level or to the overall collapse of the building. On the other hand, lateral stiffness irregularity is common in the form of soft or very soft stories, which is due to the alternation in the type of function of the first story of the building. This paper estimates the demand for the normalized separation gap (NSG) at adjacent buildings’ highest collision level that were a combination of regular and irregular frames. For this purpose, the steel moment resisting frames (MRF), compounds with a total of 700 adjacent states and their NSG, is calculated by the dynamic time history analysis. In addition, irregularity increment in lateral stiffness for the first story could lead to an increase in the NSG of 84% of the adjacent combinations. In this study, a new relationship is proposed to estimate the demand for the NSG with the consideration of the effects of irregularity of lateral stiffness in the lowest story.

The Effect of Skewness on Rotational Response of the Curved Bridge Deck under Near-Fault Motions

Sajad Heidari,Mohsen Gerami 대한토목학회 2019 KSCE JOURNAL OF CIVIL ENGINEERING Vol.23 No.11

Acquiring knowledge on the complex behaviors of bridges in an earthquake is of utmost importance for designing safe transportation systems. Earthquakes like Northridge 1994 showed that skewed angle in the bridge could lead to the failure of abutment and column because of the intensified demand for deck rotation around the vertical axis, especially in the near-fault zone. The present study attempted to examine how the skewed angle change affects the seismic response of curved and skewed bridges. To this end, a software model of two curved and skewed bridges verified through a field test was obtained. The results showed that the skewed angle change in far-fault zones has no significant effect on the seismic responses of these bridges, while in near-fault zones its effect is remarkable. In ramp bridges, the torsional force increases in the columns next to the abutment.

Experimental and Numerical Study of Pyramidal Steel Damper for Use in Frames with Diagonal Bracing

Hossein Labibi,Mohsen Gerami,Mahmood Hosseini 한국강구조학회 2022 International Journal of Steel Structures Vol.22 No.5

Convergent bracing system has long been considered by structural designers. However, the performance of this system during an earthquake has disadvantages such as high base shear and low energy absorption due to buckling of the braces. Thus, researchers have tried to improve the behavior of the structure by proposing the use of diff erent dampers in the lateral bearing section. Meanwhile, yielding dampers as low-cost dampers with easy manufacturing technology compared to visco-elastic and non-buckling dampers have always attracted the attention of researchers. However, the proposed designs have generally a one-level behavior and the yielding members cause instability of the frame in the case of the failure. Accordingly, in this research, a new type of steel yielding damper with multi-level performance has been introduced, in which the fl exural yield of the parallel trapezoidal plates has been used for the energy absorption process. Also, to ensure the stability of the braced frame in severe earthquakes, a simple support system has been included in its design. The damper has a pyramidal core that can be adjusted for stiff ness and functional levels based on the seismic requirements of the frame. To perform this research, while performing fi nite element modeling, the relevant specimens were made and subjected to cyclic loading experimental tests. Also, a comparative nonlinear time history analyses on a seven story CBF frame has been done. The results indicated the appropriate and stable cyclic behavior of the two-level pyramidal damper, tolerance of cumulative displacements of about 2000 mm, absorption of a signifi cant energy of 50 kJ and signifi cant improvement in time history responses. Finally, multi-line behavior curve and a capacity chart of the damper is presented.

Equivalent modal damping ratios for non-classically damped hybrid steel concrete buildings with transitional storey

Abbas Sivandi-Pour,Mohsen Gerami,Daryush Khodayarnezhad 국제구조공학회 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.3

Over the past years, hybrid building systems, consisting of reinforced concrete frames in bottom and steel frames in top are used as a cost-effective alternative to traditional structural steel or reinforced concrete constructions. Dynamic analysis of hybrid structures is usually a complex procedure due to variousdynamic characteristics of each part, i.e. stiffness, mass and especially damping. In hybrid structures, one or more transitional stories with composite sections are used for better transition of lateral and gravity forces. The effect of transitional storey has been considered in no one of the studies in the field of hybrid structures damping. In this study, a method has been proposed to determining the equivalent modal damping ratios for hybrid steel-concrete buildings with transitional storey. In the proposed method, hybrid buildings are considered to have three structural systems, reinforced concrete, composite steel and concrete (transitional storey) and steel system. In this method, hybrid buildings are substituted appropriately with 3-DOF system.

Equivalent modal damping ratios for non-classically damped hybrid steel concrete buildings with transitional storey

Sivandi-Pour, Abbas,Gerami, Mohsen,Khodayarnezhad, Daryush Techno-Press 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.3

Over the past years, hybrid building systems, consisting of reinforced concrete frames in bottom and steel frames in top are used as a cost-effective alternative to traditional structural steel or reinforced concrete constructions. Dynamic analysis of hybrid structures is usually a complex procedure due to various dynamic characteristics of each part, i.e. stiffness, mass and especially damping. In hybrid structures, one or more transitional stories with composite sections are used for better transition of lateral and gravity forces. The effect of transitional storey has been considered in no one of the studies in the field of hybrid structures damping. In this study, a method has been proposed to determining the equivalent modal damping ratios for hybrid steel-concrete buildings with transitional storey. In the proposed method, hybrid buildings are considered to have three structural systems, reinforced concrete, composite steel and concrete (transitional storey) and steel system. In this method, hybrid buildings are substituted appropriately with 3-DOF system.

Seismic Strengthening of Weak Bolted End Plate Connections Using Welded Haunches

Hamid Saberi,Ali Kheyroddin,Mohsen Gerami 한국강구조학회 2017 International Journal of Steel Structures Vol.17 No.2

Bolted steel connections have been frequently used after Northridge earthquake. Among the concerns regarding the poor performance of weak connections is the ability to effectively and economically rehabilitate steel moment connections in existing buildings. Strengthening of these connections without the need for changing or replacing their components is a problem that has been recently considered by engineers. This study develops and experimentally validates an innovative technique for enhancing the seismic performance of steel beam to column moment connections. The use of haunch as a way to rehabilitate end plate bolted connections with weak end plate or bolts is studied constructing 6 experimental specimens of corner connections under SAC (Structural Engineers Association of California) cyclic loading protocol. The results of the study show that this strengthening method improves the cyclic behavior of the weak connections. Besides, it provides specimens with better performance than that of the reference connection designed according to AISC. The moment capacity, initial rotational stiffness and energy dissipation of the rehabilitated connections are averagely higher than those of the reference connection by 25, 10 and 12 percent respectively. Moreover, the failure potential in the connection is reduced because the plastic hinge is kept away from connection region and is transferred to the haunch end. It should be noted that this strengthening method is more efficient for cyclic behavior of a bolted connections with weak end-plate than a bolted connection with weak bolts, since the moment capacity is more increased.

Comparison of Bolted End plate and T-Stub Connections Sensitivity to Bolt Diameter on Cyclic Behavior

Hamid Saberi,Ali Kheyroddin,Mohsen Gerami 한국강구조학회 2014 International Journal of Steel Structures Vol.14 No.3

Considering the construction limitations, proper and correct design of connections is vital and of great importance. At theconstruction stage, it is probable to use a connection bolt with a diameter less than the design assumptions. It can occur dueto an imperfection in construction or even to changes in the function of the building. The unforeseen changes can increase thestructural load, and consequently the moment and shear force demand values. Therefore, the present paper is aimed to examinethe sensitivity of the bolted connections to the diameter of bolts using a numerical method. The results show that bolted T-stubconnections are more sensitive to bolt diameter than end plate connections. Hence, the bolted end plate connections arerecommended where the imperfection in construction or changes in function of the building is probable. Moreover, inconstruction of bolted connections, if changing the number of bolts regarding the constant total cross sectional area isconsidered, it is recommended to use the symmetric arrangement of bolts on each beam flange. In design of connections fora particular frame according to AISC, the moment capacity of T-stub connection is higher than that of end plate connection. However, the total energy absorptions of these connection types are approximately equal.

Experimental Investigation on Ductile Failure in Heat Retrofitted Connections by Ultra-Low Cycle Fatigue Method

Mohammad Hossein Razmkhah,Mohsen Ghaderi,Mohsen Gerami 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.3

One of the seismic retrofit methods is applying heat to transfer the failure location from the near column into the beam. In this method, by heating a section of the beam, changes the mechanical characteristics of the beam, such as enhancing the ductility and decreasing the yield as well as the ultimate strength in that section, thus causing an altered place of failure as well as changes in the failure mode. In this study, via experimental and numerical methods an investigation was conducted on the ultra-low cycle fatigue capacity in retrofitted St-37 steel connection with three models of thermal improvement with the titles HBS1, HBS1&RIB, and HBS2 which differ in the cooling rate after the heating operation. The connection without retrofit failed in the cycle of 5% drift angle of SAC protocol loading. Retrofitted connections with HBS1 and HBS1&RIB thermal models failed in the cycle of 6% drift angle of SAC protocol loading. The results of modeling these connections in software and using the ultra-low cycle fatigue failure index also revealed a failure in a cycle with a 6% drift angle. The seismic capacity of the retrofitted connection with the HBS2 thermal model, with one percent increase in seismic capacity, failed in the cycle of 7% of the drift angle. The result of numerical modeling of this connection in the software was the same and indicated the ultra-cycle fatigue failure occurring in the cycle of 7% of the drift angle.