THE EFFECTS OF BEHAVIOUR OF A BEAM-TO-COLUMN CONNECTION WITH LIMITED STIFFNESS AND STRENGTH ON SEISMIC BEHAVIOUR OF A STEEL BUILDING

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1 THE EFFECTS OF BEHAVIOUR OF A BEAM-TO-COLUMN CONNECTION WITH LIMITED STIFFNESS AND STRENGTH ON SEISMIC BEHAVIOUR OF A STEEL BUILDING Adem Karasu 1 and Cüneyt Vatansever 2 1 Res. Ass. Adem Karasu, Civil Engineering Department, Istanbul Technical University, Sarıyer 2 Assist. Prof. Dr. Cüneyt Vatansever, Civil Engineering Department, Istanbul Technical University, Sarıyer karasuad@itu.edu.tr ABSTRACT: In seismically active regions such as Turkey, nonlinear behavior of a structure is very important and is governed by the behavior of beams, columns and their s constituting the seismic force resisting system of the structure. Of these members, beam to column s can play a considerably important role even if they have a capability of limited stiffness and flexural strength. Structural steel s are mainly classified as a pinned or a moment whose stiffness and strength do not exist or are required to define it. However, some beam-to-column s having limited stiffness and strength, which are called semi-rigid s such as header end plate, can be characterized by moment-rotation relationship. So the article presents the contribution of moment-rotation behavior of typical bolted semi-rigid s on improvement of seismic behavior of the steel buildings. This also provides a good understanding of the nonlinear response of the steel buildings including semi-rigid beam to column s. In this study, a four story steel building was selected and designed under load combinations with seismic loads. All structural members are proportioned according to Turkish Code for Design and Construction of Steel Structures 2016 by using SAP 2000 software. The fourparameter power model is used for modelling of moment-rotation relationship (M-θ) of header end plate. Nonlinear time history analyses were performed in OpeenSEES framework. Analyses results have been evaluated in terms of inter-story drifts, additional strength and energy dissipation. It is observed that interstory drifts decreased, and an additional strength was provided together with the more energy dissipation. KEYWORDS: Header Plates, Semi-Rigid Connections, Nonlinear Time History Analysis 1. INTRODUCTION Due to brittle failure of welded s of steel frames, there is increasing interest in bolted s (Çıtıpıtıoğlu & Haj-Ali, 2002). Bolted end plate steel s consist of plate, bolts and welds. Because of the large variety of configurations, stress concentrations, frictional and contact forces between components these s are classified semi-rigid s (Baei, Ghassemieh, & Goudarzi, 2012). In designing a steel framework, it is customary to represent the actual behavior by rigid moment or flexible pinned. So, most of the design engineers assume that the behavior of the s is perfectly rigid or pinned. However, this theory leads to an inaccurate prediction of the frame behavior. Because perfectly rigidity and complete flexibility are idealized forms of behavior and cannot be reached in practical s. The AISC design code, referred to as the Load and Resistance Factor Design (LRFD) designates two types of constructions in its provisions which are fully restrained (FR) and partially restrained (PR)(Abdalla & Chen, 1995). Therefore, if partially restrained construction is used, the flexibility on the behavior must be taken into account in the design procedures.

2 θ R. M Figure 1. Rotational deformation of The primary distortion of the steel beam-to-column steel s is their rotational deformation, θ r, caused by the in plane bending moment, M, (Fig.1)(Abdalla & Chen, 1995). Header plate s consist of an end plate whose length is smaller than the beam depth and connectors. Flexible end plate (header plate) is welded to beam web and bolted to column flange. The importance of header plate s in the context of overall structural behavior lies in their rotational stiffnesses. Actually, they are assumed to be pinned flexible s during the design procedures although they have pronounced rotational stiffnesses. The goal of this paper is to investigate the nonlinear response of 4-story building including header plate beam-to-column s and to show the effectiveness of these s on limiting the inter-story drifts. Behavior of the header plate s is represented by moment-rotation curve. Moment-rotation (M-θ) characteristics of bolted s are indicative for the stiffness, strength and ductility which reflect the behavior (En, Avant-projet, En, & Modifications, 2010). In determining the behavior, geometric variables such as plate thickness, bolt diameter, bolt gauge, etc. are governing. In this study, the stiffness and strength is initially calculated accurately with the component method considering these variables. 2. DESIGN OF THE MODEL BUILDING Four-story steel building whose lateral force resisting system consists of high ductile moment frames (special moment frames) has been considered to show the effectiveness of the stiffness and the strength of the header plate beam-to-column s. Plan and 3D view of the building are shown in Fig.2 and Fig.3, respectively. Model building was designed for modified version of the example building which was included in Turkish Earthquake Code 2007 (TEC-07) Structural and Seismic Design Guide-Code Application Examples (Aydınoğlu, Celep, Özer, & Sucuoğlu, 2009). The building has four and three bays in x- and y- directions, respectively. Moment frames in axes 1 and 4 and A thru E constitute the lateral load carrying systems of the building in x- and y-directions, respectively. The bay length is 6 m for both directions and the height of each story is 3m. The columns are pinned for the weak axis and fixed for the strong axis to the foundation. The floor slab system consists of the secondary beams and reinforced concrete on steel deck which carries the gravity load to columns and acts as a diaphragm. Composite action neither between slab and beams nor between slab and steel deck is considered. At first, the building has been designed assuming the header plate s act as pins. In the design, TEC-07 and Turkish Code for Design and Construction of Steel Structures 2016 (TCDCSS-16) have been followed. The seismic region is I and corresponding effective ground acceleration coefficient A 0 is equal to Local site class is taken as Z2. Direct analysis method was used to obtain the required strengths including second order effects for the structural members. For this, 3D structural model of the building has been developed and all analyses have been performed by using SAP2000. In member sizing, strong column weak beam principle is applied scrupulously. Accordingly, the columns are chosen to be. and IPE 270 are used for the girders and secondary beams, respectively. All members were produced with S235 steel with a specified minimum yield stress F y=235 N/mm 2 and minimum tensile strength F u=360 N/mm 2. Fundamental periods of the building are obtained as T 1x=0.979s, T 1y=0.885s. To verify the 2D OpenSEES model, the period corresponding to the next mode in x-direction was also

3 acquired, T 2x =0.268s. The total equivalent base shear V t was computed as kn and kn for the x- and y-direction, respectively. The analytical model is shown in Fig.3. A B C D E Semi-rigid moment y SEMI-RIGID IPE SEMI-RIGID IPE450 pinned moment x Figure 2. Plan view of the structure Figure 3. 3D Analytical model (SAP 2000) Fig. 4 shows the dimensions and elements of the typical header plate with its details. Thickness of the end plate is 15 mm and it is welded to beam web with fillet welds and bolted to column with M highstrength bolts (yield stress = 900 MPa and ultimate stress = 1000 MPa).

4 15 bolts M HE 320M x Figure 4. Details of the header plate s The prediction of the beam-column behavior can be carried out by means of the component method which is specified in Eurocode 3 (EC3) (En et al., 2010). Provided that the elementary components governing the joint behavior are properly identified, the component method can be used. It is not easy to recognize that the prediction of the overall joint behavior involves the following eight components, such as column web panel in shear, column web in compression, column flange in bending, end plate in bending, bolts in tension, column web in tension, beam flange and web in compression, beam web in tension. While the first six components govern the flexural resistance and rotational resistance, the last two components have to be considered in the evaluation of the flexural resistance only (Ciro Faella, Vincenzo Piluso, 1999). In this study, prediction of T-stub failure modes on the end plate s was applied emphasizing interaction between the bolts and end plate. Based on the component method, initial stiffness and flexural resistance of the header plate were calculated as knm/rad and as knm, respectively. It is very important to establish a simple model for estimating the nonlinear moment rotation relationship (M-θ) of the semi-rigid s to rationally perform structural analysis. In the present study four parameter model is used to investigate the moment rotation relationship since the model reflects the strain hardening stiffness. This model is proposed by Richard and Abbott (Kishi, Komuro, & Chen, 2004). The moment in this model is represented as follows: (R -R )θ ki kp r M= +R kpθr θr n 1/n (1+( ) ) θ 0 (1) where R ki = initial stiffness and M 0=reference moment obtained based on the provision of EC3, R kp = strain hardening stiffness, θ 0=reference relative rotation and it is equal to M 0/( R ki- R kp) and n is shape factor. The shape factor n was assumed 1.7 for the header plate type. The moment values are obtained according to the rotation values varying from 0.00 rad to 0.08 rad with the interval of rad. Fig. 5 shows the moment-rotation behavior of the header plate in one direction. The curve is implemented so as to create a cyclic behavior in the analysis. For this, symmetric behavior for both negative and positive moments is adopted.

5 80 Moment Rotation Curve Moment (knm) rotation (rad) Figure 5. Moment-Rotation relationship 2.1. Selection and scaling of earthquake accelerograms Due to increasing interest in structural behavior under earthquake loads of the structure and developments in computational facilities, nonlinear time-history analysis is becoming more remarkable in seismic analysis of structures. However, selection of acceleration time histories to represent the design spectrum is critical issue. The best fitted ground motion time histories are selected and classified taking into account the earthquake magnitude, focal mechanism and site conditions (Fahjan, 2008). The criteria given in TEC-07 are considered during the selecting ground motion data ( Turkish Earthquake Code, 2007) ; The duration of the strong motion part shall neither be shorter than 5 times the fundamental period of the building nor 15 seconds. Mean spectral acceleration of generated ground motions for zero periods shall not be less than A o g and the mean spectral accelerations of artificially generated acceleration records for 5% damping ratio shall not be less than 90% of the elastic spectral accelerations, S ae(t), in the period range between 0.2T 1 and 2T 1 with respect to dominant natural period, T 1, of the building in the earthquake direction considered. The ground motions that meet the above requirements are tabulated in Table 1 for the local site class Z2. The damping ratio is taken as 5% for the target spectrum curve (Durgun, Vatansever, Girgin, & Orakdogen, 2013). 3. ANALYSIS ESSENTIALS For the nonlinear dynamic analysis of the moment frame that represents the one direction behavior of the 3D structural system, OpenSEES (Open System for Earthquake Engineering Simulation) software was used. For the representation of the building for x-direction, the frame in axis-1 has been considered. First, fundamental periods of the building, T 1x=0.969 s and T 2x=0.285 s, were obtained and compared with those from the 3D model. Based on the comparison, 2D OpenSEES model shown in Fig. 6 is found to be acceptable to represent the behavior of the 3D building model in x-direction only. In order to show the contribution of the beam-to-column s with header plates on the system behavior, two analytical models have been developed; one having header plate s model defined in Fig. 5 and the

6 other with the header plate s acting as pinned as assumed in practical engineering. In both models all beams and columns are modeled using nonlinearbeamcolumn element from the element library of OpenSEES software. Beam-to-column s with the header plates are modeled with rotspring element, which is a typical definition for a rotational spring. The behavior of pinned s to the columns is simulated using uniaxialmaterialelastic with very small stiffness and strength. However, uniaxialmaterial Hysteretic was used for modelling of the actual behavior of the beam-to-column s with header plates under the reverse dynamic loading (Vatansever & Yardimci, 2011) (Mazzoni, McKenna, Scott, & Fenves, 2007). In this modeling, the moment-rotation curve employed in the rotational spring is idealized with three linear lines defined by three points, except at the origin, for each direction. Table 1. Duration and scale factors of the records Earthquake Duration(s) Scaling factor(α) Peak acceleration(g) Imperial Valley Imperial Valley Superstition Hills Northridge Imperial Valley Kocaeli Chalfant Valley A B C D E z SEMI-RIGID rotspring x SEMI-RIGID rotspring Semi-rigid moment Figure 6. 2D OpenSEES model of the Moment Frame

7 The following assumptions were made for the nonlinear dynamic analysis; The moment frame in Axis 1 was considered for 2D OpenSEES model. Probable torsional effects induced by the rotation about global z-direction were ignored. The beam-to-column s except flexible s were modeled as a rigid. The supports were fixed to the foundation. Story masses consisted of full dead loads and thirty percent of the live loads. The buildings subjected to dead loads, live loads (G+0.3Q) and earthquake loads (E) during the nonlinear time history analysis. Rayleigh damping ratio was taken to be %2 in the analysis. 4. ANALYSIS RESULTS AND COMPARISON Two analytical models were analyzed under seven ground motion histories. Mean of seven ground motion analyses results is used for the performance evaluation of the models. Fig. 7 shows the inter-story drift ratios. In this figure, the ratios were computed as the mean of seven maximum values for each story. According to the figure, interstory drift ratios obtained from the model including header plate behavior were found to be lower than those from the other model. Pushover curves, shown in Fig. 8, were also obtained for each model to indicate the contribution of the header plate s to the overall strength. As can be seen from the figure, taking the actual behavior of the header plate into account provides an additional strength. Base shear increases by 10% thanks to the actual behavior of the header plate s. Furthermore, as the actual behavior of the header plate s provide additional plastifications they also help to increase the energy dissipation. 4 3 Story Number 2 1 pin assumption actual behavior inter-story drift ratio Figure 7. Comparison of inter-story drift ratio (avg.) 1500 Base shear force-displacement curve Base shear force (kn) pin assumption actual behavior Displacement (mm) Figure 8. Pushover curve of the 2D models

8 5. CONCLUSION The goal of the paper is to present to show the effectiveness of the actual behavior of the header plate s on limiting the inter-story drifts. For this purpose, nonlinear time history analyses were performed. Analyses results show that when the actual behavior of the header plate s defined by the momentrotation relationship is utilized inter-story drifts can be limited and an additional strength can be provided for the system together with an increase in energy dissipation. Finally, considering this behavior in the analyses can also provide an economic solution for the structural systems. REFERENCES Abdalla, K. M., & Chen, W. F. (1995). Expanded database of semi-rigid steel s. Computers and Structures, 56(4), Aydınoğlu, N., Celep, Z., Özer, E., & Sucuoğlu, H. (2009). Structural and Seismic Design Manuel-Code Aplication Examples for TEC Baei, M., Ghassemieh, M., & Goudarzi, A. (2012). Numerical Modelling of End-Plate Moment Connection Subjected to Bending and Axial Forces. The Journal of Mathematics and Computer Science, 4(3), Ciro Faella, Vincenzo Piluso, G. R. (1999). Structural Steel Semirigid Connections: Theory, Design, and Software. Boca Raton London Newyork Washington, D.C. Çıtıpıtıoğlu, a. M., & Haj-Ali, R. M. (2002). Detailed 3D Modeling and Simulation of Bolted Connections. ECAS2002 International Symposium on Structural and Earthquake Engineering, October 14, 2002, Durgun, Y., Vatansever, C., Girgin, K., & Orakdogen, E. (2013). The Seismic Performance Evaluation of An Eccentrically Braced Steel Frame by Non-Linear Analyses. Pamukkale University Journal of Engineering Sciences, 19(6), En, P. N. F., Avant-projet, K., En, N. F., & Modifications, C. A. (2010). European Standard. Management. Fahjan, Y. M. (2008). Selection and scaling of real earthquake accelerograms to fit the Turkish design spectra. Teknik Dergi/Technical Journal of Turkish Chamber of Civil Engineers, 19(SPEC. ISS.), Kishi, N., Komuro, M., & Chen, W. (2004). Four-parameter power model for Moment-rotation curves of endplate s. ECCS/AISC Workshop Connections in, (c), Retrieved from _ECCS_Technical_Committee_10/-_Steel_Structures_V/-_Table_of_Contents/doc/s2-5-kishi.pdf Mazzoni, S., McKenna, F., Scott, M. H., & Fenves, G. L. (2007). OpenSees command language manual. Pacific Earthquake Engineering Research (PEER) Center, 451. Retrieved from TEC (2007) Turkish Earthquake Code. (2007). Ministry of Public Works and Settlement Government of Turkey,Ankara, Turkish Code for Design and Construction of Steel Structures (2016). Vatansever, C., & Yardimci, N. (2011). Experimental investigation of thin steel plate shear walls with different infill-to-boundary frame s. Steel and Composite Structures, 11(3),