the effects of behaviour of a beam-to … a moment connection ... turkish code for design and...
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4th International Conference on Earthquake Engineering and Seismology
11-13 October 2017 – ANADOLU UNIVERSITY – Eskisehir/TURKEY
THE EFFECTS OF BEHAVIOUR OF A BEAM-TO-COLUMN CONNECTION
WITH LIMITED STIFFNESS AND STRENGTH ON SEISMIC BEHAVIOUR
OF A STEEL BUILDING
Adem Karasu1
and Cüneyt Vatansever2
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
Email: [email protected]
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 connections constituting the seismic force resisting system of the
structure. Of these members, beam to column connections can play a considerably important role even if they have
a capability of limited stiffness and flexural strength. Structural steel connections are mainly classified as a pinned
or a moment connection whose stiffness and strength do not exist or are required to define it. However, some
beam-to-column connections having limited stiffness and strength, which are called semi-rigid connections such
as header end plate connection, can be characterized by moment-rotation relationship. So the article presents the
contribution of moment-rotation behavior of typical bolted semi-rigid connections 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 connections. 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 four-
parameter power model is used for modelling of moment-rotation relationship (M-θ) of header end plate
connection. 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 inter-
story 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 connections of steel frames, there is increasing interest in bolted connections
(Çıtıpıtıoğlu & Haj-Ali, 2002). Bolted end plate steel connections consist of plate, bolts and welds. Because of the
large variety of connection configurations, stress concentrations, frictional and contact forces between components
these connections are classified semi-rigid connections (Baei, Ghassemieh, & Goudarzi, 2012). In designing a
steel framework, it is customary to represent the actual connection behavior by rigid moment connection or flexible
pinned connection. So, most of the design engineers assume that the behavior of the connections 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 connection behavior and cannot be reached in practical connections.
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.
4th International Conference on Earthquake Engineering and Seismology
11-13 October 2017 – ANADOLU UNIVERSITY – Eskisehir/TURKEY
Figure 1. Rotational deformation of connection
The primary distortion of the steel beam-to-column steel connections is their rotational deformation, θr, caused by
the in plane bending moment, M, (Fig.1)(Abdalla & Chen, 1995). Header plate connections 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 connections in the context of overall structural
behavior lies in their rotational stiffnesses. Actually, they are assumed to be pinned flexible connections 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 connections and to show the
effectiveness of these connections on limiting the inter-story drifts. Behavior of the header plate connections is
represented by moment-rotation curve.
Moment-rotation (M-θ) characteristics of bolted connections are indicative for the connection stiffness, strength
and ductility which reflect the connection 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 connection 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 connections. 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 connections 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 A0 is equal to 0.40. 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 HE320M. IPE 450 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 Fy=235 N/mm2
and minimum tensile strength Fu=360 N/mm2. Fundamental periods of the building are obtained as T1x=0.979s,
T1y=0.885s. To verify the 2D OpenSEES model, the period corresponding to the next mode in x-direction was also
.
𝜃𝑅
𝑀
4th International Conference on Earthquake Engineering and Seismology
11-13 October 2017 – ANADOLU UNIVERSITY – Eskisehir/TURKEY
acquired, T2x =0.268s. The total equivalent base shear Vt was computed as 603.47 kN and 653.69 kN for the x-
and y-direction, respectively. The analytical model is shown in Fig.3.
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 connection 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 M24-10.9 high-
strength bolts (yield stress = 900 MPa and ultimate stress = 1000 MPa).
IPE 450 IPE 450
C D E
IPE 450 IPE 450
IPE 450 IPE 450
IPE 450 IPE 450
HE320MHE320MHE320M
Semi-rigid
connection
momentconnection
6000 6000 6000SEMI-RIGID
rotSpring
IPE 450
IPE 450
IPE 450
HE320MHE320MHE320M
HE320MHE320MHE320M
HE320MHE320MHE320M
12
34
A B C D E
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
SB
MF IPE450 MF IPE450
MF
IP
E4
50
MF
IP
E4
50
MF
IP
E4
50
MF
IP
E4
50
SEMI-RIGID IPE450 SEMI-RIGID IPE450
pinned
connection
moment
connection
y
x
60
00
60
00
60
00
6000 6000 6000 6000
2000
4th International Conference on Earthquake Engineering and Seismology
11-13 October 2017 – ANADOLU UNIVERSITY – Eskisehir/TURKEY
Figure 4. Details of the header plate connections
The prediction of the beam-column connection 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 connections was applied emphasizing interaction between the bolts and end plate. Based on the
component method, initial stiffness and flexural resistance of the header plate connection were calculated as
18178.86 kNm/rad and as 56.45 kNm, respectively.
It is very important to establish a simple model for estimating the nonlinear moment rotation relationship (M-θ)
of the semi-rigid connections 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 connection
stiffness. This model is proposed by Richard and Abbott (Kishi, Komuro, & Chen, 2004). The connection moment
in this model is represented as follows:
ki kp r
kp rn 1/nr
0
(R -R )θM= +R θ
θ(1+( ) )
θ
(1)
where Rki = initial connection stiffness and M0=reference connection moment obtained based on the provision of
EC3, Rkp = strain hardening connection stiffness, θ0=reference relative rotation and it is equal to M0/( Rki- Rkp) and
n is shape factor. The shape factor n was assumed 1.7 for the header plate connection type. The connection moment
values are obtained according to the rotation values varying from 0.00 rad to 0.08 rad with the interval of 0.0001
rad. Fig. 5 shows the moment-rotation behavior of the header plate connection 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.
HE
320M
IPE 4507
51
50
75
125 62.562.5
15x250 - 300 IPE 450
15
HE320M
bolts
M24-10.9
4th International Conference on Earthquake Engineering and Seismology
11-13 October 2017 – ANADOLU UNIVERSITY – Eskisehir/TURKEY
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 Ao 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, Sae(T), in the period range between 0.2T1 and
2T1 with respect to dominant natural period, T1, 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, T1x=0.969 s and T2x=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 connections with header plates on the system behavior,
two analytical models have been developed; one having header plate connections’ model defined in Fig. 5 and the
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.080
10
20
30
40
50
60
70
80Moment Rotation Curve
rotation (rad)
Mom
ent
(kN
m)
4th International Conference on Earthquake Engineering and Seismology
11-13 October 2017 – ANADOLU UNIVERSITY – Eskisehir/TURKEY
other with the header plate connections 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 connections with the header plates are modeled with ‘rotSpring’ element, which is a
typical definition for a rotational spring. The behavior of pinned connections 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 connections 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-06 37.815 1.831 0.402
Imperial
Valley-06 38.955 1.064 0.375
Superstition
Hills-02 22.29 1.384 0.416
Northridge
-01 39.98 1.449 0.446
Imperial
Valley-06 28.35 1.558 0.577
Kocaeli 29.995 1.598 0.351
Chalfant
Valley-02 39.975 2.141 0.375
Figure 6. 2D OpenSEES model of the Moment Frame
IPE 450 IPE 450
SEMI-RIGID
rotSpring
A B C D E
IPE 450 IPE 450
IPE 450 IPE 450
IPE 450 IPE 450
HE320MHE320MHE320MHE320M
Semi-rigid
connection
moment
connection
z
x
30
00
30
00
30
00
30
00
6000 6000 6000 6000SEMI-RIGID
rotSpring
IPE 450
IPE 450
IPE 450
IPE 450
IPE 450
IPE 450
HE320MHE320MHE320MHE320M
HE320MHE320MHE320MHE320M
HE320MHE320MHE320MHE320M
HE320M
HE320M
HE320M
4th International Conference on Earthquake Engineering and Seismology
11-13 October 2017 – ANADOLU UNIVERSITY – Eskisehir/TURKEY
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 connections except flexible connections were modeled as a rigid connection.
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, inter-
story drift ratios obtained from the model including header plate connection 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 connections to the overall strength. As can be seen from the figure, taking the
actual behavior of the header plate connection into account provides an additional strength. Base shear increases
by 10% thanks to the actual behavior of the header plate connections. Furthermore, as the actual behavior of the
header plate connections provide additional plastifications they also help to increase the energy dissipation.
Figure 7. Comparison of inter-story drift ratio (avg.)
Figure 8. Pushover curve of the 2D models
0 0.004 0.008 0.012 0.0140
1
2
3
4
inter-story drift ratio
Sto
ry N
um
ber
pin assumption
actual behavior
0 100 200 300 400 500 600 7000
500
1000
1500
Displacement (mm)
Base s
hear
forc
e (
kN
)
Base shear force-Displacement curve
pin assumption
actual behavior
4th International Conference on Earthquake Engineering and Seismology
11-13 October 2017 – ANADOLU UNIVERSITY – Eskisehir/TURKEY
5. CONCLUSION
The goal of the paper is to present to show the effectiveness of the actual behavior of the header plate connections
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 connections defined by the moment-
rotation 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 connection behavior in the
analyses can also provide an economic solution for the structural systems.
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