Dynamic response characteristics of steel portal frames having semi-rigid joints under sinusoidal wave excitation
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To demonstrate the characteristics of the nonlinear response of steel frames, an elastic dynamic response analysis of the semi-rigid frame is performed under the harmonic wave. The semi-rigid contact is represented by the alternating spring which is given stiffness by a three-parameter energy model which approaches the hysterical curve by hardening model. The properties of spectra and hysteric curves are presented. This study shows that (1) the greater the acceleration input capacitance the smaller the instant connection capability and the smaller is the response. (2) However, by allowing an extreme increase in capacitance input acceleration, response spectra can be increased as the contact stiffness results near zero.
KeywordsSteel structure Steel connection Steel frame Dynamic response analysis LS-DYNA
In the traditional analysis of the steel frame, the links between the beam and the column are assumed to be either perfectly solid or perfectly hinged.
In traditional analysis of steel frame, beam and column links are supposed to be either perfectly rigid or pinned. The real links which are specially bolted behave intermediate having nonlinear stiffness. Those connections are called semi-rigid connections. To design semi-rigid for limit state design method such as (AISC) specification (1993) for Load and Resistance Factor Design (LRFD), a prediction model on M − θ r curve for each type of connection, a nonlinear analysis method of column stability must be developed. Many researchers have been addressing these issues. The simple design procedure following AISC-LRFD specification for fully rigid frame has been proposed. However, this was based on static analyses (Durucan and Dicleli 2010; Varum et al. 2013).
A more detailed method for steel frame should be considered for dyanmic loads for earthquake load earthquake load that incorporates the seismic design concept. In recent years, the dynamic properties were analyzed by Barsan and Chiorean (1999), Bhatti (2013, 2016a, b), and Vimonsatit et al. (2012) for semi-rigid connections. However, more information on dynamic response characteristics of semi-rigid frames is required for the establishment of a seismic design procedure.
Assumptions of numerical analysis
List of parameters for power model
Connection stiffness ρ*
Moment capacity M u
Shape factor n
0.8 M p
0.60 M p
0.3 M p
Numerical results and discussions
Relationship between input acceleration amplitude and hysteretic damping effect
Natural frequency of portal frame
Natural frequency (Hz)
Figure 6 also reveals that the resonance frequency is reduced by increasing α i and by reducing M u. This reduction is attributed to the decrease in tangent connection stiffness at maximum response amplitude of displacement and the increase in hysteretic damping factor.
Behavior of semi-rigid portal frame under large amplitude excitation
To investigate the dynamic response behavior of the frame subjected to a severe earthquake, the dynamic response analyses in case of M u = 0.3 M p are performed, varying input acceleration amplitude α i from 30 to 800 gal. Figure 6 reveals that with increasing α i , the resonance displacement δ/δ st has a tendency to decrease in the region of α i ≤ 320 gal and to increase in the region of α i > 320 gal. This suggests that hysteretic damping effect of connection is limited and may not be expected under the excitation with excessive input acceleration amplitude. In contrast, the resonance frequency of the frame decreases in increment of α i . If α i = 800 gal, then frequency f i /f 0 drops to 0.55. Moreover, the distribution characteristics of response spectra are very similar to those of SDOF model, if it is assumed that the restoring force has perfect elasto-plastic relationship.
The resonance frequency of the frame must be reduced by increase in the input acceleration amplitude.
The response spectra decrease due to hysteretic damping effect of connections.
Higher acceleration magnitude results in lower ultimate moment capacity of connection and dynamic response is low.
However, by increasing acceleration, the spectra have been increased because of zero stiffness of connections.
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