Comparative Study of Linear and Nonlinear Seismic Behavior of Non-Isolated, Base-Isolated and Top Floor (TMD-Type) Isolated Structural Frames

Abstract

In this study, the linear and nonlinear seismic behavior of a 2D ten story steel model frame is investigated in four cases: Without Isolation (WI), Base Isolation (BI), Roof Isolation (RI) and Two Top Floor Isolation (2TFI). Friction Pendulum System (FPS) is used for base and top floor isolations. First, the selected model without any isolation was used as a base for comparison of the seismic behavior of models. For dynamic analysis two types of base acceleration was used namely: 1-1979 El Centro earthquake, 2-A newly proposed base acceleration similar to displacement oscillation proposed in FEMA 461 code. In the two cases, the maximum equivalent standard acceleration at DBE level spectrum with duration of 8 seconds was used. For the WI frame Nonlinear Statical Analysis (Pushover or POA) was used by applying UBC-97 loading code. The concluding result shows that in linear and nonlinear ranges BI frame shows comparatively superior seismic behavior for MCE and DBE earthquake levels. Results of POA of WI frame according to UBC-97 code shows better behavior than that of dynamical analysis. RI frame in nonlinear range shows better behavior than WI frame using POA. Two Top Floor Isolation frame shows better behavior both in linear and nonlinear ranges compared to both RI and WI frames. The details of the behavior of all frames are shown in various diagrams.

Method of Pushunder Analysis (PUA) for Approximate Seismic Analysis of Structures Using Various Patterns of Stepwise Increasing Base Acceleration and Its Comparison with Pushover Analysis (POA)

“…Where, in the above equation, T i and M i are the modal period and the mass participation of the i-th mode, respectively, and M T is the total mass of the structure. In each tooth, acceleration raises to the pick of that tooth in 0.9T ave and reduces to zero in 0.1T ave as shown in Figure 3(b) [24]. For the model structures described in Table 2 of Section (3), T ave is found to be 0.755 sec.…”

“…The number of modes that is considered in this equation is such that the sum of the percentage of mass participation exceeds 90% of the total mass of the structure (M T ) as shown by Equation (4) [24].…”

“…One of the semi-dynamic nonlinear tools which is used for testing structural members under reciprocal loading is proposed by FEMA 461. It uses incrementally increasing cyclic displacement, as shown in Figure 5 [24]. This displacement control loading is often used for experimental verification of reciprocal strength and stress analysis of structural elements [24].…”

“…It uses incrementally increasing cyclic displacement, as shown in Figure 5 [24]. This displacement control loading is often used for experimental verification of reciprocal strength and stress analysis of structural elements [24]. A similar experimental diagram for displacement control for a structural member was proposed by Razani [25] in the past which uses increasing multiples of yielding displacement in Fibonacci steps and reciprocal patterns for testing reinforced concrete members and for presentation of test results.…”

“…In this paper an innovative fictitious base acceleration pattern similar to the FEMA 461 displacement cyclic pattern shown in Figure 6 is presented which instead of applying cyclic displacement to the element being tested; it applies an analogous, similar shape cyclic acceleration with specific amplitude to the base of the frame [24]. The reason for choosing this pattern of base acceleration is the similarity of the oscillation amplitude of displacement of Single Degree Freedom (SDF) mass-spring systems with acceleration of those systems during sinusoidal cyclic vibration.…”