STUDY ON SEISMIC RESPONSE OF LARGE-SPAN CANTILEVERED ROOFS CONSIDERING VIBRATION CHARACTERISTICS OF MAIN STRUCTURES

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  • 主架構の振動特性を考慮した大スパン片持ち屋根の地震応答に関する研究
  • シュカコウ ノ シンドウ トクセイ オ コウリョ シタ ダイ スパン カタモチ ヤネ ノ ジシン オウトウ ニ カンスル ケンキュウ

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Abstract

<p> During the 2011 off the Pacific coast of Tohoku Earthquake, many buildings suffered destruction of nonstructural components, especially fall of ceiling. Cantilevered roofs with ceiling have similar risk. The building standard law of japan determines the seismic vertical design loads as 1G, but the basis is not shown. Large span cantilevered roofs may severely vibrate not only horizontally but also vertically and the response characteristics are strongly affected by the main structure.</p><p> On the above backgrounds, firstly in this study, we attempted to identify the vibration characteristics of an actual cantilevered roof by performing a microtremor measurement. The building has a 14m×26m cantilevered roof which was built in Tokyo. The cantilevered roof has a suspended ceiling and the beam section is tapered toward the tip.</p><p> By analyzing the data, we found that cantilevered roof had two major vertical vibration modes, the vibration of cantilever itself and the vibration exited by the sway movement of main building. When these two modes resonate, vertical response acceleration of cantilevered roof might be very severe even if the building is subjected to only horizontal ground motion.</p><p> Secondly, seismic response of cantilevered roofs was evaluated using parametric analyses of simplified two-dimensional frame models. Vertical deformation of the cantilever appeared commonly in the first mode of building sway and the second mode of cantilever vibration. The natural periods ratio between the first mode and the second mode is defined as T2/T1. When T1 and T2 are close to each other, the sway deformation of main building and the vertical vibration of cantilever appear the same time. For understanding the resonance in seismic response between the cantilever and the main building, a study with time-history analysis is conducted. It was found that the peak vertical response can be estimated as a function of the natural period ratio of the roof to that of the main structures, as found on shell-roofs on main structures. Amplification characteristic by the effect of the main structure is analyzed. It is evaluated as a function of RT. RT is the natural period ratio of an equivalent SDOF of main structure Teq to the cantilever’s own natural period TR. Next, we investigated the effect of RM on the response amplification of cantilever around RT=1 by changing Meq. RM factor is a mass ratio of equivalent main structure Meq to the cantilever’s total mass MR. Where RM factor is large, the amplification of the cantilevered roofs increased.</p><p> Finally, the acceleration response of cantilevered roofs is analyzed using response spectrum analysis with SRSS method of the simplified the inverted L-shaped two-degree-of-freedom model (L2DOF model). The building is converted into the L2DOF model based on push-over analysis. The two masses represent the equivalent main structure and the cantilevered roof. By using the L2DOF model, the response of large-span cantilevered roof can be expressed very simply but the calculation was found close to the numerical analysis.</p>

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