EXPERIMENTAL EVALUATION OF STRUCTURAL PERFORMANCE OF FOUNDATION BEAMS UNDER RC SHEAR WALLS WITH DOOR OPENINGS

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  • 縦開口を有するRC造連層耐震壁における基礎梁の構造性能の実験的評価
  • タテ カイコウ オ ユウスル RCズクリレンソウ タイシンヘキ ニ オケル キソリョウ ノ コウゾウ セイノウ ノ ジッケンテキ ヒョウカ

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Abstract

<p> 1. Introduction</p><p> The cross-sectional area and width of foundation beams under shear walls are required exceed the recommended Architectural Institute of Japan (AIJ) standard for reinforced concrete (RC) structures. However, the occurrence of brittle failure in the foundation beam under shear walls with door openings, similar to that in boundary beams between door openings on upper floors, is also a concern. Therefore, this study conducted static loading tests of beam specimens under shear walls with door openings to investigate their structural performance. This paper reports the experimental results upon the restoring force characteristics and cracking pattern in the beam specimens and proposes an ultimate strength evaluation method for foundation beams under a shear wall with door openings.</p><p> 2. Foundation beam specimens</p><p> The assumed foundation beams were based on the AIJ standard for RC structures. The two specimens consist of a foundation beam, two pile-caps, and two walls with boundary columns. Clevis pins were installed at the top and bottom of the specimens. The specimens were designed with the beam axial force ratio at maximum strength set to 0.13 with reference to the results of the Finite Element pre-analysis. The transverse reinforcement ratio pw of the beam was the variable investigated.</p><p> 3. Experimental results</p><p> Failure mechanisms were formed in the beam at the cycle of 0.5x10-2 rad due to tensile yielding of the main bars at the top and bottom of the beam in both specimens. However, the observed strength and deformation capacity in a specimen with pw of 0.22% were lower than those with pw of 0.66% due to occurrence of shear failure in the beam. That is, shear deformation of the beam from the opening on the tensile column side to the wall on the compression column side increased significantly in the specimen with pw of 0.22%. On the other hand, a flexural deformation of the beam increased in the specimen with pw of 0.66%. In addition, the compressive axial force of the beam decreased due to shear failure in the specimen with pw of 0.22%, but it increased until the final loading cycle in the specimen with pw of 0.66%.</p><p> 4. Ultimate strength evaluation</p><p> The ultimate bending moment of the beam was calculated using the stress block method by American Concrete Institute. The critical faces were assumed to be an end-of-the-pile cap on the compression column side and inside the opening on the tensile column side. A balance of the moment using the ultimate bending moments of the beam, vertical compressive force of the wall on the tensile column side, and tensile force of the vertical reinforcement in the wall on the compressive column side was considered. The shear strength of the beam was calculated using the Truss-Arch equation. The ratio of the observed strength in the specimen with pw of 0.22% and calculated shear strength in the beam was 1.13. On the other hand, the ratio of the observed strength and calculated bending strength in the specimen with pw of 0.66% in the beam was 1.01. This indicated that the ultimate strength evaluation methods for the beam specimens in this study showed good agreement with the observed strength of the foundation beams under the shear wall with a door opening. Furthermore, the above evaluation model of bending strength indicated that the critical face of the foundation beam on the tension column side is located 0.1 to 0.3 times inward of the beam from the opening end.</p>

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