Use of MSC Nastran/Patran to Study Seismic Behavior of Building Steel Shear Walls

Transcription

1 REFERENCE Use of MSC Nastran/Patran to Study Seismic Behavior of Building Steel Shear Walls Abolhassan Astaneh-Asl, Ph.D.,P.E., Professor, University of California, Berkeley, USA Yongjiu Shi, Ph.D., Professor, Tsinghua University, Beijing, China.

2 What is a Steel Shear Wall? Steel shear walls studied here are steel plates welded to beams and columns in a building. Boundary Beam Steel Shear Wall Boundary Column The main role of a steel shear wall is to collect the lateral forces of earthquakes or winds in a building and transfer those forces, in shear, to the foundations and the ground. 27/04/2009 2

3 An Example of Steel Shear Wall Building The 30-story Kobe, Japan City Hall building was subjected to the 1995 Kobe earthquake (M=6.8) and had only very minor damage in one floor. The old lowrise City Hall lost top 3 floors Photo: M. Kanada and A. Astaneh-Asl 1995 Photo: A. Astaneh-Asl Photos Copyright ( ): A. Astaneh-Asl and M. Kanada 27/04/2009 3

4 Performance Criteria for Design of Structures: Structures are designed for Strength and Serviceability Structures should have enough strength to withstand maximum design forces applied to them. They also should have enough stiffness not to deform under normal service loads to the level that renders them not functional during their normal life. A copy of this report can be obtained from 27/04/2009 4

5 Design Procedures for Steel Shear Walls in the ANSI/AISC , A National Standard: Only strength criteria is considered in the current ANSI/AISc standard and no consideration is given to performance during service conditions such as service wind loads and during small and moderate earthquakes. The result is that high rises built using steel shear walls designed according to ANSI/AISC have relatively small stiffness and the shear walls in them can buckle under relatively small earthquakes and low winds under an inter-story drift value of L h 27/04/2009 5

6 Objective of Studies of Steel Shear Walls: To investigate validity of the current seismic design code provisions and recommend modifications. t Moment Shear h L In design of structures to resist seismic forces, provisions of the Seismic Provisions for Structural Steel Buildings, ANSI/AISC , A National Standard is used. 27/04/2009 6

7 The Basis of Seismic Design Procedures for Steel Shear Walls in the ANSI/AISC : Moment Shear In ANSI/AISC , the steel shear wall is assumed to resist only shear and no moment. The moment is assumed to be resisted entirely by the columns. This assumption is not realistic. t L h The wall plate is assumed to act as a series of diagonal tension bracing members. This assumption for very thin walls is OK but for actual walls used in buildings is unrealistic and does not represent actual behavior 1996 of the wall. V α Tension Field Forces Acting on the Boundary Elements V 27/04/2009 7

8 Design Procedures for Steel Shear Walls in the ANSI/AISC : -- Shear Strength of the Shear wall is given as: V n = (0.90)(0.42 Fy t w L cf sin2α ) where And; t w = thickness of the web L cf = clear distance between column flanges h = distance between beam centerlines A b = cross-sectional area of a beam A c = cross-sectional area of a column I c = moment of inertia of a column taken perpendicular to the direction of the web plate line L = distance between column centerlines --- The ratio of panel length to height, L/h, shall be limited to0.8 < L/h The columns should have a moment of inertia with respect to its axis perpendicular to the shear wall equal or greater than twh4/ Lcf 27/04/2009 8

9 Objective of Our Studies of Steel Shear Walls: To study the entire range of behavior of the steel shear wall in terms of variation of shear displacement versus applied shear force. In order to do such studies we used successfully the MSC.Nastran instead of conducting expensive and time consuming actual tests of physical specimens. Photo: A. Astaneh-Asl Photo: F. Samad 1996 One of the two test specimens of steel shear walls tested at UC-Berkeley, 1995 by Q. Zhao and A. Astaneh-Asl, before (left) and after testing (right) 27/04/2009 9

10 How Realistic is Simulation of Actual Tests of Steel Shear Walls Using MSC.Nastran? During the 2007 VPD conference, we presented the validation studies to show that MSC.Nastran can simulate the complex non-linear behavior of the actual test specimens of steel shear walls very realistically; including the very complex buckling behavior of the steel wall plates and columns. Specimen 1 Lubbel et al. Tests Specimen 2 Specimen 1 Specimen 2 Driver et al. Tests 27/04/

11 Simulating Behavior of Actual Steel Shear Walls Using MSC.Nastran Prediction of maximum strength of the shear walls using MSC.Nastran was also quite close considering the many inaccuracies introduced into the specimen during the production of material as well as construction and welding process. 27/04/

12 Parametric Studies of Steel Shear After we were satisfied with verification of test results and sufficient accuracy of the predictions of MSC.Nastran simulations, we embarked on parametric studies of steel shear walls. The study frame was a 9-story steel shear wall frame as shown below. The wall is the same as given in the Design Guide 20-Steel Plate Shear Walls, a publication of the American Institute of Steel Construction. Seismic or wind pushover forces Steel shear wall studied Close-up of one floor 27/04/

13 Parameters that were varied: Type of beam-to-column connections being either moment or simple connections. Beam Column Beam-tocolumn connection Study Frame 27/04/

14 Parameters that were varied: Cross section of the boundary columns were either W, T, or, I section to represent a variety in the column bending stiffness. All sections had the same area and same grade of steel. W T I 27/04/

15 Results Comparison of Buckling modes for all four cases indicated very negligible effect of bending stiffness of the columns. Columns: W Columns: T Columns: Columns: I 27/04/

16 Results Comparison of Von-Mises Stresses for all four cases indicated very negligible effect of bending stiffness of the columns. Columns: W Columns: T Columns: Columns: I 27/04/

17 Results Comparison of Von-Mises Stresses for all four cases indicated very negligible effect of bending stiffness of the columns. Columns: W Columns: T Columns: Columns: I 27/04/

18 Conclusions: MSC.Nastran was used successfully in this research and development project to simulate the complex yielding, material and geometric nonlinearities as well as buckling behavior of steel shear walls. Using the results of the analysis, we were able to establish that the current procedures in the ANSI/AISC Standard for seismic design of steel shear walls do not represent the actual behavior and needs revisions. MSC.Nastran can be a very useful tool not only in academic research, such as this one, but it can also be used very successfully in private sector design offices as well as government agencies for Performance- Based Design of structures subjected to natural and man-made extreme loads such as hurricanes, floods, storms, earthquakes, blasts, impact as well as gravity loads. 27/04/

19 Contact Details : For further information please contact Abolhassan Astaneh-Asl University of California, Berkeley 781 Davis Hall Department of Civil and Env. Engineering Univ. of California, Berkeley, CA USA Pone: (510) astaneh@ce.berkeley.edu 27/04/