4. RESEARCH RESULTS
4.2 CASE STUDY
The case study utilized in this research was an existing high rise building of 250m height (3B+G+60) with a major transfer floor at level 11. There were two encased composite columns from the foundation up to level 10, then those two columns have been changed to tapered CFST columns from level 10 to 11 in order to withstand a significant increase in the bi-axial bending moments at the interface with the transfer slab. Level 10 was MEP floor, so it was accepted by the architect to have tapered column geometry. The encased composite column was (1400 x 1400) mm with embedded heavy I steel Section of (1000 x 1000 x 100) mm. The concrete of the encased column was confined by a closed stirrup of T16 @ every 200mm. the vertical rebar used in the encased column was 40T40. The steel tube of the tapered CFST column was varying from (1400 x 1400 x 100) mm at the interface with the encased column to (2250 x 2250 x 100) mm at the top part embedded into the transfer slab.
The size of the CFST column is (2000x2000x100) at the interface with the Transfer slab which has been considered in the design of the column under gravity and bi-axially bending.
The concrete cylinder strength used in the composite columns was C70MPa. The depth of the transfer slab was 2.50m and it is supporting about 50 floors above the transfer level.
The steel grade used in this element was S355, and the rebar has been provided with grade 500MPa.
Fig. (4.1) demonstrates the elevation of the case study of the tapered CFST column
connected to the encased SRC column with variable cross sections along the column height.
Fig. (4.2) illustrates the 3D geometry of the case study (Design vs Construction)
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Fig. (4.1) Case Study of Tapered CFST Column connected to Encased Composite Column
Detail-A
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Fig. (4.1a) Detail-A Case Study, Enlarged Column Elevation
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Fig. (4.1b) Case Study, Encased Composite Column
Fig. (4.1c) Case Study, Connection between CFST Column and Encased Composite Column
Fig. (4.1d) Case Study, Concrete Filled Steel Tube (CFST) Column
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Fig. (4.2) Case Study, 3D geometry of the case study (Design vs Construction)
Design
Construction
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The composite column is subjected to axial compression loads and bi-axial bending moments at the top edge, while it is mainly subjected to Axial forces at the bottom edge with significant reduction in the bending moments compared to the top part.
The maximum bending moments on the Encased Composite Column is not exceeding 10%
from the maximum bending moments on the top edge of the CFST column.
The axial forces on the top edge of the CFST column is 116,000 kN with corresponding moments of 100,000 kN.m in (X) direction and 55,000 kN.m in (Y) direction.
The axial forces on the bottom edge of the Encased columns is 119,000 kN with
corresponding moments of 11,900 kN.m in (X) direction and 2,400 kN.m in (Y) direction.
It is noted that the bending moments in (X) direction at the bottom of the encased section is 11.9% of the bending moments at the top of the CFST column, while in (Y) direction, the bending moments at the bottom of the Encased section is about 4.4% of the bending moments at the top of the CFST section.
Table [4.1] summarize the factored straining actions along the column height to provide a clear understanding to the straining actions diagrams inline with the changing in the tapered column cross sectional size as well as changing the composite columns type.
The CFST column is classified as compact section since b/t = 20 < 54 (2.26√E/Fy).
The ultimate axial force is equal to 33% of the nominal compressive strength of the CFST column
(Pu / Øc Pn).
As illustrated in the Literature Review, the CFST provides larger capacities to the axial and flexure compare to the encased composite section, so it was an efficient solution to change the column section from encased section to CFST section for one level only rather than having CFST column in all levels from the Foundations until the Transfer Floor.
The challenge of this idea was to assemble the CFST column components and to provide a rigid and appropriate connection details to the transfer slabs and Encased Column section to ensure a smooth load path to the tapered CFST throughout Transfer Slabs and
subsequently to the below Encased Composite Column and Foundations.
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Factored Straining Action on the Top of the CFST Column
Axial Forces Pu (kN) 116,000
Bending Moments around (X) Direction Mx (kN.m) 100,000 Bending Moments around (Y) Direction My (kN.m) 55,000 Shearing Forces along (X) Direction Vx (kN) 15,900 Shearing Forces along (Y) Direction Vy (kN) 9,100
Torsional Moments Tu (kN.m) 475
Factored Straining Action on the Bottom of the CFST Column
Axial Forces Pu (kN) 117,000
Bending Moments around (X) Direction Mx (kN.m) 17,000 Bending Moments around (Y) Direction My (kN.m) 12,900 Shearing Forces along (X) Direction Vx (kN) 15,900 Shearing Forces along (Y) Direction Vy (kN) 9,100
Torsional Moments Tu (kN.m) 475
Factored Straining Action on the Top of Encased Composite Columns
Axial Forces Pu (kN) 119,000
Bending Moments around (X) Direction Mx (kN.m) 4,100 Bending Moments around (Y) Direction My (kN.m) 2,800 Shearing Forces along (X) Direction Vx (kN) 2,100 Shearing Forces along (Y) Direction Vy (kN) 300
Torsional Moments Tu (kN.m) 0.00
Factored Straining Action on the Bottom of Encased Composite Columns
Axial Forces Pu (kN) 119,000
Bending Moments around (X) Direction Mx (kN.m) 11,900 Bending Moments around (Y) Direction My (kN.m) 2,400 Shearing Forces along (X) Direction Vx (kN) 2,100 Shearing Forces along (Y) Direction Vy (kN) 300
Torsional Moments Tu (kN.m) 0.00
Table [4.1] Straining Actions Along Column Height
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4.3 SIMPLIFIED METHOD FOR THE CFST COLUMN CAPACITY OF THE CASE STUDY