ABSTRACT
General rules for the design of CFDST structures are presented herein, including rules for the limitation of parameters in current equations.
1.The structural design of CFDST must be on the basis of national construction principles and technical economy policy, the engineering condition, material supply, transport, setup and construction of members should be fully considered. The structural plan needs to be designed reasonably to confirm that it is safe, applicable, economical, resource conservative and environmentally friendly.
2.Ultimate strength design methods based on probability theory are adopted, the design resistance is calculated using the equations with partial factors.
3.The CFDST structures need to meet the ultimate limit state and the serviceability limit state requirements.22
a.The strength limit state should consider the basic combination of load effect while the accidental combination should also be considered if necessary. The bearing capability of members should be calculated with the design value of load. For structures with requirements of seismic resistance, seismic checking based on EN 1998-1: 2004 and other relevant standards are needed as well.
b.The design for serviceability limit state should consider the standard combination of load effects.
4.The seismic design of CFDST structures should meet the provisions of EN 1998-1: 2004.
5.The diameter of the outer tube should not be less than 200 mm, the thickness of the tube should not be less than 4 mm and the maximum aggregate size of the concrete should not exceed 1/3 of the clear distance between the inner and outer tubes. The ratio of the diameter of the outer tube to the wall thickness should not exceed 1.5 times the limit value of the hollow tube without concrete infill. This limit value without concrete infill is obtained by:
D o t o ≤ 135 ε 2 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0001.tif"/>
where ε = 235 / f y https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0002.tif"/>
The ratio of diameter to thickness of inner CHS should meet the provisions for hollow tubes under axial compression in EN 1993-1-1 (Table 5.2).
6.The hollow section ratio of CFDST should be kept between 0 and 0.75. The hollow section ratio should be calculated as follows:
ψ = D i D o − 2 t o https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0003.tif"/> 23
where:
ψ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0004.tif"/> is the hollow section ratio of CFDST
D i https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0005.tif"/> is the outside diameter of inner CHS in CFDST
D o https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0006.tif"/> is the outside diameter of outer CHS in CFDST
t o https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0007.tif"/> is the wall thickness of the outer steel tube in CFDST
7.When CFDST is used in seismically active areas, the nominal confinement factor of circular CFDST members should not be less than 0.6 or greater than 4.0. The nominal confinement factor should be calculated as follows:
ξ = A so ⋅ f y ,outer A ce ⋅ f ck = α n ⋅ f y ,outer f ck https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0008.tif"/>
where:
αnis the nominal steel ratio
A sois the cross-sectional area of outer steel hollow section in CFDST
A ceis the nominal cross-sectional area of concrete in CFDST, A ce = π false( D o − 2 t o false) 2 / 4 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429440410/40d3d6a6-013b-481b-b245-30c9637e5e1e/content/TNF-CH003_eqn_0009.tif"/>
f y ,outeris the yield strength of steel of the outer tube
f ckis the characteristic compressive strength of concrete
8.Prefabricated parts should be checked for construction safety. When checking the lifting for prefabricated parts, the dead load should be multiplied by an amplification factor of 1.5.
9.The separation between the concrete and steel tube should be measured. The provisions in Sections 3.9.3 and 3.9.4 should be used to confirm if the strength reduction caused by the circumferential gap or the circular-segment gap needs to be considered in design. If necessary, the calculation of the 24bearing capacity for CFDST members should take into consideration the effect of imperfection according to Section 3.9.
