51-34 The Civil Engineering Handbook, Second Edition
strength of the column caused by the confinement effect for certain ratios of steel-to-concrete strengths,
selected values for l, and certain ratios of e/d and d/t. For the calculation, the longitudinal reinforcement
is assumed to be 4%, with a yield strength of 500 N/mm
2
. It must be recognized that for higher slenderness
and larger eccentricities, the advantage of the confinement effect is very low. Similarly, for higher diam-
eter-to-thickness ratios of the circular hollow section, and smaller steel-to-concrete-strength ratios, the
confinement effect decreases. Therefore, in the calculation of axial strength for the column, a significant
increase in strength due to the confinement effect is obtained only when the values of l are less than 0.2
and the eccentricity ratios e/d are less than 0.05.
Local Buckling
Both Eqs. (51.43) and (51.45) are valid, provided that local buckling in the steel sections does not occur.
To prevent premature local buckling, the plate slenderness ratios of the steel section in compression must
satisfy the following limits:
• d/t £ 90e
2
for concrete-filled circular hollow sections.
• h/t £ 52e for concrete-filled rectangular hollow sections.
• b/t £ 44e for partially encased I sections.
where d = the outer diameter of the circular hollow section with thickness, t
h = the depth of the rectangular hollow section with thickness, t
b = the breadth of the I section with a flange thickness, t
f
e = ÷(235/f
y
)
f
y
= the yield strength of the steel section (N/mm
2
).
Table 51.7 shows the limit values for the plate slenderness ratio for steel sections in class 2, which have
limited rotation capacity. In such cases, plastic analysis, which considers moment redistribution due to
the formation of plastic hinges, is not allowed.
For fully encased steel sections, no verification for local buckling is necessary. However, the concrete
cover to the flange of a fully encased steel section should not be less than 40 mm or less than one sixth
of the breadth, b, of the flange. The cover to reinforcement should be in accordance with Clause 4.1.3.3
of EC2-1990.
Effective Elastic Flexural Stiffness
Elastic flexural stiffness of a composite column is required in order to define the elastic buckling load,
which is defined as
(51.51)
The term (EI)
e
is the effective elastic flexural stiffness of the composite column and l is the buckling
length of the column.
The buckling length may be determined using EC3 [31] by considering the end conditions due to the
restraining effects from the adjoining members.
Special consideration of the effective elastic flexural stiffness of the composite column is necessary, as
the flexural stiffness may decrease with time, due to creep and shrinkage of concrete. The design rules
for the evaluation of the effective elastic flexural stiffness of composite columns under short-term and
long-term loading are described in the following two sections.
TA BLE 51.7 Limiting Plate Slenderness Ratios to Avoid Local Buckling
Type of Cross Section
Nominal Steel Grade
Fe 360 Fe 430 Fe 510
Concrete-filled circular hollow section (d/t)907760
Concrete-filled rectangular hollow section (h/t)52 4842
Partly encased I section (b/t)444136
NEIl
cr
e
=
()
p
22