Civil Engineering Reference
In-Depth Information
where
N
Ed,c
is the force that causes shear at the surface of the steel section
and
A
is the transformed area of the column in 'steel' units. There must be
a 'clearly defined load path . . . that does not involve an amount of slip at
this interface that would invalidate the assumptions made in design' (from
EN 1994-1-1).
There is no well-established method for calculating longitudinal shear
stress at the surface of the steel section,
τ
Ed
. Design is usually based on
mean values, found by dividing the force by the perimeter of the section,
u
a
, and an assumed 'load introduction length',
V
:
τ
Ed
=
N
Ed
,
c
/
u
a
V
(5.34)
V
should not exceed the least
of 2
b
c
, 2
h
c
(Fig. 5.10) and
L
/3, where
L
is the column length.
Design shear strengths
It is recommended in EN 1994-1-1 that
τ
Rd
due to bond and friction are given in EN
1994-1-1 for several situations. For completely encased sections,
τ
Rd
=
0.3 N/mm
2
(5.35)
This is a low value, to take account of the approximate nature of
τ
Ed
.
τ
Rd
, no account need be taken of the further
transfer of force by shear between steel and concrete as failure is
approached. The best protection against local failure is provided by the
transverse reinforcement (links) which are required by EN 1992-1-1 to be
more closely spaced near beam-column intersections than elsewhere.
In regions where
Where
τ
Ed
is less than
τ
Rd
is exceeded, shear connectors should be provided
for the whole of the shear. These are best attached to the web of a steel
H- or I-section, because their resistance is enhanced by the confinement
provided by the steel flanges. Design rules are given in EN 1994-1-1.
5.6.7
Concrete-filled steel tubes
A typical cross-section of a column of this type is shown in Fig. 5.10(c). To
avoid local buckling of the steel, slendernesses of the walls must satisfy
h
/
t
≤
52
ε
(5.36)
where
ε
=
(235/
f
y
)
0.5
and
f
y
is the yield strength in N/mm
2
units. For concrete-filled circular
hollow sections of diameter
d
the limit is more generous:
d
/
t
≤
90
ε
2
(5.37)
