Civil Engineering Reference
In-Depth Information
in Fig. 3.4(b). A new shear force diagram is constructed for a span with two
point loads only, and the same two end reactions, such that the areas of
the positive and negative parts of the diagram equal those of the original
diagram. This is shown in Fig. 3.4, in which each shaded area is 3
wL
2
/8.
The positions of the point loads define the lengths of the shear spans.
Here, each one is 3
L
/8.
Defects of the m-k method
The method has proved to be an adequate design tool for profiles with
short spans and rather brittle behaviour, which have been widely used in
North America. However, to exploit fully the ductile behaviour of profiles
now available, with good mechanical interlock and longer spans, it is
necessary to use a partial-interaction method, as explained below.
The defects of the
m-k
method and of profiles with brittle behaviour
are given in papers that set out the new methods, by Bode & Sauerborn in
Germany [29] and by Patrick & Bridge in Australia [30]
.
They are as
follows.
(1)
The
m-k
method is not based on a mechanical model, so that con-
servative assumptions have to be made in design when the dimen-
sions, materials or loading differ from those used in the tests. The
calculation of
L
s
, above, is an example of this.
(2)
Many additional tests are needed before the range of application can
be extended; for example, to include end anchorage or the use of
longitudinal reinforcing bars.
(3)
The method of evaluation of test data is the same, whether the fail-
ure is brittle or ductile. The use in EN 1994-1-1 of a penalty factor
of 0.8 for brittle behaviour does not adequately represent the advan-
tage of using sheeting with good mechanical interlock, because the
advantage increases with span.
(4)
The method does not allow correctly for the beneficial effect of
friction above supports, which is greater in short shear spans.
Partial-interaction design
This method is based on results from shear-bond tests [29]. For composite
slabs of given cross-section and materials, the result of each test on a
profile with ductile behaviour enables the degree of partial shear connec-
tion in that test to be calculated. This gives the compressive force,
N
c
,
transferred from the sheeting to the slab within the shear span of known
length,
L
s
. It is assumed that, before maximum load is reached, there is
complete redistribution of longitudinal shear stress at the interface, so a
value for the mean ultimate shear stress
τ
u
can be calculated. This is done
for a range of shear spans, and the lowest
τ
u
thus found is the basis for a
