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Steel Designers' Manual - 6th Edition (2003)
618 Composite beams
working load.This is reflected in the effect of slip on deflection in cases where partial
shear connection is used. When designing bridges 5 or structures subject to fatigue
loading, a limit of 55% of the design resistance of the shear-connectors is appropriate
for design at the serviceability limit state.
21.7.4 Partial shear connection
In plastic design of composite beams the longitudinal shear force to be transferred
between the concrete and the steel is the lesser of Rc and Rs. The number of shearconnectors
placed along the beam between the points of zero and maximum positive
moment should be sufficient to transfer this force.
In cases where fewer shear-connectors are provided than the number required
for full shear connection it is not possible to develop Mpc. If the total capacity of the
shear-connectors between the points of zero and maximum moment is Rq (less than
the smaller of Rs and Rc), then the stress block method in section 21.6.2 is modified
as follows, to determine the moment resistance, Mc:
Case 4: Rq > Rw (plastic neutral axis lies in flange):
M R
NB the last term in this expression is generally small.
Case 5: Rq < Rw (plastic neutral axis lies in web):
D R D R È q Ê Ds - Dpˆ˘Rs Rq
T
c = s + q s - Á ˜
Î
Í Rc
Ë ¯˚
˙ Rf
-
2
( - )
2 2 4
(21.11)
(21.12)
The formulae are obtained by replacing Rc by Rq and re-evaluating the neutral axis
position. The method is similar to that used in the American method of plastic
design, 10 q s p q
c s q s
Rc
R
which predicts a non-linear increase of moment capacity with degree of
shear connection K defined as:
2
È
Ê ˆ˘
= + + - Á ˜
Î
Í
Ë ¯˚
˙
w
-
2 2 4
M M R D D R
K = Rq Rs for Rs < Rc
or K = R R for R < R
q c c s
D - D R
An alternative approach, which has proved attractive, is to define the moment resistance
in terms of a simple linear interaction of the form:
Mc= Ms+ K( Mpc -Ms)
(21.13)
The stress block and linear interaction methods are presented in Fig. 21.13 for a
typical beam. It can be seen that there is a significant benefit in the stress block
method in the important range of K = 0.5 to 0.7.
In using methods based on partial shear connection a lower limit for K of 0.4 is
specified. This is to overcome any adverse effects arising from the limited deformation
capacity of the shear-connectors.
D