6

Calculation of shear stress at
composite interface:
A)Under service load:
Strain and stress distributions across composite beam
cross

section, under service and ultimate loads.
V =
V * S
I
* b
THE SHEAR STREES AT ANY POINT ALONG THE
COMPOSITE INTERFACE CAN BE COMPUTED AS:
WHERE:
V : Shear force at any section under consideration
S : Statical moment of area on one side of the composite inter
face about the neutral axis
I : Moment of inertia of the transformed composite section,
neglecting the tensile resistance of concrete
b : Breadth of the composite interface at the section under
consideration
b)
Under ultimate load:
Horizontal shear stress distribution along
composite interface, under ultimate load
V
h
=
C
1
b * L
s
The horizontal shear stress:
WHERE :
C
1
: 0.85
F
c
’ * b * t
1
L
s
:
D
istance between maximum and zero moment points.
T
1
: Depth of equivalent rectangular stress distribution over the
composite interface
7

Code Requirements:
A)
BS 8110 (1995):
i
) Average horizontal design shear stress:
the average design shear stress should then be distributed in proportion
to the vertical design shear force diagram to give the horizontal shear
stress at any point along the length of member .
Design
shear force
Beam width * length between max +
ve
or

ve
moment and zero moment
=
ii)
nominal links
:
1) should be at least 0.15% of the contact area.
2) the spacing in T

beam ribs with composite flange should not
exceed the max of :
a) Thickness of in situ concrete
b) 600 mm
3) Links should be adequately anchored on both sides of interface.
iii)
links in excess of minimum
:
when the horizontal shear stress exceeds the value given in table
all horizontal shear force should be carried reinforcement anchored
on either side of interface
Where:
A
h
(in mm
2
/m)
A
h
=
1000 * b * V
h
0.87* fy
B)
DIN 1045 (1978):
a) In case of floors, with live loads up to 500 kg/m
2
it is not
necessary to check direct shear strength at interface
provided that the contact surface is rough.
b) For
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