COMPOSITE CONCRETE CONSTRUCTION

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 considerationS : Statical moment of area on one side of the composite inter face about the neutral axisI : Moment of inertia of the transformed composite section, neglecting the tensile resistance of concreteb : Breadth of the composite interface at the section under consideration

b) Under ultimate load:

Horizontal shear stress distribution along

composite interface, under ultimate load

Vh =C1

b * Ls

The horizontal shear stress:

WHERE :

C1: 0.85 Fc’ * b * t1

Ls: Distance between maximum and zero moment points.

T1: 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:

Ah (in mm2/m)

Ah=

1000 * b * Vh

0.87* fy

B) DIN 1045 (1978): a) In case of floors, with live loads up to 500 kg/m2 it is not

necessary to check direct shear strength at interface

provided that the contact surface is rough.

b) For