Hi n9yz
Correct me if I am wrong as I am no civil or structural engineer but for a beam of two materials I would proceed as
follows:- Assuming that the steel and concrete are connected
ridgidly together the bending moment:-
M=Mc + Ms
M= Ec*Ic/(R) + Es*Is/(R)
but Es/Ec = m = modular ratio
M= (Ic/(R) + m*Is/(R))*Ec
divide thru by R
M=(Ic+m*Is)*Ec/R and Ec/R= fc/y
: M=(Ic+m*Is)*(fc/y)
where fc=stress
Is,Ic= second moments of area for steel
and concrete beam.
R = radius of curvature of beam
Ec,Es = modulus of elasticity of materials
y = distance to neutral axis
By using this method I have found the equivalent
second moment of area of a complete concrete beam by
using the modular ratio and converting the steel part
into a equivalent concrete part.So you can now
calculate stress using Ec only. Alternatively you can
turn the concrete portion into an equivalent steel beam
if you desire again using m however the formula would
look like this:-
M=(Ic/(m))+Is)*(fs/y)
hope this helps
regards desertfox
