For continuous composite beams that in normal working stage, due to concrete cracks at negative moment region are restricted by transverse tendon reinforcements at the slab, it would be reasonable to consider concrete contribution to the bending capacity of such composite beams. Based on this, a variable stiffness method is proposed in this paper to calculate the composite beam deflection. Before concrete slab cracks, the flexural stiffness (the maximum value) is calculated according to the reduced transformed beam section, with the consideration of concrete contribution; After concrete cracks, this stiffness value will be decreased as per the increase of the concrete crack, and at final stage, when concrete contribution to the flexural stiffness reach zero, the stiffness value reach it’s minimum value. The loads capacity at such maximum and minimum stiffness values could be considered as the beam capacity before concrete cracking as per current code, and the elastic moment capacity after concrete cracking respectively, while the stiffness between these maximum and minimum values could be determined by linear interpolation. The bending stiffness derived from this method has a higher precision than the method presented in current China specification in which the concrete contribution is totally neglected. Especially, this procedure would be more useful for those prestressed concrete composite beams, as they have higher crack- resistance capability.
