Experimental research on plastic design method and moment redistribution in continuous concrete beams prestressed with unbonded tendons

Determining normal ultimate load-bearing capacity is important in the design of concrete continuous beams that are prestressed with unbonded tendons. Tests on continuous unbonded prestressed concrete beams show that bending moment redistribution can occur. However, the analysis approach is far from sufficient to quantify the amount of moment redistribution to this type of beam. In this paper, an analysis approach to quantify the amount of moment redistribution is described and the concept of negative moment redistribution is introduced. Moment redistribution of two-span continuous unbonded prestressed concrete beams takes place when concrete cracking develops in the rotation of plastic hinge angle, and reaches its maximum at failure of beams. Moment considering redistribution used to calculate the ultimate load-bearing capacity at the section over the inner support is the algebraic sum of the secondary moment due to prestressing and the moment due to external loads. Two formulae of the degree of moment redistribution are proposed based on test results.