Structure II Course Code: ARCH 209 Dr. Aeid A. Abdulrazeg By :Dr. Aeid A. Abdulrazeg

Doubly-reinforced Sections When the applied design bending moment exceeds the concrete capacity (i.e. 0.156 bd2fcu) compression reinforcement is required. Consider the rectangular beam shown in Figure in which the neutral axis depth is equal to d/2 and both tension (As) and compression (As′) reinforcement are present:

As is the area of tension reinforcement, As′ is the area of compression reinforcement, Ft is the force in the tensile reinforcement, Fc is the compression force in the concrete, Fcs is the force in the compression reinforcement, ζcs is the strain in the compression reinforcement, b, d, fcu and ζs are as before.

The force in the compression reinforcement is dependent on the stress and consequently the strain ζcs. In order to ensure that this is greater than or equal to the yield stress, the ratio of (d′ / x) is limited to 0.37 as shown in Figure below:

At the yield with Fy =460 N/mm2, the stress strain ζsc = ζst = 0.00219 as specified in the code

Area of compression reinforcement Area of tension reinforcement

Example 1 The rectangular beam shown in Figure is required to resist an ultimate design bending moment of 340 kN.m. Using the data given, determine the required areas of main reinforcing steel..