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By: Edgar B.

Manubag, CE, PhD

Substituting :


Balanced design refers to a design so proportioned that the maximum stresses in concrete (with strain of 0.003) and steel (with strain of fy/Es) are 'reached simultaneously once the ultimate load is reached, causing them to fail simultaneously.

Underreinforced design is a design in which the steel reinforcement is lesser than what is requited for balanced condition. If the ultimate load is approached, the steel will begin to yield although the compression concrete is still understressed. If the load is further increased, the steel will continue to elongate, resulting in appreciable deflections and large visible cracks in the tensile concrete. Failure under this condition is ductile and will give warning to the user of the structure to decrease the load.

Overreinforced design is a design in which the steel reinforcement is more than what is required for balanced condition. If the beam is overreinforced, the steel will not yield before fai}ure. As the load is increased, deflections are not noticeable although the compression concrete is highly stressed, and failure occurs suddenly without warning to the user of the structure.


In balanced. condition, the concrete and steel yield simultaneously. In this condition, the strain in concrete reached its maximum usable value of 0.003 and the strain in steel is fy/ Es, where Es = 200,000 MPa.


By ratio and proportion iri the triangle shown in Figure 2.2:




For statically determinate T-section with flange in tension, the area As min , shall be equal to or greater than the smaller value given either by:

or Eq. 2 - 16 with bw set equal to the width of the flange .

According to Section 5.9.5 of NSCP, minimum thickness stipulated in Table2.1 shall apply for one-way construction not supporting O( attached to partitions or other . construction likely to be damaged by large deflections, unless computation of deflection indicates a lesser thickness can be used without adverse effects.

Section of NSCP states that in lieu of frame analysis, the following approximate moment and shears are permitted for design of continuous beams and one-way slabs (slabs reinforced to resist flexural stresses in only one direction), provided: (a) There are two or more spans, (b) Spans are approximately equal, with the larger of two adjacent spans not greater.than the shorter by more than 20 percent, (c) Loads are uniformly distributed, (d) Unit live does not exceed three times unit dead load, and (e) Members are prismatic.