Design Procedure Rectangular and T-shape Beam
Beams can be described as members that are mainly subjected to flexure and it is essential to focus on the analysis of bending moment, shear, and deflection.
When the bending moment acts on the beam, bending strain is produced. The resisting moment is developed by internal stresses. Under positive moment, compressive strains are produced in the top of the beam and tensile strains in the bottom.
Concrete is a poor material for tensile strength and it is not suitable for flexure member by itself. The tension side of the beam would fail before compression side failure when the beam is subjected a bending moment without the reinforcement.For this reason, steel reinforcement is placed on the tension side. The steel reinforcement resists all tensile bending stress because tensile strength of concrete is zero when cracks develop.
- Assume the depth of beam using the ACI Code reference, minimum thickness unless consideration the deflection.
- Assume beam width (ratio of width and depth is about 1:2).
- Compute self-weight of beam and design load.
- Compute factored load (1.4 DL + 1.7 LL).
- Compute design moment (Mu ).
- Compute maximum possible nominal moment for singly reinforced beam (φM n ).
- Decide reinforcement type by Comparing the design moment (M u ) and the maximum possible moment for the singly reinforced beam (φM n ). If φM n is less than Mu, the beam is designed as a doubly reinforced beam else the beam can be designed with tension steel only.
- Determine the moment capacity of the singly reinforced section. (concrete-steel couple)
- Compute the required steel area for the singly reinforced section.
- Find a necessary residual moment, subtracting the total design moment and the moment capacity of the singly reinforced section.
- Compute the additional steel area from the necessary residual moment.
- Compute total tension and compressive steel area.
- Design the reinforcement by selecting the steel.
- Check the actual beam depth and assumed beam depth.
- Compute the design moment (Mu ).
- Assume the effective depth.
- Decide the effective flange width (b) based on ACI criteria.
- Compute the practical moment strength (φM n ) assuming the total effective flange is supporting the compression.
- If the practical moment strength (φM n ) is bigger than the design moment
(Mu ), the beam will be calculated as a rectangular T-beam with the effective flange width b. If the practical moment strength (φM n ) is smaller than the design moment (Mu ), the beam will behave as a true T-shape beam.
- Find the approximate lever arm distance for the internal couple.
- Compute the approximate required steel area.
- Design the reinforcement.
- Check the beam width.
- Compute the actual effective depth and analyze the beam.