In the design of concrete beams, SAFE calculates and reports the required areas of reinforcement for flexure, shear, and torsion based on the beam moments, shear forces, torsion, load combination factors, and other criteria described in this section. The reinforcement requirements are calculated at each station
along the length of the beam.
Beams are designed for major direction flexure, shear, and torsion only. Effects resulting from any axial forces and minor direction bending that may exist in the beams must be investigated independently by the user.
The beam design procedure involves the following steps:
along the length of the beam.
Beams are designed for major direction flexure, shear, and torsion only. Effects resulting from any axial forces and minor direction bending that may exist in the beams must be investigated independently by the user.
The beam design procedure involves the following steps:
- Design flexural reinforcement
- Design shear reinforcement
- Design torsion reinforcement
1. DesignFlexural Reinforcement
The beam top and bottom flexural reinforcement is designed at each station along the beam. In designing the flexural reinforcement for the major moment of a particular beam, for a particular station, the following steps are involved:
- Determine factored moments
- Determine required flexural reinforcement
1.1 Determine Factored Moments:
In the design of flexural reinforcement of concrete beams, the factored mo- ments for each load combination at a particular beam station are obtained by factoring the corresponding moments for different load cases, with the corre- sponding load factors.
The beam is then designed for the maximum positive and maximum negative factored moments obtained from all of the load combinations. Calculation of bottom reinforcement is based on positive beam moments. In such cases the beam may be designed as a rectangular or flanged beam. Calculation of top re- inforcement is based on negative beam moments. In such cases the beam may be designed as a rectangular or inverted flanged beam.
The design procedure is based on the simplified rectangular stress block, as shown in Figure 2-1 (ACI 10.2). Furthermore, it is assumed that the net tensile strain in the reinforcement shall not be less than 0.005 (tension controlled) (ACI 10.3.4) when the concrete in compression reaches its assumed strain limit of 0.003. When the applied moment exceeds the moment capacity at this de- sign condition, the area of compression reinforcement is calculated assuming that the additional moment will be carried by compression reinforcement and additional tension reinforcement.
The beam is then designed for the maximum positive and maximum negative factored moments obtained from all of the load combinations. Calculation of bottom reinforcement is based on positive beam moments. In such cases the beam may be designed as a rectangular or flanged beam. Calculation of top re- inforcement is based on negative beam moments. In such cases the beam may be designed as a rectangular or inverted flanged beam.
1.2 Determine RequiredFlexural Reinforcement
In the flexural reinforcement design process, the program calculates both the tension and compression reinforcement. Compression reinforcement is added when the applied design moment exceeds the maximum moment capacity of a singly reinforced section. The user has the option of avoiding compression reinforcement by increasing the effective depth, the width, or the strength of the concrete. Note that the flexural reinforcement strength, fy , is limited to 80ksi (ACI 9.4), even if the material property is defined using a higher value.
The design procedure is based on the simplified rectangular stress block, as shown in Figure 2-1 (ACI 10.2). Furthermore, it is assumed that the net tensile strain in the reinforcement shall not be less than 0.005 (tension controlled) (ACI 10.3.4) when the concrete in compression reaches its assumed strain limit of 0.003. When the applied moment exceeds the moment capacity at this de- sign condition, the area of compression reinforcement is calculated assuming that the additional moment will be carried by compression reinforcement and additional tension reinforcement.
The design procedure used by SAFE, for both rectangular and flanged sections (L- and T-beams), is summarized in the text that follows. It is assumed that the design ultimate axial force does not exceed (0.1 f' c Ag) (ACI 10.3.5), hence all beams are designed for major direction flexure, shear, and torsion only.
1.2.1 Design of Rectangular Beams
In designing for a factored negative or positive moment, M(i.e., designing top
or bottom reinforcement), the depth of the compression block is given by a (see
Figure 2-1), where,
or bottom reinforcement), the depth of the compression block is given by a (see
Figure 2-1), where,
To be continued....
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