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D1.F.4 Beam Design

Beams are designed at a series of locations along their length for the moment about the local Z axis (MZ), shear force in the local Y and Z axes (FY and FZ), torsional moment (MX), and axial force (FX) for an envelope of the forces from a specified collection of load cases and combinations.

The envelope is created at each design section from a collection of the max/min forces in all degrees of freedom for all the load cases and/or combinations that have been included in the design. As there are 6 degrees of freedom (Fx, Fy, Fz, Mx, My, Mz) and to capture the maximum and minimum effects, a total of 12 sets of loads are designed for. Each set will include the maximum or minimum of one force such as Max FX along with all the other associated forces in the load case/combination.

For each member that is designed as a beam, the forces from the current LOAD LIST (see TR.39 Load List Specification) will be used to create an envelope of forces.

This envelope of forces is then used to determine a required area of longitudinal rebar at each of the key section locations. Five sections are defined on each member: the start, quarter point, mid-point, three-quarter point, and end point. This required area is then used determine an optimized bar arrangement that meets the code requirements, including detailing rules. Once the longitudinal bars have been designed and detailed, the program then designs the transverse steel based on the code provisions. Note that in case of torsion loads, the effect of the applied torsion will be catered for while designing the longitudinal steel as well as the transverse steel.

The required area of reinforcements at every cross section is first calculated. In order to then arrive at an optimized detailed bar layout, the program starts with the bar size defined in the MINMAIN parameter (or MIMT parameter for top bars and MIMB parameter for bottom bars, if specified)

The program will then iterate though increasing the number of bars or the size of bars dependent on the values of the BRDO and TRDO parameters.

If additional skin bars (i.e., side face bars) are needed for spacing requirements, these will be added.

Any detailing rules such as development lengths etc will be considered and the final design output would indicate whether a given set of bars are not be anchored or not.

Assumed Values Used

Most of the values are determined from the beam data, however, some values of note include:
  • Young's modulus of rebar (Es ) is taken as 29,000 ksi
  • Young's modulus of concrete is calculated from the given value of F'c . But if F'c > 10,000, then Ec is limited to 3,605 psi.
  • Density of concrete is taken as 145 pcf (0.083912 kip-in3)


Note that deep beams –where the span < 4× the beam depth– are not designed. A warning is reported in the output.