Learn about the design assumptions followed by the extension when using Internal Engine and Robot Structural Analysis methods.
Composite Beam Design Extension follows these assumptions:
Camber is designed as a percentage of the self-weight beam deflection (default value is 80 percent). You can also enter a specific value for camber. A beam is flagged as failing for over-camber if you specify a camber amount greater than 100 percent of the self-weight beam deflection.
If present, camber is taken into account when calculating construction deflections and post-composite total load deflections.
During design of a composite beam, the extension calculates the number of studs required to develop the moment capacity, deflection stiffness, or minimum composite action as required by code. For beams with no point loads, the studs are uniformly distributed across the beam. For girders or other beams with point loads, the studs are distributed in segments to ensure that there are enough studs between every point load and its nearest support to develop the appropriate moment capacity of the composite section.
For girders designed with segmented studs, the Design tab displays the segmented stud distribution and the uniform stud distribution, calculated using the densest stud region. You may choose to save either the uniform or segmented distribution to Robot. Segmented distributions are saved as a comma-delimited list, ordered from the start point of an analytical beam.
When the extension reads beam properties, if it encounters a segmented stud distribution on a beam, it tries to recreate the stud regions by matching them up in order from beam start to beam end. If the number of segments read from the Robot model does not match the number of regions determined by the extension, the segmented stud distribution is discarded. If a uniform stud distribution is read in for a girder, the extension applies those studs uniformly to the beam, and the segmented distribution is not available until you start a new design (but the beam will be correctly analyzed for that uniform distribution).
Lateral-torsional buckling is optionally calculated for the top and bottom flanges of beams.
The extension can automatically detect bracing against lateral-torsional buckling in some situations.
The top flange of a beam is assumed to be braced wherever there is deck resting on it, even if the beam is not designed as a composite beam. If a beam is fully covered by deck, the top flange is considered continuously braced. If the beam passes through an opening in the slab deck, the unbraced length is taken as the largest opening.
Both the top and bottom flanges of a beam are assumed to be laterally braced wherever another beam frames into it. For example, a girder is assumed to be fully braced where beams bear on it.
In addition, you can specify the maximum unbraced length of beams as a proportion of the beam length. For example, if you specify 0.5 as the maximum unbraced length ratio, a 20-foot beam is assumed to have a maximum unbraced length of 10 feet, regardless of other brace points. The engineer may use this setting to specify the maximum unbraced length that each beam should be designed for.
Since beams are only susceptible to lateral-torsional bucking when the unbraced flange is in compression, the beam is checked for bottom-flange buckling under negative moments only and top-flange buckling under positive moments only. Composite beams are required to have complete deck coverage and no negative moments, so they are never susceptible to lateral-torsional buckling.
The reduced moment capacity due to lateral-torsional buckling is computed using ANSI/AISC 360-10 section F2 for the top and bottom flanges. Cb factor for simply-supported beams or backspans may be specified by the user. The setting applies to all beams, and conservatively defaults to a value of 1.0. When checking the moment capacity for a beam in lateral-torsional buckling, the point of maximum positive moment is assumed to occur in the longest unbraced top flange section, and the point of maximum negative moment is assumed to occur in the longest unbraced bottom flange section. This may be conservative in some cases.
The extension does not check sections with non-compact flanges for compression-flange buckling. If extension selects a beam with a non-compact flange, the engineer must check the beam against local buckling.