| Product(s): | RAM Structural System; RAM Steel; Ram Frame | ||
| Version(s): | Any | ||
| Environment: | N/A | ||
| Area: | Design |
How can I set the unbraced length of the beam?
In Ram Steel, the unbraced lengths of beams are always calculated by the program. For details on exactly how this is done refer to the Ram Steel Beam Manual (RamSteelBeam.pdf), section titled Technical Notes - Unbraced Length.
Under Criteria- flange bracing the user can opt out of this by unchecking the option to Check Unbraced length, but this could be unconservative since all beams will then be assumed fully braced.
Additional brace points can also be modeled in the Modeler using Layout - Beams - Brace points.
When a shallow beam frames into a deep girder or cantilever, does it brace the top and bottom flange?
Yes, when one steel beam frames into another it braces the top and bottom flange of the support beam for bending, thus reducing the unbraced length for positive or negative bending. In situations where the girder is much deeper than the supported beam this might be unconservative. In those cases, we suggest changing the small bean into a steel joist, as steel joists only brace the top flange of the supporting beam.
Why is the unbraced length reported less than the maximum unbraced length?
The program evaluates bending for each of the various unbraced segments of a beam along with the maximum moments in that segment. The design might be controlled by a shorter unbraced segment with larger moments, or it might be controlled by a longer unbraced segment with smaller moments.
The design report shows the critical condition and indicates where from the left end the critical case occurs.
Differences in Ram Frame - Steel mode
For the member code check report in RAM Frame - Steel mode, the same is true. Only the critical condition for the unbraced segment and load combinations that produces the highest interaction ratio is reported. But, since Ram Frame also considers axial compression in beam design there are multiple unbraced length values reported.
Here's an example of a moment frame beam supporting gravity beams at 7.5' on center.
The axial unbraced length for strong axis buckling is 30 ft (used in KL/rx-x), while the weak axis unbraced length is 7.5' (KL/ry-y). 7.5' is also the unbraced length for negative bending which controls. If the beam was governed instead by positive bending the reported Lu for bending Y-Axis would be 0.00 since the top flange is continuously braced.
For details on how the unbraced lengths are determined, please refer to the program manual 17.03.01.50 section 11.3.1 General - "Compression Flange Bracing".