Changes to the default Member Bracing Type in 2022 may leave you wondering why a beam that once passed is now failing in Moment.
Dropped beams, such as garage door headers and basement beams, require extra consideration when it comes to bracing. In many cases these members are braced only at their ends with nothing to prevent lateral movement or buckling under load. The potential for buckling results in reduced load carrying capacity compared to a beam that has lateral bracing along its length to prevent buckling. Prior to 2022 new beam members defaulted to the Fully Braced setting that is more appropriate for beams flush in the floor cavity. In the April 2022 release, BC Calc’s Settings tab was modified and two new beam Usage categories were introduced for Flush and Drop Beam applications to help designers by setting initial bracing conditions based on the member’s usage (see the release notes). Drop Beam was set as the new default Usage along with the corresponding Braced at Member Ends Member Bracing Type in the Lateral Torsional Buckling settings.
While not all drop beams are only braced at the member ends, this default represents a common bracing condition for drop beam framing conditions. Designers are responsible for evaluating the bracing conditions for each of their beam designs and setting the Member Bracing Type to the appropriate selection if the default is not applicable.
What actually changed?
The Usage setting is now linked to the Member Bracing Type. Unless the bracing condition is specified differently in Analysis Settings, creating a new beam will default to a Usage of Drop Beam and Member Bracing Type will default to Braced at Member Ends. Changing the Usage to Flush Beam changes the Member Bracing Type to Fully Braced and changing Usage to any of the other options changes it back to Braced at Member Ends. Note that designers can change the Member Bracing Type independent of the Usage setting at any time to the selection that is appropriate for their beam design.
Why the change?
Beams have a reduced moment capacity when not braced for their entire length on the compression edge which is typically the top edge of the member. Ends of a beam are always required to be restrained against rotation, but bracing is typically required at 24” on-center or less for a beam to be considered fully braced. The new Drop Beam / Braced at Member Ends default will give a conservative result if the designer does not evaluate the bracing condition in their design process.
In order to consider any existing framing as bracing, framing needs to be capable of preventing the compression edge of the beam from moving laterally in both directions perpendicular to the member’s length. Cripple/knee or full height walls that run parallel to the to the beam’s length, typically are NOT able to laterally brace a beam.
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Note: The usage field is a pre-set text field and can be overwritten by selecting the field and typing over it. There are also four pre-set values available for selection: Drop Beam, Flush Beam, Wall Header, and None. With the exception of None, the usage field text appears on the member graphic and on the printed member report.