Unbraced Length

Unless specified otherwise, the program determines unbraced lengths and sizes the members accordingly. You may specify a number of parameters related to unbraced length considerations. These criteria are found in the Criteria > Design Defaults menu item. You may suppress entirely the checking of unbraced length by de-selecting the Check Unbraced Length check box. In this case, all beams are designed as if the compression flange is continuously braced for the full length of the beam. This speeds up the design process but may result in unconservative results, and is not generally recommended.

You may indicate whether or not the deck braces the top flange for noncomposite design. The effect of deck oriented parallel to the beam may be specified independently of that of deck oriented perpendicular to the beam. See Orientation for a discussion of perpendicular versus parallel deck. For the post-composite condition of composite beams, the deck is always assumed to brace the top (compression) flange in the positive moment region. For the pre-composite condition of composite beams, the assumptions are the same as for a noncomposite beam. The program assumes that the deck does not brace the compression flange of cantilevers and beams in the negative moment region.

For cantilevers, the program assumes that the ends are fully braced against twist.

One of the criteria available is an option to Consider Point of Inflection. Selecting this option does not mean that the point of inflection will necessarily be considered a brace point; rather, it affects the way the program looks at brace points of the flanges on either side of the point of inflection when determining the unbraced length. This is further explained below.

In the program, one flange brace point list is created for the top flange and another is created for the bottom flange. These brace points are determined in the following way:

Beams are assumed to be braced at supports on both the top and the bottom flange.
Cantilever ends are assumed to be braced on both the top and the bottom flange.
Beams framing into the member brace both the top and the bottom flange, regardless of the relative depth.
Steel joists framing into the member brace the top flange only. Those points are added to the top flange brace point list only.
Beams are assumed to be braced on both the top and the bottom flange wherever a column from above sits on the beam (as for a transfer girder situation). This assumes that the Engineer is going to specify some kind of bracing at these locations, as is commonly done, as such columns do not intrinsically provide bracing by themselves.
User-assigned brace points, specified in RAM Modeler with the Layout > Beams > Brace Points command, brace the specified flange.
For composite beams, the deck is assumed to brace the top flange in the positive moment region for the post-composite condition.
For noncomposite beams and for the pre-composite condition of composite beams, the deck is assumed to brace the top flange in the positive moment region according to the criteria specified for Unbraced Length in Criteria > Design Defaults. You may indicate whether or not the deck braces the top flange when the deck is oriented parallel to the beam and when it is oriented perpendicular (or at an angle) to the beam.

The unbraced length of the compression flange is then calculated as the distance between brace points. Each unbraced segment is investigated for the moments within that segment.

When determining the moment capacity associated with a moment at a given point, if the moment causes compression in the top flange and the top flange is braced by the deck, the unbraced length is set to 0.0.

Note that the calculation of the unbraced length is affected by whether or not the Consider Point of Inflection option is selected:

If the Consider Point of Inflection option is not selected, the unbraced segment length is the distance between physical brace points along the flange under consideration. For example, in the figure below, L_u2 would be the unbraced length for the Moment at the left support. This would be used even though the bottom flange is only in compression between the left support and the point of inflection; it is in tension from the point of inflection to the right support. L_u2 is the distance between bottom flange physical bracing, which in this example is the entire beam span. This is consistent with the recent work by Joseph A. Yura of the University of Texas, Austin.
If the Consider Point of Inflection option is selected, the unbraced length is the distance between points where the compression flange (whichever flange is in compression at any particular point) is braced, not the distance along a given flange where that flange is braced. For example, in the following figure, L_u1 would be the unbraced length for the moment at the left support because at the left support the bottom flange is in compression but at the point of inflection the top flange becomes the compression flange, and that flange is braced at the second top flange brace point. L_u1 is the distance between compression flange bracing. This is consistent with what has been long-standing practice; see, for example, the publication Cantilever Roof Framing Using Rolled Beams published by AISC.
Note: It is increasingly being recommended that this approach not be used as it is considered unconservative.

Points of inflection are not of themselves considered brace points. However, if the top flange is specified as continuously braced, the point at which the continuous bracing of the compression flange begins or ends coincides with the point of inflection. In the figure below, if the top flange is continuously braced and the Consider Point of Inflection option is not selected, the unbraced length would still be L_u2 as shown. However, if the top flange is continuously braced and the Consider Point of Inflection option is selected, the unbraced length would be the distance between the left support and the point of inflection (not L_u1). Thus, although the point of inflection appears to be acting as a brace point in that case, it is actually the continuous bracing that is acting as the brace point.

Note: The sign convention used is such that the moment curve is drawn on the same side as the compression flange.

PI is the point of inflection and BP are the Top Flange Brace Points

Based on these criteria and the geometry of the model, the program automatically determines the braced condition for each member.

Unbraced length is taken as the distance between brace points. At supporting columns, it is taken to column centerline, not column face.

The program checks each unbraced segment to determine the controlling condition of moment and unbraced length, and selects the beam size accordingly.