Lateral Support

Columns, beams and joists often need lateral support to keep them from buckling under heavy load.

Just like the column, a bare joist under load will probably fail by kicking sideways and buckling due to the compressive force in the top section. Side-to-side bracing of the compression edge is needed to develop published bending capacity.

Multiple Spans and Overhangs

Multiple spans and overhangs are similar to simple spans with respect to compression/tension in the span (between the supports). However, multiple spans and overhangs have a continuous member running over a support. This causes tension in the top and compression in the bottom. The stresses have switched over the support! Since we need to be aware of the compression flange (buckles like a column), the bottom flange deserves the attention near the intermediate supports.

Applying Lateral Support

Sheathing and gypsum provide lateral support of joist flanges. 1x4 strapping is also used if the joists would otherwise remain bare. Lateral support works only if it is tied to something solid, so run the lateral support across all joists and tie to an end wall.

Joists framed into the side of a beam provide adequate lateral support to the top of the beam. Since wider beams have less tendency to buckle sideways, you may notice adding lateral support to the bottom of a multiple-span beam does not always increase negative moment capacity.

Open-web trusses are designed by RedBuilt™ associates. Carefully follow the RedBuilt� installation instructions to properly brace open-web trusses both during construction and for permanent use.

See Lateral Support in the Spans Tab in RedSpec™.

Lateral Stability Analysis in RedSpec™

Ensuring lateral stability is an essential part of any design. Although the particular methods for ensuring lateral stability vary by member type in RedSpec™, they share key concepts.

• Unbraced length: The distance along the member between points of lateral bracing is called the unbraced length, or Lu.

• Critical buckling stress: As the load on the member increases from zero, it will eventually buckle, assuming it does not fail by crushing first. The level at which the member buckles is called the critical buckling stress.

• Slenderness ratio: The ratio of the effective unbraced length to a critical section dimension is called the slenderness ratio. It is used to calculate the critical buckling stress, as well as to define a practical upper limit for slenderness.

When the unbraced length is relatively long, the critical buckling stress may be quite low. To improve designs that are limited by lateral stability, the unbraced length may be decreased, the section dimensions increased, or a higher grade of material may be used, with a higher modulus of elasticity.

I-Joist

In areas of positive moment, the top flange is analyzed as a column under compression, with continuous lateral support in the strong axis of the joist. Lateral support in the joist's weak axis is controlled by the RedSpec™ user. If "Continuous" support is selected, it is assumed the top flange is braced in accordance with the minimum requirements of the product code report. Typically this consists of panel sheathing nailed along the full length of the flange. Note that multiple I-joists, such as double assemblies, must be "continuously" nailed along each flange.

When the lateral support selection is "At Supports", the unbraced length is set equal to the span. Otherwise the unbraced length is set equal to the dimension input by the user. The designer must take care to ensure that full-depth blocking or its equivalent is specified at the supports and at the maximum intervals specified by the user in RedSpec™.

The allowable moment is adjusted by the column stability factor as provided in the 2012 NDS, Section 3.7.1, with Emin per Appendix D, Section D.4, assuming a COVE of 11%. The effective length is set equal to the unbraced length.

Since the allowable moment is limited by flange tension while the stability adjustment must be applied to flange compression, the factor Cp is multiplied by the ratio of allowable flange compression to allowable flange tension. The factor cannot exceed 1.0. If the slenderness ratio, le/d, exceeds 50, the allowable moment is set to a negligible value, and the design will fail in positive moment.

In areas of negative moment, there is no reduction in allowable moment. The user cannot control lateral support of the joist bottom flange. It is assumed the bottom flange is braced in accordance with RedBuilt™ recommendations. Please contact Technical Support for bracing requirements at overhangs or on roof joists subject to net wind uplift.

Rectangular Section

Unbraced length is determined by user input as it is for I-joists. Effective length is computed according to 2012 NDS, Table 3.3.3, Footnote 1. As with I-joists, Emin is per Appendix D, Section D.4, assuming a COVE of 11%.

The user may set unbraced length of top and bottom edges of rectangular sections. Thus the allowable moment in both positive and negative load conditions is subject to the beam stability factor.

Open-Web Truss

The RedSpec™ truss analysis treats the top chord as a column braced continuously in the truss's weak axis and at panel points in the strong axis. As for I-joists it is assumed the bottom chord is braced in accordance with RedBuilt™ specifications, and the analysis proceeds in the same manner as for the top chord.