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Special Moment Frame – Beam Bracing

Strong Frame Special Moment Frame

Moment Frame Design Requirements and Assumptions

Beam Bracing

Since special moment frames are required to have the resilience to withstand large rotation at the column-to-beam connection, the beams need to be stabilized using bracing to resist buckling.

Beam Bracing Requirements

Steel special moment frame beam bracing is required by code to prevent beam torsional or flexural buckling in order for plastic hinges to form. To preclude undesirable beam buckling failure modes that may occur during the formation of plastic hinges in the beam, Section 9.8 of AISC 341 has the following requirement for SMF: Both flanges of beams shall be laterally braced, with a maximum spacing of Lb = 0.086ryE/Fy.

Per AISC 341-10, stability beam bracing shall be provided for highly or moderately ductile members (members expected to yield). In addition, unless justified by testing, beam bracing shall be provided near concentrated forces, changes in cross-section, and other locations where analysis indicates that a plastic hinge will form during inelastic deformation of the special moment frame.

Each prequalified moment connection type has different requirements for beam bracing. For RBS connections, per AISC 358-16, supplemental lateral bracing of beams shall be provided near the reduced section. In addition, the attachment to the beam shall be located no greater than d/2 beyond the end of the reduced beam section. See AISC 358-16 for additional design guidelines.

In structural steel buildings, additional steel beams connected to full-depth shear tabs with slip-critical bolts have little difficulty in satisfying SMF bracing strength and stiffness requirements. However, meeting the code-prescribed bracing requirements is far more problematic when installing SMF in light-frame construction. There are deflections in the brace caused by oversized holes in the wood, vertical deflection of the floor beam and horizontal deflection of the floor diaphragm. Each of these sources of deflection added in sequence makes it harder to achieve the minimum bracing stiffness mandated by AISC for an SMF.

Beam Bracing Requirements Diagram

Consequences of Inadequate Bracing

Currently AISC 360-10 Appendix 6 has both strength and stiffness requirements for beam bracing. If no bracing or inadequate bracing is provided (failing either the strength or the stiffness requirements), the frame designed will not achieve the expected full capacity. The beam will either buckle in torsion (Figure 1) or in flexure (Figure 2) prior to the formation of the plastic hinge in the beam at the connection region.

Consequences of Inadequate Bracing, Figure 1 - Beam Torsional Buckling

Figure 1 - Beam Torsional Buckling

Consequences of Inadequate Bracing, Figure 2 - Beam Flexural Buckling

Figure 2 - Beam Flexural Buckling

Ways to Brace a Beam

Per AISC 341, there are two methods to brace the beam: (1) lateral bracing (Figure 3) and (2) torsional bracing (Figure 4). Under lateral bracing, one can brace the beam at the compression flange (either top or bottom or both, depending on loading). Under torsional bracing, one is trying to prevent the section from twisting. To prevent twisting, typically a full-depth stiffener is welded to the SMF beam and connected to another beam nearby.

Consequences of Inadequate Bracing, Figure 3 - Beam Lateral Bracing (Concrete Slab at Top)

Figure 2 - Beam Flexural Buckling
(Photo credit: NEHRP Seismic Design Technical Brief No. 2: Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, NIST GCR 09-917-3, June 2009.)

Consequences of Inadequate Bracing, Figure 4 — Torsional Bracing

Figure 4 — Torsional Bracing

Stability Bracing at Beam-to-Column Connections

In addition to beam bracing, AISC 341-10 Section E3.4c requires connections to be braced at the column. When columns cannot be shown to remain elastic outside of the panel zone, column flanges shall be laterally braced at the levels of both the top and the bottom beam flanges. However, if the columns are shown to remain elastic outside of the panel zone, column flange bracing is required at the top flanges of the beams only. Each column flange brace shall be designed for a required strength that is equal to 2% of the available beam flange strength.

Bracing can be either direct or indirect stability bracing. Direct bracing is achieved through the use of member braces or other members (decks, slabs, etc.) attached to the column flange at or near the bracing point. Indirect bracing is achieved through connecting through the column web or stiffener plates.

Special moment frame beam-to-column connections can be unbraced also. However, the column needs to be designed for the overall height between the adjacent brace points and the following criteria need to be applied:

  1. The design strength shall be determined from the amplified seismic load combinations according to the applicable building code.
  2. The L/r for the column shall not exceed 60.
  3. The column’s required exural strength transverse to the seismic frame shall include moment from beam-bracing forces of 2% of the beam flange strength.

Strong Frame Special Moment Frame Beam Bracing

With the introduction of the Strong Frame special moment frame, the Yield-Link® structural fuses are designed as plastic hinges, where beam bracing is not required. There is no inelastic lateral torsional buckling of the beam because yielding takes place at the Yield-Link structural fuses and not in the beam itself. The beam is designed to span between the supports for the maximum load the Yield-Link structural fuse system can deliver.

Figure 1 below is a plot from our nite element analysis showing the equivalent plastic strain in the moment connection. All the yielding is concentrated (indicated by the green color) in the Yield-Link. The elastic beam behavior is supported by our testing as shown in Figure 2. Strain gauges placed on the beam’s bottom flange near the moment connection clearly show the elastic behavior in the beam. Also note the symmetry of the readings on strain gauges placed on each side of the beam. The overlapping of the red and blue lines indicate no torsional or flexural buckling occurred in the beam during testing, even at a frame drift level of 6%.

Strong Frame Special Moment Frame Beam Bracing, Figure 1 — Equivalent Plastic Strain (PEEQ) Plot of Simpson Strong-Tie® Strong Frame Special Moment Frame at
0.04 Radians

Figure 1 — Equivalent Plastic Strain (PEEQ) Plot of Simpson Strong-Tie® Strong Frame Special Moment Frame at 0.04 Radians

Strong Frame Special Moment Frame Beam Bracing, Figure 2 — Measured Strain from Testing at Beam Bottom Flange

Figure 2 — Measured Strain from Testing at Beam Bottom Flange