Special Moment Frame – Beam Bracing
Moment Frame Design Requirements and Assumptions
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 global buckling.
Beam Bracing Requirements
Steel special moment frame beam bracing is required by code to prevent beam torsional or flexural buckling as plastic hinges form. To preclude undesirable beam buckling failure modes that may occur during the formation of plastic hinges in the beam, Section D1.2.2b of AISC 341-16 has the following requirement for SMF for highly ductile members (i.e., beam element) with a maximum spacing of Lb = 0.095ryE/(Fy*Ry).
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.
Consequences of Inadequate Bracing
Currently AISC 360-16 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.
Figure 1 — Beam Torsional 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.
Figure 3 — Beam Lateral Bracing (Concrete Slab at Top)
(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.)
Figure 4 — Torsional Bracing
Stability Bracing at Beam-to-Column Connections
In addition to beam bracing, AISC 341-16 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. For the Yield-Link moment connection, if the column is designed in accordance with Section 12.9 in AISC 358 (maximum nominal flexural strength is calculated using Sx, instead of Zx), only bracing at the level of the beam top flange is required.
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:
- The design strength shall be determined from the amplified seismic load combinations according to the applicable building code.
- The L/r for the column shall not exceed 60.
- The column’s required flexural 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 to develop plastic deformations, 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 5 below is a plot from our finite 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 6. 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%.
Figure 5 — Equivalent Plastic Strain of Simpson Strong-Tie Strong Frame Special Moment Frame at 0.04 Radians
Figure 6 — Measured Strain from Testing at Beam Bottom Flange