Moment Frame Design Requirements
D1. Drift Check
Drift Check for Seismic Loads
Drift Check for Seismic Loads ASCE 7-16 Section 12.12.1 states that design story drift of a structure shall not exceed the allowable drift limit listed in Table 12.12-1. For seismic applications, the story drift limitation serves not only as a serviceability check but also as an inherent ductility requirement for seismic design related to the Response Modification Coefficient (R-value) as well as structural stability.
In the current seismic design philosophy, structures do not have to be designed for the Maximum Considered Earthquake (MCE) forces. Reduction in design forces is primarily related to the R-value in structural lateral-resisting systems. The R-value for each lateral system is related to ductility, and design codes take this relation into consideration in assigning higher R-values to more ductile systems. Reduced design forces used for drift check should be at strength level (LRFD) (ASCE 7-16 Section 12.8.6), and the deflection amplification factor (Cd) used shall correspond to the R-value used for the lateral-resisting system. Please note, for drift check, ρ shall be taken as 1.0 per ASCE 7-16 Section 188.8.131.52. In addition, drift check need not include overstrength combinations since ultimate displacement calculation already includes the Cd factor.
In calculating the minimum base shear for computing for drift (ASCE 7-16 Section 184.108.40.206), Equation 12.8.5 need not be considered. In addition, the period used for computing forces for drift check, one can use the computed fundamental period of the structure without the upper limit (CuTa) specified in Section 12.8.2. What this means is there will be two sets of seismic forces for moment frame design, one set for drift check and one set for strength design, with the drift check forces typically lower than the strength design forces.
Drift Check for Wind Loads
Currently there are no drift limit requirements for wind design. However, there are some recommendations for serviceability considerations such as Appendix C in ASCE 7-16 and AISC Design Guide 3, Serviceability Design Considerations for Steel Buildings.
Drift Check for Steel Moment Frames with Yield-Link® Moment Connections
Because the Yield-Link moment connection is considered a partially restrained connection, a new modeling and design procedure was established and verified with full-scale testing. When you are designing and analyzing partially restrained connections, the strength and stiffness of the connection need to be considered. A detailed step-by-step procedure to calculate the axial link or rotational link parameters for the Yield-Link moment connection is documented in Chapter 12 of the AISC 358-16. Once the partially restrained connection is modeled, frame drift can be calculated similar to the traditional fully restrained connections. For pushover or nonlinear time history analysis, a full nonlinear axial link or rotational link model is required. Design tools for calculating the link parameters can be downloaded from strongtie.com.
D2. Panel Zone Check
Other than drift check, another common limit state that governs the design of a moment frame is the connection panel zone shear capacity. The capacity of the panel zone depends mostly on the thickness and depth of the column web. When the demand-to-capacity ratio is greater than 1, many engineers tend to increase the thickness of the column web by welding a doubler plate to increase the shear capacity. Please note when the column web or doubler plate exceeds the geometry limit noted in E3-7 of AISC 341-16, plugwelds can be used to reduce the panel zone’s tendency to buckle out of plane. However, many fabricators are aware that increasing the column web thickness by increasing column weight approximately 75 plf (e.g., from a W14x74 to, say, a W14x145) can result in a less expensive frame because of the material handling, welding and inspection costs of using a doubler plate.
If panel zone capacity is not checked, the consequence can be column kinking due to a weak panel zone (Figure D2.1). This can lead to column flange fracture just above and below the beam flanges connecting to the column. This phenomenon has been observed after a strong seismic event (Figure D2.2) as well as reproduced in laboratory testing (Figure D2.3).
Panel Zone Check for Yield-Link® Moment Connections
For typical SMF connection design (e.g., RBS), the design shear demand on the panel zone is calculated from the summation of the moments at the face of the column by projecting the expected moment at the plastic hinge point to the column faces.
For Yield-Link moment connections, the panel zone demand is calculated from statics using the shear at the top and bottom of the beam from the link’s ultimate axial capacity (Pr-link). This demand is higher than that of a typical moment connection, where the expected moment is taken as, Mpe = Ry*Fy*Zx, where Ry = 1.1 and Fy = 50 ksi for A992 steel. For the Yield-Link moment connection, Pr-link is calculated using Rt = 1.2 and Fu; = 65 ksi. On the capacity side, the panel zone’s shear capacity is calculated using a ϕ = 0.9, whereas a typical seismic connection uses a ϕ = 1.0. Panel zone capacity check is required by AISC 341 and is provided in the design tools supplied by Simpson Strong-Tie.
D3. Strong-Column, Weak-Beam Check
The moment ratio between the columns and beams in Section E3.4a of AISC 341-16 is one of the requirements that distinguish a steel SMF from an IMF or OMF. For SMF, plastic hinges are expected to form in the beams (Figure D3.1a). If plastic hinges occur in the columns (meaning the beams are stronger than the columns), there is a potential for the formation of a weak-story mechanism (Figure D3.1b).
Yield-Link Moment Connection Strong-Column, Weak-Beam (Weak-Link) Check
A moment frame with Yield-Links is unlike the typical SMF. Typical SMFs would either have a reinforced connection (e.g., bolted flange plate connections) or weakened beam connection (e.g., RBS connections) where the plastic hinges are formed by the buckling of the beam flange and web (Figure D3.2). In the Yield-Link moment connection, the stretching and shortening of the links at the top and bottom of the frame beams are the yielding mechanisms (Figure D3.3). Instead of a strong-column, weak-beam check, the Yield-Link moment connection design procedure checks for a strong-column, weak-link condition where the ratio of the column moments to the moment created by the Yield-Link couple is required to be greater than or equal to 1.0.
D4. Beam Bracing
Since special moment frames are required to have the resilience to withstand large rotation at the column-to-beam connection, the beams must be stabilized using bracing to resist buckling.
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 (prior to reaching the target capacity) that may occur during the formation of plastic hinges in the beam, Section D1.2b of AISC 341-16 has the following requirement for highly ductile members (i.e., SMF): Both flanges of beams shall be laterally braced, with a maximum spacing of Lb = 0.095ryE/(RyFy).
Moment Frame Design Requirements
Per AISC 341-16, 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 farthest from the face of the column, where d is the depth of the beam. See AISC 358-16 for additional beam bracing requirements.
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 D4.1) or in flexure (Figure D4.2) prior to the formation of the plastic hinge in the beam at the connection region.
Ways to Brace a Beam
Per AISC 341, there are two methods to brace the beam: (1) lateral bracing (Figure D4.3) and (2) torsional bracing (Figure D4.4). 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.
Beam Bracing for Steel Frame with Yield-Link Moment Connections
With the introduction of the Yield-Link moment connection for moment frames, the Yield-Link structural fuses are designed as the
yielding mechanism. 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. If the beam is designed to span between the supports for the maximum load the Yield-Link structural fuse system can deliver, then beam bracing is not required.
The elastic beam behavior is supported by our testing as shown in Figure D4.5. 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%. Both physical testing and FEA validates beam bracing is not required for beams using the Yield-Link moment connection.
D5. Beam-to-Column Connection Bracing
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.
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 slenderness (L/r) for the column shall not exceed 60.
- The column’s required flexural strength transverse to the seismic frame shall include moment from beambracing forces of 2% of the beam flange strength.
For the column with Yield-Link moment connections, same beam-to-column bracing requirements are required by AISC 341. However, instead of designing for 2% of the available beam flange strength, the required bracing force for Yield-Link moment connections is 2% of the yield strength of the Yield-Link.
D6. Connection Design
The Yield-Link® moment connection incorporates the capacity-based design approach, wherein energy dissipation is confined predominantly within the reduced region of the Yield-Link structural fuse. Member and connection design is based on the probable maximum tensile strength, Pr-link, of the reduced region of the link (see Figure D6).
The following are steps for the Yield-Link moment connection design:
- Model and analyze the moment frame to get demand loads (moment, shear and axial) using code level forces.
- Design the link yielding area to resist the maximum axial force from all the load combinations. This means the
Yield-Links are designed to remain elastic under code force load combinations including lateral plus gravity loads.
- Once the yielding area is known, calculate the probable maximum tensile strength, Pr-link, of the link as:
Pr-link = Ay-link x Rt x Fu-link
- Ay-link = specified area of the reduced Yield-Link section, in2.
- Rt = ratio of the expected tensile rupture strength to the minimum tensile stress of the link stem material, 1.2
- Fu-link = specified minimum tensile strength of the link stem material, 65 ksi
- It is worthwhile to point out that we are using Rt and Fu for this calculation where other SMF connections typically use Ry, Fy and a Cpr factor that is less than or equal to 1.2. Using Ry of 1.1, Rt of 1.2, Fy of 50 ksi, Fu of 65 ksi and Cpr of
- 1.2. The difference in demand can be seen below:
Simpson Strong-Tie® Yield-Link Moment Connection Design Demand: 1.2 x 65 ksi = 78 ksi
Standard SMF Connection Design Demand: 1.1 x 50 ksi x 1.2 = 66 ksi
The reason for this approach is to accurately capture the ultimate strength of the Yield-Link structural fuse,
since we want to make sure this is the only region where inelastic action occurs.
- After Pr-link has been determined, design the rest of the connection to exceed this Pr-link demand load:
- Link stem-to-beam flange connection bolts
- Link flange-to-column flange connection bolts
- Link-flange thickness to prevent prying
- Beam-to-column shear tab connection
- Column panel zone
- Column flange thickness
- Stiffener/continuity plate (if required)
D7. Member Design
Similar to the connection design, members (beam and column) are designed for frame mechanism forces, assuming that links at both ends of the beam are at their probable maximum tensile strength. Tested as unbraced, the beam can be designed as unbraced or can be designed assuming beam bracing per the AISC specifications. There are no requirements for stability bracing of the beams at the link locations. For designing members using elastic analysis software, the amplified moment at each end of the beam need not be greater than probable maximum moment capacity of the Yield-Link® moment connection (Mpr-link).
Columns can be designed so bracing is only required near the top flange of the beam. Since the frame members are not dissipating energy (i.e., plastic hinges do not form), members are designed in accordance with AISC Steel Construction Manual (AISC 360). This means b/t and h/tw ratios in AISC 341 are not applicable to our beam and column members in the frame when designed using a pinned-base design. However, if the base is designed as fixed or partially fixed, (i.e., the columns may yield at the base), then AISC 341 slenderness ratios will need to be met for the columns at the base level. Please note that the design of moment frame columns using Yield-Link moment connections must consider the interaction of amplified moment and amplified axial loads. This exceeds the AISC 341-16 requirement to design for only the amplified axial load and neglect the moment in the column design for overstrength load combinations.
As part of our FEMA P695 study for connection qualification, extensive study shows following the above design procedure(s) for the Yield-Link moment connection leads to improved performance in reducing the amount of expected damage and residual frame drift. A comparison of two 12-story frames using RBS and Yield-Link moment connections can be seen in Figure D7.1. The difference in the hinging distribution in the Yield-Link frame versus the RBS frame is evident in the following figures.
D8. Protected Zones
According to the AISC 341-16 Section E3.5.c, the region at each end of the beam subjected to inelastic straining (plastic hinge formation) shall be designated as a Protected Zone. Each prequalified moment connection in AISC 358-16 has its own section on what is considered a Protected Zone. A clear marking denoting the protected zone is required, as well as a sign prohibiting penetrations and welds to this zone as it would negatively affect the performance of the moment connection. AISC Code of Standard Practice for Steel Buildings and Bridges (ANSI/AISC 303-16) also has a similar requirement where the Fabricator shall permanently mark the protected zones designated in accordance with AISC 341. If markings are obscured in the field after application of fire protection, then it shall be re-marked.
Figure D8.2 shows the protected zone for an RBS connection. As can be seen in these examples, the protected zone encompasses the beam flange and the beam web, because this is the location where the expected inelastic deformation will occur.
This means that during construction, the owner’s designated construction representative will have to confirm with the mechanical, electrical and plumbing (MEP) trades that no penetrations will be made through the beam web at these locations. In addition, someone will have to physically mark these locations on each moment connection, as seen in Figure D8.2.
Protected Zone for Yield-Link® Moment Connections
Figure D8.3 shows the protected zone for the Yield-Link moment connections. Since the beam is not the yielding element, the protected zone only includes the elements in contact with the link at the beam flanges and shear tab at the beam web.
- Protected zone includes the following elements:
- Link flange and link stem
- BRP plates
- Beam flange areas connected to the link stem
- Column flange areas connected to the link flange
- Shear tab and beam web at shear tab (two inches away from edges of shear tab)
- Link-to-beam connection bolts
- Link-to-column connection bolts
- Shear-tab-to-beam connection bolts
- No attachment shall be made to the protected zone.