A Webinar: Design Resilient Steel Structures with Yield-Link® Moment Connections in RAM Structural System was presented February 4, 2021, and is available on-demand. This wiki includes links to the webinar and to other useful resources on the topic. Several questions were submitted during the webinar, the Q & A is included below.
Tim Ellis, PMP, Market Segment Manager, Simpson Strong-Tie
Brandon Chi, P.E., S.E., Lateral Systems Engineering Manager, Simpson Strong-Tie
Allen Adams, P.E., S.E., Chief Structural Engineer, Bentley Systems, Inc.
Presented Live: February 4, 2021
Link to Webinar Recording, On-demand:
Link to Presentation PDF:
Webinar Slides: Design Resilient Steel Structures with Yield-Link Moment Connections in RAM Structural System - Bentley Communities
Time limitations on the webinar only allowed for an introductory presentation on the Yield-Link connection and its analysis and design in the RAM Structural System. Several resources are available for more comprehensive information on the Yield-Link moment frame connection and to assist in modeling, analyzing and designing the connection in the RAM Structural System.
For more information on the Yield-Link connection, see the Simpson Strong-Tie Yield-Link Moment Connection Design Guide:
For a detailed step-by-step guide to Yield-Link in the RAM Structural System, see this wiki, “Specifying and Designing Yield-Link in the RAM Structural System” on Bentley Communities:
Yield-Link is a prequalified connection. See Chapter 12 Simpson Strong-Tie Strong Frame Moment Connection of AISC 358-16 Prequalified Connections for Special and Intermediate Moment Frames for Seismic Applications.
RAM Structural System:
Does a stiffness reduction need to be considered for the drift check?
For moment frames using RBS’s, it is recognized that the reduction in frame stiffness due to the reduced cross-sections in the beams results in greater drifts. This can be accounted for either by amplifying the calculated drifts or by modifying the stiffness of the beams at the location of the reduced beam sections; RAM Structural System has the option of doing either. For moment frames using Yield-Link, in the RAM Structural System the beam/column joint panel is automatically represented in the analysis by a spring that explicitly accounts for the reduced stiffness of the Yield-Link connection. The reported drifts therefore include the impact of those reduced stiffnesses, and do not need to be further amplified.
The prequalified method given in AISC 358 does not calculate and consider the bending of the column flange in frame drift. Depending on beam depth, could there be a significant difference between calculated and actual drift?
For the overall requirement of 4% story drift in AISC 341, deflection from the contribution of the beam, column and connection are all included. However, as part of the test reporting, portions of the total inelastic rotation (total rotation minus elastic rotation) contributed by each component of the test specimen are reported. In the earlier versions of AISC 341, this inelastic rotation requirement was 0.03 radians. From our testing, we met this 0.03 radian inelastic rotation requirement. In addition, we just finished a validation testing in Virginia Tech using a “T-Configuration” setup with a beam length of 16’-0” and column height of approximately 12’-0”. Performance between the portal frame test setup and “T-configuration” setup is very similar. In Chapter 12 of AISC 358s2-20, using the equations in Step 11, one can calculate a rotational spring to represent the Simpson Strong-Tie Yield-Link Connection. (See the Simpson Strong-Tie Yield-Link moment connection modeling guide for comparison of test result and calculated deflection using a rotational spring to model the Yield-Link connection: F-L-SMFYLCMG). From an analysis perspective, this effect is included in the panel spring stiffness used by RAM Structural System.
RBS’s yield slighter higher drifts as compared to WUF-W, for the same member sizes. What would be the comparison for the Yield-Link moment connection?
For the Yield-Link Connection, in addition to the beam and column, the Yield-Link connection also contributes to the overall drift. Depending on beam, column and Yield-Link connection sizes, the drifts using Yield-Link Connectioncan vary from approximately 5% - 10% greater than those for RBS, and up to 10%-20% greater than a WUF-W.
What Rigid End Zone setting should be specified for this connection for strength and drift analysis?
The option to Ignore Effects should be selected. The panel spring stiffness used by the program accounts for the reduced stiffness of the panel zone.
How are the gravity shears and moments at the ends of the beam accounted for in the Yield-Link connection?
The beam shear due to seismic, wind, and gravity loads are transferred to the column through the shear tab. Required shear strength Vu, of the beam and column shear tab connection is noted in AISC 358s2-20 EQ 12.9-34 (see excerpt below). Not only is the shear tab/beam web required to resist all the gravity loads, but it is also required to resist the shear loads from the Yield-Link Connection when the Yield-Link connection at both ends of the beam have reached its maximum probably moment capacity (Mpr). This is similar to the RBS beam web shear design requirement at the beam web-to-column connection.
The Yield-Link connector is only designed for axial forces, so the beam end moments are resolved into seismic, wind, and gravity axial loads on the top and bottom connectors (the gravity moments generally induce an axial tensile force in the top connector and an axial compressive force in the bottom connector). These are automatically calculated and considered in the design in the RAM Structural System.
Does the vertical line of bolt in the shear tab have horizontal slots to allow for rotation?
Except for the bolt at the mid-depth of the beam, the holes are slotted to allow the beam to rotate around the center bolt (as the Yield-Link connectors shorten and elongate) without inducing any moment on the shear tab.
If the shear tab bolts have horizontal slots to allow rotation, how are axial/collector loads transferred into the columns? Is that captured in the analysis using RAM Structural System?
The vertical line of bolts (with horizontal slots in the shear tab) are designed to resists the vertical shear loads (Vu). The horizontal line bolts (those aligned with the centerline of the beam) with vertical slots in the shear tab are designed to resist the axial loads. To increase the axial load capacity, more horizontal bolts can be provided. Both vertical and horizontal slotted holes in the shear tab are designed to accommodate a connection rotation of +/- 0.07 radians (see AISC 358s2-20 Step 15.2). Both vertical and horizontal forces in the shear tab and beam web are captured in the analysis and design in the RAM Structural System.
Are the bolts slip critical?
The Link stem-to-beam flange bolts are pre-tensioned bolts only, not slip critical. The contact surface between the Yield-Link and the beam flange are required not to be painted (to match testing conditions). However, no surface preparation is required. All other bolts in the Yield-Link connection can be installed snug-tight.
What does the composite deck attachment detail look like at the connection? How does the deck bear on the top flange if the Yield-Link connector is in the way?
For composite deck applications, Simpson Strong-Tie offers a cover plate that goes over the Yield-Link connection (see https://www.strongtie.com/structuralsteel/ylc_cover/p/yield-link-cover-plate).
What is the “protected zone”?
A protected zone is the region where inelastic deformation/movement is expected to occur. For the RBS connection, the protected zone includes the reduced beam section region and the distance from the radius cut to the face of the column in the beam. For the Yield-Link connection the protected zone is shown in the figure below:
Is this available for Special Moment Frames (R = 8)?
The Yield-Link connection can be analyzed and designed in the RAM Structural System for SMF (R=8), IMF (R=4.5), and MF (R=3).
What are the beam bracing requirements for Yield-Link compared to RBS and WUF-W?
Because the connection is designed such that the Yield-Link connector is the fuse element while the beams and columns remain elastic, the stability beam bracing requirements of Section D1.2 of AISC 341-16 are not required for the Yield-Link moment frames. The bracing merely needs to conform to the bracing requirements of AISC 360.
How good is the building’s re-centering ability with the Yield-Link moment frames?
From our FEMA P695 study, because of the better hinge distribution along the height of the structure, less residual drift was observed. However, the Yield-Link connection does not have any re-centering abilities, same as any other standard moment connection.
How do you replace the top Yield-Link connector if there is a concrete deck above the beam?
Limited concrete removal above the Simpson Strong-Tie concrete cover is expected in order to replace the Yield-Link at the top of the beam flange. There is no metal deck directly on top of the Yield-Link connection, only on the light gage Yield-Link cover. In addition, the Yield-Link cover is only attached at the end of the Yield-Link connectorat the beam flange. After removal of the concrete above the Yield-Link cover, the cover can be pried open to expose the Yield-Link at the top of the beam.
Why are Yield-Link connections designed with combinations with the overstrength factor, Ω0, as it is not a code requirement for RBS?
In Yield-Link moment frames, the Yield-Link connector is the yielding element. In order to assure that all of the yielding takes place in the connector and that the rest of the system remain elastic, the beams, columns and connections are designed for omega-level forces. For RBS’s, the connections must be designed for the shear given by Eq. (5.8-9) of AISC 358-16, which is based on the probable maximum moment, Mpr, at the center of the reduced beam section, to ensure that the connection doesn’t fail before the reduced beam section.
It was indicated that we need to over-design the columns and beams for the Yield-Link moment frames. How much higher do we need to design?
The members need to be designed for combinations that include seismic forces that have been amplified by the overstrength factor, Ω0. This is described in Section 12.4.3 Seismic Load Effects Including Overstrength in ASCE 7-16. Ω0 factors are given in Table 12.2-1 and, for steel moment frames, has a value of 3.0.
Does RAM Structural System perform the Column Seismic checks for the Ω0 load combinations?
As required for Yield-Link moment frames, RAM Structural System does perform the member and connection checks using the load combinations with the overstrength factor, Ω0.
Can the RAM Structural System run the analysis and select the minimum acceptable links automatically?
No, not currently. The engineer must assign initial Yield-Link sizes. The default sizes are a good starting point. Using the View/Update feature, the engineer can quickly and easily
Have there been any parametric studies on how Yield-Link would perform against other proprietary moment frame connection systems?
Each system has its own strengths and weaknesses, and some of those will be more or less important on any particular project. As is the case for all framing systems, least-weight is not necessarily the best. Performance, drift, retrofit, repair, initial cost, lifetime cost, etc., are all considerations. Owners don’t all view these the same. As is the case for all framing systems, least-weight is not necessarily the best. RAM Structural System has extensively implemented the analysis and design requirements of several moment frame connection systems. Engineers are invited to perform those comparisons in RAM Structural System on their specific project to help determine the most economical system for that project.
Is there a Simpson catalogue that details each of the Yield-Link sizes?
See page 11 of the Simpson Strong-Tie Yield-Link Moment Connection Design Guide, listed at the beginning of this document: https://www.strongtie.com/structuralsteel/smf-yl_link/p/yield-link-moment-connection
Are CAD files generated so we can draw up the connection for interferences?
Detailed CAD files (DXF) are created using the Export Yield-Link DXF command. These include frame elevations and joint connection details. A comprehensive set of typical details and notes is also created. These files are suitable for any drafting or detailing program.
How is the AISC 360 Direct Analysis Method’s stiffness reduction considered, for strength vs drift check?
This question is applicable to all frames, not only to the Yield-Link moment frame connection. The AISC 360 Direct Analysis Method requires that a 0.8 stiffness reduction be applied to all stiffnesses that contribute to the stability of the structure, and that an additional reduction factor, tau_b, be applied to the flexural stiffnesses. This is required for the analysis used to perform the member strength checks, but not the drift checks. Due to the short time allotted for the demonstration of the RAM Structural System in the webinar, and to avoid a lengthy discussion on a tangential topic, this issue was not addressed. However, for a real project the following procedure should be performed in the RAM Structural System. When checking drift and to get the building periods and seismic story forces, unselect the Criteria option to include the Direct Analysis Method stiffness reduction; select the gravity, wind, and drift seismic load cases to be analyzed; analyze; and then check the drifts compared to the appropriate wind and seismic drift limits. Also check for Torsional Irregularity. Modify sizes as necessary. When drifts are acceptable, proceed with the strength checks. Select the Criteria option to include the stiffeness reduction; select gravity, wind, and strength seismic load cases; analyze; and perform the standard and seismic code checks.
In the RAM Structural System, are the selected drift control points the only ones used in the calculation of drift ratios and torsional irregularity determination, or does the program consider every point on the diaphragm? If the Torsional Irregularity check reveals an irregularity, then to what part of the design process would you have to cycle back to to fix the loading to comply?
This question applies to all frames, not only to the Yield-Link moment frame connection. In order to keep the Drift report from being unduly long, the program only investigates drift at the control points specified by the engineer. Since only the center and the extreme corner points are of interest (the rest won’t control), the user typically only needs to specify a very few points, which can be done very quickly. In the example used in the demonstration it was only necessary to specify and consider three points.
If the structure is Torsionally Irregular, ASCE 7 requires that frames be designed for the seismic load cases that include the accidental torsion, that is, the plus and minus 5% eccentricity cases (four cases total, two in each axis), and requires that frames be designed for the wind load cases that include torsion (12 cases total). If the structure is not Torsionally Irregular, ASCE 7 allows the frames be designed for the seismic load cases that don’t include the accidental torsion (two cases total, one in each axis, through the center of mass – see Section 188.8.131.52), and allows the frames to be designed for only the wind load cases that don’t include torsion (four cases total – see Appendix D). RAM Structural System has options to automate each of these.
Has Yield-Link been used for any projects under the jurisdiction of California DSA? Were there any issues getting approval?
Yield-Link connection has been used in at least 5 completed DSA projects that have gone thru DSA review from various DSA offices and locations. The latest currently under construction is the Fremont Unified High School District Office building in California. The entire lateral system for the structure utilized Yield-Link Moment Connections.
Originally the connection approval would have to go through the alternate means process for beam sizes larger than what was listed in AISC 341-16. However, with the release of AISC 358s2-20, W36 columns/beams and 6 in2 Yield-Links are now permitted to be used in DSA projects without the additional requirements of the alternate means process.
Is the Yield-Link feature available in STAAD.Pro?
No, it is not currently implemented in STAAD.
Is there a yield link option in RAM Elements?
No, it is not currently implemented in RAM Elements.