The Legacy of Loma Prieta

Every year on October 17, we take a moment to reflect on the 1989 Loma Prieta earthquake. The 6.9-magnitude earthquake was one of the most powerful and costly quakes to shake the San Francisco Bay area since the 7.9-magnitude earthquake of 1906. The quake caused an estimated $6 billion in damage and, tragically, resulted in 63 deaths and 3,757 injuries.

Many of those casualties were due to failing infrastructure when sections of the Nimitz Freeway collapsed.

Simpson Strong-Tie was founded in Oakland, California — practically in the heart of the Bay Area. Earthquakes were never far from the minds of our founders. It’s why even before Loma Prieta our mission was to provide solutions that help people design and build safer, stronger structures. However, it’s safe to say the earthquake not only reinvigorated our mission but inspired countless structural engineers who would go on to define the next 30 years of Simpson Strong-Tie research and development into community resilience.

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Project Profile: Special Moment Frame Dropped-Beam Solution Saves Day(light) by Preserving Window Openings

Simpson Strong-Tie recently had the opportunity to work with MAK Construction to come up with a rather unique solution for a residential project in Phoenix, Arizona. Our Strong Frame® Moment Frame Selector software was used by the engineer on record, L.R. Nelson Consulting Engineers, LLC, to design this truly “special” special moment frame (SMF). The challenge for this particular moment frame design was figuring out how to work around a large garage door opening on the bottom floor without obstructing the window openings on the next floor, because the standard SMF design would cause the beam to cross right through the middle of the windows as they were situated. The solution required dropping the beam below the top of the columns, something seldom seen in moment frame designs. However, our engineering services, in collaboration with L.R. Nelson Consulting Engineers, were able to determine that dropping the beam to the needed level would be quite feasible in this case, and within 24 hours a new design was sent to the EOR and to the contractor for final approvals, which were granted.

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The New Way to Connect with Strong Frame®

The April SE blog article, What Makes Strong Frame® Special Moment Frames So Special, explained the features and benefits of the Yield-Link® structural fuse design for the Strong Frame® special moment frame (SMF) connection. In this blog, I will be introducing the Yield-Link end-plate link (EPL) to the Strong Frame connection family.

What is the EPL?
The EPL connection (Figure 1) is the latest addition to the Strong Frame Strong Moment Frame (SMF) solution. The new EPL connection can accommodate a W8X beam which is approximately a 33% reduction in beam depth from a W12X beam. The frame is field bolted without the need for field welding which means a faster installation. The snug-tight bolt installation requirement means no special tools are required. The EPL SMF connection has the same benefit of not requiring any additional beam bracing as the T-Stub connection. The frame can be repaired after a large earthquake by replacing the Yield-Link connection. Since the shear tab bolts will be factory installed, installation time for the frame is reduced by 25% making the EPL connection one of the most straightforward connections to assemble.

Figure 1: New Yield-Link EPL connection

Why Did We Develop the EPL?
The development of the EPL came from strong interest and numerous requests to offer a solution with more head room for clearance of retrofit projects or enhancement for new construction using a shallower beam profile. The original T-stub link design has the shear tab welded to the column flange. The geometry of the shear tab meant that a W12X beam is required to accommodate the Yield-Link Flange. In Figure 2, you can see that a shallower beam profile will bring the Yield-Link flange closer to each other and limit the attachment of the shear tab. A new connection was needed.

Figure 2: Yield-Link flange interference with shear tab
Figure 3: 3 Bolt configuration with notched flange plate. (The 3rd bolt is on opposite side of beam.) The asymmetric layout produced uneven force distribution in the bolts.

How Did We Develop the EPL?
Multiple configurations were studied, including a notched flange plate with 3 bolts (Figure 3) to avoid interference with the shear tab connection to the column. In the end, a compact end plate link combining the shear tab and Yield-Link stem in a single connection was the final design. However, many questions loomed over the prototype. How will the single end plate design perform in a full scale test? Will the new configuration change the limit state? These questions needed to be studied prior to launching an expensive full-scale test program with multiple samples and configurations. Numerous Finite Element Analysis (FEA) models were studied and refined prior to full scale testing of a prototype. Modeling included ensuring the stem performs as a fuse (Figure 4) as discussed in the April blog and the integrity of the shear tab is maintained in the compact design. Figure 5 shows a graph comparing the analytical model to the actual full scale test. The full scale test with a complete beam and column assembly was performed to the requirements under AISC 341 Section K. The full scale test passed the requirements for the SMF classification as can be seen in Figure 6 for the specimen with 6-inch columns and 9-inch beam.

Figure 4: Equivalent Plastic Strain Plot of Yielding-Link Stem
Figure 5: Full Scale Test vs. Analytical model
Figure 6: Moment at Face of Column vs. Story Drift

Where Can I Get More Information?
The EPL is now recognized in the ICC-ES ESR-2802 code report as an SMF. EPL solutions are also offered in the Strong Frame Moment Frame Selector Software. Want to see how the new connection and member sizes can expand your design options? Visit www.strongtie.com to download the new Strong Frame Design Guide or contact your Simpson representative for more information.

What Makes Strong Frame® Special Moment Frames So Special

In a Structural Engineering Blog post I wrote last October, “Soft-Story Retrofits Using the New Simpson Strong-Tie Retrofit Design Guide,” one item I barely touched on at the time was the benefit of using Simpson Strong-Tie® Strong Frame special moment frames to retrofit vulnerable soft-story wood-framed buildings commonly found on the West Coast. In this post, I will be diving into more detail on a few features that make the Strong Frame special moment frame truly special.

In the recent release of the ANSI/AISC 358-16 (AISC 358-16), the Simpson Strong-Tie Strong Frame moment connection has been included as a prequalified special moment frame (SMF) connection.  Prequalified moment connections are structural-steel moment connection configurations and details that have been reviewed by the AISC Connection Prequalification Review Panel (CPRP) and incorporated into the AISC 358 standard. What’s unique about this newly prequalified connection is that it’s the first moment connection to be prequalified in AISC as a partially restrained (PR-Type) moment connection.

prequalified-connections

With this recent inclusion into AISC 358-16, we’ve also developed our newly released Strong Frame Design Guide  to help designers understand the differences in design and detailing between the Strong Frame connection and traditional SMF connections. The following are just a few of the key differences discussed in this guide.

SMF Yielding Elements

Traditional prequalified moment frames most often require a welded connection with either a weakened beam or a stiffened connection. SMF connections are designed so that the beam will yield as necessary under large displacements that may occur during a seismic event. The yielding of the beam section provides energy dissipation and is designed to ensure that the fully restrained beam-to-column connection isn’t compromised. The current design philosophy is the product of extensive testing of SMF connections based on studying the effects of the 1994 Northridge and 1989 Loma Prieta earthquakes in California. Figures 1, 2 and 3 below depict test specimens that demonstrate yielding at the designated areas of the beam.

special-moment-frame-development

The Strong Frame SMF has taken a different approach to the traditional connections by utilizing a Yield-Link® structural fuse designed to provide the energy dissipation for the beam-to-column moment connection. This is a modified T-Stub that has a reduced section in the stem. The yielding during a seismic event has been moved from the beams to the Yield-Link structural fuse. The fuse can be replaced after a major event, very much like an electrical fuse when overloaded. A traditional moment frame may require a much more invasive structural repair.

yielding-area-strong-frame

Beam Lateral Bracing

The traditional types of prequalified connections, as along with other proprietary connections included in AISC 358, all require the beam to yield so as to dissipate energy as discussed above. These types of connections require that the beam be braced to resist the lateral torsional buckling per code. However, it is difficult to meet the bracing requirements in the case of a steel SMF in a wood structure.

stiffness-model-beam-stability-wood-construction

With the Strong Frame SMF connection, the energy dissipation is moved from the beams to the Yield-Link structural fuses, with the connection following a capacity-based design approach. This allows the connection to remain elastic under factored load combinations. With the yielding confined to the structural fuses, inelastic deformation is not expected from the members and lateral beam buckling braces are not required. The beam can be designed to span the entire length without beam bracing. See also this blog post.

Column-Beam Relationship Requirements

Traditional SMF follow a strong column – weak beam requirement to ensure plastic hinging occurs in the beams and not the columns. If the energy dissipation takes place within such hinging in the beams, the column members will remain elastic so as to provide stability and strength for the above stories. If plastic hinges occur in the columns, there is a potential for the formation of a weak-story mechanism.

weak-story-mechanism

The Strong Frame special moment frame is unlike the traditional SMF, where the plastic hinges are formed by the buckling of the beam flange and web. In the Strong Frame SMF, the stretching and shortening of the links at the top and bottom of the Strong Frame beams are the yielding mechanisms. So instead of a strong column – weak beam check, the Strong Frame 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.

yielding-strong-frame-links

Installation

Traditional moment frame connections typically require welding in the field. Where bolted SMF connections are used, pretensioned bolts are necessary. Both welding and pretensioned bolts require third-party special inspection.

The Strong Frame SMF has been designed and tested as a 100% field-bolted connection. Unlike other bolted options, the Strong Frame’s field-bolted connections only need to be made snug tight. No onsite bolt pretensioning or special inspections are required with this system. This allows the beams and columns to be maneuvered into place, erected and installed in a fraction of the time needed for the welding, lateral-beam-bracing installation and additional inspections or repairs that traditional moment frames typically require.

T-Stub-link-installationv2

Design

One last item I’d like to discuss is the design service that Simpson Strong-Tie provides for the Strong Frame special moment frame. Whether you design moment frames only once in a while or on a regular basis, the Strong Frame design team will provide you with No-Equal design support at no additional cost. Designers receive a complete package that includes drawings and calculations, which are submittal-ready. This ensures that you’ll have a frame connection design meeting the latest codes and design requirements. Contact strongframe@strongtie.com for more information or to request design support.

To learn more about the special benefits and uses of Strong Frame moment frames, check out the following links:

“You Cannot Escape Responsibility Tomorrow by Evading it Today”

While the contents of this blog are certainly not what Abraham Lincoln had in mind when he made the statement that I’m using to title this blog post, it does speak volumes to the pertinence of what will be discussed today. “Design by others” or some variation of this appears in many parts of Simpson Strong-Tie details.Continue Reading

Breaking News: Simpson Strong-Tie® Strong Frame® Special Moment Frame Testing Today

Today the NEES-Soft project has begun testing the steel Simpson Strong-Tie® Strong Frame® Special Moment Frame as a retrofit option for soft-story buildings at the NEES outdoor shake table facility at UC San Diego. The testing is focused on validating the FEMA P-807 design procedure, which attempts to create a least-cost retrofit solution by only retrofitting the garage areas of problem buildings.Continue Reading

Soft-Story Retrofits

In February 2007 I had the opportunity to volunteer for a Soft-Story Sidewalk Survey for the San Francisco Department of Building Inspection. The purpose of the survey was to inventory buildings in San Francisco that appeared superficially to have soft or weak first stories. The volunteers were given a list of addresses to review and we recorded if the building was more than three stories tall, had five or more dwellings, and estimated what percentage of the ground level had openings in the walls. No structural analysis going on, just counting stories, mailboxes, doors and windows.

San Francisco soft-story structure. Photo credit: USGS.
San Francisco soft-story structure failure. Photo credit: USGS.
A collapsed house in San Francisco from the 1989 Loma Prieta earthquake. Photo credit: Adam Teitelbaum, AFP, Getty Images.
A collapsed soft-story in San Francisco from the 1989 Loma Prieta earthquake. Photo credit: Adam Teitelbaum, AFP, Getty Images.

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