Frequently Asked Questions: Strong Frame® Special Moment Frame

What is a structural fuse?

Most people are familiar with electrical fuses.  When an electrical circuit gets overloaded, an easy-to-replace fuse intentionally fails before a bigger problem is created elsewhere.  A structural fuse is very similar.  Most seismic-force resisting systems have some aspect to them that is designed to be the failure mechanism, or fuse, during an earthquake. The problem is that the damage is typically not constrained to some easy-to-replace structural element.  A structural fuse, similar to that electrical fuse, can be defined as passive energy dissipation (PED) device that has been implemented to enhance structural performance by absorbing seismically induced structural damage. This allows the primary structure to remain intact and with minimal damage.  If designed accordingly, only the fuses would need to be replaced following a severe seismic event, making repair work easier and more expedient.

The structural fuse concept can be implemented in new or existing structures using various kinds of PED elements. The Simpson Strong-Tie® Strong Frame® special moment frame Yield-Link® structural fuse fits the definition of a structural fuse. The Strong Frame special moment frame links are designed to be inexpensive, disposable and easy-to-replace structural elements that bear the brunt of inelastic (or permanent) deformation while the remainder of the steel frame remains elastic or with minor inelastic deformations.

Is the Strong Frame® Yield-Link® a structural fuse?

Yes.  Because virtually all inelastic demand is confined to the Yield-Link elements of the connection, making it a structural fuses.

Is the base connection pinned or fixed?

All Strong Frame moment frames utilize a pinned base connection where the column base is not a fully rigid connection as if it was embedded in a concrete foundation.  Please contact Simpson Strong-tie if you have any projects that required a fixed based solution to control frame drift.

Is lateral torsional beam bracing typically required for an ordinary, intermediate or Strong Frame special moment frame?

Beam bracing is typically required for intermediate and special moment frame.

Why not just use an Ordinary Moment Frame for wood structures in high seismic areas?

Because of the limited inelastic capacity (ability to yield and absorb large lateral movements) of ordinary moment frame systems, their use in high seismic regions has limits enforced by code provisions, which can eliminate the ordinary moment frame as a potential solution. This will force the designer to directly address the beam bracing challenges for special moment frames in wood and cold-formed steel light-frame structures. Therefore, other more difficult and/or costly solutions may have to be employed.

Is lateral torsional beam bracing typically required for an OMF, IMF or SMF ?

Beam bracing is typically required for intermediate and special steel moment frame systems because the beam is designed to take the inelastic deformation. Lateral bracing of steel moment frame beams can be difficult or impossible to achieve in wood buildings due to the AISC 341 required brace stiffness requirement. Typically, beam bracing is not required for an OMF because the beam is designed to span between the columns and the beam is expected to remain elastic during a seismic event.

Does the Strong Frame® special moment frame require beam lateral bracing?

No. Our proprietary Yield-Link® Structural Fuse is designed to be the yielding mechanism, not the beam itself.  The beam and columns are all designed to develop full strength of the connection, while themselves experiencing little, if any, inelastic demand.  This permits them to be designed without beam bracing, thus no beam bracing is required.  This capability enables the Strong Frame special moment frame to be used for a wider range of wood or cold-formed steel light-frame structures.

How does the special moment frame connection work?

The beam-to-column moment connection is a proprietary partially restrained moment connection in lieu of a fully restrained moment connection, typically used for Strong Frame special moment frame steel Special Moment Frames. The connection utilizes a welded “T” section (Yield-Link® Structural Fuse) bolted to the column flanges.  The “T” contains a reduced section to control axial stiffness, yield strength and tensile strength.  A buckling restraint plate is placed over the yielding section to prevent buckling when in compression. The buckling restraint plate is bolted on either side of the Yield-Link® Structural Fuse reduced area to the beam flange, with the bolts passing through a spacer plate with the same thickness as the stem of the Yield-Link® Structural Fuse.  Beam shear is transferred to the column flanges from the beam web via a single plate shear connection (shear tab).  The top and bottom holes of this 3-hole single plate shear connection are slotted horizontally to avoid moment transfer through the shear plate. 

What is the definition of a partially restrained (PR) connection?

According the AISC 360, there are two types of moment connection: Fully Restrained and Partially Restrained.   These terms restrained refers to the degree of moment transfer and the associated deformation in the connection. Typically for special moment frames, the beam-to-column moment connection is a fully restrained connection. In that analysis, the engineer is permitted to assume there is no rotation between the beam and column at the beam-column connection (i.e., a 90-degree angle is maintained in the joint even as the frame sways back and forth in an earthquake).  A partially restrained connection has more than negligible rotation at the beam-to-column moment connection. The Strong Frame special moment frame is considered a partially restrained PR connection, and as such the force-deformation response characteristic of the connection shall be included in the analysis and design of the frame.

How can the frame be repaired after a large seismic event?

Because the Yield-Link® Structural Fuses are bolted, not welded, to both the beam and the column they can be removed and replaced if needed.  The shear tab remains undamaged and continues to carry gravity loads so the frame can remain in place while the Yield-Link® Structural Fuses are replaced. 

What is AISC?

The American Institute of Steel Construction, often abbreviated AISC, is a not-for-profit technical institute and trade association promoting and supporting the use of structural steel in the U.S. construction industry. Headquartered in Chicago, IL., they develop specifications, participate in code development, provide technical assistance, provide quality certification, etc. See www.aisc.org. AISC is to the steel construction industry as NDS is to the wood construction industry.

Were the Strong Frame Special Steel Moment Frame Connections tested or calculated like OMF?

All Strong Frame® special moment frame connections are required to be tested in accordance with AISC 341-05 Chapter K or be listed in AISC Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications AISC 358.  

Is the Strong Frame® special moment frame a prequalified moment connection under AISC 358?

Simpson Strong-Tie has submitted the results of nine, full-scale Strong Frame special moment frame tests to the Connection Prequalification Review Panel (CPRP) for consideration of inclusion in AISC 358. At the time of this writing, it is in the final stages of approval. Once approved, our Strong Frame® special moment frame will be part of the supplemental release in 2013 as a prequalified connection and be referenced in 2015 IBC.

Can engineers design their own Strong Frame® special moment frame Yield-Link® Structural Fuses using the AISC 358 methodology and then manufacture them?

Yes and no.  Engineers will be able to design Special Moment Frames using our technology; however.  as with other proprietary prequalified connections in AISC 358, the Yield-Link® Structural Fuses can be purchased from Simpson Strong-Tie or a licensed and approved fabricator. See worksheet (page 111 of the Strong Frame Moment Frames catalog) to fill out calculated and designed dimensions of the Yield-Link® Structural Fuses. Please contact Simpson Strong-Tie for manufacturing and sale.

Does Strong Frame® special moment frame CONNECTION have a code report?

Yes, Strong Frame® special moment frame Yield-Link® Structural Fuse is listed in ICC-ES ESR-2802.

How many types of links will Simpson Strong Tie offering at this time?

Our Strong Frame® special moment frame standard product line offers three links at this time: light, medium and heavy (L,M,H).  However, other size links are available for special engineered frames.  Please contact Simpson Strong Tie if your solution falls outside of our Catalog or Strong Frame Selector Software offerings.

How can we purchase Strong Frame® special moment frame?

Simpson Strong Frame Special Moment Frames are offered three ways. First, a frame can be chosen from the 192 pre-engineered catalog solutions. Second, non-catalog sized frames (custom 1-story frames) can be designed using the Strong Frame® Selector software. Third, an engineer of record can detail their own frame using our technology and either purchase one of our three links or provide a design within our design limitations and Simpson Strong-Tie® will manufacture and provide the links to the installer.

What is the R value of the Simpson Strong Frame® special moment frame?

Strong Frame special moment frames designed with the Simpson Strong Frame Yield-Link® Structural Fuse gets an R value of 8 for the lateral structural steel systems and an R value of 6.5 when used with wood- or cold-formed-steel-framed shear wall systems.

Will the Strong Frame® special moment frame fit in a standard 2x6 wall like the Strong Frame Ordinary Moment Frame?

No. Strong Frame® special moment frame uses standard W-shapes, not built-up plate girders, so profiles are larger. All standard Strong Frame Special Moment Frames will fit in a 2x8 wall space.

How can the frames ship?

The frames can be shipped in three pieces (flat) or, for large jobs, can be pre-assembled in our factory and shipped to the jobsite ready to install (no dimension can be greater than 14 feet for this option).

Will the Strong Frame® special moment frame be a good solution for soft-story or weak-story retrofits?

YES! The bolted connection and ease of installation make it the best choice. There are no weight or height limitations imposed on its use in seismic areas since the Strong Frame is a Steel special moment frame. In addition, the unique ductile properties of the Simpson Strong-Tie® Yield-Link® Structural Fuse connection make it an ideal solution for older, less ductile, wood structures.

What is a tension control bolt and how does it work?

A tension control bolt is a heavy duty steel bolt used in structural steel construction. The head is usually domed and is not designed to be driven. The end of the shank has a spline on it, which is engaged by a special power wrench that prevents the bolt from turning while the nut is tightened. When the appropriate bolt tension is reached, the spline will shear off. Tension Control Bolts are used for attaching the Yield-Link® Structural Fuse to the steel beam flanges. All inspection is done in our factory in a controlled environment installation.

What is the definition of a snug-tightened bolted connection?

A snug-tightened bolted connection is defined in the RCSC Specification for Structural Joints Using High-Strength Bolts. The definition has recently been updated to the condition that exists when all of the plies in a connection have been tightened sufficiently to prevent the removal of the nuts without the use of a wrench. The old definition, which is known to most engineers, is the tightness attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the plies into firm contact. All field installed bolts in the Simpson Strong Frame requires snug-tight bolted connections only.

Is special field inspection required for Strong Frame® special moment frame?

Since the link-to-column connection is allowed as a snug-tight connection, no additional field inspection beyond what's required by code or the use of direct tension indicating washers are required. Special inspection might be required for the placement of the non-shrink grout between the top of concrete and bottom of Strong Frame® special moment frame base plates. This should be in the general notes of the project's plans under "Special Inspections." All tension control bolts are lot inspected and continuously inspected during installation. Lot and inspection info can be found here or by scanning the QR code on the stickers adjacent to each moment connection on the frame.