Special Moment Frame – Protected Zones
Moment Frame Design Requirements and Assumptions
D4b. 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. Figure D4b.1 shows the requirements from the Los Angeles Department of Building and Safety (LADBS). 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 341-16. If markings are obscured in the field after application of fire protection, then it shall be remarked.
Figure D4b.2 shows the protected zone for an RBS connection. As can be seen, 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 D4b.3.
Figure D4b.1 — LADBS Protected Zone Marking Requirements
Figure D4b.2 — Protected Zone for an RBS Moment Connection
Figure D4b.3 — Protected Zone Marking for an RBS Connection in the Field (Sprayed on Top of Fireproofing)
Strong Frame Special Moment Frame Protected Zone
Figure D4b.6 shows the protected zone for the Strong Frame SMF connection. Since the beam is not the yielding element, the protected zone only includes the elements in contact with the Yield-Link® at the beam flanges and shear tab at the beam web.
- Protected zone included the following elements:
- Yield-Link flange and Yield-Link stem
- BRP plates
- Beam flange areas connected to the Yield-Link stem
- Column flange areas connected to the Yield-Link flange
- Shear tab and beam web at shear tab (2" around shear plate on 3 sides)
- Yield-Link-to-beam connection bolts
- Yield-Link-to-column connection bolts
- Shear tab-to-beam connection bolts
(a) T-Stub Yield-Links
(b) End Plate Yield-Links
Figure D4b.4 — Protected Zone for Strong Frame SMF Connection