Section A4 requires that design documents and specifications identify specific items as listed in this section. Items that are not listed in Section A4 that are required for a specific project should be similarly identified.
A4.1 General
- (a) To ensure proper understanding of the contract requirements and the application of the design, it is necessary to identify the specific types of seismic force-resisting system (SFRS) or systems used on the project.
- (b) The special design, construction, and quality requirements of Chapter J, compared to the general requirements of Specification Chapter N, are applicable to the SFRS. The additional quality control and quality assurance requirements of Chapter J are prepared to address the additional requirements for the SFRS, not the structure as a whole. Therefore, it is necessary to clearly designate which members and connections comprise the SFRS.
- (c) Floor and roof decks may be designed to serve as diaphragms and transfer seismic loads, and additional connection details may be needed to provide this load transfer. Consideration should also be made for other floor and roof deck connections when the deck has not been specifically designed and detailed as a diaphragm, as the system may behave as one.
A4.2 Steel Construction
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(a) It is necessary to designate working points and connection types, and any other detailing requirements for the connections in the SFRS.
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(b) Information should be provided as to the steel specification and grade of the steel elements that comprise the connection, the size and thickness of those elements, weld material size, strength classification, required Charpy V-notch (CVN) toughness, and bolt material diameter and grade, as well as bolted joint type.
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(c) Where special concentrically braced frame (SCBF) brace connections are designed to provide rotation capacity to accommodate buckling in accordance with Section F2.6c.3(b), special detailing is required. These connections must be identified in the structural design documents.
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(f) The majority of welded connection applications in buildings are in temperature-controlled settings. Where connections are subjected to temperatures of less than 50°F (10°C) during service, additional requirements for weld filler metals are necessary for demand critical welds to ensure adequate resistance to fracture at the lower service temperatures.
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(g) The protected zone includes locations anticipated to undergo significant inelastic deformations and often the adjacent regions. Unanticipated connections, attachments, or notches may interfere with the anticipated location and distribution of inelastic deformations or initiate a fracture. Because the location of the protected zone may vary depending on member and connection configuration, the extent of the protected zone must be identified.
Fabricators commonly have shop drawings that show the locations of the protected zones with the piece during the time on the shop floor. Those working on the piece are expected to be knowledgeable of protected zones and their restrictions. Similarly, the locations of protected zones are shown on the erection drawings. Should the fabricator’s or erector’s personnel fail to heed the protected zone restrictions, the quality control inspector is expected to identify the error. When required, quality assurance inspection of protected zones also is performed, using the design drawings that identify the protected zones.
AISC Code of Standard Practice for Steel Buildings and Bridges (ANSI/AISC 303) (AISC, 2022a) Section 1.12 requires that protected zones be permanently marked by the fabricator and re-marked by the owner’s designated representative for construction if those markings are obscured in the field, such as by application of fireproofing. Marking and re-marking is important because the structural steel quality control inspector and quality assurance inspector have finished their tasks and are no longer present as the work of other trades (e.g., curtainwall, plumbing, electrical, HVAC, column covers, and partitions) is being performed. It also is important for subsequent remodeling or renovation of the structure over its life, particularly when design drawings are no longer available.
- (h) Connection detailing
Connection-related items are collected in this section.
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(1) Demand critical welds are identified in the Provisions for each type of SFRS. Demand critical welds have special CVN toughness and testing requirements to ensure that this notch toughness will be provided.
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(2) The presence of backing may affect the flow of stresses within the connection and contribute to stress concentrations. Therefore, backing removal may be required at some locations. Removal of backing should be evaluated on a joint-specific basis, based upon connection prequalification requirements or qualification testing. AWS Structural Welding Code—Seismic Supplement (AWS D1.8/D1.8M) (AWS, 2021), hereafter referred to as AWS D1.8/D1.8M, provides details for weld backing removal, additional fillet welds, weld tab removal, tapered transitions, and weld access holes.
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(3) Where steel backing remains in place in tee and corner joints with the load applied perpendicular to the weld axis, a fillet weld between the backing and the flange element of the tee or corner joint reduces the stress concentration at the weld root. The requirement for this fillet weld should be evaluated on a joint specific basis, based upon connection prequalification requirements or qualification testing for moment connections, and the requirements of the Provisions for column-to-base plate connections. AWS D1.8/D1.8M provides details for additional fillet welds at weld backing.
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(4) The presence of weld tabs may affect the flow of stresses within the connection and contribute to stress concentrations. In addition, weld starts and stops made on weld tabs typically contain welds of lesser quality and are not subjected to nondestructive testing. Therefore, complete or partial weld tab removal may be required at some locations. Removal of weld tabs should be evaluated on a joint-specific basis, based upon connection prequalification requirements or qualification testing. AWS D1.8/D1.8M provides details for weld tab removal.
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(5) AWS D1.8/D1.8M provides details for tapered transitions when required for welded butt joints between parts of unequal thickness and width.
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(6) Analysis and research regarding the use of weld access holes have shown that the shape of the weld access hole can have a significant effect on the behavior of moment connections. The selection of weld access hole configuration should be evaluated on a joint-specific basis, based upon connection prequalification requirements or qualification testing. The use of different weld access holes other than those prescribed by AWS D1.1/D1.1M (AWS, 2021) or the Specification has not been found necessary for specific moment connection types, nor necessary for locations such as column splices and column-to-base plate connections. Care should be exercised to avoid specifying special weld access hole geometries when not justified. In some situations, weld access holes are undesirable, such as in end-plate moment connections.
Section A4.2(i) as contained in the 2016 Provisions (AISC, 2016) is removed from the 2022 edition. In the years that immediately followed the 1994 Northridge earthquake, there was some thought that reinforcing fillet welds may be needed for some connections to reduce through-thickness loading on column flanges; Dexter and Melendrez (1999) showed that such fillet welds were not necessary. None of the prequalified moment connections listed in AISC Prequalified Connections for Special and Intermediate Steel Frames for Seismic Applications, ANSI/AISC 358 (AISC, 2022b), require reinforcing fillet welds, and these Provisions do not mandate such fillets. Accordingly, this section has been deleted. The deletion of this item does not preclude the engineer of record from specifying such fillet welds if needed for a specific design.
Section A4.2(m) as contained in the 2016 edition of these Provisions is removed from the 2022 edition. In the years that immediately followed the 1994 Northridge earthquake, there was some thought that specific assembly orders, welding sequences, welding techniques, or potentially other special precautions were required for moment connections. Johnson (2000) showed that different welding processes, including self-shielded flux-cored arc welding, were appropriate for seismic moment connections when proper electrodes are selected. AWS D1.8/D1.8M addresses welding requirements, including welding sequence issues. The deletion of this item does not preclude the engineer of record from specifying a special welding requirement if needed for a specialized project.
A4.3 Composite Construction
Structural design documents and specifications, fabrication documents, and erection documents for composite steel-concrete construction are similar to those given for all-steel structures. For the reinforced concrete portion of the work, in addition to the requirements in ACI 318, Chapter 26, attention is called to the ACI Detailing Manual (ACI, 2020), with emphasis on Section 2.10, which contains requirements for seismic design of frames, joints, walls, diaphragms, and two-way slabs.