Built to Resist Earthquakes The Path to Quality Seismic Design and

Briefing Paper 1 Building Safety and Earthquakes Part D: The Seismic Load Path

Introduction classified as either flexible or rigid, and the method of distributing earthquake from This Briefing Paper 1, Building Safety and the diaphragm to the resisting elements depends Earthquakes, consists of four parts describing on that classification. Wood-framed diaphragms earthquakes and their effects on buildings. can be considered either flexible or rigid and Parts A and B describe the causes of - concrete diaphragms are considered rigid. quakes and resulting ground motions and explain how earthquake motions create various forces Shear walls and frames are primarily lateral- acting on a building. Part C describes typical -resisting elements but can also perform structural systems and lateral-force-resisting force-transmitting functions. For example and elements used in buildings. This Part D defines while not necessarily desirable, an upper-story the seismic load path elements, describes their interior shear wall may not continue to the base functions and the necessary interconnections of the building and therefore must transmit its between them to resist earthquake forces. Also forces to a floor diaphragm. Also, at the base of included in this part of Briefing Paper 1 are a frame or a shear wall, forces are transmitted resources for additional reading on building into a element. The primary struc- safety and earthquakes. tural elements that participate in the earthquake load path are shown in Figure 1. Primary Load-Path Elements Foundations form the final link in the load path by Within every building, there are multiple ele- collecting the base shear and transmitting it to ments that are used to transmit and resist lateral the ground. Foundations resist lateral forces forces. These transmitting and resisting ele- through a combination of frictional resistance ments define the building’s lateral-load path. along their lower surface and lateral bearing This path extends from the uppermost roof or against the depth of in which they are embed- parapet, through each element and connection, ded. Foundations must also support additional to the foundation. Load-path elements vary in vertical loads caused by the overturning forces scale from massive multi-story moment-resisting from shear walls and frame columns. frames to individual nails connecting wood members. An appreciation of the critical importance of a complete load path is essential for everyone involved in the design, construction, Roof Diaphragm and inspection of buildings that must resist earthquakes. Floor Diaphragm There are two orientations of primary elements in the load path: those that are vertical, such as Braced shear walls, braced frames, and moment frames, Shear Frame Bay and those that are essentially horizontal, such as Wall the roof, floors, and foundation. The roof and floor elements are known as diaphragms. Diaphragms serve primarily as force-transmit- ting or force-distributing elements that take Foundations horizontal forces from the stories at and above their level and deliver them to walls or frames in Figure 1. Primary structural load path the story immediately below. Diaphragms are elements.

Briefing Paper 1, Part D 1 ATC/SEAOC Joint Venture Training Curriculum Secondary Load-Path Elements Boundary Chord in Compression Within the primary load-path elements, there are individual secondary elements needed to resist specific forces or to provide specific pathways Boundary Collector along which lateral forces are transmitted. Transferring Shear Particular attention must be given to transmitting Lateral Force forces between horizontal seismic elements (diaphragms) and vertical seismic elements. Two Boundary Chord in important secondary elements are chords and collectors. A chord is a structural member along the boundary of a diaphragm that resists tension Figure 2. Function of diaphragm chords and and compression forces. A collector is a struc- collectors. tural member that transmits diaphragm forces into Collectors are needed when an individual shear shear walls or frames. Figure 2 depicts the wall or frame in the story immediately below the overall function of chords and collectors. diaphragm is not continuous along the diaphragm In the case of floors and roofs, the perimeter boundary (See Figure 3). This is a very common edges or boundaries are critical locations be- situation because shear walls are often inter- cause they form the interface between the rupted by openings for windows and doors, and diaphragms and the perimeter walls. The because resisting frames are normally located in perimeter is typically the location for vertical only a few of the frame bays along a diaphragm seismic elements, although many buildings also boundary. A path must be provided to collect the have shear walls or frames at interior locations. lateral forces from portions of a diaphragm An interior line of resistance also creates a located between vertical resisting seismic diaphragm boundary. Boundary elements in elements and to deliver those forces to each diaphragms usually serve as both chords and individual shear wall or frame. The collector collectors, depending on the axis along which member provides that path. Collectors are lateral loads are considered to be applied. commonly called drag struts or ties. Collectors are also needed when an interior shear wall or As shown in Figure 2, the forces acting perpen- frame is provided (see Figure 3). In this case, dicular to the boundary elements tend to bend the the collector is placed in the diaphragm, aligned diaphragm, and the chord member must resist the with the wall or frame, and extends to the associated tension and compression. Similar to a diaphragm edges beyond each end of the wall or uniformly loaded , a diaphragm experiences frame. Collectors can occur in wood-framed the greatest and largest deflection walls using headers and top plates to transfer the at or near the center of its span between vertical forces and can occur in spandrel beams, of resisting seismic elements. The chord on the side concrete or masonry construction, that link of the diaphragm along which the forces are sections of shear walls together. being applied is in compression, and the chord on the opposite side is in tension. These tension and Connections compression forces reverse when the earth- quake forces reverse. Therefore, each chord The following statements contained in the 1997 must be designed for both tension and compres- UBC clearly require that a complete load path be sion. provided throughout a building to resist lateral forces. Walls that structurally support diaphragm edges must also resist out-of-plane forces caused by “All parts of a structure shall be intercon- diaphragm bending. In wood-frame walls, the nected and connections shall be capable of double top plates usually act as chords for the transmitting the seismic force induced by diaphragm at that level. In concrete and masonry the parts being connected.” walls, reinforcing steel is placed at the diaphragm “Any system or method of construction level to resist the out-of-plane bending in the wall. shall be based on a rational analysis... Such

2 Briefing Paper 1, Part D ATC/SEAOC Joint Venture Training Curriculum Diaphragm Briefing Paper 1 Part D: Boundary The Seismic Load Path Collector

Interior Shear Wall

Interior Shear Wall Lateral Force

Isometric View Plan View

Figure 3. Use of collector element at interior shear wall.

analysis shall result in a system that pro- array of materials, products, and methods of vides a complete load path capable of construction. For example, forces are resisted in transferring all loads and forces from their wood-framed diaphragms by the action of nails point of origin to the load-resisting ele- or other fasteners used to attach structural ments.” sheathing to the joists, , beams, ledgers, and blocking that make up the diaphragm To fulfill these requirements, connections must framing. Nails, bolts, and prefabricated metal be provided between every element in the load connectors are used for diaphragm chord and path. When a building is shaken by an earth- collector splices of wood members. In steel quake, every connection in the lateral-force load construction, metal deck diaphragms use welds path is tested. If one or more to resist diaphragm forces and chord connections fail because they and collector beams are connected by were not properly designed or When a building is bolts, welds, or a combination of both. constructed, those remaining in In concrete construction, diaphragm parallel paths receive additional shaken by an earthquake, every reinforcing steel resists forces in the force, which may cause them to diaphragm and chord tension stresses, become overstressed and to fail. connection in the and reinforcing dowels are generally If this progression of individual lateral-force load used to transfer forces from the connection failures continues, it path is tested. diaphragm boundaries to concrete can result in the failure of a walls or frames. complete resisting seismic element and, potentially, the Connection capacity is determined by entire lateral-force-resisting system. Conse- performing a detailed analysis of the individual quently, connections are essential for providing forces the connection must transfer. The adequate resistance to earthquakes and must be capacity actually provided, however, is highly given special attention by both designers and dependent on the implementation of the specific inspectors. details of its construction. Therefore, two specimens of the same connection can have Connections are details of construction that significantly different capacities, even when the perform the work of force transfer between the differences in construction are imperceptible. individual primary and secondary structural Some common examples can illustrate this point. elements discussed above. They include a vast The capacity of a 3/16” fillet weld is 25 percent

Briefing Paper 1, Part D 3 ATC/SEAOC Joint Venture Training Curriculum less than that of an equal length of 1/4” fillet Steel Moment-Frame Structures, prepared by weld; the capacity of an 18-gauge strap to resist the SAC Joint Venture (Report SAC-95-02), tensile forces is 36 percent less than for a 14- published by the Federal Emergency Manage- gauge strap of equal width; the shear capacity of ment Agency, FEMA 267 Report, Washington, a 1/2”-diameter foundation bolt in a 2×4 wood D.C. (Other SAC publications are available sill is 33 percent less than that of a 5/8”-diam- through the Applied Technology Council; see eter bolt; and the shear capacity in wood of an box.) 8d box nail is 22 percent less than that of an 8d common nail. Stratta, J.L., 1987, Manual of Seismic Design, Prentice Hall, Englewood Cliffs, New Jersey. Construction tolerances play an equal role in determining the actual capacities of connections. Yanev, P., 1974, Peace of Mind in Earthquake Parameters such as minimum edge and end Country, Chronicle Books, San Francisco, distances, required embedment or penetration California. depths, round versus slotted holes for bolts, spacing of reinforcing ties in concrete, and misalignment of parts causing eccentric loads, can all significantly reduce connection capacity. Careful design and detailing on the drawings and thorough inspection of every connection in the load path is necessary to avoid creating weak links that lead to excessive earthquake damage.

References ICBO, 1997, Uniform Building Code, Interna- tional Conference of Building Officials, Whittier, California.

Resources for Additional Reading Algermissen, S.T., 1983, An Introduction to the Seismicity of the United States, Earthquake Research Institute, Oakland, California. ATC, 1997, NEHRP Guidelines for the Seismic About this Briefing Paper Series Rehabilitation of Buildings, prepared by the Briefing papers in this series are concise, easy-to-read Applied Technology Council for the Building summary overviews of important issues and topics that Seismic Safety Council, published by the Federal facilitate the improvement of earthquake-resistant building Emergency Management Agency, FEMA 273 design and construction quality. Report, Washington, D.C. This briefing paper was prepared by the ATC/SEAOC Joint Venture, a partnership of the Applied Technology Council Dames & Moore Inc., 1988, How Earthquakes (ATC) and the Structural Engineers Association of California Affect Buildings - A Video, Final Cut Video, (SEAOC). Funding for the series was provided by the California Dames & Moore, Inc., Walnut Creek, Califor- Seismic Safety Commission, Proposition 122 Retrofit Practices nia. Improvement Program.

FEMA, 1998, Seismic Retrofit Training for Copies of Briefing Papers can be downloaded from ATC’s Inspectors and Contractors, FEMA Emer- World Wide Web site (www.atcouncil.org), or are available gency Management Institute, Emmitsburg, from: Maryland. ATC/SEAOC Joint Venture c/o Applied Technology Council SAC, 1995, Interim Guidelines: Evaluation, 555 Twin Dolphin Drive, Suite 550 Repair, Modification, and Design of Welded Redwood City, California 94065

4 Briefing Paper 1, Part D ATC/SEAOC Joint Venture Training Curriculum