B. Definitions.

BASE is the level at which the earthquake motions are considered to be imparted to the structure or the level at which the structure as a dynamic vibrator is supported.

BASE SHEAR, V, is the total design lateral force or shear at the base of a structure.

BEARING WALL SYSTEM is a structural system without a complete vertical load carrying space frame. See Section ID6a.

BOUNDARY ELEMENT is an element at edges of openings or at perimeters of shear walls or diaphragms.

BRACED FRAME is an essentially vertical truss system of the concentric or eccentric type which is provided to resist lateral forces.

BUILDING FRAME SYSTEM is an essentially complete space frame which provides support for gravity loads. See Section ID6b.

COLLECTOR is a member or element provided to transfer lateral forces from a portion of a structure to vertical elements of the lateral force resisting system.

CONCENTRIC BRACED FRAME is a braced frame in which the members are subjected primarily to axial forces.

DIAPHRAGM is a horizontal or nearly horizontal system acting to transmit lateral forces to the vertical resisting elements. The term "diaphragm" includes horizontal bracing systems.

DIAPHRAGM CHORD is the boundary element of a diaphragm or shear wall which is assumed to take axial stresses analogous to the flanges of a beam.

DIAPHRAGM STRUT (drag strut, tie, collector) is the element of a diaphragm parallel to the applied load which collects and transfers diaphragm shear to vertical resisting elements or distributes loads within the diaphragm. Such members may take axial tension or compression.

DRIFT. See STORY DRIFT.

DUAL SYSTEM is a combination of a Special or Intermediate Moment Resisting Frame and Shear Walls or Braced Frames designed in accordance with the criteria of Section ID6d.

ECCENTRIC BRACED FRAME (EBF) is a steel braced frame designed in conformance with Section 4H.

ESSENTIAL FACILITIES are those structures which are necessary for emergency post earthquake operations.

FLEXIBLE ELEMENT or system is one whose deformation under lateral load is significantly larger than adjoining parts of the system. Limiting ratios for defining specific flexible elements are set forth in Section IE6a or lG2b.

HORIZONTAL BRACING SYSTEM is a horizontal truss system, that serves the same function as a diaphragm.

INTERMEDIATE MOMENT RESISTING FRAME (IMRF) is a concrete frame designed in accordance with Section A.9 of ACI 318-83.

LATERAL FORCE RESISTING SYSTEM is that part of the structural system assigned to resist lateral forces.

MOMENT RESISTING FRAME is a frame in which members and joints are capable of resisting forces primarily by flexure.

ORDINARY MOMENT RESISTING FRAME (OMRF) is a moment resisting frame not meeting special detailing requirements for ductile behavior.

ORTHOGONAL EFFECT'S are the effects on the structure due to earthquake motions acting in directions other than parallel to the direction of resistance under consideration.

P-DELTA EFFECT is the secondary effect on shears and moments of frame members induced by the vertical loads acting on the laterally displaced building frame.

PLATFORM is the lower rigid portion of a structure having a vertical combination of structural systems for use in Section ID8a.

SHEAR WALL is a wall designed to resist lateral forces parallel to the plane of the wall (sometimes referred to as a vertical diaphragm or a structural wall).

SOFT STORY, as used in this document, is one in which the lateral stiffness is less than 70 percent of the stiffness of the story above.

SPACE FRAME is a three-dimensional structural system without bearing walls composed of members interconnected so as to function as a complete self contained unit with or without the aid of horizontal diaphragms or floor bracing systems.

SPECIAL MOMENT RESISTING FRAME (SMRF) is a moment resisting frame specially detailed to provide ductile behavior and comply with the requirements given in Chapters 3 or 4.

STORY is the space between levels. Story x is the story below Ievel x.

STORY DRIFT is the displacement of one level relative to the level above or below.

STORY DRIFT RATIO is the story drift divided by the story height.

STORY SHEAR, Vx, is the summation of design lateral forces above the story under consideration.

STRENGTH is the usable capacity of an element or a member to resist load as prescribed in these Recommendations.

STRUCTURE is an assemblage of framing members designed to support gravity loads and resist lateral forces. Structures may be categorized as building structures or non-building structures.