In seismic design, a structure which has significant physical discontinuities in plan or vertical configuration or in its lateral force resisting system. Since irregular structures have less favorable and predictable seismic response characteristics than regular structures, specific design requirements are prescribed for each type of irregularity.
The speed with which seismic waves move in a given direction, in inches or centimeters per second.
Describing the condition when the lines of action of several forces pass through a common point.
The moisture content at which a soil starts to change from a plastic to a semi-liquid state.
The level below which the subsoil is completely saturated with water. Also called the water table.
A structural steel connection using high-strength bolts, in which some slip can occur and bearing stresses are considered.
A structural system consisting of spaced members solidly sheathed on one or both sides, in which the sheathing forms the flanges and resists flexure while the spaced members comprise the webs and resist shear.
a horizontal system which distributes lateral forces, caused by wind or earthquake, the the vertical resisting elements.
radius or gyration (r)
A term used in column design equal to the square root of I/A, where 'I' is the moment of inertia of a member, and 'A' is its cross-sectional area.
The location in the earth's crust where rock slippage begins during an earthquake. Also called the 'focus'.
In seismic design, a combination of moment-resisting frames and shear walls or braced frames.
modulus of rupture
The unit bending stress calculated from the flexure formula, for the maximum bending moment resisted by a beam before rupture.
Replacing a force with two or more other forces (components) which will produces the same effect on a body as the original force.
A type of diagonal bracing in which each end of each brace frames into a beam or column, not a beam-column joint. K-bracing is considered undesirable for seismic resistance and is generally prohibited.
Concrete which is permanently loaded so as to cause stresses opposite in direction from those caused by dead and live loads.
A material which is able to unite nonadhesive substances into a solid mass. The cement most commonly used in concrete construction in Portland cement.
A series of trusses which intersect in a consistent grid pattern and are rigidly connected at their points of intersection.
one-way concrete slab
A concrete slab designed to span in one direction and whose main reinforcement runs in that direction.
special wind region
An area where local records and terrain features indicate wind speeds greater than those shown in the building code.
basic wind speed
The fastest mile wind speed which has a 2 percent probability of occurring in any one year measured at a point 33 feet (10 meters) above the ground.
A steel beam and a concrete slab connected so that they act together as a single structural unit to resit bending stresses.
The rate of change of velocity, usually expressed as a fraction or percentage of 'g', the acceleration of gravity.
A method of isolating a structure from the ground by specifically designed bearings and dampers which absorb earthquake forces. Also called 'base isolation'.
The ratio of the lateral unit strain to the longitudinal unit strain, when a member is subject to a uniform longitudinal stress. For steel, the value of Poisson's ratio is about 1/4.
The assumed distribution of earthquake forces to various levels of a structure.
The vertical load caused by the use and occupancy of a building, not including wind, earthquake, or dead loads.
The method generally used for reinforced concrete design, formerly called ultimate strength design.
A method of framing wood stud walls in which the studs are one story in height and the floor joists bear on the top plates of the wall below.
bearing wall system
In seismic design, a structural system without a complete load-carrying frame. Gravity loads are resisted by bearing walls or bracing systems, and lateral loads are resisted by shear walls or braced frames. Bearing wall systems are designed for relatively high seismic forces.
A type of pile consisting of a tapered steel shell which is driven into the ground using a mandrel and then filled with concrete after the mandrel is removed.
A weld placed in the right angle formed by lapping or intersecting plates and generally subject to shear stress.
A frame consisting of two columns and two inclined beams which meet at the ridge, in which the joint between each column and beam is rigid.
Method 2 (projected area method)
A wind design method in which the horizontal pressures are assumed to act on the full vertical projected area of the structure, and the vertical pressures are assumed to act simultaneously on the full horizontal projected area.
natural period (t)
The time it takes for a structure to go through one complete back-and-forth motion under the action of dynamic loads. Also called 'fundamental period of vibration or period'.
Transformation of soil into a liquefied state, similar to quicksand, as a result of earthquake vibrations.
The horizontal movement of one level of a building relative to the level immediately above or below, caused by wind or earthquake.
A localized, violently destructive windstorm characterized by a long funnel-shaped cloud. Building code requirements for wind design do not usually include the effects of tornadoes.
A story whose lateral stiffness is less than 70 percent of the stiffness of the story above. Such an abrupt change of stiffness should be avoided if possible.
rw or r
A numerical coefficient used in seismic design which depends on the type of lateral force resisting system used.
The level below which the subsoil is completely saturated with water. Also called 'groundwater level'.
An exterior column footing joined by a concrete beam to an interior column footing. It is also called a 'strap footing'.
A membrane enclosing a pressurized occupied space, which must be held down to its foundation.
A threaded metal fastener with a pointed end which forms its own matching thread in the wood member into which it is inserted.
A retaining wall in which the stem and base are connected at intervals by transverse walls called counterforts.
welded wire fabric
A type of reinforcement used in reinforced concrete, consisting of a grid of steel wires perpendicular to each other and welded at all points of intersection.
A roof structure whose shape is that of an arch rotated about its vertical axis to form a curved surface.
A hammer used to drive piles into the ground. Pile hammers may drop by gravity, or may be operated by steam or compressed air.
Deepening an existing foundation or building a new foundation for an existing building. Underpinning is usually required when excavation for a new building is adjacent to and deeper than an existing foundation.
The separation between two adjoining buildings, or parts of the same building, to permit these adjoining elements to move independently when subject to earthquake motion. The amount of separation should be sufficient to prevent the adjoining elements from battering each other during an earthquake. Also called 'seismic separation'.
A jointed structure designed to support vertical or horizontal loads and composed generally of straight members forming a number of triangles.
seismic zone factor (z)
A factor used in seismic design which depends on the seismic zone in which a site is located.
The property of a structure which has multiple paths of load resistance, so that if one element fails, the load will be redistributed to other elements. Lateral force resisting systems should be as redundant as possible.
Describing a material which returns to its original size and shape when load is removed. Also describes structural behavior in which members are stressed below the yield point.
An ocean wave produced by displacements of the ocean bottom as the result of an earthquake or volcanic activity. Tsunamis can affect areas thousands of miles from their origin.
fastest mile speed
The highest sustained average wind speed, based on the time required for a mile-long sample of air to pass a fixed joint.
The physical law that states that up to a certain unit stress, called the elastic limit, unit stress is directly proportional to unit strain.
A member which collects seismic load from the diaphragm to which it is attached and delivers it to a shear-resisting element. Also called a 'strut' or 'drag strut'.
special moment-resisting frame (SMRF)
As used in earthquake design, a moment-resisting frame made of structural steel or reinforced concrete which has the ability to absorb a large amount of energy in the inelastic range, that is, when the material is stressed above its yield point, without failure and without deforming unacceptably.
A logarithmic scale used to measure the magnitude of an earthquake (the amount of energy it releases). The largest earthquake ever recorded had a magnitude of about 8.9. The scale is named after its inventor, Dr. Charles Richter.
soil boring log
A log showing the types of soil encountered in a test boring and other relevant information.
method of joints
An analytical method for determining the force in the members of a truss, in which each joint is isolated and the unknown forces determined using the equations of equilibrium.
glued laminated beam
An assembly of laminations of lumber in which the grain of all the laminations is approximately parallel longitudinally. The laminations are bonded with adhesives and fabricated in accordance with certain accepted standards.
normal weight concrete
Concrete made with standard aggregates, usually weighing about 150 pounds per cubic foot.
A pipe for placing concrete under water. A hopper for filling is provided at the top, while the lower end is kept submerged in fresh concrete.
a concrete slab reinforced in two directions which brings its load directly to supporting columns without any beams, girders, column capitals (widened tops of columns), or drop panels (thickened slab around columns).
A flat plate cast at grade around columns and then lifted to position with hydraulic jacks.
A structural steel connection using high-strength bolts, in which no slip can occur.
A high-strength timber connector used in wood-to-steel connections (using one shear plate) or wood-to-wood connections (using two shear plates), where demountability is desired.
method of sections
An analytical method for determining the force in the members of a truss, in which the truss is cut by an imaginary section and a free body diagram drawn of the portion of the truss thus isolated.
A roof structure comprising a series of parallel arches, skewed to the axes of the building, which are intersected by another series of skewed arches, so that they interact with each other.
A joint formed when a concrete surface hardens before the next batch is placed against it.
A method of prestressing concrete in which the concrete is cast and then the steel tendons are stressed by jacking.
The curve assumed by a cable hung between two supports, when the only load acting on it is its own weight. The stresses in the cable are pure tension.
A high-slump concrete, consisting of Portland cement, sand, hydrated lime, water, and sometimes pea gravel.
Reference points offset a given distance from the building line and set prior to excavation.
In wind design, a pressure coefficient for the structure or portion of the structure under consideration.
lateral force resisting system
The part of the structural system assigned to resist lateral forces from wind or earthquake.
Reinforced concrete design in which there is simultaneous crushing of concrete and yielding of the reinforcing steel. To assure that yielding of the steel occurs before crushing of the concrete, the amount of reinforcing is limited to 75 percent of that which would produce a balanced design.
A large footing under an entire building, which distributes the building load over the entire area. It is also known as a 'mat foundation'.
The chemically inert element of concrete, usually consisting of sand, gravel, and/or other granular material.
A type of cement (ASTM Type III) which provides earlier strength in concrete than ordinary cements. It is used when forms must be removed quickly or when the structure must be put into service quickly.
A reinforced concrete beam cast on or in the ground and used to provide support for the superstructure by spanning between piles or footings.
A large footing under an entire building, which distributes the building load over the entire area. It is also known as 'raft foundation'.
A foundation which spreads the load over a large area of soil. It is also called a 'spread footing'.
A graphical method for determining the forces in the members of a truss. It is also called a 'Maxwell diagram'.
effective depth (d)
In reinforced concrete design, the distance from the extreme compression fiber to the centroid of the tension reinforcement.
The maximum units of stress permissible in a structural member. It is also called 'working stress'.
Steel floor decking with embossed ridges, bonded to a concrete slab so that they act together as a single structural unit.
The line on a beam cross-section which has zero bending stress when the beam is loaded.
A basic equation which applies to all columns and gives the maximum stress a slender column can resist without failing by sudden buckling.
The resistance to the movement of a retaining wall provided by the earth in front of the wall and its footing.
An end-bearing pile, the bottom of which may be belled, which is constructed by pouring concrete into a drilled shaft.
point of inflection
The point in a beam or other flexural member where the bending moment changes sign and has a value of zero.
A structural roof system consisting of inclined planes which support each other and a function as deep beams.
A laboratory compaction test to determine the optimum moisture content and density for a soil.
A method used to provide the bolt tension specified for high-strength bolts, in which the bolts are first brought to a 'snug tight' condition and then tightened additionally by a specified amount of nut rotation.
The incorporation of tiny air bubbles into concrete to improve its workability and resistance to frost.
Unequal settlement of the various parts of a building, which may cause excessive stresses in the structural frame or tilting of the building.
The product of an area and the distance from the centroid of the area to a given axis.
A prefabricated, lightweight wood truss used to support roof loads and other small structures.
A longitudinal load which acts at a distance from a member's centroid, thereby producing bending moment in addition to axial stress.
stub girder system
A steel framing system in which beams sit on top of a girder and short lengths of "stub girders" the same depth as the floor beams are welded to the top of the girder to provide for composite actions.
A thin shell saddle-shaped surface formed by moving a vertical parabola with downward curvature along and perpendicular to another parabola with upward curvature.
impact hammer test
A nondestructive test to determine the strength of hardened concrete, by measuring the rebound of a plunger after striking the concrete surface. This test is not accurate enough to be a substitute for standard compression tests.
Bending moment which produces tension in the upper part of a beam and compression in the lower part.
A retaining wall which depends entirely on its own weight to resist the pressure of the retained earth and provide stability.
A curved line on the surface of a dome, usually circular, which is formed by the intersection of a vertical plane with the dome, when the plane passes through the top of the dome.
The part of a building's structure which transmits the building's load to the underlying soil.
A reinforced concrete column, usually square or rectangular, containing longitudinal reinforcing bars and separate lateral ties.
A foundation which spreads the load over a large area of soil. It is also called a 'footing'.
Describing a material or structural system which tends to fail suddenly without warning when subject to high stresses, as opposed to ductile material or system, which can absorb energy without failure.
moment-resisting frame system
In seismic design, a structural system with an essentially complete frame which provides support for vertical loads. Lateral loads are resisted by moment-resisting frames.
Modified Mercalli scale
A scale used to measure the intensity of an earthquake, that is, its effects on people and buildings. The scale varies from I (not felt except under especially favorable circumstances) to XII (damage nearly total).
An exterior column footing joined by a concrete beam to an interior column footing. It is also called a 'cantilever footing'.
angle of repose
The steepest angle with the horizontal at which a pile of loose earth will stand without sliding.
California bearing ration (CBR)
A ratio used to determine the bearing capacity of a soil, based on a standard test.
A member which collects seismic load from the diaphragm to which it is attached and delivers it to a shear-resisting element. Also called a 'collector' or 'drag strut'.
An inside corner which occurs in buildings with L-, T-, U-, and cross-shaped plans. Reentrant corners cause high stress concentration during an earthquake unless they are strengthened or a seismic separation is provided.
The separation of laminations of a glued laminated beam caused by failure of the adhesive.
The ratio of water to cement in a concrete mix, the main factor which determines concrete strength.
strength reduction factor (ø)
A factor used to reduce the capacity of reinforced concrete members to account for possible variations in quality control.
The moisture content at which a soil starts to change from a semisolid to a plastic state.
Increased earth pressure against a retaining wall caused by vertical load behind the wall or a sloping ground surface.
A high-strength connector used for wood-to-wood or wood-to-steel joints. Types include one split ring (wood-to-wood), two shear plates (wood-to-wood), and one shear plate (wood-to-steel).
A small hole near the bottom of a retaining wall, usually backfilled with gravel, to allow water to drain to the outside of the wall and thus avoid hydrostatic pressure against the wall.
horizontal bracing system
A horizontal truss system which distributes lateral forces, caused by wind or earthquake, to the vertical resisting elements.
In seismic design, a structure which has no significant physical discontinuities in plan or vertical configuration or in its lateral force resisting system. Regular structures exhibit more favorable and predictable seismic response characteristics than irregular structures.
A seismological instrument which is normally inoperative, but becomes activated when subject to strong earth motion, records the earth motion, and then shuts off.
section modulus (s)
The ration of the moment of inertia of a beam (/) to the distance from its neutral axis to the most remote fiber (c). Thus, section modulus (s) = I/c. The S-value of a beam is a measure of its ability to resist bending moment.
A hole drilled into the ground at the site of a proposed structure in order to obtain samples of the subsurface soil for examination and testing in a laboratory. Based on these tests, the soils engineer recommends the type of foundation and the allowable soil bearing pressure.
A medium carbon steel, rolled in a variety of shapes and sizes for use as load-bearing structural members.
fundamental period of vibration (t)
The time it takes for a structure to go through one complete back-and-forth motion under the action of dynamic loads. Also called 'period' or 'natural period'.
A story whose strength is less than 80 percent of that of the story above. Such an abrupt change of strength should be avoided if possible.
The time it takes for a structure to go through one complete back-and-forth motion under the action of dynamic loads. Also called 'fundamental period of vibration' or 'natural period'.
In wind design, the nature of the terrain at a given site, varying from B (the least severe exposure) to D (the most severe exposure).
The sudden application of load from a moving object, such as a crane or elevator, which causes stresses much greater than those caused by a static load.
The arbitrary limits which define the boundaries between the different states of rigidity or fluidity of fine-grained soils.
The most widely used fastener for structural steel connections made in the field. A very high tensile stress is developed in the bolt, thus tightly clamping together the connected parts. The resulting friction between the clamped parts resists the applied load.
A graphic representation of the value of the vertical shear at any point along the beam.
A test to determine the compressive strength of concrete by subjecting a standard cylinder of hardened concrete to compression in a testing machine.
An end-bearing pile constructed by pouring concrete into a drilled shaft, the bottom of which is enlarged (belled) to provide a larger bearing area.
Describing a structure whose reaction can be determined from the equations of equilibrium. Examples are simple beams, cantilever beams, and overhanging beams that rest on two supports.
Concrete containing adequate reinforcing steel and designed on the basis that the concrete and steel act together in resisting forces. The concrete is usually assumed to resist compression, while the reinforcing steel is assumed to resist tension.
Reinforcing steel embedded in the compression face of a reinforced concrete beam.
The most usual welding process used in building construction, in which intense heat is produced by an electric arc between the members to be joined and a metal wire or rod, called the electrode.
importance factor (i)
A factor used in earthquake and wind design, whose value varies between 1.0 and 1.5. This provides that certain essential facilities, such as hospitals and fire and police stations, be designed for seismic and forces greater than normal. In this way, such emergency facilities are expected to be safe and usable following an earthquake or sever windstorm.
A graphic representation of the value of the bending moment at any point along a beam.
The ratio I/r or KI/r used in column design, where 'l' is the length, 'r' is the radius of gyration, and 'K' is an effective length factor.
The amount of energy released by an earthquake, as measured by the Richter scale. Magnitude refers to an earthquake's energy, while intensity refers to an earthquake's effects.
ultimate load factor (u)
Factor used to increase loads, shears, and moments to their ultimate values in reinforced concrete design.
free body diagram
A diagram obtained by making an imaginary cut through a structure and applying the equations of equilibrium to the remaining portion, called the "free body." By this means, the internal forces of a structure may be determined.
A structure with a curved surface that supports load by tension, compression, and shear in the plane of its surface, but which is too thin to resist bending stresses.
A spread footing, generally square or rectangular in plan, used to support a single column.
moment of inertial (I)
The sum of the products obtained by multiplying each unit of area by the square of its distance to the neutral axis. Moment of inertia of a beam is a measure of its stiffness, or resistance to deflection.
An alloy of iron and carbon, with a carbon content between 0.1 and 1.7 percent (more than that of wrought iron and less than that of cast iron).
reinforced concrete block masonry
A type of wall construction consisting of hollow concrete masonry units, with certain cells continuously filled with grout in which reinforcing bars are embedded.
The rotation caused in a diaphragm by lateral load from wind or earthquake, when the center of mass does not coincide with the center of rigidity. Torsional effects are most significant in unsymmetrical buildings.
A shop-fabricated steel truss that supports evenly-spaced steel joists along its top chord.
A frame with rigid joints, in which the members and joints are capable of resisting vertical and horizontal forces primarily by flexure. It is also called a 'rigid frame'.
The most common accelerating admixture used for concrete. It can be used safely in amounts up to 2 percent of the Portland cement weight.
A horizontal wood member used to support a structure above, such as one of the wood strips between a concrete slab and a finished wood floor.
The stress diagram used in the graphical solution of a truss, which consists of the combined force polygons for all the truss joints.
An underground wood, concrete, or steel member, usually vertical, and usually driven into place, which is used to support building loads.
An assembly of steel plates, or plates and angles, which are fastened together to form an integral member.
The principal cause of structural damage, injury, and loss of life during an earthquake. The provisions of the Uniform Building Code provide resistance to earthquake ground shaking, without settlement, slides, subsidence, or faulting in the immediate vicinity of the structure.
A wide, flat reinforced concrete member, usually horizontal, which is supported by beams or walls.
A factor used to modify the allowable unit stress in bending for the curved portion of glued laminated members.
A method of joining two pieces of metal by heating their surfaces until they are molten or plastic, with or without applying pressure, and with or without the use of additional filler material.
Consolidating freshly poured concrete by using an oscillating vibrator. Immersion-type (spud) vibrators are lowered into the concrete, while form vibrators are attached to the exterior of forms.
eccentric braced frame (EBF)
A braced frame in which at least one end of each brace is eccentric to the beam-column joint or the opposing brace. The intent is to make the braced frame more ductile and therefore able to absorb a significant amount of energy without buckling the braces.
building frame system
In seismic design, a structural system with an essentially complete frame providing support for gravity loads. Lateral loads are resisted by shear walls or braced frames. Building frame systems are designed for seismic forces which are lower than those for bearing wall systems.
A severe tropical storm which occurs mainly along the coasts of the Gulf of Mexico and the South Atlantic. The design of buildings to resist wind in accordance with building code requirements includes the effects of hurricanes.
The vertical load due to the weight of all permanent structural and nonstructural components of a building, such as walls, floors, roofs, and fixed service equipment.
A type of diagonal bracing in which one end of each brace frames into a beam-column joint and the other end frames into a beam. There are two configurations, V-bracing and inverted-V-bracing. Because of various potential problems with chevron bracing, the bracing members must be designed for increased seismic loads.
The effects of an earthquake on people and structures at a particular place, as measured by the Modified Mercalli scale. Intensity refers to an earthquake's energy.
A standard designation for a structural steel, wide, flange shape. Also, the total dead load used in earthquake design.
A smooth dense concrete surface, produced by steel troweling after the concrete has partially hardened.
The decrease of vibration caused by the absorption of energy. Buildings contain a number of elements, both structural and nonstructural, which absorb energy during an earthquake and thereby diminish the earthquake-induced vibrations.
Method 1 (normal force method)
A wind design method in which the wind pressures are assumed to act simultaneously normal (perpendicular) to all exterior surfaces.
A wall consisting of small, closely spaced members usually sheathed on both faces with a wall material.