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post-and-beam system

A simple structural system consisting of beams simply supported on posts.

irregular structure

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.

frost line

The expected maximum depth of frost penetration in the ground in a given area.

braced frame

A vertical truss used to resist lateral forces.


A device used to measure wind speed.


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.

liquid limit

The moisture content at which a soil starts to change from a plastic to a semi-liquid state.

waffle slab

A flat slab which is ribbed in two directions, resulting in a waffle-like appearance.

groundwater level

The level below which the subsoil is completely saturated with water. Also called the water table.

bearing-type connection

A structural steel connection using high-strength bolts, in which some slip can occur and bearing stresses are considered.

stressed skin

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'.

dual system

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.


Pertaining to earthquakes and the shock waves within the earth which they produce.

resolving forces

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.

prestressed concrete

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.

wall footing

A continuous spread footing supporting a uniformly loaded wall.

control joint

A groove in a concrete structure made to predetermine the location of cracks.

space frame

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.

composite beam

A steel beam and a concrete slab connected so that they act together as a single structural unit to resit bending stresses.

framing anchor

A metal device used for connection members in wood frame construction.


The rate of change of velocity, usually expressed as a fraction or percentage of 'g', the acceleration of gravity.

seismic isolation

A method of isolating a structure from the ground by specifically designed bearings and dampers which absorb earthquake forces. Also called 'base isolation'.

frost heave

Uplift of the soil surface or foundations caused by freezing of moisture in the soil.

needle beam

A short beam passed through a wall to provide temporary support.

Poisson's ratio

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.

triangular distribution

The assumed distribution of earthquake forces to various levels of a structure.


A thin sheet which can resist tension, but cannot resist compression, bending, or shear.

hydrostatic pressure

The pressure exerted by a liquid against every surface it contacts.

live load

The vertical load caused by the use and occupancy of a building, not including wind, earthquake, or dead loads.

strength design

The method generally used for reinforced concrete design, formerly called ultimate strength design.

bearing pile

A pile which supports a vertical load.

skin friction

The friction between the surface of a pile and the surrounding soil.

platform framing

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.

Raymond pile

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.

fillet weld

A weld placed in the right angle formed by lapping or intersecting plates and generally subject to shear stress.

gabled frame

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.

story drift

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.


The effect produced on a structure by earthquake ground motion.

soft story

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.

windward side

The side of a building facing the direction from which the wind is blowing.


The combined height, exposure, and gust factor used in wind design.

rw or r

A numerical coefficient used in seismic design which depends on the type of lateral force resisting system used.

water table

The level below which the subsoil is completely saturated with water. Also called 'groundwater level'.

cantilever footing

An exterior column footing joined by a concrete beam to an interior column footing. It is also called a 'strap footing'.

air-supported structure

A membrane enclosing a pressurized occupied space, which must be held down to its foundation.

wood screw

A threaded metal fastener with a pointed end which forms its own matching thread in the wood member into which it is inserted.

counterfort wall

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.

cantilever wall

A retaining wall in which the stem, heel, and toe act as cantilever slabs.

pile hammer

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 designation of the quality of a manufactured piece of wood.


Stress which tends to make two members, or two parts of a member, slide past each other.


Another term for bending.

building separation

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.

Hooke's Law

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.

Richter scale

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.

sheepsfoot roller

A large-toothed roller used for the compaction of soil.


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 prepared substance added to concrete to alter or achieve certain characteristics.

flat plate

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).

lift slab

A flat plate cast at grade around columns and then lifted to position with hydraulic jacks.

slip-critical connection

A structural steel connection using high-strength bolts, in which no slip can occur.


A horizontal member which extends around the circumference of a dome.

shear plate

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.

lag screw

A large wood screw with a head similar to that of a bolt and without a nut.

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.


An effective length factor used in the design of structural steel columns.

cold joint

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.

batter boards

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.


The chemical reaction which combines cement and water to form a hard, solid mass.


One force which will produce the same effect as two or more other forces.

factor for safety

The ratio of the ultimate strength of a material to its working stress.

lateral force resisting system

The part of the structural system assigned to resist lateral forces from wind or earthquake.

balanced design

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.

raft foundation

A large footing under an entire building, which distributes the building load over the entire area. It is also known as a 'mat foundation'.


A very lightweight volcanic rock used as an aggregate in lightweight concrete.


The chemically inert element of concrete, usually consisting of sand, gravel, and/or other granular material.

high-early-strength cement

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.

grade beam

A reinforced concrete beam cast on or in the ground and used to provide support for the superstructure by spanning between piles or footings.

jack rafter

A short rafter between hip rafter and eave or between valley and ridge.


A main beam which supports secondary beams.

fixed end beam

A beam that is restrained (fixed) against rotation at both ends.

mat foundation

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'.


The ease with which concrete can be placed and consolidated in forms.

stress diagram

A graphical method for determining the forces in the members of a truss. It is also called a 'Maxwell diagram'.

bar chair

A device used to support reinforcing bars during the placing of concrete.

effective depth (d)

In reinforced concrete design, the distance from the extreme compression fiber to the centroid of the tension reinforcement.

allowable stress

The maximum units of stress permissible in a structural member. It is also called 'working stress'.

working stress design

The theory used for most reinforced concrete design until the middle 1960s.

composite deck

Steel floor decking with embossed ridges, bonded to a concrete slab so that they act together as a single structural unit.


In concrete, the cement past in which the aggregate particles are embedded.


A mixture of fine and coarse aggregates, Portland cement, and water.

neutral axis

The line on a beam cross-section which has zero bending stress when the beam is loaded.

Euler's equation

A basic equation which applies to all columns and gives the maximum stress a slender column can resist without failing by sudden buckling.

passive pressure

The resistance to the movement of a retaining wall provided by the earth in front of the wall and its footing.


Referring to soil which is easily crumbled or reduced to powder.

drilled caisson

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.

folded plate

A structural roof system consisting of inclined planes which support each other and a function as deep beams.

Proctor Test

A laboratory compaction test to determine the optimum moisture content and density for a soil.

turn-of-net method

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.

slump test

A test for mixed concrete to determine consistency and workability.

air entrainment

The incorporation of tiny air bubbles into concrete to improve its workability and resistance to frost.

web members

The interior members of a truss, which connect the chords.

differential settlement

Unequal settlement of the various parts of a building, which may cause excessive stresses in the structural frame or tilting of the building.

cable roof

A curved structure in which the internal stresses are pure tension.


A fine-grained soil, whose particles are larger than clay and smaller than sand.

dry pack

To pack a damp concrete mixture into a confined space.


The change in size of a body caused by external forces. It is also called 'deformation'.

ultimate strength

The maximum unit stress that can be developed in a material.

statical moment

The product of an area and the distance from the centroid of the area to a given axis.


The change in size of a body caused by external forces. It is also called 'strain'.

trussed rafter

A prefabricated, lightweight wood truss used to support roof loads and other small structures.

eccentric load

A longitudinal load which acts at a distance from a member's centroid, thereby producing bending moment in addition to axial stress.

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