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Contents | 1. Introduction to structural design | 2. Loads | 3. Wood | 4. Steel | 5. Reinforced concrete

Unit abbreviations and conversion | References |

**about** [as in: compute moment about the neutral axis] *prep.* the term "about" is equivalent to saying: "by measuring moment arms perpendicularly from each force to."

**aggregate** *n.* Stone selected for use in concrete, consisting of various grades, or sizes, from course (gravel) to fine (sand).

**axial force** *n.* A force parallel to the longitudinal axis of a structural element.

**balloon frame** *n.* A type of light wood-frame construction characterized by the use of dimension lumber for continuous
vertical studs (2x4 or 2x6 at 16 in. or 24 in. on center) and closely spaced joists and rafters; largely superseded by
platform frame construction.

**beams and stringers** *n.* A subcategory of "timbers," refers to lumber whose smaller nominal cross-sectional dimension exceeds 4 in. and whose larger nominal cross-sectional dimension is at least 4 in. bigger than the smaller dimension, thereby forming a rectangular shape appropriate for use as a beam, but not limited to that use.

**bearing** *ger.* The force exerted, in compression, by the surface of a structural element in contact with (that is, pressing
against) the surface of another element.

**bending moment** *n.* An effect on a structural element caused by the action of at least two parallel force components
that are not co-linear, and resulting in a distribution of stress within the element's cross section characterized by maximum stress at the "extreme fibers" (opposite edges) and zero stress at the neutral axis.

**bolt** *n.* A type of fastener used in both wood and steel construction consisting of a head and threaded shank, onto which
is placed a nut; bolts are first inserted into a bolt hole before being tightened.

**brittle** *adj.* Lacking ductility, that is, lacking the ability to absorb energy, therefore being susceptible to catastrophic and
sudden failure, especially under dynamic loading.

**cantilevered beam** *n.* A beam which extends beyond one or both of its supports.

**cast iron** *n.* An alloy of iron with very high carbon content; used most famously in the nineteenth century for structural
columns, but largely superseded by steel in the early twentieth century.

**cellulose** *n.* An organic compound consisting of hundreds of linked and linear glucose units comprising about half the content of wood.

**compact section** *n.* Steel shapes proportioned so that, when used in bending, the strength and reserve capacity of the element will not be compromised by local flange or web buckling within those portions of the cross section subjected to compressive stress; this is primarily a function of the relative thickness of flanges and webs; for noncompact shapes, the available strength is reduced.

**conifer** *n.* A type of cone-bearing tree, one of the gymnosperms, including the most common structural lumbers: firs, pines, hemlocks, redwoods, larches, etc.

**continuous beam** *n.* Any beam that extends over more than two supports, and is therefore statically indeterminate.

**couple** [of equal and opposite forces on a cross section in bending] *n.* Two equal and opposite forces, *F*, separated by a moment arm, *τ* (that is., two such forces that are not co-linear), and therefore causing a moment, *M* = *F* × *τ*.

**curing** [of concrete] *ger.* The chemical process by which concrete hardens; the reaction of portland cement and water within the concrete mix.

**cylinder strength** [of concrete] *n.* The compressive stress at which a 6 in. × 12 in. cylinder of concrete, which has cured for 28 days, fractures.

**decay** *n.* The deterioration of wood caused by fungi; occurs when wood is wet or moist, temperatures are suitable, and oxygen (air) is available.

**deflection** *n.* The movement measured perpendicular to the longitudinal axis of a structural element under load, typically
a beam; the term usually refers to the maximum deflection, often at midspan.

**determinate** *adj.* Pertaining to a class of structures whose reactions can be determined using only equations of equilibrium; includes simply-supported beams, cables, three-hinged arches, and pinned trusses formed from simple triangles.

**development length** *n.* For reinforcement in reinforced concrete structures, the minimum bar length such that any tendency for the bar to slip relative to the concrete is counteracted.

**dimension lumber** *n.* Lumber whose smaller nominal cross-sectional dimension is 4 in. or less; used extensively in light wood framing.

**ductile** *adj.* Having the capacity to absorb energy without fracturing; a quality of steel, but not of cast iron; of wood (in
compression), but not of unreinforced concrete or masonry; see *brittle*.

**effective length** *n.* The length of a compressive element with simple (pinned) constraints whose strength is identical to
an otherwise similar compressive element with at least one non-pinned constraint.

**elastic** [behavior of material] *adj.* A material property characterized by a return to the initial shape after a load is first applied and then removed; associated with linear stress-strain behavior.

**elastic moment** *n.* The largest bending moment that can be sustained by a structural element such that all stresses within a given cross section are within the elastic range; in steel, the distribution of stresses coinciding with an elastic moment are linear, with a maximum value equal to the yield stress, *F _{y}*.

**equilibrium** *n.* A state of "rest," or balance, characterized by the sum of all forces in any direction being zero and the sum of all moments about any axis being zero; in a "plane" (two-dimensional) structure, conditions of static equilibrium
are met when all forces in the x- and y-directions (that is, those axes that define the plane in which the structural element exists) equal zero, and all moments about the z-axis equal zero.

**free-body diagram** *n.* [FBD] A diagram of a structural element (or portion thereof) abstracted from its context, together with all forces and moment acting on the element, both externally (ordinary loads and reactions) and internally (at cross sections where the element has been "cut," representing internal shears, axial forces, and bending moments).

**force** *n.* A vector with magnitude and direction represented by an arrow, ordinarily described as a load or weight, and measured in units of pounds or kips.

**graded** [lumber] *adj.* Having a mark that describes the quality of a given piece of lumber; typical grades include select structural, No. 1, No. 2, No. 3, stud, construction, standard, and utility.

**grain** [of lumber] *n.* The directional pattern observed on the surface of lumber (or manufactured products such as plywood) corresponding to the groups of cellulose fibers originally running longitudinally up the trunk of the tree.

**hardwood** *n.* Wood from broad-leafed, deciduous angiosperm trees, such as oak, elm, and maple; not necessarily harder than "softwoods."

**hydraulic** *adj.* Referring to cement, having the ability to harden due to a chemical reaction with water.

**indeterminate** *adj.* Pertaining to a class of structures whose reactions cannot be determined using only equations of
equilibrium; analysis of such structures requires, in addition to equilibrium, consideration of compatibility of displacements, and therefore of the relative stiffness of structural elements; such structures are also described as redundant, in that they contain elements, or constraints, beyond what is required for equilibrium.

**influence area** *n.* The area in plan within which a load will have an effect upon (that is, influence) a structural element, formerly used in the calculation of live load reduction; not to be confused with tributary area, but rather equal to the tributary area times the live load element factor, *K _{LL}*.

**internal hinge** *n.* A connecting device within a structural form that prevents translation (vertical or horizontal moment) of one side relative to the other, but allows rotation; present in three-hinged arches and multi-span determinate beams.

**joist [steel or wood]** *n.* One of a series of closely spaced and parallel beams supporting a floor; in wood-frame construction,
joists are commonly made from dimension lumber and spaced at 16 in. or 24 in. on center.

**knot [in lumber]** *n.* A defect in a piece of lumber characterized by the interruption of the board's parallel grain by circular rings corresponding to the former position of a branch.

**lag screw** *n.* A type of fastener used in wood construction consisting of a head, shank, and tapered tip; part of the shank and tapered tip are threaded; sometimes called lag bolts.

**leeward** *adj.* Referring to the side of a building on the far side relative to the direction of the wind; see windward.

**lignin** *n.* The "glue" binding cellulose fibers together within a wood cross section.

**linear** [for example, stretching and shortening on a cross section subjected to bending] *adj.* In a straight line; referring to the straight-line stress-strain (or load-deformation) curves of certain materials, within their elastic ranges.

**live load reduction** *n.* The permitted reduction of live loads assumed to be present on relatively large areas, justified
by the probabilistic argument that the worse-case live load values found in building codes (determined for relatively small areas) are increasingly less likely to be valid as the areas being considered get larger; calculations for live load reduction were formerly based on the so-called influence area, but now are based on the tributary area multiplied by a live load element factor, *K _{LL}*.

**live load element factor** [see *influence area*]

**main member** *n.* Where two structural elements are connected using nails or lag screws, the member into which the fastener end terminates; with bolts, the thicker of the two members, if any; or the middle member in three-member (usually bolted) connections.

**modulus of elasticity** *n.* A material property defined as the change in stress divided by the change in strain; therefore, the slope of a stress-strain curve; implicated in the "stiffness," but not the strength, of a material.

**moisture content** *n.* A measure of the water within a piece of wood, defined as the weight of water divided by the dry weight of the wood and expressed as a percentage; the moisture content (m.c.) separating dry ("seasoned") and wet ("green") lumber is about 19%.

**moment of inertia** *n.* For structural elements subjected to bending, a cross-sectional property indicating the section's contribution to stiffness; calculated by finding the sum of the products of areas and the square of their distances to the centroidal axis of the section.

**nail** *n.* A type of fastener used to connect two pieces of wood consisting of a head, shank, and tapered tip; typically
driven into the wood by means of a hammer or pneumatic device.

**penetration** *n.* For nails and lag screws, the length of the fastener within the main member.

**plane structure** *n.* A structure or structural element that can be modeled as existing, and moving under the application
of loads, on a two-dimensional (plane) surface.

**plastic** [behavior of material] *adj.* A material property characterized by a failure to return to the initial shape after a load is first applied and then removed; steel, for example, has a distinct plastic range beyond its elastic range.

**plastic moment** *n.* In steel, the bending moment at a cross section within a structural element corresponding to a stress distribution in which all stresses are assumed to be equal to the yield stress, *F _{y}*, half in tension and half in compression.

**platform frame** *n.* A type of light wood-frame construction characterized by the use of dimension lumber for 1-story-high
vertical studs (2x4 or 2x6 at 16 in. or 24 in. on center) and closely spaced joists and rafters which, after a subfloor has been installed, provide a "platform" for the construction of additional stories; derived from, but has largely superseded balloon frame construction.

**point of inflection** *n.* A point along a structural element subject to bending marking the transition from positive to negative moment; a point of zero moment between regions of bending with opposite curvature.

**ponding** *ger.* A phenomenon associated with flat or low-slope roofs in which rain water, collecting in the deflected areas at the midspan of roof beams, causes increased deflection as it accumulates, leading to progressively large deflections and, potentially, structural failure; can be prevented by providing adequate slope, proper drainage, and camber for large spans.

**portland cement** *n.* The most commonly used cement used to make concrete.

**posts and timbers** *n.* A subcategory of "timbers," refers to lumber whose smaller nominal cross-sectional dimension exceeds 4 in. and whose other nominal cross-sectional dimension is the same or no more than 2 in. bigger than the smaller dimension, thereby forming a relatively square shape appropriate for use as a column (post), but not limited to that use.

**reaction** *n.* For any structural element, the forces and moments at its supports necessary to resist the action of applied loads, thereby maintaining a condition of equilibrium.

**redundant** [see *indeterminate*]

**reinforcement (steel) ratio** [in reinforced concrete] *n.* The ratio of the area of reinforcing steel to the gross area, for columns; for beams, the ratio of the area of reinforcing steel to the area defined by the beam width times the beam depth measured from the face of concrete in the compression zone to the centerline of tensile steel reinforcement.

**relative stiffness** *n.* The stiffness of one element (stiffness defined for elements subjected to bending as the modulus of elasticity times the moment of inertia; for elements subjected to axial force, as the modulus of elasticity times the cross-sectional area) compared to that of another; where two or more elements combine to resist the same loads, those loads are resisted by each element in proportion to its relative stiffness. Stiffness is sometimes used as a convenient shorthand to describe a load-deformation relationship which includes both the actual element stiffness (involving only material and cross-sectional properties) as well as element length or span.

**residual stress** *n.* Stress "locked in" to a structural element, usually as an unintended but unavoidable result of heating and cooling during the manufacturing process (for example, hot-rolled steel sections), but sometimes as a deliberate technique for improving material qualities (for example, tempered glass).

**sag** [of a cable] *n.* The vertical distance measured from the low-point of a cable to the level of the supports.

**sag point** *n.* The position along the length of a gable corresponding to the lowest point; see sag.

**seasoning** *ger.* The process of drying out wood after it has been cut into boards (lumber); both air-drying or kiln-drying are used.

**section modulus** *n.* A cross-sectional property indicating that section's relative strength in bending; equals the moment of inertia divided by half the height of the section (for symmetrical sections).

**shear force** *n.* An internal force within a cross section perpendicular to the longitudinal axis of the structural element.

**shear lag** *n.* A phenomenon encountered when a connection is made to only a portion of a steel element in tension, so that the cross section in the vicinity of that connection is only partially, and incompletely, stressed.

**side member** *n.* Where two structural elements are connected using nails or lag screws, the member into which the
fastener is first inserted; with bolts, the thinner of the two members, if any; or the two outside members in three-member (usually bolted) connections.

**sign** [of a bending moment or shear force] *n.* An arbitrary assignment of "positive" or "negative" corresponding to rotational direction (for a bending moment), or vertical direction (for shear in a beam); for beams, positive bending corresponds to tension on the bottom and compression on the top of the cross section, with a counterclockwise moment
acting on the right side of a free-body diagram; while positive shear corresponds to an downward-acting force on the same free-body diagram.

**simply supported beam** *n.* A beam supported by a hinge and a roller, the hinge preventing all translation but allowing rotation, and the roller preventing translation perpendicular to the longitudinal axis of the beam while allowing "horizontal" translation and rotation; such a model is commonly applied to ordinary steel and wood beams and joists, which both approximates their actual behavior, and allows them to be analyzed as statically determinate structures.

**slenderness ratio** *n.* A dimensional property of a structural element subjected to axial compression indicating its susceptibility to buckling, and defined as the effective length divided by the radius of gyration; the more "slender" the element, the greater the tendency to buckle.

**softwood** *n.* Wood from cone-bearing gymnosperm trees, such as pines, firs, larches, etc., and comprising the majority of structural lumber; not necessarily softer than "hardwoods."

**spandrel** [beam or girder] *adj.* At the outside face of a building.

**specific gravity** *n.* A material property equal to the relative density of the material compared to that of water.

**spiral** [in a reinforced concrete column] *n.* A continuous steel wire in the shape of a spiral used to confine both longitudinal reinforcing steel and concrete within a round cross section.

**stagnation pressure** [see *velocity pressure*]

**statically determinate** [see *determinate*]

**statically indeterminate** [see *indeterminate*]

**stud** [steel or wood] *n.* One of a series of closely-spaced and parallel posts comprising a wall; in wood-frame construction, studs are commonly made from dimension lumber and spaced at 16 in. or 24 in. on center.

**tension-controlled member** *n.* A reinforced concrete element in which failure is initiated by yielding of reinforcing steel in tension, rather than by crushing of concrete in compression.

**thickness** [of wood cross section] n. The smaller dimension of a wood cross section.

**tie** [in a reinforced concrete column] *n.* One of a series of steel reinforcing bars placed around the perimeter of reinforced concrete columns and used to confine both longitudinal reinforcing steel and concrete within rectangular
cross sections.

**timbers** *n.* Lumber whose smaller nominal cross-sectional dimension is greater than 4 in.

**torsion** *n.* An effect on a structural element caused by the action of a moment about the element's longitudinal axis; also referred to as torque or twisting.

**tributary area** *n.* The area in plan assigned to each structural element, measured from the centerlines between those elements; used to determine the distribution of loads; results in accurate load values only in special cases without cantilevers or continuous (indeterminate) elements, and with symmetrical placement of loads; otherwise still useful
as an approximate means for assigning loads.

**unbraced length** [between lateral supports on a beam] *n.* The distance between lateral supports on a beam, used to determine the beam's susceptibility to lateral-torsional buckling, and therefore the reduction in allowable bending
stress.

**under-reinforced** [concrete beams] *adj.* Having the desirable property that failure is initiated by yielding of reinforcing steel in tension rather than by crushing of concrete in compression; such behavior is implemented by requiring a minimum steel strain at failure of 0.004 (or 0.005 to take advantage of the highest "strength reduction" factor for bending).

**uniformly distributed** [load] *adj.* Spread out evenly over a floor or roof (in which case it is measured in units of pounds per square foot), or over a linear element such as a beam (in which case it is measured in units of pounds or kips per linear foot).

**unserviceable** *adj.* Not useful or adequate for its intended purpose, due to such things as excessive vibration or deflection under normal loads.

**velocity (or stagnation) pressure** *n.* The pressure (uniformly distributed load) assumed to act on the surface of a building, caused by the force of a constant wind; proportional to the square of the wind's speed.

**weld** *n.* A type of fastening used in steel construction in which molten steel deposited by an electrode cools and joins two structural steel elements together. *v.* To engage in the activity of depositing such electrode-steel in order to connect two steel structural elements together.

**width** [of wood cross section] *n.* The larger dimension of a wood cross section.

**windward** *adj.* Referring to the side of a building directly in the path of the wind; see *leeward*.

**withdrawal** *n.* The capacity of a nail or lag screw to remain in place when subjected to a tension force that would otherwise
pull it out.

**workability** [of concrete] *n.* Being of a consistency that permits proper mixing and placement; not too stiff.

**wrought iron** *n.* An alloy of iron with very low carbon content; in many ways a precursor to modern steel, but no longer commonly used.

**yielding** [of steel] *ger.* A characteristic property of steel in the plastic range in which the material is able to strain without any increase in stress, that is, deformations can increase at a constant load; the stress at which yielding occurs, marking the end of the elastic range, is called the yield stress.

© 2020 Jonathan Ochshorn; all rights reserved. This section first posted November 15, 2020; last updated November 15, 2020.