Why These Terms Matter

Structural engineering has its own precise vocabulary, and much of it is defined — or implied — by the codes and standards that govern design: ASCE 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures), ACI 318 (Building Code Requirements for Structural Concrete), AISC 360 (Specification for Structural Steel Buildings), and the International Building Code (IBC), which adopts all three by reference. A term like "factored load" or "moment frame" carries a specific technical meaning that determines how a structure is analyzed, detailed, and inspected. Using these terms loosely, or confusing closely related concepts, leads to miscommunication between engineers, contractors, and building officials, and in the worst case to design errors.

This glossary covers 55 of the most frequently used structural engineering terms, organized alphabetically, with the governing standard or code section noted where applicable. It is written for engineers early in their careers, students preparing for the FE or PE Structural exam, and anyone who needs to read structural drawings and specifications without stumbling over the jargon.

A

Allowable Strength Design (ASD) — AISC 360
A design method in which nominal member strengths are divided by a safety factor (Omega) and compared against service-level (unfactored) loads. ASD is one of two design philosophies permitted by AISC 360, the other being LRFD. Under ASD, the check is Ra ≤ Rn/Omega, where Ra is the required strength from unfactored loads. Most engineers today default to LRFD for new steel design, but ASD remains common in wood design (NDS) and is still fully valid under AISC 360.
Anchor Bolt — ACI 318 Chapter 17
A bolt embedded in concrete, used to attach a steel base plate, ledger, or other structural element to a concrete foundation or wall. Anchor bolt design (embedment depth, edge distance, breakout capacity) is governed by ACI 318 Chapter 17, which addresses concrete breakout, pullout, and steel strength failure modes — all of which must be checked, not just the bolt's tensile capacity.
Axial Load — general
A load applied along the longitudinal axis of a member, causing pure tension or compression without bending. Columns primarily carry axial load; the term is used to distinguish this loading from bending moment and shear, which act transverse to the member axis.

B

Base Shear — ASCE 7 Chapter 12
The total lateral (horizontal) force a structure's seismic force-resisting system must be designed to resist at its base, resulting from earthquake ground motion. Base shear (V) is calculated from the seismic response coefficient, the structure's weight, and its fundamental period, then distributed vertically to each floor level using the equivalent lateral force procedure of ASCE 7 Section 12.8.
Beam — general
A horizontal (or near-horizontal) structural member that primarily resists loads through bending and shear, transferring load to its supports (columns, walls, or other beams). Beams are the most common flexural member in building structures.
Bearing Wall — IBC
A wall that supports vertical load from the structure above, in addition to its own weight, as opposed to a non-load-bearing (partition) wall that only supports itself. Identifying which walls are bearing is critical before any renovation — removing a bearing wall without an engineered header or beam can cause structural failure.
Bending Moment — general
The internal moment (rotational force) at a section of a member, caused by loads applied transverse to the member's axis, that tends to bend the member. Bending moment is typically expressed in units of force times length (kip-ft, kN-m) and varies along the length of a beam, reaching a maximum at the location of peak flexural demand.
Braced Frame — AISC 341
A structural system that resists lateral loads primarily through axial forces in diagonal bracing members, rather than through bending in beams and columns. Braced frames (concentric or eccentric) are generally stiffer and more economical in steel than moment frames but place diagonal members that can interfere with architectural openings.
Bracing — general
Secondary structural members added to resist lateral loads, prevent buckling, or provide stability to primary members during construction and in service. Lateral bracing on a beam compression flange, for example, reduces the unbraced length and increases the member's flexural capacity.

C

Cantilever — general
A structural member that is fixed (or rigidly connected) at one end and unsupported (free) at the other, projecting horizontally beyond its support. Cantilevers carry the full moment and shear demand at the fixed end and experience zero moment at the free end.
Composite Construction — AISC 360 Chapter I
Construction in which a steel member and a concrete slab act together as a single structural unit, typically through shear studs welded to the top flange of a steel beam and embedded in the concrete deck. Composite beams achieve significantly greater stiffness and strength than the steel section alone, allowing shallower, lighter framing.
Concentrated Load — general
A load applied at a single point (or over a very small area) on a structural member, as opposed to a distributed load spread over a length or area. A column reaction transferred to a supporting beam is a common example of a concentrated load.
Connection (Structural) — AISC 360 Chapter J
The detail joining two or more structural members — bolted, welded, or a combination — designed to transfer forces (shear, moment, axial, or a combination) between the members. Connections are classified as simple (shear-only, pinned), moment (fully restrained), or partially restrained, each assuming a different degree of rotational rigidity in the overall structural analysis.

D

Dead Load — ASCE 7 Chapter 3
The permanent, essentially constant load on a structure, consisting of the weight of the structure itself and all permanently attached components — framing, floor and roof decking, fixed partitions, finishes, and permanently installed mechanical equipment. Dead load is calculated from actual material weights and is one of the two baseline loads (with live load) combined in every structural load combination.
Deflection — general
The displacement of a structural member from its unloaded position, caused by applied loads. Deflection limits (commonly expressed as a fraction of span, such as L/360 for floor live-load deflection) are serviceability checks, separate from strength checks — a member can have adequate strength but still deflect enough to crack finishes or feel "bouncy."
Design Load — general
The load a structural member or system is designed to resist, determined by applying the appropriate load factors (LRFD) or using service loads directly (ASD) per the governing load combinations in ASCE 7 Section 2.3 or 2.4.
Diaphragm — ASCE 7 Chapter 12
A horizontal (or near-horizontal) structural element, typically a floor or roof deck, that transfers lateral (wind or seismic) forces to the vertical elements of the lateral force-resisting system (shear walls, braced frames, or moment frames). Diaphragms are classified as rigid, flexible, or semi-rigid depending on their in-plane stiffness relative to the vertical elements, which affects how lateral force is distributed.
Drift (Story Drift) — ASCE 7 Section 12.12
The lateral displacement of one floor level relative to the floor below it, caused by wind or seismic loads. Story drift limits control damage to non-structural elements (cladding, partitions, glazing) and are checked separately from strength requirements, particularly important in taller or more flexible structures.
Ductility — general
The ability of a structural material or system to undergo significant inelastic (plastic) deformation before failure, absorbing energy rather than failing suddenly (brittle failure). Ductile detailing is central to seismic design — codes require ductile connections and members in higher seismic design categories so a structure can deform and dissipate earthquake energy without collapsing.

E

Eccentricity — general
The distance between the line of action of an applied load (or the resultant force) and the centroid (or shear center) of a structural section, or between a structure's center of mass and center of rigidity. Eccentricity between mass and rigidity in a building plan causes torsional response under seismic load, an important irregularity check under ASCE 7.
Effective Length Factor (K) — AISC 360 Chapter C
A factor applied to a column's actual unbraced length to determine its effective length for buckling calculations, accounting for the end restraint conditions (pinned, fixed, or somewhere between) and whether the frame is braced or unbraced (sway permitted). K = 1.0 for a pinned-pinned column; K can be less than 1.0 for fully fixed ends or greater than 2.0 for an unbraced cantilever column.
Elastic Design — general
Design based on the assumption that a structure remains within its elastic range (stresses proportional to strain, per Hooke's Law) under design loads, so it returns to its original shape when load is removed. Contrasted with plastic (inelastic) design methods that permit and account for yielding.
Envelope Analysis — general
An analysis approach that considers multiple load cases or combinations and designs each member for the worst-case (maximum) demand from any combination, rather than a single governing case. Structural software typically performs envelope analysis automatically across all required ASCE 7 load combinations.

F

Factor of Safety — general
The ratio of a structure's or member's actual (ultimate or yield) capacity to the load it is required to resist, providing a margin against uncertainty in material properties, load estimation, and construction tolerances. In modern LRFD design, factor of safety concepts are embedded in resistance factors (phi) and load factors rather than expressed as a single overall ratio.
Factored Load — ASCE 7 Section 2.3
A service (unfactored) load multiplied by its corresponding load factor in an LRFD load combination — for example, 1.2D + 1.6L, where D is dead load and L is live load. Factored loads represent a statistically unlikely but possible combination of loads at their maximum expected values, used to size members under LRFD.
Fixed Support — general
A structural support condition that restrains both translation (vertical and horizontal movement) and rotation, capable of resisting moment, shear, and axial force. Contrasted with a pinned support, which restrains translation but allows rotation (no moment resistance).
Flexural Strength — ACI 318 / AISC 360
The maximum bending moment a member can resist before failure, governed for concrete by the tension steel reinforcement ratio and concrete compressive strength, and for steel by the section's plastic or yield moment capacity and its resistance to lateral-torsional buckling.
Foundation — general
The structural element that transfers loads from a building's superstructure into the supporting soil or rock. Foundations are broadly classified as shallow (spread footings, mat/raft foundations) or deep (driven piles, drilled piers/caissons), the choice driven by soil bearing capacity, settlement tolerance, and load magnitude.

G

Girder — general
A large primary beam, typically supporting other beams (joists or secondary beams) that frame into it, rather than supporting the floor or roof deck directly. The distinction between "beam" and "girder" is one of relative size and role rather than a precise code-defined term.
Grade Beam — general
A reinforced concrete beam that transfers load from a wall or column above down to isolated foundation elements (piles or piers), typically spanning at or near grade level and, in expansive soil conditions, designed to span over the soil without bearing directly on it.

I

Importance Factor (Ie, Iw) — ASCE 7 Table 1.5-2
A multiplier applied to seismic (Ie) or wind (Iw) design forces based on a structure's Risk Category, increasing design forces for buildings whose failure would pose a greater life-safety or societal risk — hospitals, emergency response facilities, and buildings holding large numbers of people receive higher importance factors than ordinary occupancies.
Inelastic Deformation — general
Permanent deformation that remains after a load is removed, occurring once a material or member is stressed beyond its elastic limit. Seismic design deliberately permits controlled inelastic deformation in designated ductile elements (such as moment frame beams) to dissipate earthquake energy.
Interaction Equation — AISC 360 Chapter H / ACI 318
A design equation that checks a member's combined resistance to two or more simultaneous force effects — most commonly combined axial load and bending moment in a beam-column. AISC 360 Equations H1-1a and H1-1b are the standard steel beam-column interaction checks; concrete columns use interaction (P-M) diagrams for the equivalent check.

L

Lateral Force-Resisting System (LFRS) — ASCE 7
The structural system — moment frames, braced frames, shear walls, or a dual system combining two of these — specifically designed and detailed to resist lateral (wind and seismic) loads and transfer them safely to the foundation. Every building requires a defined, continuous LFRS with a clear load path from roof to foundation.
Live Load — ASCE 7 Chapter 4
A load produced by the use and occupancy of a building — people, furniture, movable equipment, and stored materials — that varies over time and is not permanently attached to the structure. ASCE 7 Table 4.3-1 specifies minimum uniformly distributed live loads by occupancy type (e.g., 40 psf for residential, 100 psf for assembly areas without fixed seats).
Load Combination — ASCE 7 Section 2.3/2.4
A specific factored (LRFD) or unfactored (ASD) sum of different load types — dead, live, wind, seismic, snow, rain — that a structure must be checked against, representing plausible simultaneous loading scenarios. ASCE 7 lists seven basic LRFD combinations and a parallel set of ASD combinations; the governing (worst-case) combination controls the design of each member.
Load Path — general
The continuous route by which a load travels from its point of application through the structure to the foundation and ultimately into the supporting soil. A "complete load path" — for gravity, wind, and seismic loads alike — is a fundamental requirement of structural design; any break in the path (a missing connection, an unbraced discontinuity) can cause localized or progressive failure.
LRFD (Load and Resistance Factor Design) — AISC 360 / ACI 318
A design method in which loads are multiplied by load factors (generally greater than 1.0, reflecting the probability of exceeding expected values) and nominal member strength is multiplied by a resistance factor phi (generally less than 1.0, reflecting uncertainty in capacity), with the check phi·Rn ≥ Ru. LRFD is the default method in ACI 318 for concrete and one of two permitted methods in AISC 360 for steel.

M

Mat Foundation (Raft Foundation) — general
A large, continuous reinforced concrete slab foundation that supports an entire building footprint (or a large portion of it) as a single unit, used where soil bearing capacity is low, where individual footings would be so large they overlap, or where differential settlement must be minimized.
Moment Frame — AISC 341 / ACI 318 Chapter 18
A structural system that resists lateral loads through bending (flexural) action in rigidly connected beams and columns, without diagonal bracing. Moment frames (ordinary, intermediate, or special, in increasing order of required ductility) provide open floor plans free of bracing but are generally less stiff and more expensive than braced frames for the same lateral demand.
Moment of Inertia (I) — general
A geometric property of a cross-section that measures its resistance to bending, calculated from the distribution of the section's area relative to its centroidal axis. A larger moment of inertia means greater stiffness and lower deflection for a given bending moment; it is a purely geometric property, independent of material.

N

Nominal Strength (Rn) — AISC 360 / ACI 318
The theoretical capacity of a structural member or connection, calculated using specified material strengths and section properties without any reduction for uncertainty. Nominal strength is reduced by a resistance factor (phi, LRFD) or safety factor (Omega, ASD) to obtain the usable design strength.

O

Overturning Moment — general
The moment tending to tip (rotate) a structure or retaining element about its base or toe, caused by lateral loads (wind, seismic, or lateral earth pressure) acting above the base. Overturning is resisted by the restoring moment from the structure's own weight and any counterweight, checked with a required minimum factor of safety (commonly 1.5) against overturning.

P

P-Delta Effect — ASCE 7 Section 12.8.7
The secondary (second-order) increase in member forces and displacements caused by gravity loads (P) acting through the lateral displacement (delta) of a structure, magnifying the primary first-order effects. P-delta effects become significant in taller, more flexible structures and must be checked or amplified per code-specified stability coefficients.
Pile — general
A long, slender deep foundation element — driven, drilled, or augered — that transfers building loads down to competent bearing strata or develops capacity through skin friction along its length, used where shallow foundations cannot achieve adequate bearing capacity or where settlement must be minimized.
Plastic Hinge — general
A location in a structural member where the material has yielded across the full section and continues to rotate under roughly constant moment (the plastic moment capacity), effectively behaving like a hinge for further load increase. Plastic hinges are the intended mechanism for ductile energy dissipation in seismic moment frames, deliberately located and detailed away from connections that must remain elastic.
Progressive Collapse — general
A structural failure in which the loss of a single member or connection triggers a chain reaction of additional failures disproportionate to the original local damage, potentially resulting in the collapse of a large portion of a structure. Alternate load path analysis and redundancy requirements are used to reduce progressive collapse risk in structures where it is a design consideration.

R

Rebar (Reinforcing Bar) — ACI 318 / ASTM A615
Steel bars embedded in concrete to carry the tension forces that plain concrete cannot resist (concrete is strong in compression but weak in tension). Rebar is specified by ASTM A615 grade (yield strength) and bar size number, and its placement (cover, spacing, development length) is governed by ACI 318.
Redundancy — ASCE 7 Section 12.3.4
The degree to which a structural system has multiple, independent load paths so that failure of a single element does not lead to overall instability or collapse. ASCE 7 assigns a redundancy factor (rho) that increases seismic design forces for structures with limited redundancy (few lateral-resisting elements), reflecting the greater consequence of a single element failing.
Resistance Factor (phi) — AISC 360 / ACI 318
A factor less than 1.0 applied to nominal member strength in LRFD design to account for variability and uncertainty in material properties, workmanship, and analysis assumptions. Typical values range from about 0.65 to 0.90 depending on the failure mode and the consequence of underestimating capacity.
Response Modification Coefficient (R) — ASCE 7 Table 12.2-1
A factor that reduces elastic seismic design forces to account for a structural system's expected ductility and overstrength, reflecting that most structures are not designed to remain fully elastic during a design-level earthquake. Systems with greater inherent ductility (special moment frames) are assigned higher R values than less ductile systems (ordinary systems), resulting in lower calculated design forces but requiring more rigorous ductile detailing.
Rigid Diaphragm — ASCE 7
A diaphragm assumed to be infinitely stiff in-plane compared to the vertical lateral-resisting elements, so it distributes lateral force to those elements in proportion to their relative stiffness (rather than tributary area). Concrete slabs are typically treated as rigid diaphragms; the assumption significantly affects how lateral loads are distributed among shear walls or frames of differing stiffness.

S

Seismic Design Category (SDC) — ASCE 7 Section 11.6
A classification (A through F, in increasing order of required seismic detailing rigor) assigned to a structure based on its Risk Category and the mapped seismic ground motion parameters (Ss, S1) at its site. SDC determines which structural systems are permitted, height limits, required analysis procedures, and the level of ductile detailing required throughout the design.
Shear — general
An internal force acting parallel (transverse) to a member's cross-section, tending to cause one part of the section to slide relative to the adjacent part. Shear and bending moment together are the two primary internal force effects checked in flexural member design.
Shear Wall — ACI 318 Chapter 18 / AISC 341
A wall (concrete, masonry, or steel-plate) designed specifically to resist in-plane lateral (wind or seismic) loads, transferring them down to the foundation through combined shear, flexure, and overturning resistance. Shear walls are among the stiffest and most economical lateral systems, commonly used in residential and mid-rise construction.
Simply Supported — general
A support condition in which a member is supported at its ends by pinned or roller supports that resist vertical (and sometimes horizontal) reaction but provide no rotational restraint (no moment transfer). A simply supported beam is the most basic and commonly analyzed structural configuration.
Slenderness Ratio — AISC 360 Chapter E
The ratio of a column's effective length (KL) to its radius of gyration (r) about the governing axis, a key parameter in determining whether a column fails by yielding (short, stocky columns) or by elastic/inelastic buckling (slender columns). Higher slenderness ratios correspond to lower buckling capacity for a given cross-section.
Spread Footing — general
An individual shallow foundation element that "spreads" a column or wall load over a sufficient soil area to keep bearing pressure within the soil's allowable bearing capacity. Spread (isolated) footings are the most common and economical foundation type where soil conditions and settlement are not limiting factors.
Story Drift — see Drift.
See "Drift (Story Drift)" above.
Strength Design — ACI 318
ACI 318's term for its LRFD-based design methodology, in which factored loads are compared against reduced nominal member strength (phi·Mn, phi·Vn, phi·Pn). "Strength design" and "ultimate strength design" are used interchangeably with LRFD in concrete design terminology.

T

Tributary Area — general
The area of floor, roof, or wall surface whose load is assumed to be carried by a particular structural member, typically taken as halfway to the adjacent parallel members in each direction. Tributary area is the standard simplified method for distributing distributed loads to individual beams, columns, and footings.
Truss — general
A structural framework composed of straight members connected at joints (traditionally assumed pinned) arranged in triangular patterns, so that each member carries primarily axial force (tension or compression) rather than bending. Trusses efficiently span long distances with relatively little material, common in roof framing and bridge structures.

U

Ultimate Strength — general
The maximum stress or load a material or member can sustain before failure or rupture, as distinct from yield strength (the stress at which permanent, inelastic deformation begins). For ductile steel, there is a significant margin between yield and ultimate strength; for brittle materials, the two may be close together.
Unbraced Length — AISC 360 Chapter E/F
The distance between points along a compression member or the compression flange of a beam that are braced against lateral movement or twist. Unbraced length directly affects buckling capacity — flexural buckling for columns, lateral-torsional buckling for beams — and is a key input to both AISC 360 column and beam design equations.

W

Wind Load — ASCE 7 Chapters 26–31
The lateral (and sometimes uplift) pressure a structure must be designed to resist from design wind speeds at its site, calculated from basic wind speed maps, exposure category (terrain roughness), building height, and enclosure classification. ASCE 7 provides several analysis procedures of increasing complexity (simplified, analytical/directional, and wind tunnel) depending on building geometry and risk category.

Y

Yield Strength — general
The stress at which a material begins to exhibit permanent (plastic) deformation rather than returning fully to its original shape when unloaded. Yield strength (Fy) is the primary material property used to size steel members under both ASD and LRFD, and marks the practical onset of ductile structural behavior.