Definitions

Structural Design · FE Reference Handbook section

Structural Design

28 formulas

10 exam-style examples

~60 min

All Structural Design lectures

Learning objectives

What you must be able to do before leaving this section.

This chapter section covers Definitions within Structural Design. Read it the way you would read a textbook chapter: the theory first so the relations mean something, then every equation with its use and its trap, then 10 fully worked examples with the arithmetic shown line by line, and finally a self-check you should be able to answer without notes.

Explain, in your own words, what definitions describes physically and when it applies.
State every one of the 28 relations the handbook lists here and name each symbol with its unit.
Select the correct relation from the wording of an exam stem within 20 seconds.
Carry a complete solution from givens to a "most nearly" answer with the correct unit.
Recognise the distractors generated by the unit trap: f'c and Fy in ksi with areas in in² give kips.

Lecture

Why this section exists. Definitions is the part of Structural Design that lets you connect a reinforced concrete or steel member being checked to a number you can defend. Before any equation is useful you must be able to picture the physical situation it describes; the schematic below is that picture.

How the theory is built. The handbook prints results, not derivations. Each relation in this section comes from one governing principle applied to the idealised system: state the principle, impose the stated assumptions, and the printed equation follows. Knowing which assumption each relation rests on is what lets you reject a wrong answer choice in seconds.

How it is examined. Items from this page are written as a factored demand compared against φ times a nominal capacity. Roughly two thirds are direct substitution, one third require one intermediate quantity from a neighbouring relation, and a small number are conceptual — testing whether you know the assumption, not the arithmetic.

The habit that earns the points. Unit discipline. f'c and Fy in ksi with areas in in² give kips. Every relation below is dimensionally consistent only when that rule is honoured, and the distractor set is deliberately built from candidates who ignored it. Write the unit next to every number you substitute, every time.

How to study this page. Read the theory, then cover the formula cards and try to reproduce each relation from its description. Then work the examples with the solution hidden, revealing one line at a time. Finish with the self-check questions; if you cannot answer one, return to the matching formula card.

Bolted connection at the bottom chord of a steel truss bridge. — Photo 1. Where this shows up in practice: definitions.
HAER / Library of Congress, public domain

Structural Design — Definitions: reference schematic for orienting the symbols used in this section.

Theory, developed

Read this before the equations — it is what makes them memorable.

The physical situation

Every item from this section describes a reinforced concrete or steel member being checked. Sketch it before you compute — a labelled sketch with the givens on it converts a wordy stem into a solvable problem and exposes the quantity the examiner left out on purpose.

The governing principle

The 28 relations on this page are consequences of one principle applied to that idealised system. Identify which quantity is conserved, balanced, or defined, and the correct equation follows without memorisation.

Assumptions and limits of validity

Each printed relation carries silent assumptions — linearity, steady state, uniformity, small deformation, or standard conditions, depending on the subject. Conceptual exam items are written by violating exactly one of these, so read the sentence above the equation as carefully as the equation itself.

Solution procedure you should automate

1) Read the last sentence of the stem to identify the requested quantity. 2) Locate the relation on this page whose left-hand side is that quantity. 3) Tabulate the givens with units and mark the missing symbol. 4) If a symbol is missing, find the one relation that produces it. 5) Rearrange symbolically, substitute once, evaluate, and round only at the end.

Notation used in this section

a	Quantity produced by "a = depth of equivalent rectangular stress block (in.)" — read its definition and unit from the handbook line directly above the equation.
Ag	Quantity produced by "Ag = gross area of concrete section (in2)" — read its definition and unit from the handbook line directly above the equation.
As	Quantity produced by "As = area of longitudinal tension reinforcement (in2)" — read its definition and unit from the handbook line directly above the equation.
Ast	Quantity produced by "Ast = total area of longitudinal reinforcement (in2)" — read its definition and unit from the handbook line directly above the equation.
Av	Quantity produced by "Av = area of shear reinforcement within a distance s (in.)" — read its definition and unit from the handbook line directly above the equation.
b	Quantity produced by "b = width of compression face of member (in.)" — read its definition and unit from the handbook line directly above the equation.
β1	Quantity produced by "β1 = ratio of depth of rectangular stress block a to depth to neutral axis c" — read its definition and unit from the handbook line directly above the equation.
c	Quantity produced by "c = distance from extreme compression fiber to neutral axis (in.)" — read its definition and unit from the handbook line directly above the equation.
d	Quantity produced by "d = distance from extreme compression fiber to centroid of longitudinal tension reinforcement (in.)" — read its definition and unit from the handbook line directly above the equation.
dt	Quantity produced by "dt = distance from extreme compression fiber to extreme tension steel (in.)" — read its definition and unit from the handbook line directly above the equation.
Ec	Quantity produced by "Ec = modulus of elasticity (psi)" — read its definition and unit from the handbook line directly above the equation.
εt	Quantity produced by "εt = net tensile strain in extreme layer of longitudinal tension reinforcement" — read its definition and unit from the handbook line directly above the equation.

Handbook notes for this section

Definitions and conditions exactly as the handbook states them.

f cl _ psii b1
2500 # f cl # 4000 0.85 (a)
4000 1 f cl 1 8000 0.05 ` f cl - 4000 j (b)
0.85 - 1000
f cl $ 8000 0.65 (c)

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1

ASCE 7-16 load combinations and the required nominal strength — Definitions

A floor beam carries D = 0.9, L = 2.9, L_r = 0.5 and W = 1.1 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 0.9 kip/ft
L = 2.9 kip/ft
L_r = 0.5 kip/ft
W = 1.1 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (0.9) = 1.260 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 5.970 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 5.330 k i p / f t$
Governing
$w_{u} = 5.970 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 5.970/0.90 = 6.633 k i p / f t$

Answer: w_u = 5.97 kip/ft governs; required nominal capacity = 6.63 kip/ft

Why the other options are there

4.30 kip/ft (service loads, unfactored)
5.37 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 2

ASCE 7-16 load combinations and the required nominal strength — Definitions (2)

A floor beam carries D = 2.3, L = 2.0, L_r = 0.4 and W = 1.3 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 2.3 kip/ft
L = 2.0 kip/ft
L_r = 0.4 kip/ft
W = 1.3 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (2.3) = 3.220 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 6.160 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 6.260 k i p / f t$
Governing
$w_{u} = 6.260 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 6.260/0.90 = 6.956 k i p / f t$

Answer: w_u = 6.26 kip/ft governs; required nominal capacity = 6.96 kip/ft

Why the other options are there

4.70 kip/ft (service loads, unfactored)
5.63 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 3

ASCE 7-16 load combinations and the required nominal strength — Definitions (3)

A floor beam carries D = 1.9, L = 2.3, L_r = 0.6 and W = 1.1 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 1.9 kip/ft
L = 2.3 kip/ft
L_r = 0.6 kip/ft
W = 1.1 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (1.9) = 2.660 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 6.260 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 5.980 k i p / f t$
Governing
$w_{u} = 6.260 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 6.260/0.90 = 6.956 k i p / f t$

Answer: w_u = 6.26 kip/ft governs; required nominal capacity = 6.96 kip/ft

Why the other options are there

4.80 kip/ft (service loads, unfactored)
5.63 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 4

ASCE 7-16 load combinations and the required nominal strength — Definitions (4)

A floor beam carries D = 2.1, L = 2.6, L_r = 0.4 and W = 0.4 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 2.1 kip/ft
L = 2.6 kip/ft
L_r = 0.4 kip/ft
W = 0.4 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (2.1) = 2.940 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 6.880 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 5.720 k i p / f t$
Governing
$w_{u} = 6.880 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 6.880/0.90 = 7.644 k i p / f t$

Answer: w_u = 6.88 kip/ft governs; required nominal capacity = 7.64 kip/ft

Why the other options are there

5.10 kip/ft (service loads, unfactored)
6.19 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 5

ASCE 7-16 load combinations and the required nominal strength — Definitions (5)

A floor beam carries D = 0.7, L = 2.4, L_r = 0.8 and W = 0.4 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 0.7 kip/ft
L = 2.4 kip/ft
L_r = 0.8 kip/ft
W = 0.4 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (0.7) = 0.980 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 5.080 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 4.040 k i p / f t$
Governing
$w_{u} = 5.080 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 5.080/0.90 = 5.644 k i p / f t$

Answer: w_u = 5.08 kip/ft governs; required nominal capacity = 5.64 kip/ft

Why the other options are there

3.90 kip/ft (service loads, unfactored)
4.57 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 6

ASCE 7-16 load combinations and the required nominal strength — Definitions (6)

A floor beam carries D = 0.8, L = 1.5, L_r = 0.7 and W = 1.4 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 0.8 kip/ft
L = 1.5 kip/ft
L_r = 0.7 kip/ft
W = 1.4 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (0.8) = 1.120 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 3.710 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 4.210 k i p / f t$
Governing
$w_{u} = 4.210 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 4.210/0.90 = 4.678 k i p / f t$

Answer: w_u = 4.21 kip/ft governs; required nominal capacity = 4.68 kip/ft

Why the other options are there

3.00 kip/ft (service loads, unfactored)
3.79 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 7

ASCE 7-16 load combinations and the required nominal strength — Definitions (7)

A floor beam carries D = 1.9, L = 1.6, L_r = 0.6 and W = 1.1 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 1.9 kip/ft
L = 1.6 kip/ft
L_r = 0.6 kip/ft
W = 1.1 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (1.9) = 2.660 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 5.140 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 5.280 k i p / f t$
Governing
$w_{u} = 5.280 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 5.280/0.90 = 5.867 k i p / f t$

Answer: w_u = 5.28 kip/ft governs; required nominal capacity = 5.87 kip/ft

Why the other options are there

4.10 kip/ft (service loads, unfactored)
4.75 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 8

ASCE 7-16 load combinations and the required nominal strength — Definitions (8)

A floor beam carries D = 2.4, L = 1.1, L_r = 0.6 and W = 1.2 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 2.4 kip/ft
L = 1.1 kip/ft
L_r = 0.6 kip/ft
W = 1.2 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (2.4) = 3.360 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 4.940 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 5.480 k i p / f t$
Governing
$w_{u} = 5.480 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 5.480/0.90 = 6.089 k i p / f t$

Answer: w_u = 5.48 kip/ft governs; required nominal capacity = 6.09 kip/ft

Why the other options are there

4.10 kip/ft (service loads, unfactored)
4.93 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 9

ASCE 7-16 load combinations and the required nominal strength — Definitions (9)

A floor beam carries D = 2.4, L = 1.9, L_r = 0.5 and W = 0.4 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 2.4 kip/ft
L = 1.9 kip/ft
L_r = 0.5 kip/ft
W = 0.4 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (2.4) = 3.360 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 6.170 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 5.430 k i p / f t$
Governing
$w_{u} = 6.170 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 6.170/0.90 = 6.856 k i p / f t$

Answer: w_u = 6.17 kip/ft governs; required nominal capacity = 6.86 kip/ft

Why the other options are there

4.80 kip/ft (service loads, unfactored)
5.55 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Example 10

ASCE 7-16 load combinations and the required nominal strength — Definitions (10)

A floor beam carries D = 1.2, L = 1.2, L_r = 0.2 and W = 1.1 kip/ft. Evaluate the governing LRFD load combination and, with a resistance factor φ = 0.90, determine the nominal strength the member must provide.

Given

D = 1.2 kip/ft
L = 1.2 kip/ft
L_r = 0.2 kip/ft
W = 1.1 kip/ft
φ = 0.90

Find

Governing factored load and required nominal strength

Start with the thinking

Every ASCE 7-16 combination must be evaluated; the largest result governs the limit state.
LRFD requires φRn ≥ Ru, so the nominal demand is the factored load divided by φ.

Step-by-step solution

Combination 1
$1.4 D = 1.4 (1.2) = 1.680 k i p / f t$
Combination 2
$1.2 D + 1.6 L + 0.5 L_{r} = 3.460 k i p / f t$
Combination 3
$1.2 D + 1.0 W + 1.0 L + 0.5 L_{r} = 3.840 k i p / f t$
Governing
$w_{u} = 3.840 k i p / f t$
Formula — φR_n ≥ R_u → R_n ≥ R_u/φ
Substituting
$R_{n} = 3.840/0.90 = 4.267 k i p / f t$

Answer: w_u = 3.84 kip/ft governs; required nominal capacity = 4.27 kip/ft

Why the other options are there

2.60 kip/ft (service loads, unfactored)
3.46 kip/ft (multiplied by φ instead of dividing)

Reference: FE Reference Handbook — Structural Design → Definitions

Self-check

Answer these without notes before moving on.

Without looking, state the relation on this page whose left-hand side is the quantity most often requested, and name every symbol in it.
Which assumption, if violated, makes the main relation of this section invalid?
Given a reinforced concrete or steel member being checked, what is the first quantity you would compute, and why that one first?
Which unit conversion in this subject most often produces a wrong answer choice, and what is its numerical factor?
Rework Example 1 above from the givens alone, without reading the solution lines.

Chapter summary

Definitions contains 28 relations; you must be able to find this page in under 15 seconds.
Exam style: a factored demand compared against φ times a nominal capacity.
Unit rule: f'c and Fy in ksi with areas in in² give kips.
Work the 10 examples until the solution path, not the answer, is automatic.

Common traps in this section

f'c and Fy in ksi with areas in in² give kips
Answering the intermediate quantity instead of the quantity requested.
Rounding intermediate values before the final step.
Using a relation from an adjacent handbook section that shares a symbol.
Skipping the sketch — most lost points on this page start with a misread geometry.

Definitions

Learning objectives

Lecture

Theory, developed

Notation used in this section

Handbook notes for this section

Core formulas for this FE topic

a=depthofequivalentrectangularstressblock(in.)

Ag=grossareaofconcretesection(in2​)

As=areaoflongitudinaltensionreinforcement(in2​)

Ast=totalareaoflongitudinalreinforcement(in2​)

Av=areaofshearreinforcementwithinadistances(in.)

b=widthofcompressionfaceofmember(in.)

β1​=ratioofdepthofrectangularstressblockatodepthtoneutralaxisc

c=distancefromextremecompressionfibertoneutralaxis(in.)

d=distancefromextremecompressionfibertocentroidoflongitudinaltensionreinforcement(in.)

dt=distancefromextremecompressionfibertoextremetensionsteel(in.)

Ec=modulusofelasticity(psi)

=33w1​c.5fcl

εt = net tensile strain in extreme layer of longitudinal tension reinforcementNeeds technical review

flc=compressivestrengthofconcrete(psi)

fy=yieldstrengthofsteelreinforcement(psi)

Mn=nominalflexuralstrengthatsection(in.−lb)

φMn= design flexural strength at section (in.-lb)Needs technical review

Mu=factoredmomentatsection(in.−lb)

Pn=nominalaxialcompressiveloadstrengthofmember(lb)

φPn = design axial compressive load strength of member (lb)Needs technical review

Pu=factoredaxialforce:tobetakenaspositiveforcompressionandnegativefortension(lb)

ρg = ratio of total reinforcement area to cross-sectional area of column = Ast /AgNeeds technical review

s=centertocenterspacingoflongitudinalshearortorsionalreinforcement(in.)

Vc=nominalshearstrengthprovidedbyconcrete(lb)

Vn=nominalshearstrengthatsection(lb)

φVn = design shear strength at section (lb)Needs technical review

Vs=nominalshearstrengthprovidedbyreinforcement(lb)

Vu=factoredshearforceatsection(lb)

Worked exam-style examples

Self-check

Chapter summary

Common traps in this section