Required and maximum-permitted stirrup spacing s

Structural Design · FE Reference Handbook section

Structural Design

0 formulas

10 exam-style examples

~45 min

All Structural Design lectures

Learning objectives

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

This chapter section covers Required and maximum-permitted stirrup spacing s 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 required and maximum-permitted stirrup spacing s describes physically and when it applies.
State every one of the 0 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. Required and maximum-permitted stirrup spacing s 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: required and maximum-permitted stirrup spacing s.
HAER / Library of Congress, public domain

Structural Design — Required and maximum-permitted stirrup spacing s: 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 0 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.

Handbook notes for this section

Definitions and conditions exactly as the handbook states them.

Vu # 2 c : No stirrups required
Vu > 2 c : Use the following table (Av given)
ASTM STANDARD REINFORCEMENT BARS
BAR SIZE DIAMETER, IN. AREA, IN WEIGHT, LB/FT
#3 0.375 0.11 0.376
#4 0.500 0.20 0.668
#5 0.625 0.31 1.043
#6 0.750 0.44 1.502
#7 0.875 0.60 2.044
#8 1.000 0.79 2.670
#9 1.128 1.00 3.400
#10 1.270 1.27 4.303
#11 1.410 1.56 5.313
#14 1.693 2.25 7.650
#18 2.257 4.00 13.60

Core formulas for this FE topic

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

This section is conceptual; there are no equations to memorise.

Worked exam-style examples

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

Example 1

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s

A beam has b_w = 18 in., d = 20.0 in., f′c = 5000 psi and carries Vu = 50.9 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 18 in.
d = 20.0 in.
f′c = 5000 psi
Vu = 50.9 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 25000 (18) (20.0) = 50, 912 l b = 50.9 k i p$
Design capacity — φVc = 0.75(50.9) = 38.2 kip
Compare — Vu = 50.9 kip > φVc = 38.2 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 38.2 kip → stirrups required

Why the other options are there

50.9 kip (φ omitted)
50,912 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 2

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (2)

A beam has b_w = 14 in., d = 23.0 in., f′c = 5000 psi and carries Vu = 45.5 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 14 in.
d = 23.0 in.
f′c = 5000 psi
Vu = 45.5 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 25000 (14) (23.0) = 45, 538 l b = 45.5 k i p$
Design capacity — φVc = 0.75(45.5) = 34.2 kip
Compare — Vu = 45.5 kip > φVc = 34.2 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 34.2 kip → stirrups required

Why the other options are there

45.5 kip (φ omitted)
45,538 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 3

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (3)

A beam has b_w = 14 in., d = 25.5 in., f′c = 4000 psi and carries Vu = 49.7 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 14 in.
d = 25.5 in.
f′c = 4000 psi
Vu = 49.7 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 24000 (14) (25.5) = 45, 157 l b = 45.2 k i p$
Design capacity — φVc = 0.75(45.2) = 33.9 kip
Compare — Vu = 49.7 kip > φVc = 33.9 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 33.9 kip → stirrups required

Why the other options are there

45.2 kip (φ omitted)
45,157 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 4

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (4)

A beam has b_w = 16 in., d = 18.5 in., f′c = 5000 psi and carries Vu = 58.6 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 16 in.
d = 18.5 in.
f′c = 5000 psi
Vu = 58.6 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 25000 (16) (18.5) = 41, 861 l b = 41.9 k i p$
Design capacity — φVc = 0.75(41.9) = 31.4 kip
Compare — Vu = 58.6 kip > φVc = 31.4 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 31.4 kip → stirrups required

Why the other options are there

41.9 kip (φ omitted)
41,861 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 5

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (5)

A beam has b_w = 16 in., d = 16.5 in., f′c = 4000 psi and carries Vu = 50.1 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 16 in.
d = 16.5 in.
f′c = 4000 psi
Vu = 50.1 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 24000 (16) (16.5) = 33, 394 l b = 33.4 k i p$
Design capacity — φVc = 0.75(33.4) = 25.0 kip
Compare — Vu = 50.1 kip > φVc = 25.0 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 25.0 kip → stirrups required

Why the other options are there

33.4 kip (φ omitted)
33,394 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 6

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (6)

A beam has b_w = 16 in., d = 21.0 in., f′c = 4000 psi and carries Vu = 59.5 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 16 in.
d = 21.0 in.
f′c = 4000 psi
Vu = 59.5 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 24000 (16) (21.0) = 42, 501 l b = 42.5 k i p$
Design capacity — φVc = 0.75(42.5) = 31.9 kip
Compare — Vu = 59.5 kip > φVc = 31.9 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 31.9 kip → stirrups required

Why the other options are there

42.5 kip (φ omitted)
42,501 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 7

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (7)

A beam has b_w = 15 in., d = 17.0 in., f′c = 5000 psi and carries Vu = 57.7 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 15 in.
d = 17.0 in.
f′c = 5000 psi
Vu = 57.7 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 25000 (15) (17.0) = 36, 062 l b = 36.1 k i p$
Design capacity — φVc = 0.75(36.1) = 27.0 kip
Compare — Vu = 57.7 kip > φVc = 27.0 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 27.0 kip → stirrups required

Why the other options are there

36.1 kip (φ omitted)
36,062 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 8

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (8)

A beam has b_w = 16 in., d = 21.5 in., f′c = 4000 psi and carries Vu = 52.2 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 16 in.
d = 21.5 in.
f′c = 4000 psi
Vu = 52.2 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 24000 (16) (21.5) = 43, 513 l b = 43.5 k i p$
Design capacity — φVc = 0.75(43.5) = 32.6 kip
Compare — Vu = 52.2 kip > φVc = 32.6 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 32.6 kip → stirrups required

Why the other options are there

43.5 kip (φ omitted)
43,513 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 9

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (9)

A beam has b_w = 16 in., d = 16.0 in., f′c = 4000 psi and carries Vu = 42.1 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 16 in.
d = 16.0 in.
f′c = 4000 psi
Vu = 42.1 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 24000 (16) (16.0) = 32, 382 l b = 32.4 k i p$
Design capacity — φVc = 0.75(32.4) = 24.3 kip
Compare — Vu = 42.1 kip > φVc = 24.3 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 24.3 kip → stirrups required

Why the other options are there

32.4 kip (φ omitted)
32,382 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

Example 10

Concrete shear capacity check — Required and maximum-permitted stirrup spacing s (10)

A beam has b_w = 16 in., d = 19.0 in., f′c = 5000 psi and carries Vu = 60.2 kip. Is shear reinforcement required (φ = 0.75)?

Given

b_w = 16 in.
d = 19.0 in.
f′c = 5000 psi
Vu = 60.2 kip

Find

φVc and the stirrup requirement

Start with the thinking

Vc = 2√f′c·b_w·d in psi units — divide by 1,000 for kips.
Stirrups are required once Vu exceeds φVc/2.

Step-by-step solution

Concrete capacity — Vc = 2√f′c·b_w·d
Substituting
$V c = 25000 (16) (19.0) = 42, 992 l b = 43.0 k i p$
Design capacity — φVc = 0.75(43.0) = 32.2 kip
Compare — Vu = 60.2 kip > φVc = 32.2 kip
Conclusion — designed stirrups are required

Answer: φVc ≈ 32.2 kip → stirrups required

Why the other options are there

43.0 kip (φ omitted)
42,992 kip (psi/kip conversion missed)

Reference: FE Reference Handbook — Structural Design → Required and maximum-permitted stirrup spacing s

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

Required and maximum-permitted stirrup spacing s contains 0 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.