Understanding Internal Forces, Stress, and Strength in Structural Analysis

Name: 03 Internal Forces, Stress, and Strength
Uploaded: 2026-01-14T18:12:46.986203+00:00
Channel: stm ALeisawy
Description: Summary and key takeaways on Understanding Internal Forces, Stress, and Strength in Structural Analysis, covering Introduction The lecture builds on the

stm ALeisawy

Jan 14, 2026

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4 min read

YouTube video ID: RYtCQKr_8Uc

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Introduction

The lecture builds on the roadmap of structural analysis, moving from external forces (loads and reactions) to the internal concepts of force, stress, and strength. It explains how external loads generate internal forces within structural elements, how those forces are quantified as stress, and how material strength determines failure.

Historical Development of the Simple Tension Test

Leonardo da Vinci (Renaissance): First systematic tension test using an iron wire, a sand‑filled bucket, and a hanging weight. Concluded incorrectly that strength depended on length.
Galileo Galilei (1638): Demonstrated that tensile strength is independent of length and proportional to cross‑sectional area, correcting da Vinci’s error.
Robert Hooke (1650s, published 1678): Measured load versus elongation for iron wires, discovering a linear relationship now known as Hooke’s Law (F ∝ ΔL). He did not yet link external load to internal stress.

Cross‑Sectional Area and Strength

The cross‑section is the 2‑D shape obtained by cutting a member perpendicular to its longitudinal axis.
Strength of a tension member is directly proportional to its cross‑sectional area (e.g., a 10 in² area is twice as strong as a 5 in² area of the same material).
Common shapes: circular (pipes), rectangular (2×4 lumber), I‑shaped (flanges and web), and hollow tubes.

From External Load to Internal Force

Free‑body diagram of a tension specimen shows the external load (e.g., 4 lb) and the reaction at the cut.
By cutting the member conceptually, the internal tension force at the cut must balance the external load for equilibrium.
In simple axial loading the internal force equals the external load, but in more complex configurations (e.g., angled wires) internal forces differ from the applied load.
Internal force originates from molecular resistance to deformation, not from the applied weight itself.

Stress and Strain

Stress (σ) = internal force (F) ÷ cross‑sectional area (A). Units: psi, Pa, etc.
Strain (ε) = deformation (ΔL) ÷ original length (L₀). Dimensionless (often expressed as a percentage).
Stress reflects intensity of internal force; strain reflects intensity of deformation.

Stress‑Strain Curve – The Material Fingerprint

Construction: Plot stress (vertical) vs. strain (horizontal) using data from a hydraulic tension test.
Elastic Region (linear up to ~50,000 psi for steel): Stress and strain are proportional; material returns to original shape when load is removed.
Yielding (~55,000 psi for steel): Curve flattens; permanent (plastic) deformation begins.
Ultimate Strength: Highest stress the material can sustain before necking.
Fracture: Final break; for structural steel strain at fracture ≈ 0.25 (25 % elongation).
Material Comparisons: Steel shows a clear yield plateau and high ductility; cast iron lacks a pronounced yield region and fails at much lower strain.
Key properties read from the curve:
Ultimate strength – height of the curve.
Ductility – width of the curve (strain capacity before fracture).
Stiffness (Young’s modulus) – slope of the initial elastic portion.

Real‑World Applications of Tension Members

Cable‑stayed bridges (e.g., Sunshine Skyway Bridge) use tensioned cables radiating from towers.
Suspension bridges (e.g., Golden Gate Bridge) rely on vertical suspenders that carry large tensile forces (≈ 500,000 lb each) while staying within the elastic range.
Everyday examples: chandeliers, swings, dog leashes, and rope in tug‑of‑war games.
Historical note: Widespread use of tension members became feasible after mass‑produced wrought iron (18th century) and later steel.

Design Implications

Engineers size and shape members so that actual stress < material strength for all anticipated loads.
The stress‑strain curve provides the necessary data to select appropriate materials and ensure safety.

Summary of Core Concepts

External loads → internal forces → stress.
Strength is the stress level at which a material fails.
The simple tension test, refined over centuries, yields the stress‑strain curve, the definitive fingerprint of a material.

Structural analysis hinges on converting external loads into internal forces, quantifying those forces as stress, and comparing stress to a material’s strength. The stress‑strain curve, obtained from a simple tension test, uniquely characterizes a material and guides engineers to design members that stay safely within elastic limits.

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Full Transcript YouTube

you
welcome back last lecture we began our
exploration of the science of
engineering mechanics the study of
forces acting on bodies and to guide
that exploration we developed this
roadmap diagram that shows the major
concepts used in structural analysis
last lecture we focused on the first
block of this diagram external forces
and we saw how two types of external
forces loads and reactions are related
by the all-important principle of
equilibrium along the way we met two men
who pioneered the principle of
equilibrium Isaac Newton who formalized
the principle in his first law of motion
in 1687 and Newton's predecessor Simon
Stevin that really unique Renaissance
era mathematician who developed the
world's first proof of equilibrium using
this ingenious thought experiment
consisting of a string of beads draped
over an inclined plane
stevan was a evidently quite pleased
with this insight because in his book
underneath the diagram he wrote what
seems mysterious can be understood a
phrase he later adopted as his personal
motto well in the spirit of Simon Stevin
let's focus this lecture on
understanding the remaining major
concepts of structural analysis on the
roadmap internal force stress and
strength during this lecture we'll see
that loads which are external to the
structure cause internal forces to
develop within the structural elements
that compose the structure and we'll see
that these external and internal forces
have to be in equilibrium with each
other we'll see how internal force is
manifested as stress a measure of the
intensity of that internal force and
we'll also learn once again about
strength the characteristic value of
stress at which a particular material
fails we'll see where strength comes
from how it's determined experimentally
in the lab and then finally we'll
compare stress with strength to evaluate
structural performance these concepts in
total describe how structures carry load
and so they'll lie at the very heart
of our analysis of the world's great
structures through the remainder of this
course when all is said and done I hope
that Simon's Devon's motto what seems
mysterious can be understood will ring
true for you as well now there's a good
reason why all this mysterious stuff can
be understood much of what we know about
the strength of engineering material is
gleaned from a very simple laboratory
test that scientists and engineers have
been using for centuries today we call
this the simple tension test and it's
done by simply taking a specimen of
material mounting it in a testing
machine and then using that machine to
slowly pull on the specimen and stretch
it in tension until it breaks into
pieces that's it if you can understand
this test and the information we glean
from it then you'll understand the
concepts of internal force stress and
strength the subjects of today's lecture
oh and by the way the test specimen that
I just used for our simulated simple
tension test is in effect a structural
element that's subjected to pure tension
in other words it is one of our six
basic types of structural elements so by
the end of today's lecture
you won't just understand these key
concepts from our roadmap you'll also
understand how tension members work in
great structures great structures like
the sunshine skyway bridge across the
Tampa Bay in Florida shown here the
structure is called a cable-stayed
bridge and the cable stays are all
tensioned members which radiate out from
the two towers in a way that's intended
to evoke the appearance of a sailboat
clearly the designer of this bridge
wanted us to appreciate these tension
members because he had them painted
yellow just to make them a little bit
more prominent so let's begin our
exploration of tension members by
looking at the historical development of
that all-important laboratory experiment
the simple tension test that tells us so
very much about key structural analysis
concepts in doing so we'll see some of
the the seminal discoveries that really
ultimately helped foster the era of
science-based design
during the Renaissance Leonardo da Vinci
performed a number of very
well-conceived mechanics experiments and
then recorded him in his notebooks so we
know about him one of those sketches
which I've replicated here is entitled
testing the strength of iron wires of
various lengths the setup for this test
as DaVinci depicted it a consisted of an
iron wire which is the test specimen
attached to a support at the upper end
which you can see here in this diagram
and has a bucket hanging from the lower
end to do the experiment then Leonardo
hung a bin filled with sand above the
bucket he opened a hole in that bin and
allowed sand to pour out into the bucket
gradually until the wire broke he then
weighed the bucket to determine the
force at which the wire had broken this
is a measure of the strength of the wire
now as one of the earliest systematic
experiments on materials Leonardo's
tension test was a noteworthy effort but
as science
it was mostly unsuccessful because
Leonardo erroneously concluded from his
experimental data that the strength of
iron wire depends on its length in fact
what we know today is that the strength
of any tension member is actually
independent of its lake a fact that
ultimately was proved by our next great
pioneer of engineering mechanics Galileo
Galilei Galileo published the world's
first comprehensive scientific study of
the mechanics of materials in 1638 and
in an interesting indirect way we
actually owe this publication to the
Catholic Church six years earlier
Galileo had defied the church by
publishing his assertion that the
Copernican theory of planetary motion
was indisputable truth as a result he
was called to Rome he was condemned
ultimately placed under house arrest in
his villa in Tuscany for the remaining
eight years of his life throughout this
final period these final eight years
Galileo focuses work primarily on
mechanics probably because the subject
was likely to be significantly less
controversial than cosmology as a result
his great work was published during this
period it was titled to new sciences and
really was his seminal treatise on
engineering mechanics Galileo's
experiments established the fact that
the strength of a structural element is
independent of its length and thereby he
disproved da Vinci's earlier
experimental results but more
importantly Galileo showed convincingly
that the strength of a member in tension
depends on that members cross-sectional
area okay so what's a cross-section in
what's cross-sectional area well the
cross-section of a structural member is
really just the two dimensional shape
that's formed by cutting through the
member on a plane perpendicular to its
long or longitudinal axis so if this
were a concrete member it's it's
cylindrical in shape that long or
longitudinal axis of the member is
running this direction and when I make a
cut I expose this shape it's a circle
and that circular two-dimensional shape
is the cross-section the cross-sectional
area then is simply the area of that
circle PI R squared many other types of
configurations of structural members are
possible and you've seen them in the
world all around you this is a standard
two by four piece of lumber that's used
often in construction and it has a
rectangular cross-section in fact the
the very definition of the member the
two-by-four describes the dimensions of
that rectangular cross-section in steel
we often see structural elements made
with an eye shaped cross-section now
this is really just a short segment of
member in practice the real steel beam
probably runs about 20 or 30 feet in
this direction for our purposes here
we're really just concerned with the
cross-section its itself which forms an
eye shape as you see here and later on
we'll talk in more detail about I shaped
sections but for now recognize that it's
composed of three elements the top and
bottom horizontal planes we call flanges
the vertical plane we call a web and
steel cross-sections are also available
and often used in hollow tube
configurations this one happens to be
square but we also often see hollow
tubes that are rectangular or circular
in geometry
okay so back to Galileo now based on his
experiments Galileo discovered that the
strength of a member is directly
proportional to its cross-sectional area
for example a member with a
cross-sectional area of 10 square inches
is twice as strong as a member with a
cross-sectional area of 5 square inches
as long as they're made of the same
material
this discovery eventually led to the
development of the concept of a stress
but it took a long time almost 200 years
for engineers and scientists to
ultimately get to that conceptual
understanding
in the mean time around 1650 and an
English scientist named Robert Hooke
also performed an extensive series of
tests on iron wire these were similar to
Leonardo's and Galileo's experiments in
that a piece of iron wire was subjected
to tensile loading but hook went a
crucial step farther rather than simply
determining the breaking point of the
wire he also investigated how it
elongated under the full range of load
from zero all the way up to the breaking
point let's simulate Hookes experiment
here just to give you a physical sense
of what he did instead of iron wife wire
for this demonstration I'm using this
piece of rubber tubing just so it'll be
a little bit easier for you to see to
run his tests Hookes suspended a piece
of iron wire from a frame and then he
hung a pan from the bottom of that wire
okay once the pan was in place he
recorded its position using some sort of
a measuring device like this and then
instead of applying a steady stream of
sand to load the member as da Vinci did
he applied the load in a series of
increments using known waves so he took
a known weight let's say it's equal to
one pound he laid it in the pan and he
measured the amount of deformation the
amount of elongation of the specimen
that occurred as a result of that known
discrete load he recorded that data the
load and the amount of deformation then
he added another known weight watched
the deformation once stabilized he
recorded the amount of displacement that
is how much the memory elongated as a
result of that additional increment of
load
continue to repeat that process each
time recording load and deformation
until the specimen finally broke at this
point he plotted the data load versus
deformation and when he plotted the data
an amazing result emerged as you can see
here the experimental values of load and
deformation plotted as a near-perfect
straight line on a graph almost all the
way up to the point of fracture hook
probably made this discovery sometime in
the 1650s but he didn't actually publish
it until 1678 because the discovery
actually gave him the capability to
build very accurate clock Springs and he
didn't want to give away his competitive
advantage when he did publish the
discovery he used the simple Latin
phrase hoot tensio sic visa as the
extension so the force the discovery of
this linear relationship between load
and deformation is so important to
modern engineering mechanics that even
today we call it Hookes law
groundbreaking as it was Hookes
discovery was fundamentally limited by
the fact that he only ever considered
the external forces acting on the test
specimen he never actually made the
conceptual leap from external force to
internal force and then stress to see
what Hooke overlooked let's return to
our tension experiment and draw a free
body diagram of the test setup first we
isolate the member the simple tension
member we then add the low down at the
bottom four pounds oriented in the
vertical Direction downward up above
will add the reaction that keeps the
body in equilibrium we'll label that R
and since the body is in equilibrium we
know that because it's not moving then
all the forces acting on it must be in
balance clearly then the magnitude of
that reaction has to be four pounds we
have four pounds up and four pounds down
this configuration by the way is often
called axial loading because both the
load and its reaction are aligned with
the long or in this case the
longitudinal axis of the member but both
the load on the reaction are external
forces and what we are interested in
understanding is what's happening inside
the member to do this we have to do
another thought experiment what we're
going to do is imagine cutting
completely through the member to expose
that internal force and then we'll use
the principle of equilibrium to figure
out what that force actually is let's
use our free body diagram as a way to
see how this process works here's the
free body diagram once again of our
tension test specimen and now here's the
location of the cut it's just a
conceptual cut but it's used as a
problem-solving tool now we can isolate
the portion of the member that falls
below the cut and draw a separate free
body diagram just of that segment the
four pound load is still applied on the
end of the segment and so the only way
that this segment can possibly be in a
equilibrium is if there's some
additional force acting up there on the
face of the cut we know this little
segment is in equilibrium because it's
not moving and therefore there is a
force on the face of that cut and that
is the internal tension force which you
you can see here clearly the principle
of equilibrium tells us that this
internal force F must be exactly equal
to the external load four pounds okay
now at first glance this analysis might
seem trivial we apply to four-pound
external load and it resulted in a
40-pound internal tension force what's
the difference well there is a
difference it's very important to
recognize that the external load and the
internal force are two very different
things in the simple tension test they
happen to be equal but in many other
situations they won't be equal for
example if I just reconfigure this
experimental setup that is if I take the
load off for a moment and rather than
having the load suspended from a single
wire in one uniaxial test setup suppose
the load is suspended from two wires
which are oriented such that they form
an angle now in this circumstance there
is not one internal force but two
different internal forces and both of
them will turn out to be substantially
different from the externally applied
load more importantly the external load
and the in
Curnow force are fundamentally different
phenomena the external load is a
concentrated force that's transmitted to
the member or members through some sort
of physical connection to another object
in this case the connection is the pan
holding that four pound weight the
internal force on the other hand is an
attraction between the molecules that
constitute this material it is internal
when we apply a load to a structural
element it stretches slightly and as a
result the individual molecules that
make up the material have to move away
from each other ever so slightly that
chemical bonds between these molecules
are very strong and they resist being
deformed this resistance accumulated
over the entire cross section of our
conceptual cut is the internal force
that keeps the lower portion of this
member as you see it in a Freebody
diagram in equilibrium now this idea of
internal force as an accumulation of
molecular level resistance to
deformation is a really powerful concept
that actually leads us directly to the
concept of stress this concept was
vaguely suggested indirectly by
Galileo's experiments of 1638 but it
wasn't until 1822 that the French
mathematician Agustin Koshi
actually formulated the concept of
stress as we know it today stress is the
intensity of internal force in a member
and in an axially loaded member it's
calculated simply as the internal force
divided by the cross-sectional area an
expressed in units of force per area for
example pounds per square inch Newton's
per square meter stress is independent
of geometry and material type for
example if a long steel bar with a
cross-sectional area of two square
inches as you see here is loaded with
the tensile force of 2,000 pounds then
the stress in the bar is 1,000 psi 2,000
divided by two now if a short concrete
cylinder that has a cross-sectional area
of 10 square inches is loaded with the
tensile force of 10,000 pounds
well the stress is still exactly the
same it's 1,000 psi in this case 10,000
divided by
10 the concept of stress is so powerful
because the strength of a material is
defined in terms of stress for example
the strength of standard grade
structural steel is about 36,000 pounds
per square inch it doesn't matter if the
piece of steel is long or short wide or
narrow circular I shaped the material
will fail when the stress reaches 36,000
psi
okay that's stress next Agustin ko she's
second contribution to today's lecture
is the Associated concept of strain just
as stress is the intensity of internal
force strain is the intensity of
deformation so we saw in robert hooke's
experiments a member loaded in tension
elongates strain is a way of
characterized this in characterizing
this elongation in a particularly useful
way now beware that in everyday usage
the word stress and strain tend to be
thrown around in a very imprecise way
and often are used more or less
synonymously in engineering these are
distinctly different concepts and you
have to use the terms very precisely
stress is a measure of internal force
strain is a measure of how much the
member deforms under load to calculate
strain and a tension member we simply
divide the amount of deformation by the
original length of that member for
example if the member is originally 100
inches long and it deforms 2 inches
under load then the strain is just 2
divided by 100 or point 0 2 now notice
that strain has no units in the
calculation inches are divided by inches
so the units cancel out you can think of
strain as if it's a percentage in this
example a strain of point zero 2
actually means that the member has
stretched 2 percent beyond its original
length so it's really not a complex
concept now that we have a basic
understanding of both stress and strain
let's put these concepts to work
following the example of Leonardo
Galileo and Robert Hooke let's go to the
laboratory and do a simple tension test
will run essentially the same tests as
Robert Hooke did back
the 1650s but with two important
differences first we're gonna use a much
more sophisticated piece of testing
equipment this large hydraulic testing
machine that's capable of applying tens
of thousands of pounds of load to a test
specimen so we'll be able to test much
larger specimens rather than just thin
wires the machine also has a built-in
measuring device that automatically
records load and deformation of the
specimen very accurately so we don't
have to do the manual measurements that
we saw in my earlier simulation second
rather than plotting our results in
terms of load and deformation as Robert
Hooke did we're gonna plot our results
in terms of stress and strain now
remember the conversion is quite simple
to plot stress all we do is take the
measured value of load and divide it by
the cross-sectional area to plot strain
we take the measured deformation and
divide it by the length of the original
specimen the result of this test is a
graph of stress versus strain which is
commonly called the stress-strain curve
and here we see a typical example of a
stress-strain curve for structural steel
straight out of the laboratory the
stress-strain curve is an
extraordinarily powerful tool because it
characterizes not just the specimen that
we tested but the material that that
specimen was made of in other words if I
test ten different specimens of
different shapes and sizes that are all
made of the same type of steel then the
stress-strain curves for all ten of
those tests will look exactly the same
just like this one and the stress-strain
curve tells us almost everything we need
to know about a materials mechanical
properties and about its suitability for
a particular structural task think of
the stress-strain curve as the
fingerprint of an engineering material
and you're pretty well along the way to
understanding why it's so valuable to us
let's examine this particular
stress-strain curve for steel a bit more
closely we're gonna read this graph like
a book from left to right so the curve
begins logically enough down at the
lower left-hand corner the point where
we have zero stress and zero strain
makes sense at the start of the test
there's no load and the
specimen hasn't undergone any
deformation so both stress and strain
should be zero before we begin to apply
any load now let's switch on that big
hydraulic testing machine and slowly
increase the load which corresponds to
increasing values of stress load over
area at the same time the specimen is
beginning to stretch to elongate which
means that there's a corresponding
increase in strain hey and guess what
just as Robert Hooke predicted the
relationship between stress and strain
is almost perfectly linear up to a
stress of about 50,000 psi this linear
region of the stress-strain curve has a
particularly important characteristic if
we load that specimen say up to 40,000
psi so it's it's still within that
linear range and then we remove the load
entirely the specimen will then return
all the way back to its original length
with no permanent deformation at all we
call this elastic behavior and just to
illustrate the point if we come back to
my simple tension test specimen and we
remove the load you'll notice that the
specimen does in fact return all the way
back to its original length there's no
permanent deformation this is elastic
behavior now continuing our tracing of
the path of the stress-strain curve as
the load increases beyond this point at
a stress up around 55,000 psi something
very interesting happens the
stress-strain curve abruptly Peaks then
it drops off a bit and then it becomes
horizontal
meaning that large strains are occurring
with no increase in stress at all the
material is actually stretching like
taffy this behavior is called yielding
and it means that the metal is beginning
to fail its crystal structure is
actually starting to break down the key
characteristic of yielding is that it
involves permanent deformations now if
the loads removed the specimen won't
return to its original length but you
know all about yielding it happens to
you all the time when you go into your
closet and you
out of hangars and there's only one left
and you hang three suits on the hangers
this happens and when the hanger bends
beyond the point when it Springs
naturally back to its original shape
what you have done is to cause the
material to yield and as you see the
deformations that occurred as a result
are permanent they didn't come back out
after the load was removed we call this
plastic behavior beyond that region of
yielding on our stress-strain curve the
curve now begins to rise a bit though at
a much shallower slope than the elastic
region of the curve
eventually the curve Peaks it drops off
and then the specimen finally fractures
that means that it physically breaks
into two pieces just like that earlier
simulation I did now note that the value
of strain at the point of fracture for
structural steel is over 0.25 think
about what that means that means the
steel has elongated more than 25% beyond
its original length before it finally
failed it's a very important quality of
Steel that we'll talk more about later
the stress-strain curve that's shown
here on this graph is actually quite
typical for structural steel but the
curves of other materials can be vastly
different for example this is a typical
stress strain curve for cast iron notice
that it doesn't have that distinctive
horizontal yield region and the strain
at failure turns out to be much much
much lower than it was for steel you
have changed the size of the horizontal
axis of the graph remember that the
maximum strain for steel at failure was
up close to 0.3 for cast iron is
somewhere in the neighborhood of point
zero zero three that's one hundred times
smaller remember the stress-strain curve
is the fingerprint of the material the
differences between these two
fingerprints tell us a tremendous amount
about how these materials work in
structural applications and they're
going to allow us to distinguish between
the important mechanical properties for
the purpose of design the three most
important properties of an engineering
material actually can be read directly
from the stress-strain curve no matter
what its shape is here again is our
typical stress for
curr for steal and the first of those
critical qualities we want to evaluate
is the height of the stress-strain curve
shown here and that height corresponds
to the largest stress that the material
can endure before it finally fractures
this stress is called the ultimate
strength of the material and it's
measured in units of stress since it's
measured vertically on this graph such
as psi pounds per square inch second
important material property is
represented by the width of the curve
and it corresponds to the materials
capacity to undergo large permanent
plastic deformations before it finally
fails this capacity is called ductility
we'll talk a lot more about it next
lecture and finally the third property
is represented by the slope of the lower
elastic portion of the stress-strain
curve to measure of the resistance of
the material to elastic or non permanent
deformation this property is called
stiffness we're gonna revisit these
ideas of ductility and stiffness in more
detail next lecture for today I just
want to focus on strength which is by
far the most important of the three
material properties as our roadmap
reminds us the strength is the principle
basis for evaluating the performance of
a structural element the actual stress
in an element is greater than the
strength then the material will fail
when the stress is less than the
strength the material won't fail
Engineers use this fundamental
evaluation as the basis for designing
structures that is determining the size
and shape of each individual member in
the structure to ensure that the stress
is always less than strength no matter
what the externally applied loads are
now our exploration of the structural
analysis roadmap is complete and I hope
that Simon Stefan's motto has proved to
be true for you as well what seems
mysterious can be understood remember
that in every one of the experimental
tests that we've just looked at the test
specimen was a tension member it was
actually loaded in tension with a
tendency to a lot elongate
so our exploration
the concepts of internal force stress
and strength has also taught us
everything we actually need to know
about the first of our six basic
structural element types tension members
are all around us they hold up the
chandelier in my dining room and the
swing in my backyard my dog's leash is a
tension member and as the rope in this
particularly vicious looking tug-of-war
indicates it's also represented by that
rope but most importantly we find many
tension members in great structures for
example here's the beautiful Golden Gate
Bridge and if we look closely at the
bridge we'll see these vertical cables
the suspenders that I referred to back
in lecture 1 are excellent examples of
tension members they're called
suspenders because they suspend the
bridge deck from the main cable so
they're clearly loaded in tension the
Golden Gate Bridge has five hundred such
suspenders arranged in bundles of four
as you see and spaced at fifty foot
intervals along the entire length of the
bridge the suspenders look tiny in this
photo but in fact each one is a bit over
two and a half inches in diameter and
can carry about 500,000 pounds of
internal force note that these members
behave just like specimens in a simple
tension test except that they've been
designed such that their actual stress
always remains well within the elastic
range of material behavior in this
lecture we've looked at two great
structures up front we saw the Sunshine
Skyway Bridge and here we see the Golden
Gate Bridge you might have noticed that
both of these examples are relatively
modern dating from the 20th century this
is no coincidence
our main construction materials
available today have significant
strength and tension but the materials
available to earlier engineers stone
brick concrete and wood were much more
limited in their application of those
four only Wood is capable of carrying
significant tension forces later in this
course we'll see some tension members in
early wooden bridges and roof trusses
but in general the extensive use of
tension members in structures only
became viable with the advent of
mass-produced wrought iron in the 18th
century
we'll learn much more about this
development next lecture
we apply our newfound knowledge of basic
mechanics to explore the wonderful world
of materials and when we'll see how the
choice of materials contributes to just
about everything we see around us in the
built environment until then thank you