Lateral Force-Resisting Systems – braced frame, shear wall, and moment-resisting frame
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Lateral Force-Resisting Systems – braced frame, shear wall, and moment-resisting frame

October 11, 2019

So we got to experiment a little bit
with lateral force-resisting systems. The goal was to try to figure out
pros and cons of different systems and specifically, to
look at their stiffness. So which of them was stiffer? Which deflected less as
you try to push on them? The three main kinds, again, are
a braced frame, a shear wall, and a moment-resisting frame. So every building really needs to
have some type of one of those three systems. And sometimes they have
multiple of those systems. And we’re looking at
how stiff– how they’re going to resist those
applied loads, whether they come from earthquakes or from wind. So you probably found that
brace frame was pretty stiff. So using that truss system tends to
be very stiff, also very efficient. Shear walls can also be very stiff
but they tend to use more material, so there’s pros and cons there. From our single degree of
freedom system experiments, when we experiment with the
single degree of freedom model, we found that stiffer isn’t always
better– that there’s pros and cons. Sometimes we need the stiffness. When we get buildings that are really
tall, if they’re not stiff enough, there’s too much motion. There are certain buildings
in the world that you get too much motion at the top in a
big wind and that’s not comfortable. And if your beams are not stiff
enough, they deflect too much. So we have to balance how much is
enough stiffness versus not enough. So why would we choose a
brace frame versus a share well or a moment-resisting frame? Stiffness is one piece of the
puzzle but there’s other reasons. So a moment-resisting frame tends to
be the choice for a lot of buildings because it allows, at least on
the exterior, allows windows. So we have a lot more
open space for windows. It is one of the least
stiff options but it provides other things that are useful. Shear walls are the opposite. No options for windows
in a shear wall but we often use them in the
core of a building. So around elevators and
staircases, we’ll put shear walls. And they’re very stiff, and
so they’ll help us stiffen up the interior of the building. But engineering is all about
all these competing criteria, so it’s not just stiffness. It’s also strength. But it’s aesthetics and functionality. And cost is a big factor. So how you pick different systems
will depend on all those criteria. So I want talk a little bit
about how structures– so putting all these pieces together–
how it’s changed over history. So we’ve talked a little bit about
the early structures and stone. We talked about beams. We talked about stone. Stone was used in many
of the early structures. Stone is very strong in
compression so it does a great job with arches and vaults and domes. It is very weak in tension. People did use them for beams
and they used them for buildings but the spans of the beams tended
to be very small, very short. Again, because it doesn’t have very
much tension capacity and those beams rely on both tension and compression. So if you look at a plan
view of ancient buildings– so examples would be the
Parthenon, Temple of Zeus, any of the large stone structures. If you look at a plan view,
you’ll see lots of columns very closely spaced
throughout the building. That’s because stone, if it’s used
for the beams, cannot span very far. It could span a little bit farther
when they started using arches, so in many of the cathedrals,
they would use arches and vaults. And so the spans got a little
greater, heights got a little higher. But still, you were
limited by your material. It wasn’t until the 1700s, with
the advent of steel and iron, that we started to be
able to span further. So columns would be able
to be spaced further apart. So we started to get more open space. We started to get taller buildings. So one of the first examples of
a steel moment-resisting frame– moment-resisting frames were the most
common system used in early buildings– was the Wainwright
Building in Saint Louis. So there were a lot of big
buildings going up in Saint Louis. If you remove the outer skin of the
building, you’ll see steel columns and beams. It was designed by Louis
Sullivan and think Dankmar Adler. They went on to design
many buildings together. That was one of the first
moment-resisting frames. There was around that
time– around the 1900s– an unofficial skyscraper
competition began. It was mainly New York
and Chicago competing, trying to get the tallest structure. So the Woolworth Building
is an example of that. It was commissioned
by Franklin Woolworth. The entire goal was to
build the tallest building. So Franklin Woolworth paid $13.5
million in cash for the building. It was completed in 1913. It was 241 meters tall. And it was the tallest until 1930. It was designed by engineers
Gunvald Aus and Kort Berle. Again, like many of the early buildings,
it used a steel moment-resisting frame. The was again to get a lot of windows. That was one of the first
systems that was used. It also was the first
building that would include an elevator, which is the
key to making buildings go taller. So if you want to get buildings taller,
we started to have to think about fires and getting people in and out. The next building– completed in
1931– that I want to talk about, because it took over as the tallest,
is the Empire State Building. It was completed in 1931. It was the tallest until 1972. It was designed by William
Lamb and Homer Balcom. One of the keys about this building
is the speed with which was built. It was designed and built in 20 months. So it was when people are starting
to use an assembly line approach and construction was also using
an assembly line approach. It’s an iconic building. It was designed in art deco style
so many people recognize it. It’s often referred to as the most
famous skyscraper in the world. Internally, it’s using a steel
moment-resisting frame, again, to resist those lateral loads. But it was the tallest
for many, many years. Next building I want to talk
about is the John Hancock tower. And this never was actually
the tallest in the world, but it introduced a new type of framing
for structures that actually helped structures in general become taller. So it introduced a tubed structure. So up until then, buildings
were traditionally very regular. So all the columns were space regularly. The John Hancock tower uses a trussed
tube, so if you look at the exterior you’ll see X’s on the outside. And that’s a truss on the
outside or a tube structure. It was designed by engineer Fazlur
Khan with the help of architect Bruce Graham. They both work for Skidmore
Owings and Merrill. Fazlur Khan– he’s a famous engineer. He’s known as the father
of the tube structure. He’s a Bangladeshi American
engineer and he introduced the tube. He is the one that invented and
started using a tube structure. So instead of that traditional grid
of columns placed very regularly throughout the building,
he moved everything to the outside of the
building– not everything, but most the columns and the
lateral force-resisting system was moved to the outside. This turned out to be much
more efficient and economical. It has to do with the stiffness. So if we go back and think
about beams– beams that have all the mass concentrated
right at the center were not nearly as stiff as beams
that had the mass further away. That’s why we use I-beams. That’s why we go to forms that
have everything moved away from that central axis. It was the same with buildings. So we’re moving the columns and
the lateral force-resisting system to the outside– it was more efficient. Fazlur Khan also designed
the Willis Tower– formerly known as the Sears Tower. It’s again a tubed frame
system, not with a truss, but still tubed frame, so
everything on the outside. It was completed in 1973 and it was
the tallest for almost 25 years. So the Burj Khalifa
in Dubai is currently the tallest building in the world. It’s got a height of almost
830 meters, which is huge. It’s three times the height of the
Eiffel Tower, to give you perspective. Another way to give you perspective
is the way the concrete that was used. The weight of the
concrete in the building is equal to the weight
of 100,000 elephants. Another statistic that
I found interesting was that 12,000 people
worked on the Burj Khalifa during the peak of construction. The Burj Khalifa was designed by
engineer Bill Baker and architect Adrian Smith. They’re both at Skidmore
Owings and Merrill. It’s interesting to
read about this design. It’s based on a flower. It also uses a bundled
tube construction. But it’s got these three arms,
so it’s got a very strong core. And then as it goes up, less and less it
tapers, somewhat like the Willis Tower. The reason it has these three arms is
so it can resist wind in any direction. So it has these three
arms– it gives them a nice moment of inertia at the base. So you have a strong core
and then these arms that extend out to give it some stiffness. So as tries to bend, it’s got
stiffness in any direction. That goes back, again, to beam
theory and moment of inertia. There was extensive
wind tunnel testing that was done on models of the Burj Khalifa
and also detailed computer analyses. It’s just an amazing feat and it
brings all of our systems together. So we have another video coming up on
using glass as a structural material. Go ahead and check that out. And then a couple final
activities for the course.

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