this video is sponsored by Ren more
about them later in the video there's a
lot of different materials and methods
that you can choose to build a building
out of just a steel framing Timber
Institute concrete and composite deck
but quite often Precast is the chosen
solution due to the many benefits that
it brings to the table however there's a
lot of design considerations that you
need to go into when correctly detailing
a Precast structure and I'll be going
through the key elements that you need
to watch out for and why you should
choose different elements and where they
fit best my name is Brendan your
structural engineer now let's get into
it so why would they choose a Precast
solution for a design that's due to the
benefits that it brings such as design
Freedom it's both a structural element
and an architectural element you see you
can have a structural that goes up that
provides the structural strength that
you need but you can also have a nice
finish on the outside it's called
textures and colors to it as soon as it
comes in it's both architectural and
structural in one piece it also offers a
durable solution you don't need to treat
it as is the concrete protecting on the
outside you put the steel internal to
provide the strength that you need so it
provides both a durable and
architecturally free element Precast
also offers an out of the back solution
that is also great for both thermal and
acoustic properties as it's made out of
concrete and dents takes a lot of effort
and heat Mass to heat up a building from
the outside so it can provide that
thermal barrier and it's the same reason
why it's good for the sound as well so
it can either be a partition wall
between two areas or between egg
internal and external you don't need to
have additional treating most of the
time but the main reason why people pick
a Precast solution is due to its speed
of construction and cost you see it's a
lot faster to build a Precast building
typically especially to have a lot of
repetition you can build the elements
offside undercover so it's not affected
by temperature weather or rain as
opposed to its in situ solution where
you are affected heavily by the weather
if it's too hot need to call it off if
it's raining you can't work so it can be
a lot of delays from an in-situ design
which you don't have with a Precast
design it also speeds up through
construction as it can also act as a
formwork solution as you can have slabs
that go down that provide both the
structural elements and the formwork
solution in one hit but has a lot of
benefits here however there's key
considerations that you need to be
careful of you see it has the both the
complexity of Steel design in the
connections and the complexity of
concrete how it behaves over time
bringing them into the both together and
mashing together really it's the worst
of two worlds so you need to be really
careful when you're designing the
elements in Precast Precast also offers
you a lot of different solutions as it
can either be a column wall beam or a
flooring solution now when you're
looking at columns and walls it pretty
much directly replaces them despite a
little bit of connection which we'll go
about later whereas if you're looking at
a Precast floor or beam typically you do
need to pour additional concrete over
the top to finish it off as you need to
have it integral across the whole
structure where solution comes in
different bits the benefit really comes
in a prolonging solution is the fact
that it replaces formwork as you can
have big areas coming down and finishing
it off we're not needing a lot of
reinforcement and we do need to put
concrete over the top now this is where
the drawbacks first start with your
Precast solution do the time nature of
how concrete actually behaves you see
the Precast has already come down it's
already shrunk where if you put the
concrete on top it's fresh so it's going
to shrink a lot more what this causes is
a differential between the top and the
bottom so essentially you're compressing
the top they'll Force deflections down
and also add additional stress in the
element underneath so when you're
looking at a Precast solution it's not
just looking at that first solution of
where you've got the Precast design as a
solid box as it will deflect a little
bit more so you need to watch out for
those curvatures and the additional
deflection that it's going to apply to
your design now that's whether it's
replacing a Shelby or a flooring
solution now the shell beams are really
great as typically you can have this U
and have it spanning a long way but we
do have a lot more concrete over the top
so it can have a greater effect on that
curvature now flooring solution can form
up the whole structure you can have
shell beams that create deep beam
structures you can have planks such as
Hollow core or solid Planks on top
but the consideration we're saying is
you need to pour concrete on top to make
it integral this is due to diaphragm
forces needed transmitting from one to
the other and the fact that the Precast
solution comes in little bits as it's
not joined together so that topping side
needs to do a couple of things now you
need to make sure that you've got enough
reinforcement through that tired both in
those lateral forces but also those
robustness forces now this is really
where the first key problem with a
Precast solution comes in now this
doesn't matter if you're doing a zoning
a column wall flooring it all has the
same problem as is in individual bits it
doesn't have a good job of tying each
other together and this is where the
rules behind disproportionate collapse
came back now there's a great example of
what actually happens if you haven't
designed correctly this is Ronan point
it's all very low rise now but 50 years
ago today at just before 6 a.m one
corner of a tower block collapsed we'll
actually see oh it's completely eyes Rey
still lives on this estate now rebuilt
beyond that was Ronan point and he was
just 21 when he watched the collapse
from his living room in a neighboring
Tower block oh this big explosion
and uh I see this flat side coming down
and everyone in our block was screaming
and shouting these are fire brigade
pictures taken inside the flat on the
18th floor where there was a gas
explosion it blew out an external wall
and once it had gone all the other walls
above and below progressively collapsed
killing four people 50 years on Rey
still isn't convinced Beverly never
learned at all did let this go on Ronan
point was rebuilt but then demolished
along with neighboring blocks because of
structural weakness you see they had a
small explosion on the lower floors what
this caused was all the flaws above it
to come down on top of each other now
it's considered that that little failure
at the bottom shouldn't have caused such
a catastrophic event up the whole height
of the tower the problem was it was the
fact you had Precast slabs and walls
stacked on top of each other there
wasn't any direct connection to the
building adjacent to it so what that's
mean as soon as it loses the low path
underneath it everything comes down so
it's minimal requirements of how you
need to tie things together whether it
be through the flooring elements or the
walls directly to each other
and you need to be allowed to hang down
and not have that disproportionate
collapse Solution that's where you need
to make sure you've got enough
continuous reinforcement through your
slopes you need to make sure you're
connecting walls and columns effectively
and this is something you need to be
carefully detailed making sure that you
don't have a critical element that if
you lose it you lose the whole building
the next drawback which is specifically
for slabs and floors is the fact that
you can get locked in stresses in your
design we talked about when we pulled
the concrete over it provides that
additional curvature forcing the slab
down but as it's there in its in situ
State it's not in its final element as
you have a beam that's a specific depth
there's a potentially locked in stresses
in your design as a smaller element is
trying to prevent the wet weight of the
concrete and when you do your math a lot
of the time the wet weight of the
concrete is a greater proportion of your
design loads you can have these locked
in stresses that cause critical actions
so first up you need to put the Precast
down and have a look at it with only the
additional weight of the wet concrete on
top of it as it sets those stresses get
locked into place so Stills at a certain
ratio so it's already reduced in
strength before you've even started so
when you put the additional loads on top
of it you'll have additional stress
however it's on the deeper element so
you need to add both the stress at the
start removing any of that self weight
because it's already locked in then
adding the additional load on top in the
bigger section to make sure you haven't
over stressed your elements so you just
need to be careful about how you do your
loads it's not just looking at the final
State saying okay it's 300 deep the
reinforcement is this therefore it's
fine it's going okay what is it in its
first state and it's reduced depth and
what is it in its final state in the
greater depth and adding the two loads
on top of each other to make sure you
have an overstressed your structure now
this additional stress is not only just
for flooring and slabs however that's
where it's most critical but it can also
have the same problems with beams now if
we're talking about that first solution
where you've got shell around the
outside of your beam to have the most
depth of concrete it provides a lot of
solution to bring to sight but sometimes
you can have big beams pre-formed up
with Priestess in it so it means it's
got a bow in it before it's even started
this allows you to have big structures
and quite often you more see it applied
in the Civil space such as building
roads or Bridges you have these big key
culverts or beams pre-formed up with
pre-stress already bowing it out as it
has a lot of control it means it can
have a better quality structure however
it still has that locked in stresses
that you need to consider it's becoming
more and more brief to reduce the
embodied carbon in outer buildings that
we're designing today as Engineers we're
constantly trying to improve
Technologies and go down that path
however quite often in our home life we
don't think about the embodied carbon
that we actually use and the impact it
has on the environment and this brings
us perfectly into the sponsor of this
video Ren Ren offers a simple and
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let's get back to learning about Precast
the walls have one key element they also
have this same problem with Precast
columns it's the fact that where you
join it between either two walls or
floor and a wall is a factory
undergraded joint the reason behind this
is the fact that you will not
necessarily pull that slab wall 100
flush so when they come together they'll
have local areas that they'll bear
against and cause local stresses they
can cause damage so you need to grout
that element now it needs to be careful
consideration of that grounded element
because not only does it bring the
forces in causing local dispersion
forces but it also means that that joint
is also not well waterproof so typically
you need to put especially as an
external area there's a water beating on
the outside to make sure water
propagation can't come through that
joint so you need to look at the ground
joint look at where the forces are going
and making sure you're carefully
considering that in your design and as
it is a greater joint it's created a pin
solution which is you typically tie that
together with dowels it does have a
couple of key aspects but they need to
tie the building together firstly if
they are a rotational wall they need to
be long enough and strong enough to
prevent those rotational forces but also
they're typically there for sheer load
as well
as you will have sliding against it
there's a reduced sliding effect it
typically pre-cuts is smooth and the
slab underneath it is smooth as well so
you need to look for that sheer friction
on top of it that has two components
both the load that's coming on top of it
and the dowels together that are
clamping together and looking at how
much of resistance you can get in that
sliding effect and sometimes you may
need to add additional dowels in there
to help resist that sliding force all
those tension forces in your design it's
also critical of when you grab up that
joint you see typically when a Precast
comes down it's not graded up straight
away or sometimes you'll mud it up and
it will come down but sometimes it'll be
granted up later now this is due to a
couple of reasons that when you put it
down having a good smooth seal on the
outside is very hard especially if
you're mudded up because you don't
necessarily know how much material you
need to put down so as well if you are
putting it down there you need some way
of creating a gap to fill up that
position so you typically put little
spaces in there those spaces do hold the
building up temporarily and allows you
to stabilize the structure but you need
to make sure that you haven't gone too
far before you grab that joint and as
we're saying there was two ways you can
either mud it up straight away have it
bearing down and then seal it off around
the outside making sure it's nice and
smooth so it's only really waiting for
that grout to set but what happens a lot
of the time into the Precast solution
you need to fill them with grout to make
sure you're transmitting those lateral
forces you can have a well around the
outside filling up with grout while
you're solely filling up that joint now
this typically can mean a stronger joint
as flowable grout is typically a higher
strength however you need to make sure
that you do have a problem well and you
don't have stuff rubbing down the
outside what a lot of precastors do is
they put a concrete donut dowel around
the outside which is a bit of foam it
helps them seal off that joint make sure
that the grout doesn't flow out where
this does coat a local stress area that
you need to carefully consider in highly
stressed elements it may cause
additional local stresses that you need
careful consideration of so typically I
try and prevent the use of these dowels
and using them as an integral piece to
make sure you've got enough solution
there if you do come to site is making
sure if they are using Precast dials
you've actually carefully considered it
in your design if we've seen learning
more about some of the design rules that
you go into when designing a Precast
building I've got a link to a video here
if you're more interested in still
design I've got a link to a video here
both of these would be relevant for a
Precast solution and if you're
interested in supporting the channel
there's two ways that you can do this if
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without the support of my YouTube
members and patreons this type of
content will not be possible as always
stay safe keep learning and I'll see you
next week bye