there's always a love-hate relationship
between engineers and architects however
you need to have both of them to come
together to have a successful design in
a building so it's important to
understand both sides and i had the
privilege to be invited to rmit to talk
some of the undergraduate architects
through the design projects that they're
currently working on
yeah so we'll start off by introducing
ourselves so
mackenzie so therefore we are mba
architects
and um we've chosen to do a concrete
structure with a rigid system
[Laughter]
[Music]
this gives me great insight into what
they're thinking about when they're
approaching a design but it allows me to
impart some of my knowledge that they're
able to make their designs better so
whether you're on the engineering side
or the architecture side there'll be
something in it for you my name is
brendan your structural engineer now
let's get into it a lot of the issues
that they were having in their designs
was like how big does the column need to
be where should i place it what type of
spans do i need to have what are the
depth of the slabs and structures that
are spanning between those columns and
if we're going into the ground what is
that detail need to be how much
allowance do i need to have in my design
so let's just head outside and have a
quick look at what happens when we go
into the ground and how it comes
together so here we can see a perfect
example of what our retention system is
and the makeup of build of it so we can
see we've got the pile here which is
doing the primary work for retaining the
soil then we've got the capping beam on
top
that makes it span between the piles and
giving us a little bit of tolerance and
then we've got the shock creek wall
so there's a couple of purposes like the
pile itself is roughly about 600 in
diameter on average if you've got a big
high low that might be 750
but then you can see they're at certain
centers you can see they're about 1.5 to
2 meter centers depending on the soil
they're restraining then between them
you've got a shock crit wall that helps
retain the soil back between them so
this is what's doing the primary
retention of the wall but then why do
you have a pile cap well the pile cap
does a couple of things helps span
between the piles but also allows you to
support the structure over you see
you've got a structure over
that needs to come back onto these piles
it also gives you tolerance if we look
underneath we can see there's a gap here
now not all the piles are going to be
100 accurate they can be outwards of 50
mils plus or minus either way because
the rig is not that sensitive so the
pile cap is here and it's built up with
a 50 mil gap plus a 650 pile
so the pile cap over on the 650 pile
needs to be 750 wide so that means
that's the minimum width you need off be
off the boundary the additional one is
that the pile also needs to be about 150
clear if you've got a boundary element
really close as piling rigs are really
big you need some gap to be able to
drill dive up to the side of the wall
when you drill down there might be
another couple of things that you see
which you don't see on this one but you
may have sole anchors now the soil link
has helped australia install before you
get the additional basements installed
now that we're back in the studio we can
see that we're all about that location
and volume that needs to take up for
that retention system so we can see how
it comes together and how much the
basement will shrink so when talking to
architects we can say this is how much
you need to be off the boundary this is
roughly how much area will take up but
there's also another thing that we need
to carefully consider and discuss with
them so we've got multiple basements you
can see the slab goes all the way to the
outer edge and that retention system
will act as a big restraining agent we
all know that in slabs slabs tried to
shrink and if we restrain it it means
we'll potentially get some cracking so
if we can what we want to try and do is
place a column really close to the
retention system this allows us to
detach it it's not completely detached
it will come up and bare against it
however there won't be a direct
connection so only a bearing connection
to allow for the piles to bear against
that location by only bearing against it
as well it means it can move and shrink
which will only be a little bit so the
basement will move slightly but it means
that we won't get that heavier restraint
meaning that we need heavy cracking
reinforcement in the slab talking about
column placement this can be quite new
ones and this brings us perfectly into
the next part that needs careful
consideration through the height of the
buildings that they're designing so if
we're looking at where that column needs
to go there's a number of things that we
need to consider first up is what is the
type of slab that we're going to use are
we going to use reinforced concrete or
post tension slabs and how deep does
that slab need to be as this depth of
the slab not only adds weight making the
columns bigger but also adds depth to
the overall profile and potentially the
height of your building if we're looking
at the different span difference between
reinforced concrete and post-tensioned
slabs we can see that we get a lot
better spent with ratios without
post-tensioning where roughly we're
getting about twice the distance so
where we've got reinforced concrete we
can spend span on 25 when we go to the
post tensioning you can get upwards of
span on 50. now there is minimum
considerations here as we not only need
to these different span to depth ratios
we also need minimum for shear and fire
constraints so if we're talking about
fire constraints roughly around the
center of the columns in those punching
season zones so adjacent the column we
need a minimum of roughly 200 deep
so typically the most efficient slab is
that 200 thick flat plate slab in the
post tensioning it is allow us to
achieve those greater spans upwards of
nine meters but still achieve that
minimum depth so when we are looking at
the slabs the other consideration is
where do we put our set downs now in the
designs we were looking at didn't have
any showers or set downs in the
locations that were critical but if we
do need them we need to make sure we've
got at least 200 deep around the slabs
so trying to move those set downs away
from the column grids wouldn't lead to a
more efficient design so this is where i
typically like to create what is i call
a traffic light system where you say red
green and orange so this is where we're
trying to locate our set downs and
penetrations in our slab so the red
location is around that column so we're
trying to avoid at all costs any set
downs or penetrations through the slabs
in these locations as their critical
shears where all the stress is trying to
consider so all that load is trying to
come towards that column to drag it down
the building where we've got orange
which is between the columns you can
sort of think they're acting sort of
like a design strip if we do need a
penetration we can put them in those
locations but trying to minimize it and
then we've got the green location which
is between columns so there's no real
direct load path going through there so
we can have as bigger penetration or set
downs in those locations within reason
this allows the architect to have
careful consideration on where they're
putting their set downs and penetrations
and trying to avoid those locations
they're going to increase the structure
and reduce efficiency of the design in
our discussions we quickly discussed
roughly what they should be doing and
the column grids that they're having so
trying to keep within that 9 to ten
meters wherever possible it allows us to
have a terrific flat plate slab which is
really quite efficient it's just the
flat plate slab down minimal
reinforcement and the post tensioning
doing the majority of the work the one
thing that needs careful consideration
though is there is a minimum width that
you need to have for post tensioning so
in one direction you may need to go for
reinforced concrete which may increase
the depth or just make sure that you're
reducing the span a little bit the other
consideration as well is when you've got
multiple bays that middle span is really
super efficient so it allows you to
achieve those greater span to depth
ratios however that end bay becomes
super critical and there's not a
continuation on the other side it means
that it typically needs to be deeper or
more reinforcement in there so wherever
possible is trying to reduce that end
base span slightly so five to six meters
and allow you to have that continuous
greater spans beyond that but through
the height of the building what should
we do it's always about maintaining that
verticality we don't want to have
transfer structures wherever possible or
moving columns so we need to look at the
height of the building overlay all the
plans and see where we can get columns
through the whole height of the
structure now that does not mean that
you don't need the slabs to move in and
out as you can do that and the more we
move the slab out the more efficiency
that we have so we can have a column
grid running all the way straight up the
height of the building while the slabs
are going in and out so it looks like
it's got a curvy profile but actually
vertical structure is vertically aligned
in the most efficient solution so how
far can we cantilever we can go back to
those spent today ratios there's also
another consideration is what is that
backspin we can achieve greater
cantilevers with bigger back spans so
roughly you want that the cantilever to
be one third of the backspace so we look
at the backspin how far it back is
divided by three and that's how far we
want to go but we don't want to exceed
roughly about two to two and a half
meters as well but depending on the
profile of the structure that may not
always be able to be achievable as when
we're going up and we have got a bigger
kind of say three and a half meters part
way up what do we do here well we can
have walking columns and we've got to
realize that walking column will add
additional stress into the building but
it's still achievable and you want to
try and make sure you're walking that
column at roughly one to six where
possible and it's over as many flaws as
possible so we can have it at the bottom
we put it on a slight tilt so we can
make sure we're maintaining that two and
a half meter grid at the top then we can
either go vertically up depending on the
profile of slabs or walk that column
back in now over the floors that that
column is walking there's a number of
things you need to consider that column
will need to be a little bit bigger at a
slower rate but how big does that column
need to be and there's a really simple
way that you can work it out whether
you're an architect or engineer to see
how big that column needs to be and need
to work out what load it is on it now no
architects well
why do i need to worry about the load
that's for the engineer but don't be
scared about it some simple maths that
we can go through so we know roughly the
weight of any floor somewhere between 10
kpa and 15 kpa now you don't need to
really worry about what that kpi unit is
it's just a measure of the size and the
stress in the structure but if you do
want to know what it is here's the units
here typically you're between on the
lighter end at 10 kpa so you've got a
really light structure which is
residential with some close columns you
may be able to achieve that 10 kpa on
the tributary area where if you've got a
heavier structure with planet platforms
maybe officers blocks be close to 15. so
for this purpose and first pass it'd be
better to take the higher end because
the column size can always be reduced
and we've got the load we need to work
out the number of floors so we can at
the number of floors and typically we're
measuring it by the bottom of the column
so the number of floors above it will
then give us the number of floors that
we have so we've got end floors in this
case we've got 10. after that we need to
work out the load that's actually
applied based on the slab so we just
look at the column grid and we go
halfway between the different supporting
structures for this example we've got a
column here and we've got a column here
so go halfway in we draw two lines we go
the other way and we've got the edge of
the wall so we move halfway in from the
edge of the wall so this gives us a
tributary area in this location now if
you are on the edge you roughly want to
increase the load by 20 there's this
thing called shear throw don't need to
worry about it but it roughly increases
the load that's applied to that column
so now we've got a tributary area we've
got the number of floors and we've got
the load so this allows us to give an
overall load in the bottom of that
column now that we've got the load how
do we design it do we need to plug it
into a design software no as we roughly
know the maximum pressure on a column is
based on its f c now if that she is the
strength of the concrete as we're going
up the height of the building the f dash
will change to at the top of the
building you'll be 40 mpa for the
roughly the first five to six stories
and for the next couple of stories
you'll be 50 npa now if you're upwards
of 10 15 stories you'll be upwards of
65. when you're going beyond that you
may go to 80. with this situation let's
keep it at 65. so now we've got the
number of stories we've got the tribute
area we've got the load we've got the
strength of the concrete at the bottom
and we times it by a factor of 0.3 which
is roughly maxing out the maximum load
on the structure now we've got the at
the bottom of that bracket we've got 0.3
times f c now this is for your typical
three meter floor to floors if you've
got bigger than that you may need to be
careful but upwards of four meters
you're probably okay if you're going too
far you have this thing called buckling
and boeing of columns you've got double
height columns you want to maybe reduce
that to 0.15 or 0.1 and it may need the
engineer's input at this point we've got
those typical floors 0.3 times f dash g
will give us the rough area that that
column needs to be whether it be a
circle square or rectangle the next
thing you need to carefully consider is
the width of the column if you're going
to a rectangular shape as it will be
governed by two things it's a really
long blade column you may not be able to
put the additional reinforcement in
there so it may need to be designed as a
wall or if it's too thin it may not meet
fire requirements so you roughly want to
make sure you're maintaining at least
250 thick now talking about walls if you
do need those long blade walls you can
do the same thing with this type of
situation but roughly putting 0.1
instead of 0.3 and this will give you
the rough thickness of that wall so this
allows you to either quickly scheme up a
building or the architect can quickly go
through their design and size up the
specific elements in their structure
next part that needs careful
consideration is not the fact that we
just need vertical arms we also need
stability elements stability elements
are much preferred in the center of the
building the reason behind this is walls
are typically lightly stressed so we
have it out to one edge it will
potentially cause a sway in the building
putting additional stress in your design
now it's not the end of the world we can
design for it but something needs
careful consideration to push it into
the center of the building with those
stability elements there's another thing
that needs careful consideration are you
doing in situ or precast
typically in your precast designs you
want to make sure you're maintaining at
least 250 through the height of the
building you won't better reduce it as
it will be clashing with the dowels so
where possible is trying to make sure
your central stability cores are 250
thick or make it much easier on design
and much easier to build beyond that
potentially with higher loads such as in
brisbane or high earthquake areas you
may need thicker buildings but you can
get that from experience and when you're
working with the engineer about the
different areas that you have but the
typically the bigger and more walls you
have the more likely you are to reduce
those stability elements if you want to
get more into detail about how to size
up buildings and a different size of
your elements go no further than my
detailed rules of thumb of concrete
design if you're more interested in
steel i also have this one here about
the rules of thumb of steel structures
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week bye