changing from a reinforced concrete
structural design to a post-tensioned
structure is arguably one of the best
ways to achieve structural efficiencies
not only does it reduce the amount of
reinforcement you need it can also
reduce the amount of concrete by
achieving greater spans at thinner
depths so you can significantly reduce
the carbon footprint of your structure
by adopting a post tension solution i
break down some of the basics of post
tensioning design so you'll be more
comfortable in adopting in your design
and achieving these structural
efficiencies my name is brennan your
structural engineer now let's get into
it why is it beneficial to adopt post
tensioning in your structure it really
breaks down one of the key
characteristics of concrete it's great
under compression but really poor under
tension so what post engineering does it
pre-compresses the slab putting a
compression force that needs to be
overcome before the concrete sees any
tension you're really gaining the
benefit what concrete's good at that
compression action so post tensioning
doesn't just compress the slab either
you just don't have a straight tendon
pre-compressing it to overcome that you
also drape the tendon the reason for
draping the tendon is that it has a
secondary action the drape or curvature
of the tendon is pushing the slab up and
dragging the loads closer to the
supports of where you need that load so
it adds that additional benefit of
dragging the force up from places where
it's most critical say in the middle of
the slab and dumping it adjacent
supports will have minimal impact while
you're learning about the section of
post tensioning design don't forget to
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content to get out to more people so how
should you drape the tendons in your
structure what's really quite simple you
just look at where the tension force is
on your structure so when you're looking
in a bending moment diagram we can see
where the tension forces are so at the
top or over a cantilever it's at the top
of the slab so the top surface of the
slab is in tension where if we look at
the bottom of the slab we can see the
curvature goes down and now we're below
so the bottom half of the slope is in
tension so it's beneficial to have the
tendons high over supports and low at
mid span
so you can limit deflections quite well
so you're making the load go from where
it's most critical in the midpoint of
the slab pulling it up and dumping it to
where it's less critical and have less
impact on your design over the supports
so with post tensioning you can tweak
the slabs up and down and really get a
really low deflection across your whole
slab by moving these drapes up and down
will determine on how much secondary
actions you're pulling the slab up and
down so how can you work out where the
critical locations are in your slab
design there's a number of different
ways the first way that i think about is
the slab acting as two-way or one-way is
through yield line what yield line does
it puts a number of columns on your slab
and drawing the lines across at 45
degrees which is a line of equilibrium
so where the slab is going one way
loading on top of a column strip or the
other way to the other column strip so
there's a two-way action within the
triangular line in the middle it's just
a straight line as there's no other
critical way for it to go just spanning
top to bottom so the midpoint of that
slab is acting in one way but of course
we've got the column strips between each
of the columns as well so that's where
you've got a strong band that will need
to have the lowest point as it's taking
the most load it's dumping the load from
the slab onto the column strip that then
needs to be taken back to the column
line so between columns you typically
need your lower straight as much as
possible then over the top of columns
you want your highest rate so you want
to have as big a variation between your
high and your bottom along those column
strips how about beams though beams can
have a lot bigger depth as they can go
from the top of the beam to the bottom
of the beam whereas they've got a
greater depth they're not as critical so
typically you'll have your slab tendons
go up and over and have your beam drapes
below it
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over there let's keep learning when
you're looking at your sub design you've
got some random columns you've tried
some yield lines so you don't
specifically know where your column
strips are and where your middle strips
are you can throw it into an fa package
like ram and through a landing placement
of columns you can see where your high
and low points are based on the
deflection plot so you don't need any
tendons here when you're looking at this
so when you're looking at your design
you can see where the ridgelines are
where the top of the slab is that's
where your strong bands are and where
your very low values that's your middle
strips so from this you can see which
directions you need your strong bands to
be in now with tendons you don't want to
be angling them along these column
strips so you will have some way of two
way actions on those random strips as
you want to keep them in a grid pattern
as you will have tendons going in one
way and tendons going in the other way
but it allows you to drape them in
certain ways to make sure the loads are
dumping onto the column strips instead
of onto the middle strips where they'll
be least effective one pro tip when you
are draping your tendons over a band
beam
is the fact that you want a high point
on either end of them as you want to
have as great a curvature as possible in
the midpoint of the slab the more
curvature you have the more load that
air gets pulled up and dumped onto your
column strips so if you want to help
with deflection you have a high point on
the left hand side of your band beam and
on the right hand side of your band beam
and that will allow you to dump more
load on that band beam support sometimes
as well if you've got enough
reinforcement this will also work for
your column and middle strip arrangement
some basics about how much post tension
you have in design there's really a
range here first up you want to make
sure you're not over balancing it much
like when you're pre-cambering a beam
you don't want to precamp any more than
about 80 percent so when you're doing
your first numbers you need to calculate
how much load you're actually balancing
out so how much load you're actually up
and making sure you're not balancing out
any more than about 80 of dead load
because any more than that you may have
a permanent hog of your slab which is
not a good thing you're using it for
crack control depends on what level of
crack control so if you're on a
non-critical area such as you're going
to have carpets down in a car park or
other areas where it doesn't matter if
you've got some minor cracking from that
shrinkage movement this is where you
might have a pna around about 0.7 this
is really a low degree of control and
really the minimum where you should be
starting if we're moving up a little bit
which is your moderate level you're
roughly at about 1.4 mpa we're moving up
to our high end such as we've got a
podium slab that may be affected by
thermal movements or if we need to have
a watertight structure we'll be trying
to focus our post tensioning around 2 to
2.5 as the greater the level of pna the
more crack control that we're going to
achieve one of the biggest enemies of
this post tensioning is restrained as
restraint will pull the load into those
restraint locations instead of into the
slab so we need to make sure we're
carefully detailing our temporary or
permanent movement joints such that the
slab can shrink into one area another
thing that you may miss because the slab
is shrinking you've got to make sure
that you're shrinking all the same way
you don't want to lock it up so if
you've got a moving joint that needs to
move in location you want to make sure
you locate it such that it will move
away and not lock up there because if it
does lock up there it's putting
additional stresses in those loads will
dissipate out of the slab and you won't
achieve the post hinging that you're
expecting post tension cables are also
quite long as they come out of a reel
they roll into the duct means you can
have them as long as you want as opposed
to reinforcement which you can normally
have about 12 max meters post tension
cables can go upwards of 20 30 or even
40 meters so how long is it too long and
how long is too short well roughly a
post engine cable of about 5 meters or
less is really ineffective in those
locations you will need your
reinforcement in your structure
optimally you want to try and aim
between 12 to 20 meters this is really
the optimal range for your post-hinching
as you'll have enough length for it to
go through and forward pre-compression
to really form however when you're
getting up near 40 or 50 meters you've
really had a lot of losses as the tendon
will be dropping up and down you'll have
drag forces on it so the post tensioning
in the cable will become less and less
effective the longer it gets when you're
achieving up near those 40 or 50 meters
you want to make sure you're flipping
your live and dead ends as the stress
starts from that live end location so
you can get somewhat of an even amount
of post tensioning across your whole
slab so what are you trying to aim for
when you're looking at your designs
what's that post tension structural
first up are you hitting the minimum
pnas to achieve that crack control and
how much reinforcement are you actually
displacing just post tensioning doesn't
mean you won't have no reinforcement in
your design so where should you be
trying to put it well a lot of time
you're trying to drape your tendons such
that you've got enough pna and drape
that you don't need any bottom
reinforcement wherever possible only
over some supports locally so when
you're looking at your designs making
sure that you don't have any bottom
reinforcement will mean that you've
achieved your optimal design another
critical aspect of post tensioning is
the fact that you typically have a
straight run but sometimes you may need
to diverge around openings so when
you're diverging around your openings
you need to be really careful that
you're not applying additional stresses
on the structure as that divergence will
add a load similar to the drape of the
tendon so you don't want to make sure
you're racking the tendon any more than
about one to six you want to make sure
also that occurs before the opening so
if you've got any thrust forces they're
not going directly to the opening that
you're trying to diverge around this is
also goes for the same thing as steps
and folds steps and folds can make post
tensioning really ineffective and you
need to be careful about how much
grading you're going through has that
same problem that if you need to change
from one height to another you
potentially will need a bigger fold as
it goes down now when you're going down
below the fold you want to make sure
your tendon is not too high if it's too
high it's pushing up and it can cause a
cracking at that location so you want to
make sure that you're either flat or
dropping it down below so the post
tension cable might burst out another
thing about post sensing it doesn't
develop straight away it's like a point
load on a slab if you've got the
compression load you'll see a bond
moisture stress just locally to that
point load the same thing happens with
post tensioning and post tensioning
doesn't really come effective for about
the first meter to a meter and a half so
you've got this zone of reinforced
concrete that does not have any comp if
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