Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres CAMPBELL ACT 2612 Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
What are
the Pros and Cons associated with using Ultrafloor on a multistorey residential
project?
Mal first used Ultrafloor in
Since that time Ultrafloor has expanded into most states and has carried
out a large amount of research on their product. This research has been both on strength
verifications as well as fire and acoustic performance. Acoustic performance, which had been a
problem area in their earlier years in the multistorey residential market is
now one of their strengths.
They have also expanded their product range to include band beam
construction although they are essentially acting as formwork for the bands,
which are stressed in most cases. Mal has used Ultrafloor on a number of
projects here and in
The Ultrafloor option is always at its most competitive when formwork is
expensive but they believe they can compete in any market. We seriously consider their use in the
following circumstances.
·
Where there is a large degree of repetition on the
project.
·
The building has a rectangular grid.
·
There are not too many balconies (or the balconies
rest within the building envelope).
·
There is a necessity to build walls at less than 8m
centres that can be used as load bearing walls.
·
Lateral loads can be adequately handled by the service
shafts (rarely a problem under 10 stories).
·
Form workers are busy into the future (and therefore
prices high).
·
There is a tight building program*.
·
The builder wants to minimise site labour and/or union
disputes.
·
Detailing can achieve a tight floor envelope.
The last point is the one most often missed as some of the recent
developments in Canberra have sat the Ultrafloor beams on top of deep steel
transfer beams at ground floor which has resulted in solutions 300mm or more
deeper that the equivalent prestressed banded solution. This is simply lazy detailing as there are
alternatives available to minimise the problem.
If you do require 300 of extra basement to ground floor structural depth
you have to incur cost for
- 300mm of
additional excavation.
- 300mm more
height to retaining walls.
- Extra
strength to retaining walls (additional bending).
- Extra
height and strength for all columns (additional slenderness).
- Extra 4
metres of ramp to basement (and the loss of amenity/land).
- Longer
stair runs
- Additional
SW depths (pumping dewatering etc)
To give a feel for what this means on a cost per square metre basis 1 to
4 alone (in rippable rock) will amount to around (10.5+3.1+1.4+0.7) =
$15.7/m^2, 5 and 6 may bump this up to $20/m^2 and 7 can be large or negligible
depending on water table heights. The
extra basement depth can also make it more difficult to comply with natural
ventilation requirements which can be a very heavy cost penalty if mechanical
ventilation is required.
The same also applies to beams spanning between support walls on typical
floors. It is far preferable to have the
Ultrafloor beams and the support beams in the same plane. To get around this problem it is tempting to
go for a steel solution with concrete filling but there are a number of fire
rating concerns that need to be properly addressed. It is our view that a number of solutions
currently being used in the ACT cannot be justified to fulfil their design fire
rating requirements due to poor detailing.
The other construction issue that you may not be aware of is the fact
that the Ultrafloor shop drawings are not dimensioned. The philosophy here is
that they cater for major penetrations such as AC risers but they do not cater
for smaller penetrations such as those required for toilet pans. This can cause quite a few headaches if the
plan geometry is complex as Hindmarsh found out on the Forum. When the beams and FC are all in place and
your surveyor goes around and sets out all of the penetrations every time a
penetration lines up with a beam you have a problem and need to add an extra
beam. This is not such a problem if all
the beams are the same size but is much more of an issue when they are varying
in length. This problem can be
minimised by having a penetrations drawing signed off prior to construction but
we can’t say we’ve seen that done for quite some time.
Another issue to think about is cranage and speed of erection. Many of the Ritek or reinforced block wall
solutions being used have starter bars protruding from the erected wall. These can cause a problem whenever a beam
lines up with a bar and serious thought should be given to drilling and
epoxying splice bars on site. If the
walls are free of obstacles you can often speed the process by craning more
than one beam at a time.
Be careful not to store any FC in wet conditions as it severely weakens
the product and concreters have been injured stepping through weakened planks.
Cement leaking through holes in the formwork can also be a costly issue
to address later so sealing the joints is normally carried out prior to
pouring. Ramset’s ‘Fomo Plus’ is one of
a number of products that can be used for this purpose. The trick is to use just enough if you don’t
want to have to come back with a knife and cut off the excess.
The Ultrafloor manual suggests handling cantilevered balconies in
certain circumstances using conventional formwork but we believe it is more
cost effective to use alternative means and have come up with a number of
standard details that eliminate the need for conventional formwork.
Basically you need to do everything possible to avoid mixing Ultrafloor
with conventional formwork as the formwork price per m2 will double or triple
and the clear workspace under can be lost to scaffolding and form props. This point seem to have been lost on a number
of designers in recent times with any economies or benefits being swamped by
the inefficiencies involved in mixing trades.
Some thought also needs to go into how steel columns are supported on
the top floor as far too often riggers are tearing their hair out trying to
support columns on 50mm concrete slabs and in extreme cases being told to
scabble out a set down for the base plate.
This issue should be addressed on the shop drawings with thicker slabs
provided in appropriate areas.
In brief
The benefits are
- Speed of
construction*
- Minimised
site labour – union disputes
- Cost (in
the right market)
- Less
material wastage on site
The drawbacks are
- Monopoly
supplier (although notoriously reliable)**
- Unattractive
soffit (if exposed)
- No
dimensions on shop drawings
- Complex to
place if support geometry is not regular (need to cut all of the infill
panels and all beams are a different size).
- Fixing
steel columns to thin topping is can cause headaches.
* This is perhaps more true on paper than it is in real life as a well
designed stressed deck using one of the bigger formworking companies can have a
floor cycle of less than a week on a 500-600m2 deck which is very hard to beat.
**It is perhaps not fair to say they have a monopoly as Rescrete also
supply a somewhat different precast decking system but once you’ve signed up if
they don’t perform you don’t have many options.
Ultra-floor generally quote a price for design and installation of the
planks and Hardiform infill and provide shop drawings of beam layouts out of
their Sydney Office after a fixed price is agreed to. Shop drawings take a minimum of two weeks to
produce and they are happy to do deals well into the future if your project has
a long lead time. ASD can produce the
shop drawings if you prefer, as can one or two other local consultants. In this case we are paid $2.50/m2 by
Ultra-Floor and the cost to the developer/Builder is unchanged.
Costs per square metre start at around $60 (March 2002) for their
smaller beams but this does not include the mesh or concrete topping.
If you need to know more about how it’s done or need a proposal put
together by an experienced structural engineer in Canberra or the surrounding
region call Mal Wilson from Advanced Structural Designs on Ph 61612171.