ACN 097 789 87
92 Vasey Cres CAMPBELL ACT 2612
Stripping the formwork and falsework from a concrete Deck
the formwork under slabs is an issue close to every builder’s heart because it
directly affects when other trades can get onto the floor that is blocked up
with falsework and start hanging the various services
that may influence the timing of the strip are:
- Are there any unusual heavy
loads such as materials stacking or erection cranes that may require
support in future construction works?
It is sometimes more economical to leave formwork and false-work in
place a little longer and strip out in one hit?
- Will floors over need to be
carried by the false-work and is there a set stripping and re-shoring
procedure? If this is the case it
is the responsibility of the design engineer to provide sketches showing
the minimum amount of shoring required on each floor and the timing of the
stripping operation. This
information is normally requested by the managing contractor and is
influenced by his site management needs.
If the projects structural engineer wants to charge you for the
advice we suggest you point them to AS3610 section 2.3 and ask when they
stopped complying with code requirements.
- Will any scheduled building
activities be physically in the way of the stripping operation (blocking
access etc)? We have seen money
wasted on high strength concrete where the stripping could not be
completed due to blocked access ways.
- Will cranage and manpower be
available at the time to take advantage of an early strip time? Systems such as Table-forms will need a
crane booked and ready.
- Is the formwork needed
elsewhere on site or at another site?
Sometimes very competitive formwork rates are contingent upon a
specified cycle of form reuse.
- Is it cost effective to spend
the money on higher strength concrete to allow early stripping? This question may be a function of
project programming, staff salaries, early finish incentives and/or
- Is the slab stressed? Stressed slabs can often be stripped
earlier as concrete strengths form the only criteria if the future loads
are within the floors capacity. We
have allowed complete stripping of single storey stressed car park decks
in four days using high early strength concrete. On a job we did in Asquith in October
2002 we allowed stripping of each floor of a multistorey warehouse 5 days
after each floor was poured. The trick is ensuring that the site cured
cylinder strengths are truly representative of the concrete in the deck
(but that is another storey).
- How stiff is the reinforced
concrete slab? If the slab is
conventionally reinforced it would normally need to be supported after the
concrete has reached its design strength because if the props are taken
away too early the slab will deflect beyond recommended limits. This additional deflection is known as
creep deflection and is more pronounced when the concrete is relatively
“green”. Where normal class early
age strength concrete is used with reasonable stiffness parameters, guidance
can be taken from AS3600 (2001) Tables 220.127.116.11 and Table 18.104.22.168.
contractor rang us recently on a project where the Lift Motor Room Cantilevered
over a Foyer just above roof level on a project running into $5000/day
liquidated damages. They had programmed
12 days to strip the formwork (Table 22.214.171.124 AS3600) and wanted to know if
there was any possibility of reducing this.
After a few deflection calculations on what were fairly stiff slabs and
beams I rang back to say they could strip completely in 7 days with high
strength concrete and 5 days if we add 20 mm to the slabs.
always worth remembering that your structural engineer is often in the best
position to offer solutions to programming and construction problems. If the above contractor had rung one month
earlier with the problem we could have designed steel propped cantilevers off
the lift shaft as supports to beams carrying Bondeck slabs. The whole system could have been left in
place except for the props to the shaft, which could have been removed by
hand. This cheap and easy solution would
have taken the LMR off the critical path and a couple of weeks off the program.
and certify formwork systems so if you need a structural engineer for advice on
construction solutions in Canberra ACT give Mal Wilson a call on (02) 61612171.