Advanced Structural Designs

  

ACN 097 789 87    92 Vasey Cres CAMPBELL ACT 2612

Telephone  61612171         Facsimile 61612170

Email   mal@structuraldesigns.com.au

 

 
 
                                                                                                           

                                                             

 

Stripping the formwork and falsework from a concrete Deck

 

Stripping 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

 

Considerations that may influence the timing of the strip are:

 

  1. 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?
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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 liquidated damages.
  7. 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). 
  8. 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 19.6.2.4 and Table 19.6.2.5.

 

A 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 19.6.2.5 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.

 

It is 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.

 

We design 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.

 

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