Holdfast Training Services Trial Blog

I am currently working for the Design Team in Multiplex Queensland.  Having saved the company a bit of cash on the Jewel project I was asked to have a look at my old site Brisbane Casino Towers.  They were having a spot of bother with a sub-contractor.

The project is running over budget on steel reinforcement.  On further inspection it appears that the post tension (PT) concrete slabs are needing double the reinforcement that they were originally budgeted for. While there has been an increase in load by 9.52% it doesn’t explain the doubling of the conventional reinforcement.

The PT contract was let as a design and build and the PT designer works for the sub-contractor.  The consultants will act in the best interests of their clients which is not Multiplex but the sub-contractor.  Compounding the problem is that Multiplex pay for the conventional reinforcement and the sub-contractor pays for the PT.  So an unscrupulous contractor could put in more conventional reinforcement and less PT and pocket the money.  In order to prevent this the contract has a limit upon the amount of conventional and a sum for the  PT see below.

Level	Area (m2)	Post-Tensioning Price (Design, Supply  & install)	Reinforcement in PT Slab Design (t)
PT slab Design	Design	$118,000.00
Level 1- Ground Floor	1350	$51,000.00	16.2
Level 1 Mezzanine	350	$3,000.00	10.0
Level 2	1535	$55,000.00	39.9
Level 3	1275	$41,000.00	10.2
Level 4 Transfer	1525	$120,000.00	62.7
Level 5	725	$20,000.00	5.8
Level 6	1200	$31,000.00	12.0
Level 7 to 24 (18 Levels)	21060	$594,000.00	179.0
Levels 25 to 29 (5 Levels)	5850	$161,000.00	55.6
Level 30	1165	$27,000.00	10.5
Roof Level	935	$27000.00	12.2
Total	36,970m2	$1,248,000.00	414.0t

Having pounded my head against my desk for the last week trying to re-design the next slab (the Level 4 Transfer), and prove that they are conning us. I have come up with this simpler method to spotting you are being conned when the loads have changed.

So what do you do?

A tonne of PT steel is not the same as a tonne of conventional steel.  You need to find the equivalent ratio of capacity per tonne.  For 15.2 mm strand (PT) and a 16 mm bar (Conventional) this is approximately 3.5:1 at a cost of (2:1), so it is more cost effective to put in more PT if you need to increase capacity (why would the sub-contractor do this if it costs him money).

• Calculate the equivalent capacity at contract.
  • Assume $5K per tonne for PT.
  • Equivalent tonnage – 139.1 T
• Find increase load and percentage. – 9.52 %
• Calculate equivalent capacity needed. 152.3 T
• Find the current equivalent capacity.  This is painful and involves counting bars and strands.  -187.2 T.
• Calculate Difference = 34.9 T
• Calculate Cost ($2k per tonne for laying conventional reinforcement) – $70K owed by sub contractor to Multiplex.

Other stuff I have found out along the way.

The deflection they are using does not correspond to the 1/1000 (deflection to span) in the Australian codes and is currently twice what it should be.  I was having a real issue bringing the deflection down – it appears they didn’t bother.  The reason the best way to do this is add more PT!

The sub-contractor has increased the strength of the concrete in the transfer beams from 40 to 65 KPa.  Presumably to account for the punching shear problems that I was having.  However, Multiplex pay for the concrete and not the sub-contractor so this is another additional expense that Multiplex should not be liable for.

Potential talking points

• Benefits/risk of design and build
• Benefits of technical expertise in decision making
• Oversight of sub-contractors
• Ethics in the construction industry.
• The sub-contractor was repeatedly asked what were the impacts on the load changes but failed to respond.