Initial Service Design in RAM Concept

Question

Hi, 
 I have a transfer beam with unexpected reinforcement in Initial Service Design. A simply support transfer beam 2000Wx600D, with 3000 kN transfer load in the middle, provided with 6x5s 15.2mm bonded PT system (need to provide this much tendon since the deflection limit is tight). Everything seem fine except bottom reinforcement in Initial Service Design, which the moment from Initial Service LC is -1070kNm but the reinforcement from RAM show is 16N36 on the bottom layer . (I checked the Calc Log with no warning or Error). 
 - This is PT layout:

- And the Bottom Initial Service Design result, along with Bending Moment diagram: 
 
 - To have a better view, a have the result in Strength Design on bottom layer (The moment is +4970kN and 16N36 on bottom layer, which is expected!) 
 
 Is there anyone face the same issue? 
 And could there be a reasonable explanation? 
 Thanks in advanced!

Seth Guthrie · Accepted Answer

In AS3600-2009, the Initial Service code check for a PT slab is code section 8.1.6.2. In the Concept manual, section "60.6.6 8.1.6.2 Transfer Compressive Stress Limits" we give some details about who this is implemented. Here's a copy of that, I think the 4th bullet is probably most relevant for you: 
 
 The second clause in 8.1.6.2 is implemented. 
 The cross section is analyzed as cracked. This analysis is somewhat more conservative than the gross section calculation suggested in the code. 
 The concrete compressive stresses are limited to 0.5 fcp for all cases. 
 If the limit stress is exceeded then reinforcement is added as required to limit the concrete stress; depending on the bending moment and axial forces, either compression reinforcement, tension reinforcement or both will be added. 
 For sections with multiple concrete strengths, the section limiting stress is reported as the smallest (absolute value) stress limit of all of the individual concrete areas.