Prestressed Concrete- Stresses at transfer

[kww2008]

AS3600 Clause 8.1.6.2 (Prestressed beams at Transfer) refers to limiting stress at transfer depending on the distribution of stressed used in the numerical model - rectangular distribution or triangular distribution. The triangular distribution means the section analysis is uncracked SLS. Does the rectangular distribution means the section analysis uses a cracked section analysis at ULS? The code is not clear on this.

In the old NAASRA 1970 standard (for bridges), Clause 6.9.3 stipulates an upper limit of 0.60 F'cp for a triangular or approximately triangular distribution of prestress, and an upper limit of 0.50F'cp for a uniform and approximate uniform distribution of prestress, where F'cp is the minimum compressive strength of concrete required by the designer at transfer. This shows that the current "deemed to satisfy" requirement in AS3600 has been in Australian Standards as early as 1970. Does anyone know where these limits come from ?

 

[IDS]

I agree it isn't very clear, but my understanding is it is supposed to be a simple approximation using unfactored loads, intended to ensure that under ULS conditions the design capacity (with 0.6 reduction factor) will be adequate.

My copy of Warner Rangan Hall and Faulkes (Concrete Structures) suggests doing the ULS design check anyway, which seems like a good idea to me, at least for final design.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

This is not a SLS check on tension at transfer. It is an ULS check to ensure the strength as a combined compression and bending situation is ok. This will normally only be a possible problem in highly compressed members such as bridge I beams.

A separate check should be made on transfer stress at ULS. This is not specifically mentioned in AS3600 but is a logical service load combination stress check that should be made.

 

[IDS]

Hi rapt,

Have we had this discussion before? (I vaguely remember it, or something similar, but I can't find the thread).

On reflection, I think it makes sense that this check should be at ultimate loads, but it really should say so. To me "maximum stress ... under the loads at transfer" implies the maximum actual stress under unfactored design loads. But it's a bit academic since the easiest way to handle it is to do a ULS check at transfer. The commentary to AS 5100 (current version) says much the same (extract attached).

Further comments:
- Under rectangular distribution the 0.5fcp stress is a little lower than the concrete stress at ultimate load (0.6 x 0.85 = 0.51 fcp), but it is hard to see how a uniform stress will ever be critical, because there will be other sections or load cases with approximately the same prestress force but a non-uniform stress distribution and hence a higher peak stress.

- 0.6fcp with a triangular distribution will have a higher peak stress than the ultimate load, but the total concrete force will be lower for low - medium strength concrete, but higher for high strength concrete, where alpha2 and gamma2 both reduce down to 0.67.

- The uncertainties in the "deemed to comply" provisions are another reason to just do the ULS check.

- But it makes sense to limit maximum stress at the level of the tendons under SLS loads to 0.5fcp to avoid excessive creep anyway (see Cl. 3.1.8.3(b) of AS 3600).

- I note that the ULS load factor on prestress force at transfer is 1.15 in AS 3600 (Cl 2.4.2), but 1.00 in AS 5100, which seems odd.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/