ACI 318 Load combinationS for POST-TENSIONING: Where? 1

[DavidStructural08]

I am using ADAPT and SAFE PT to design post-tensioning and found discrepancies for the load combinations for serviceability.

ADAPT:
1.0D+0.3L+1.0PT

SAFE:
1.0D+0.5L+1.0PT
1.0D+1.0L+1.0PT

Both programs refer the comb. to ACI chapter 18. However, after checking ACI 02, 05, 08, I must say I could NOT find any specific clauses in ACI which clearly list the load combinations, either service or strength states.

Does anybody know where in ACI 318 we can find the load combinations for PT?

 

[structSU10]

From the ADAPT website

Technical Note on Impact of ACI 318-2002 and 2005 for PT Design
Explanation of reference to 0.3 Live Load Coefficient for the Service Load Combination

This addendum explains the background behind the use of 0.3 as the live load coefficient in the Technical Note published in the January 2005 ADAPT Newsletter. The paragraph in which it was referenced is reproduced below:

(i) Up to 1999 code, the prestressing force had to meet tensile stresses for the service load combination (1xDL + 1xLL + 1xPrestressing). The live load coefficient is now reduced by 70% to 0.3. As a result, a lesser amount of prestressing will satisfy the stress requirements of the code.

ACI 318-02 background to load combinations.

1 - Most major non-North American building codes, such as the European Code (EC2) have two load combinations for the gravity design of post-tensioned members. One is referred to as "quasi-permanent," and the other as "frequent." The "quasi-permanent" is the loading that the structure is likely to experience in service, under normal conditions. This is the loading that is used to predict the creep and long-term deflection of the structure. The load combination for the quasi-permanent load case is:

1xDL + 0.3xLL + 1xPT

2 - ACI 318-02 (Chapter 18 - allowable stresses) has introduced the concept of multiple load combination for service condition, namely "sustained" and "total" load cases. ACI 318 associates the "sustained" load case with the one to be used for the determination of creep deflection, and specifies a different allowable compressive stress for it. However, it leaves it to the judgment of the designer to select the appropriate live load factor.

3 - ACI 318-02's "sustained" load condition, in concept, is the same as the European's "quasi-permanent" load condition, with the difference that the European code explicitly specifies the load factor.

4 - In ADAPT's Newsletter, the specified load factor of the European Code (EC2) was intended to be used for the condition, where ACI 318-02 leaves the selection to the engineer's judgment. But, since ACI 318-02 does not explicitly extend the dual load combination to the "tensile" stresses, the load combination given in the Newsletter can not be applied universally without extending the intent of the ACI code. For this reason, it is prudent to use the service load combination of 1, 1 and 1 that has been the practice among design engineers for many years.

Therefore, when designing post-tensioning according to ACI 318-02, use the following load combinations:

1xDL + 1xLL + 1xPT for "Service Condition"
1.2xDL + 1.6xLL + 1x(Hyperstatic forces from PT) for "Strength Condition"

 

[DavidStructural08]

Thanks structSU10. After reading this note, my understanding is that:

the 0.3 factor in ADAPT and the 0.5 or 1.0 factor in SAFE are the program selection. In other words, they are the corresponding engineers' selection, NOT something ACI 318 explicitly specified.

That being said, SAFE might yield more conservative design than ADAPT.

Am I right???

 

[RobertHale]

For what it is worth the SAFE numbers seem to be in alignment with the serviceability load combinations suggested by ASCE 7-05 (or 7-10) Appendix C commentary. When I use ADAPT I use 1.0DL + 0.5LL + 1.0PT for the sustained load combo, and 1.0DL + 1.0LL + 1.0PT for the total load combo.

 

[rapt]

Clause 8.2 covers this referring you to ASCE/SEI 7 and the general building code!

My understanding for ACI318 design for service crack control design is it is based on a short term (Frequent)loading of
1.0 SW + 1.0 SDL + 1.0 LL + PT effects

Deflection then uses short term and permanent loading cases if it is done properly (not so in Adapt) with the permanent loading used to determine creep and shrinkage effects and the short term loading for the final total deflection.

The Eurocode combinations do NOT apply to ACI code and cannot be used for it!