ACI Unified Design 2

[SKJ25POL]

May I ask is the concrete/ACI "Unified Design", exactly the "Working Stress Method"?
I believe it is not the USD (Ultimate Strength Design), right?

Any articles or source you know or you can share? Just never heard of this design philosophy!

Thank you

 

[rapt]

My understanding is that it is a unified approach to the design of RC, PT and compression controlled members using USD.

The old steel ratio logic for ductility does not work well with mixes of steel types and for compression members, so now it is based on strain limits.

 

[BAretired]

Unified Design is said to yield the same results as Ultimate Strength Design, but is thought by some (not by me) to be more intuitive than current methods. I have never used it, so don't really know much about it, but the following is a link to an article on the subject.

https://www.sciencedirect.com/science/article/pii/S2214399815000077

BA

 

[SKJ25POL]

Anybody has used ACI Unified Design Method in their design?

I am still confused that how it differs USD and WSD?
Where do we do different ?

Thank you

 

[BAretired]

Read the article I posted. It explains the differences.

BA

 

[RPMG]

Working stress design is the ASD method for CMU, previously used for concrete design. It assumes an elastic distribution with a triangular compression block and allowable service level stresses in steel. It was last published in the appendix of ACI 318-99, which you can probably find for free online.

The current method, LRFD, focuses on failure modes, strength level (not elastic), and assumes a rectangular Whitmore compression block.

 

[r13]

In my opinion, the concept of unified design approach (strain centric) can be applied to both ASD and USD, however, this paper focus on formulations to replace USD only.

 

[IDS]

BAretired - thanks for the link, but according to that, "unified design" is just the approach taken in recent versions of ACI 318, i.e. design for reinforced beams, compression members, and prestressed members are all covered by the same basic procedures, based on strain limits for strength design.

The paper provides a number of closed form solutions for reinforcement requirements which are quite useful, and seem to be basically the same as procedures I had developed for myself, but I can't see that he provides any great intuitive insight.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BAretired]

IDS,

I haven't looked into it in detail, but would agree there does not seem to be anything very unique about it, yet if the author is accurate in his predictions, it would seem to be about to replace traditional design in concrete codes of the future.

His concluding remarks:

A series of ideas and formulas for the flexural design of reinforced concrete sections have been presented. They are an attempt to reconcile the so-called unified design method (UDM) with the ACI traditional approach to design. The UDM relies on strain limits for the characterization of the behavior of reinforced concrete sections. The traditional ACI approach to design relies on the simple concept of reinforcement ratios. The formulas and ideas presented in this paper bridge the gap between these two approaches by allowing the treatment of reinforced concrete sections in a much simpler way. These formulas and ideas should also prove useful in the classroom since the concept of reinforcement ratios is more intuitive and therefore more pedagogically appealing than that of strain limits. Many of the formulas presented in this paper are well known, but some are new. In particular, the formula for the compression-controlled reinforcement ratio limit and the procedure for the design of transition-zone sections are new.

I guess we ought to give it a chance. Perhaps we will grow to love it.



BA

 

[IDS]

BA - I'm certainly no authority on design to ACI standards, but the way I read it was that the UDM was already in place (and has been since 2002), and he was comparing that to the approach in previous versions.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

Before the UDM, ACI had separate rules for RC members, PT members and Compression members. But there is always an overlap and the rules were often inconsistent.

Eurocode and Australian code (since 1988) have had unified rules for RC and PT members. They just did not see the need to give it a name. The respective codes were for the design of structural concrete members. Not Reinforced Concrete Members and prestressed Concrete Members. They were simply concrete flexural members with different types of reinforcing and partially prestressed members contained both types. Where modifications to the rules are required for one member type or another in a specific area (eg minimum reinforcemennt and crack control), that was simply covered by an exception.

Similarly, flexural members can have significant levels of compression. This now gives a way to cover that type of member using the common rules without calling them Compression Members.

It is not rocket science. Just logical.

 

[BAretired]

IDS,

I was not aware that UDM was introduced into the ACI code as early as 2002 but the abstract confirms that. The paper was dated December 2015, so I assumed it was more recent. Having never used the UDM, I am not familiar with its details.

BA