Allowable strength/stress design 1

[Settingsun]

Allowable str* design came up recently in the structures forum. Purely out of interest since I'm not in America: can you still use the older allowable stress method? AISC 9th edition IIRC from casually observing other topics on this forum. Or would you always use the allowable strength method from the current version of AISC 360 these days?

Also, does anyone in the US use allowable stress methods for concrete design, or is that fully limit state now?

 

[JAE]

In the US the Standard Operating Procedure is this:

1. In major cities, the city will adopt a model building code. This is usually some version of the International Building Code (IBC)
2. Each version of the IBC has a reference section (Chapter 35) where it lists all of the external standards, codes and specifications for the various materials and topics.
3. Each of these references includes the particular year or edition of the document that is applicable.
4. One of these external references is the AISC steel specification and it will designate the applicable version/edition of the spec that is dependent on that code.
5. So for example - the IBC 2012 code might be adopted by a city. In Chapter 35 of that code is refers to AISC 360-10 as the governing steel specification. That is what you'd use.

Now having said all that, the building code is a minimum standard and an engineer is always OK to use whatever they'd like to use - providing they still meet that minimum standard.

In AISC's specification, they still equally maintain both ASD and LRFD - both based on a nominal strength. They no longer include general provisions for ASD (allowable STRESS design)

IN ACI for concrete - there is only LRFD as ACI has walked away from continued support of Allowable Stress Design. ASD used to be in the code, then moved to the appendix, then only included in a commentary referring back to an earlier version in ACI 318-99 I believe. Now it isn't included or mentioned at all as far as I know.



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[Settingsun]

Thanks, JAE. Sounds like the authorities are on board with limit states but some resistance from practising engineers. I haven't come across this elsewhere but maybe the change to limit states was more recent in the US. 1990 in Australia for steel.

 

[BUGGAR]

We no longer use elastic analysis for deflection calculations.

 

[Settingsun]

How do you calculate deflection?

 

[ajh1]

generally speaking you should always be using the latest edition of the standard. There were a number of issues with the older allowable stress design as that standard was not kept up to date for many years until the introduction of the first ASD/LRFD AISC standards. That said, the real difference overall between allowable stress design and allowable strength design is simply "are you working with stress or are you working with total capacity?"

 

[BUGGAR]

We're designing suspension parts for off-road racers. Doesn't matter how much the part bends under normal conditions, it's how it bends in failure (we mostly design to energy input requirements). Think earthquakes.

 

[Settingsun]

OK Buggar. Possibly could have included that information in your first post.

 

[BUGGAR]

Sorry. The Navy also designs prestressed concrete piles for plastic deformation considering the stress-strain curves of the actual prestress steel as tested, and "special" hi range and low range values for the concrete, depending on its lateral restraint in the pile.

 

[Settingsun]

Surely that's ultimate limit state though. Can't imagine they want service loads traversing that territory.

 

[JoshPlumSE]

I believe there may still be some "specialty" codes that are based on ASD 9th edition. Codes that don't update very often. Maybe tank design, or offshore or such.

I don't work in any of those specialty areas, but my guess is that even these industries have not mostly come around to strength design. They may not update their codes very often, but it's hard to still justify basing your code off a steel code that came out 30 years ago.

Serviceability checks still are done to unfactored loads, so my guess is any remaining hold out industries are the ones that are more concerned with serviceability issues (deflection restrictions, slip, maybe fatigue) than strength.

 

[Settingsun]

That's similar here in Australia. The old allowable stress steel building code (AS1250) was withdrawn when the limit state code was introduced and our national building code refers to the limit state code, so allowable stress design is completely gone for buildings - no ambiguity. AS1250 was however renumbered as AS3990 and that code is still current for mechanical design, and rarely updated (if ever).