When to start using new AISC manual

[steve1]

There has been some discussion in our office over when to start using the new AISC manual. There are two schools of thought. The first is that we should start using the new manual NOW. That is we should specify steel sizes and follow the specification in the "latest" edition availiable. The second thought is that the "correct" edition of the manual is the one specified in the building code that we are designing to. It may be acceptable to use the new manual for "sizes" but that the steel specification to follow is the one called out in the aplicable building code. Anyone else have these "conversatitions", and how did you resolve them.

 

[WillisV]

In my experience we typically use the edition that is called out in the applicable building code with the caveat that if we know a certain provision has been revised due to the fact that it was unconservative then we will use the revised provision to provide a better standard of care.

 

[JAE]

For us...same as WillisV - and we do take note of the current size availability which is especially key in this latest manual.

 

[UcfSE]

I agree with Willis and JAE, and add that I will use the new manual in the event it covers an issue not addressed by the referenced manual.

 

[steve1]

One of our engineers sent a message on this subject to the AISC and received the following reply:

"That depends on your contract documents. If the 2005 AISC Specification is cited in your contract documents, then one should use the 13th edition AISC Manual.

If you are not bound to contract documents (the owner is your company, not an outside entity), then it is your choice of when to upgrade. AISC always recommends the use of the latest available design information.
You may find that the latest requirements will lead to more economical designs in some cases."

 

[aggman]

I received my new manual a couple of weeks ago. I have been working ASD 9th more or less side by side checking it. Technically I am still using the 9th edition and will continue to do so until the codes specify otherwise. But I am working with the new edition and designing using it. To be honest I am not in love with the new beam design algorithm. It's not using the plastic section modulus that's bothering me. (I clearly understand that's what we have always been doing) I don't care too much for the moment capacity verses unbraced length equations. They are a little long for my taste. Probably not much longer than the 9th edition, but I was used to them. I also am not in love with the bolt charts either. Allowable bolt capacities are a little higher than the 9th edition and that will take a little getting used to.

 

[archeng59]

does anyone know if the new AISC manual will be referenced in the 2006 IBC? the jurisdictions I'm working in are still using the 2003 IBC, so I have not looked at the 2006 IBC yet.

 

[JAE]

Yes, the 2005 AISC Specification will be referenced in the IBC 2006. But the manual is not a code/spec entity so it is simply a tool to assist you in meeting the AISC specification which is a legal document once a city/county/region adopts the IBC 2006.

 

[SlideRuleEra]

This discussion is very interesting. The steel design that I perform these days is very limited, (a job last month had exactly one 20 ft. beam, a light-weight W12). Since Steve1 has learned from AISC that "...the latest requirements will lead to more economical designs in some cases", this implies that 9th Edition ASD calculations will give conservative results. If this is true, then it may not be worthwhile (for me) to make the transition to the new manual. I would interested in your comments on this.

http://www.SlideRuleEra.net

 

[ctcray]

I think the big difference in adopting the 2005 AISC Spec is that the code no longer allows the choice of using ASD (9th edition) spec instead of the latest LRFD {something that many engineers have been clinging to, no doubt}. The code checks are now integrated in the spec so whether you prefer ASD or LRFD, you do the same calcs then apply a Load Reduction Factor or a Safety Factor depending on whether you are using Factored or Service Loads.

That being said, if you design by ASD (9th) for one beam per job, it's probably not going to cause you any problems.

 

[JAE]

The new manual contains both ASD and LRFD and the specification within the manual (which is an independent document as well) also deals in both design methods.

The case of economy varies depending on the ration of Dead load to Live load. LRFD being more economical for low live load conditions than the ASD and visa-versa for high live load conditions.

The "choice" as to when to use the new specification is not really up to the engineer, but rather, depends on the location of the project and what particular code governs the design. Each bulding code will reference a particular AISC specification and this will be the deciding factor as to which to use.

 

[JAE]

That should be "ratio" not ration....finger slipped.

 

[archeng59]

several years ago I did a test during a project to see which gave the most economical design when everything was considered: stresses and deflection/lateral drift. I sized the steel framing using both ASD and LRFD using 50 ksi steel. LRFD gave me smaller members most of the time. but when I checked deflections and lateral drift, I had to upsize several members to increase stiffness. those members were closer to what was sized using ASD. anyone else experienced that?

 

[JAE]

Well, the servicability requirements are independent of which system you design by so I would agree that the results would be very similar, especially if the spans and loads were such that deflection controlled a lot of the designs.

 

[archeng59]

you are correct, JAE. my point is, we select a refined and sometimes tedious design method planning to save steel for economy then are often forced to add steel for servicablity.

 

[HgTX]

My understanding was that it was not economy but some notion of better reflecting structural behavior that drove the revisions to the codes. As others have pointed out, whether this makes for heavier or lighter members depends on the situation.

Strength-based design under ASD could involve some serious beefing up for serviceability as well.

Hg

Eng-Tips policies: faq731-376

 

[DRC1]

Just went to an AISC seminar for the new manual. If you are planning to switch over, I would recomend the seminar. It is a very good overview and the concept behind what AISC is trying to accomplish is explained. If you are a member of AISC then for the price of the manual you can go to the seminar and get a copy of the manual, detailed notes on applying the new code and some other freebies. The quality of the presentation was very good. If thats not enough, they throw in lunch. Dates and locations can be found at the AISC website.

 

[JAE]

I attended it too - pretty good as far as seminars go. The part on Chapter C will need some study, though. I thought it (the Spec in Chapter C) was not set up too clear as you have three analysis methods and these are combined with two analysis types...hmmmm

 

[WillisV]

JAE - the following article does a good job of explaining the intent of what I agree is a confusing subject:

http://www.aisc.org/MSCTemplate.cfm...Management/ContentDisplay.cfm&ContentID=30513

 

[JAE]

WillisV - thanks - I've not time to read it right now but that's the sort of thing I'm looking for. I've saved a copy for later.

What I tried to resolve is how to use one of their three methods via a Pdelta analysis on one of our programs without the extra work of calculating those B factors or messing with the EA and EI of the members since the program also checks/designs the members and if I change their properties the analysis may be right but the design won't be.

For some structures, there are perhaps over 20 load combinations to check, and with N number of members, two directions of moment (x, y), and two B factors (B1 and B2) that would require 40 x N x 2 x 2 different B factors (160 x N).

Sometimes I don't think that the academians who write these specs realize what they are doing to the practicing engineer.