ASD

Allowable Stress Design (1989 9th Edition AISC Manual) or Allowable Strength Design (2005 13th Edition AISC Manual). Both use service level loads and a safety factor to member strength.

WSD

Working Stress Design (not used in design anymore). Uses services level loads and a safety factor to member strength.

LRFD

Load and Resistance Factor Design. Uses factored loads and applies a reduction factor to member strength.

LSD

Limit States Design. A design methodology where different failure mechanisms or states are checked and allowable strengths for each failure mechanism or state are determined. The controlling limit state is normally the one that results in the least available strength. This is more of a general term and includes ASD ’89, ASD 2005 and LRFD.

Strength Design = Generally refers to LRFD however the most new manuals which include ASD could be considered strength design methods as well. Meaning stresses are typically not calculated anymore…well they are but the end result is usually in terms of a members strength. In concrete you may also hear the term Ultimate Strength Design (where the old ’63 code used Working Stress Design) which is referring to LRFD.

Ultimate or Strength Level = Generally strength or ultimate level loads refers to Factored Loads in LRFD design. Ultimate capacity is generally the Factored Resistance or Capacity of the member being designed with LRFD.

Service Level = Generally service level loads are used with ASD methods. They are also used when checking deflection for serviceability.

Nominal Strength = This is the strength of the member for a given limit state before any safety factor or reduction factor is applied to the member. This is used with ASD or LRFD and is normal given in manuals that present a “Unified Approach” aka they give you a nominal capacity then  the user applies a safety factor or resistance factor.

Available Strength = This is the strength of the member based on the nominal strength reduced by the applicable safety factor or reduction factor. In LRFD it is common to refer to this as the Ultimate Strength. In ASD it is commonly referred to as the Allowable Strength.

Required Strength = This is the strength required based on the applicable ASD or LRFD combination. The required strength should always be less than the available strength.

Resistance Factor = The reduction factor applied to the nominal strength as used in LRFD.

Safety Factor = This is the factor which reduces the nominal strength as used in ASD.

These terms can be confusing when your fresh out of school. Most likely in school you predominantly used LRFD design. However when you show up to work you may find some who still use a lot of ASD. Or you may see alot of old ASD example problems or even need or want to use it in your design. I will try to clear some of this up for you.

ASD can mean either Allowable Stress Design or Allowable Strength Design. The Allowable Stress Design is the older or original designation which was used in the 9th Edition of the AISC Steel Construction Manual (1989 AISC) and the old ACI Concrete code (called Working Stress Design. Side note: working stress design can be helpful in reducing cracks and crack size. Therefore the method is sometimes still used in water applications). In these codes service level loads where applied to members. The stresses in the members where found and then checked against an allowable stress value which had a safety factor incorporated into it. Many ‘old timers’ will say that this used to give you more of a feel for the design as you better understood how the material and members where stressed. Allowable Strength Design (2005 AISC) – was mostly developed so that engineers who did not want to use LRFD could still use ASD and service level loads therefore both the ’89 ASD and ’05 ASD both use the same load combinations. It differs from the allowable stress design in that it is a ‘Strength Design’ methodology. The ’05 ASD uses safety factors on the nominal strength of the member based the particular limit state. The 05′ ASD allowable strength values maybe transformed into 89′ ASD stress values by factoring out the appropriate section property. Both ASD methods utilize Limit States Design however they are ‘hidden’ in the ’89 ASD code. Meaning that in the ’05 ASD each limit state is checked (i.e. yeilding, local buckling, lateral-torsional buckling, etc.). In the ’89 ASD code the allowable stress is reduced to the lowest applicable limit state. They also both take advantage of inelastic behavior in some limit states.

LRFD refers to Load and Resistance Factor Design which is also a Limit States Design methodology. This method uses a load factor to ‘factor up or down’ service level loads and also reduce member strength based on reliability and statistical data. When using LRFD you must design the strength based on the LRFD load combinations and factors however deflection should be based on service level loads, so you must keep track of your loads!

In the 2005 AISC both the ASD and LRFD methods for determining nominal strengths are presented side by side. The nominal strength will be the same for both methods and only the allowable strength will differ due to the fact that the safety factor applied for ASD and the reduction factor applied for LRFD will be different.

So why the switch, whats behind it? LRFD is a more reliable and statistical based method for predicting loads and material strengths. Whereas the allowable stress saftey factors where based on engineering judgement and past experiences. It is debated which will give you a more efficient design however it seems in most situations LRFD will produce a smaller sized beam based on strength but not always. Also serviceability and deflection control many designs, in which case both methods will yield the same result as the design is not base on strength at that point.

Check the code you are using for ASD safety factors/combinations and LRFD factors/combinations i.e. IBC, ASCE, ACI, etc.

Also see Chapter 2 of the 2005 AISC manual for further discussion.

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There are probably three major or general rules to follow:

1.) Visualization. Visulize the load path and visulize the detail. You must understand where the load is going and how you plan to resist it. Also your details must be construct-able. Visulize how the building will be built as you are performing your design.

1.) Be consistent. Try to have a consistent design. Be consistent in your in notes, in your  nomenclature and details, and even in your calcs. Make a decision and follow through. Don’t use one method one time and for the same situation use another. Keep it as consistent as possible.

3.) CYA – I hate this part, really do, but you must Cover Your Ass! Engineers are forced to be part lawyer. It sucks but it’s the reality of it. Follow company protocol and learn how to handle different situations. If you work at a small firm there is a good chance you will get direct calls from the contractor even when they are not supposed to talk to you.

The following are other hints and tips:

1.) Is there torsion on the that beam? Many times lintels will be supporting brick which is offset from the center-line of the beam. This torsion must be accounted for. For steel beams see AISC Design Guide 9.

2.) What is the unbraced length of that beam? For all beams write down the unbraced length down so that you do not over look beam stability.

…more to come soon.

An interesting read:

http://www.aisc.org/WorkArea/showcontent.aspx?id=18544

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Various References by Subject

I will try to include texts and other articles which I have found handy thus far in my ‘career’. I will update this from time to time as we cover more topics.

Earth Retention and Soil Mechanics:

One of my favorite general references, especially for basic concepts in earth retention is “Basics of Retaining Wall Design” by Hugh Brooks and John P. Nielsen.

http://www.amazon.com/Basics-Retaining-Wall-Design-Edition/dp/0976836416

http://www.vulcanhammer.net/ maybe the only reference your really need.

Winterkorn and Fang, “Foundation Engineering Handbook”

Buy here: http://www.amazon.com/Foundation-Engineering-Handbook-Hans-Winterkorn/dp/0442295642

Bowels 3rd and 4th editions are also very good I’m sure they all are. Fifth Edition (I think): http://www.amazon.com/Foundation-Analysis-Design-Joseph-Bowles/dp/0071188444/ref=sr_1_1?s=books&ie=UTF8&qid=1332558674&sr=1-1

The problem is that in soil mechanics many theories apply sometimes and this may seem awkward at first but the more you get into it the more this makes sense. In my opinion engineering judgement is really of top importance in geotechnical work. For this reason I own more geotechnical/earth retention/soil mechanics books than any other subject.

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