# Plastic section modulas vs Plastic design

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QUESTION
I am looking for some clarity on Section F2 of the 13th edition of the AISC. I am designing a compact I shape with the Mn/omega = FyZx for yielding and my unbraced length lies between Lp and Lr. It is a simple beam with governing wind loads applied to the strong axis.

Reading further commentary, I understand that with the unbraced length falling between the Lp and Lr you are looking at inelastic LTB where as if your unbraced length was above Lr you would be using elastic LTB analyis.

I also understand that the yield equation uses the plastic modulas as opposed to the section modulas. My understanding is that you can use the plastic modulas in ASD which is typically elastic design, but by doing so you are designing above the yield pt. In speaking to another engineer, he said that using the plastic section modulas is:

1. not permitted in a determinate design???
2. meant to be used in plastic design analysis only with plastic hinges?

I understand his position but it seems as though the 13th ed is intermingling the use of plastic design and elastic design in bending moment design by applying the plastic modulas in an ASD design based on material limit states. So does this mean that I can use plastic design on a determinate structure?? Where is the line drawn between the two design criterias?

I looked further into inelastic (plastic) design and it states that inelastic design is not permitted for ASD except as provided in section 1.3 (moment distribution) which goes on to say that gravity loaded compact sections satisfying unbraced lengths in section 1.7 and with stipulations of increasing the positive moment by .1 of the average neg moment is permissible.

Since it is a lateral wind load design, I can’t use the plastic modulas? Is this a correct assumption given the information provided above? Is there any other information that may provide me with a different conclusion?

REPLIES

Lion06
Even in the old 9th ed. ASD (REAL ASD) the design equations took into account the extra capacity of the plastic section modulus. The basic allowable bending stress was always 0.6Fy. AISC recognized, however, that for compact sections that were adequately laterally braced that you can take advantage of the plastic section modulus. Typical shape factors (Z/S) for rolled shapes are in the 1.12 to 1.18. AISC said it will use 1.10. This is where the allowable bending stress of 0.66Fy comes from - 1.1*0.6Fy = 0.66Fy.

Now, in the 13th edition, AISC is saying you can use the real shape factor (or real Z) instead of a generic 10% bump, but that only applies if you’re unbraced length is adequate to develop the full plastic moment.

For your reference, here is a link to AISC’s 14th version of “DESIGN EXAMPLES For Use in First Semester Structural Steel Design Classes”: