Safety factors for compression 1

[sedesigner06]

AISC 13th edition table 4-4 uses 0.9 for safety factor in compression. Table 4-15 uses 0.75 for safety factor in compression. Does anybody know the reasoning behind this?

I have a 4 x 4 x 1/2 column that is real close to capacity, my thought was if I filled it with concrete it would give me some added comfort. Guess that's not that case, granted I know it will have more capacity just not by design calculations.

Thanks in advance for any feedback.

 

[sekwahrovert]

You're looking at the design aids rather than the specification section of the AISC manual. You can confirm the phi factor for steel compression members = 0.9 in section E1 (page 16.1-32). The phi factor for composite steel & concrete compression members is given as 0.75 in sections I2.1b (page 16.1-79). Note that this section treats concrete encased sections, but the code section treating concrete filled sections refers right back to it (page 16.1-82). I didn't find any explanation in the commentary for the phi factors, but it seems reasonable that since your section includes different materials, the member's performance may have something to do with how well the concrete and steel portions of the section are connected, so the phi factor may need to be a bit lower than for steel compression members. That's just speculation, though.

 

[Hokie93]

The following comment on the safety factor for composite column members is found in Chapter 16 of "Steel Structures: Design and Behavior", fifth edition, by Salmon, Johnson, and Malhas.

"The resistance and safety factors adopted for composite columns are rather conservative in order to account for the uncertainty of composite columns and the use of ultimate strength of two different materials in defining the capacity".

I suspect this matter is addressed in AISC Engineering Journal papers if you are interested in further explanation. Look for papers by Richard Furlong and SSRC Task Group 20.

 

[RPMG]

The capacity increase should be small. You're adding 3" square concrete that is non-composite (it shrinks) with the steel, and decreasing the radius of gyration.

 

[dhengr]

Sedesigner06:
Are safety factors and Phi factors (resistance factors) the same thing? What are their definitions and purposes, and the differences btwn. the two?

 

[Thawkes]

In ASD design, nominal capacities are divided by safety factors (omega in AISC). In LRFD design, nominal capacities are multiplied by phi factors. They serve essentially the same purpose, but they are not interchangeable. If you have calculated your demand using ASD load combinations, you have to compute ASD capacity by dividing the nominal capacity by omega. I hope that made sense.