## Errors while designing a column (aci SP17)

## Errors while designing a column (aci SP17)

(OP)

Hello all,

I have a question, I want to solve this problem using the same procedure as SP17 but once I'm done and want to verify my answer and find the column axial load capacity it comes out as inadequate. I'm aware if i dont multiply by 0.65 and 0.8 and rather use phi = 0.7 it comes at at the borderline. but how did aci solve it like that in their design aid? am i missing something?

so is it correct like that at 4% reinf? please help

I have a question, I want to solve this problem using the same procedure as SP17 but once I'm done and want to verify my answer and find the column axial load capacity it comes out as inadequate. I'm aware if i dont multiply by 0.65 and 0.8 and rather use phi = 0.7 it comes at at the borderline. but how did aci solve it like that in their design aid? am i missing something?

so is it correct like that at 4% reinf? please help

**my problem:**

**used interaction diagrams:**

## RE: Errors while designing a column (aci SP17)

Similarly the 60 ksi bar strength is only 413.7MPa, not 420MPa. The interaction diagram way of working out strength is quite crude by its very nature, the errors in estimating a reinforcement ratio and slight differences in setout of reinforcement, etc, all have a corresponding effect on the calculated capacity when compared to a first principles analysis of the cross section accounting for the location of each bar and the actual strength of each bar. For example based on your K_n value of 1.4 the reinforcement ratio is actually more like 0.039 or so, little under or over estimates will mean you'll potentially never match exactly.

I think if you divided the axial load by phi = 0.7, then you should be using the same phi for working out the axial strength (i.e. not the 0.65 x 0.8). Otherwise you're comparing a reinforcing content worked out using 0.7 to a capacity applying alternative 0.52 factor which makes no sense. This is why using 0.7 gets you pretty close.

I find generic interaction diagrams are only useful for preliminary estimates of strength, ballpark calculations to establish initial member sizes if you like.

They are simply too inaccurate for real design or are basically useless for biaxial loading scenarios in my view.

You can however write a spreadsheet to work out the capacity based on a given reinforcement arrangement and column orientation, or resort to commercial software, you can even use this to generate your own interaction diagrams for say 420MPa vs the 413MPa. It's a useful exercise to familiarise yourself with the process of determining the section capacity.

## RE: Errors while designing a column (aci SP17)