ETABS Column "To be checked" vs "To be designed" options 5

[CloudSan1]

Hi everyone!

I have slight problem with etabs column design result. When I designed my columns using the "to be designed" option, etabs gives an area of steel of around 3,400mm^2 on a 250x500mm column, but when I use the "to be checked" option and I input 10-16mm rebars equivalenting to around 2011mm2 of steel area, the column still passes the design.

How is that possible? The "to be designed" requires 3400mm2 of steel area but the "to be checked" with 2011mm2 of steel area still passes.

Is the "to be checked" method reliable?

Thank you in advance for your replies.

 

[milkshakelake]

ETABS is good for steel design, but it gives weird designs for concrete columns and beams. For concrete, I only use ETABS to get the forces and moments, and I design columns using those forces in my own spreadsheet.

 

[JoshPlumSE]

I don't yet have sufficient knowledge of how the reinforcement design routines in ETABS work. Therefore, I can't comment with any authority on the reason for the behavior you describe.

However, I was pretty involved in creating the design routines for reinforcement when I worked at RISA. Based on that knowledge, I can say pretty definitively that this sort of thing is MUCH more difficult to do. Some thoughts on why:
1) There is a near infinite way to get a concrete member to pass. Maybe it would pass with 14 relative small bars, 8 intermediate sized bars or 4 very large bars. Which one of these is the most efficient arrangement? This is a tricky decision for a computer to make.

2) Multiple design code requirements that can be tough to reconcile programmatically. You've got minimums and maximum areas of reinforcement. You've got minimum spacing to allow for lap splices and aggregate size and such. You have maximum spacing for temp / shrinkage, a different one for crack control, another if you're talking about skin reinforcement. In what order do you enforce these requirements? Because that can really affect the final design the program picks.

3) At RISA, the design routines had a bias towards larger bar sizes with fewer numbers of bars. However, after the fact, there was a check for "crack control" requirements.... which is based on spacing. The design routine could start with a choice between 5 # 5 bars or 2#8 bars and decide that 2#8s was more efficient. Then, check to see that crack control requirements require a tighter spacing and settle in on 5#8 bars! when 5#5 bars would have worked perfectly well! I don't know if RISA still does this (hopefully not), as it's been almost 4 years since I worked for them. However, I mention this because it points out a real life example of what I was saying with items 1 and 2.

Caveat:
As I said before, I used to work for RISA. After I left (and after a 1 year attempt at doing OSHPD work) I went to work for CSI (which is the company that writes ETABS).

 

[MSUK90]

CloudSan1, I also face the same issue with etabs. Update here if you get the solution for this issue. JoshPlumSE reply helps but I am not sure if etabs works the similar way..

 

[skeletron]

Maybe a good thing that it is unclear, so that you can perform secondary calculations to check & finalize design.

 

[CloudSan1]

Thanks for all the comments.

MSUK, I haven't find the exact answer to the question yet, but what I did was perform a secondary calculation like skeletron said, using either manual calculation or other designing software like ASDIP Concrete where you can check a column individually.

 

[MSUK90]

CloudSan1, that is possible for simple and small projects but in case of projects of considerable size with a lot of columns, it will be almost impossible to make manual checks for each and every column.

 

[MotorCity]

The program is not always as smart as a human when it comes to optimizing the design. The program provides a design that works. The human provides the optimal solution. This is a great example of trust, but verify computer output.