Stiffness Modifier 1

[rummaan17]

Dear Members,
I would like to have your feedback on the use of stiffness modifier. We generally use the value dictated by ACI committee as to model the cracked behavior, which are
Compression Member : Wall, Column, Pier = 0.7
Flexure Member : Beam, Spandrels = 0.35
Slab = 0.25

What if i want to use 0.5 for columns or 0.2 for beams. Are there any restrictions. I have played around with it on software like ETABS and noticed change in flow of forces as it transfer towards more stiff element. Your kind comments would be highly appreciated.

Thanks

 

[JAE]

You can certainly do a parametric design where you alter the stiffnesses and use the worst-case results.

Using "more cracked" less stiffness in your overall frame increases your second order effects and usually results in a more conservative design.
But as you note, there could be conditions where you alter the stiffness of one element an actually underestimate the load that it receives...so be careful that you don't under design something by making it a wet noodle in your analysis.

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[rummaan17]

Dear JAE,
Thanks for your response. As you mentioned that i should be careful that I don't end up under-designing something. How about walls, mostly when I assigned opening I usually find small piers corner failing at the edges. What If want to alter its stifness so that It can transfer load to the other end. Possible ? Wall core is a stiff element and will reducing stiffness at one corner can result in safety issue ?

 

[JAE]

ACI provides a general measure of member stiffness (beams, columns, walls) using the Chapter 9 equation for modified moment of inertia, Ie.
This is an attempt to measure, as close as practical, the stiffness as it is reduced under moment, and possibly helped under axial compression.

The approximations you list in your first post are general reductions found in frames which can be used to account for second order effects of drifting.

For walls, as you mention, at openings and irregularities, it probably isn't wise to just randomly change these values to get a design to "work".
Rather, I think you can adjust groups of members down, and up, to get a worst case scenario.

This means you don't adjust relative Ie values between members to get a particular design to work.
Instead, due to uncertainty in knowing what Ie may actually be, you would set Ie values at very low, and also at very high values and design each member for the worst case in that range.

I'm not sure if the above answers your specific question or not. If you provide more information on what you are doing perhaps we can further discuss.



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[KootK]

JAE's got the measure of this. When you make stiffness alterations, do so because it makes sense structurally, not merely because you're trying to game the system to make things work that don't. If you can accomplish both purposes, then great. We do similar things with exterior columns all the time. But, in those instances, there is genuine logic to support the change in stiffnesses assumed rather than simply a desire for a different outcome.

How about walls, mostly when I assigned opening I usually find small piers corner failing at the edges. What If want to alter its stifness so that It can transfer load to the other end.

This is actually a pretty common problem. What can be done about it rationally is a function of what the failure mode(s) is for the small pier. If it's primarily an axial load failure, then I would re-proportion the pier to make it work. If it's primarily a shear or bending failure, I'd consider allowing the small pier to redistribute it's bending to other, stiffer wall piers. Then, capacity design the pier for shear base on the axial load and the vertical reinforcing provided.

Another alternative is to use non-ACI recommendations for stiffness. Canada's concrete code, for example, presents flexural stiffness on a sliding scale based on what the axial demand is.

See pages 21 & 26 of this document for some related discussion. It doesn't answer your specific question but discusses similar issues in a highly rational manner.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rummaan17]

Dear Jae and Kootk,
Thanks alot to both of you for adding value to this post with your kind comments. Was really helpful.

JAE/KootK my case is that sometime I see walls piers Or spandrel failing in shear, when I am using modifier PIER=0.7 and Spandrel=0.35. Then I lower these value for PIEr to 0.35 Or spandrel to 0.25 which result in a safe design. Is this correct?

According to me I am using the value mentioned in the code. ACI TABLE 6.6.3.1.1.(b) dictates that the minimum value also that can be used

 

[KootK]

Then I lower these value for PIEr to 0.35 Or spandrel to 0.25 which result in a safe design. Is this correct?... ACI TABLE 6.6.3.1.1.(b) dictates that the minimum value also that can be used

My interpretation of the alternate table is different. I believe that, if you use the alternate table, you have to use the values in the "I" column based on the the axial load / reinforcement ratio etc. You only get to use the minimum value if those formulas predict stiffnesses equal to or less than the minimums. That said, historically, 0.25 has been a pretty common value to use for coupling beams.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rummaan17]

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If it's primarily a shear or bending failure, I'd consider allowing the small pier to redistribute it's bending to other, stiffer wall piers.

Can you please tell how your redistribute. Thanks in Advance.

 

[KootK]

Can you please tell how your redistribute.

Take the shear that cannot be resisted without failing the pier in flexure and relocate it to some other pier(s) in a logical manner.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rummaan17]

Take the Shear without failing the pier ? can you explain a bit in detail, would be helpful.

 

[KootK]

1) Figure out how much flexure the pier can take.
2) Capacity shear design the pier for the shear associated with the moment from #1.
3) Figure out the lateral shear associated with the moment from #1.
4) Take the shear demand on the pier, subtract the shear from #3, and assign that shear to other wall segments.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rummaan17]

Thanks for help KootK.

 

[KootK]

You're most welcome rummaan.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.