6.7—Linear elastic second-order analysis 2

[SPR Baker]

Dear Everyone
I am not sure about this section in ACI318-19"6.7.1.3 The cross-sectional dimensions of each member used in an analysis (Linear elastic second-order analysis) to calculate slenderness effects shall be within 10 percent of the specified member dimensions in construction documents or the analysis shall be repeated.". it means the dimension of all member need to reduce to 10% (bnew=0.1b or hnew=0.1h)? if Yes how do we reduce the dimension in CSI Etabs?
thank you

 

[JoshPlumSE]

That's not even close to how I interpret that section.

I interpret it as meaning the following:
"If you've already run your linear elastic 2nd order analysis and the member sizes change slightly, you are not REQUIRED to redo your analysis / design so long as the member sizes changed by less than 10%"

 

[SPR Baker]

Oh thank Josh ,my English bla bla... xD
it means in case our section design fail then we try to enlarge less than 10%, so we don't need to re-analysis. yet if the section change more than 10% then we need to re analysis.

 

[IDS]

JoshPlum's interpretation seems reasonable, but I'm not sure this allowance is a good idea. Increasing section dimensions by 10% in both directions increases the flexural stiffness by 46%, or reducing the dimensions reduces the stiffness by 34%.

If the section dimensions are changed, why not just repeat the analysis with the new dimensions?


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JoshPlumSE]

it means in case our section design fail then we try to enlarge less than 10%, so we don't need to re-analysis. yet if the section change more than 10% then we need to re analysis.

That is how I would interpret this provision in the ACI code.

However, IDS makes an interesting point. I would suggest that if the AREA of a member increased by more than 10%, then that would violate the provision and force you to re-run the analysis. Though a 10% increase in area can still cause a 33% increase in moment of inertia. So, my tendency would be to re-analyze the structure even if the code doesn't explicitly require me to.

Perhaps this section is intended to be "helpful" for cases where the contractor hasn't been as precise with their construction as you'd like. Then, you could use this provision to assert to the building official that you don't need to re-submit all your calculations with the new "as built" dimensions. Now, you might still run some numbers and make sure it's still safe. But, you don't have to put together a whole new calc package.

 

[EZBuilding]

I would be curious to know how far back this provision has been in ACI. I would assume it has been a holdover from back when running an analysis took a longer period of time than it currently does now with today's computing capabilities.