2nd order over 1st order of 1.4 limit

[structural87]

Hello,

i would like to have your opinions regarding section 10.10.2.1 of ACI318-11 where it states "Total moments inluding second-order effects in compression members , restraining beams, or other structrural members shall not exceed 1.4 times the moment due to the first order effects" if it is applicable to a single element or to a whole story of a building.
my own interpratation when reading the commentary is that it is applicable to a story rather one single element.

Thank you.

 

[HTURKAK]

I understand that the code requires , the total moment including secondary moments in compression members shall not exceed 1.4 times the first-order moments calculated for that member for sway and non-sway frames. Otherwise, the column dimensions and if necessary structural system must be modified.

 

[JAE]

my own interpratation when reading the commentary is that it is applicable to a story rather one single element.

Look up at 10.10.2 where it is explicit that the provisions of 10.10.2.1 and 10.10.2.2 are for member sizing.
That said, the analysis of the structure to determine the ratio between first and second order moments is, of course, the general structure...not some isolated part of the structure.

The analyses for both first and second orders (of the whole structure) will result in two moments in any one member you look at. The ratios between them are what is used to check for stability.

 

[klaus.]

This factor between first and second order just shows how sensitives the structure ( or member) is for nonlinear behavior
Some codes say if this factor is lower than (for example) 1.4 then you do not need to do second order theory calculation
first order is enough
but doing second order calculation is never wrong and noways it is easy do nonlinear calculation (second or third order) with fast computer

 

[JAE]

klaus, just a clarification, if I may.

Your statement seems to imply:
...that if the factor is over 1.4 then you must do a second order analysis, and...
If under 1.4...then you do not need to do a second order.

This is not what ACI is stating here.

What they are stating is that, if your ratio is over 1.4, you have an unstable structure and must alter the design to make it stiffer.
This is not a threshold between 2nd order or not 2nd order but rather a measure, as you state, of the over-sensitivity of the given structure to excessive PDelta effects....which can cause collapse.

You must generally always take into account second order effects, either by a 2nd order analysis or by using ACI's moment magnification method.

 

[structural87]

ok thank you for your answers.

so if I understand well, the ratio of 1.4 is for the general structure.
i am asking this because the new version of Etabs calculates the moment magnification factor delta-ns and displays a warning message stating that it exceeds the limit of 1.4.