P-delta and slenderness

[hyper]

Hi

I want to know if my concrete structure has sway or non-sway behavior.

According ACI318-95 R10.11.4 In a story of a frame there is non-sway behavior
if the increase in the lateral load moments resulting from PDelta effects
dos not exceed 5% of the first-order moments.

I am doing the P-delta analysis in SAP2000

According R10.13.4.1 P-delta analysis must be representative of the stage
immediately prior to the ultimate load using cracked bahavior in the structural
analysis.

Then , Do i have to use cracked properties to the P-Delta but normal properties
to the first-order analysis???

When do I have to consider cracked properties??



Thank you for your time.
Regards.
Franz

 

[Polecat]

You need to consider concrete as being cracked whenever the applied moments cause the extreme fiber stress to exceed the modulus of rupture (which is 7.5*sqrt(F'c)). It doesn't matter whether this occurs in the first order or the second order stage, or both. It's the magnitude that matters, not when it occurs.

http://www.spiraleng.com

 

[Gtoyo]

hyper!
What am I understanding you should not go further with P-D analysis without checking approx. means proposed by ACI. such as Q-stability index- first
You should prelim. anlysis your frame with first order by reducing stiffness of columns and beams. ( 0.7 and 0.35 ). and check Q.They are many basis explain in ACI commentary to use these figures.
According to ACI., you should use these reducing sfiffness even in static analysis to get more reliable redistribution of internal forces in frame due to crack sections. Because under USD design most of concrete sections especially beams will crack under service load, while columns with effect of axial load ( comp. )they would have less reduction in stiffness.
I do not comfortable to analyse my concrete frame with second order analysis such as P-D, cause it is very difficult to predict exactly their stiffness which load-depending.

- hope this helps-

 

[Murali27]

Hi Gtoyo

You are right. I need same clarification if my structure is a postensioned flat slab system. Can you suggest me what is the reduction in stiffness applied on post tensioned slab system . Can we take as 0.5Ig. or still more. Any reference to the same will be appreciated.

In one situation, Q > 0.05 but drift ratio is being very less say 0.0010 (limit is 0.15"/Story height). Can we say that this is sway story?.

Please explain on this