Core wall out of plane design

[saburo]

Hi

I am designing a box shaped core wall surrounded by two rows of perimeter columns for a 40 storey building. The wall is predominantly taking in-plane shear/moment from the lateral force. The flexural and shear demand is not significant and easily dealt with. My question will be about the out of plane flexural design.

The wall support 40 levels of floor and the gravity axial force at lower levels is quite big. I am allowing the minimum eccentricity that ACI states e=(15+0.03h) for out of plane flexural action.

I am using RAPT for concrete section design. And my question is when you input M1 and M2, would you input N*e at both ends to calculate the slenderness/moment magnification effect? RAPT calculates moment magnification factor based on cracked section curvature.

Same sigh at both ends means single curvature, and from buckling point of view it will be conservative. But if you consider the minimum eccentricity is intended to take account for construction tolerance, it could happen. Imagine only wall in one level is out of position along the height of the building, in this case wall receive single curvature moments at both ends.

I was also reading foundation section of ACI and found that the minimum eccentricity moment does not need to be transferred to the footing. The implication is that we could input M1=N*e, M2 = 0 to consider the slenderness effect?

Wondered how everyone here deals with wall out of plane check. Please advise.

Thanks in advance

 

[asilmozamil]

reinforced masonry design of out of plane

 

[ishvaaag]

Certainly if one can´t follow the actual layout of the bending moments the sound practice would be to assume single curvature. As well, any geometrical imperfection must be assumed occurring in the way that more worsens the situation, so increasing the single curvature moment. Respect the question at first level over footings, I wouldn't neccesarily assume it as waving the general requirement of minimum eccentricity and less to assume Moment zero. Codes are not consistent nor with science nor within themselves, simply mandate what they think must be done, hopefully with sound basis.

Anyway, of the concern of that the eccentricity must be taken the worse one can't get rid off, that is the intent. On the assumed law of moments one at least can get relief by taking the pain of following the real moments; better if the program does, of course.

 

[rowingengineer]

sorry Saburo but you really have me confused as to your question. I am going to give you two answers let me know if either match your question.

1. For a isolated wall carrying vertical load, an out of plan bending due to eccentricity must be taken into account. As per ishvaaag post

2. Box core walls pour as monolithic can with "lots of simplification" be assumed to act as a single member for global axial effects. Effectively this is treated like a large concrete column with a void in the centre (There is some problems with this analysis you will need to review publications like "Deficiencies in the Wide Column Analogy for Shear wall Core Analysis" ect). Thus using this analysis the eccentric effect on the global reaction is minimised. However local effects must be taken into account as well, and these will have the appropriate eccentricity at the local level.