RC blade column biaxial bending

[QSIIN]

Hi all,

When designing an RC blade column, does the biaxial bending check in Section 10 cover the out of plane bending/buckling that the compression block may face due to the in plane bending? Or is it necessary to design the compression block from the in plane bending condition for out of plane bending separately, almost as a separate, isolated column?

Thanks for you input

 

[Agent666]

Section 10 of what standard?
AS3600, NZS3101, AS5100??

Generally I'd say any biaxial check on a column (M*x/M*y/N*) obviously should check the combined actions together on the section, i.e. essentially for a diagonal resultant moment + axial load.

Checking them separately is possible, some codes allow this and then do some combination of the design ratios, I don't believe this is really the intent of AS/NZ codes though (steel design to AS4100 & NZS3404 use this approach for combined actions for example). But its more correct for concrete to check the section for the resultant diagonal moment with the axial load and compare to the capacity/interaction surface for this angle of bending and axial load.

 

[rscassar]

Personally I don't apply the biaxial moment particularly with blade columns, one axis is really stiff and the other is weak. The columns governing failure mode is buckling about the weak axis so I apply the eccentricity in that direction.

 

[Trenno]

I believe it's common practice in the UK (Eurocodes) to consider bending on the critical/minor axis only. This situation is for braced columns subjected to minimum moments.

 

[QSIIN]

Thanks for the replies.
Yes, sorry, I'm referring to AS3600.
Im interested in understanding how to design isolated shear walls and blade columns that are subject to in-plane bending moments (wind load) and then a minimum moment in the weak axis.

If the wall/blade is long enough, even a significant in-plane moment may be within capacity in the strong axis interaction diagram, however isn't there a risk of the compression block buckling out in the weak axis?

Is this covered by a biaxial bending check, or is the only choice to divide the walls into segments and design each segment about its weak axis for the load from the in-plane moment? To me, this secondary method is logical, but potentially too conservative, however I don't know if the biaxial bending check covers this in one calculation.

Thanks

 

[Agent666]

I'm not overly familiar with AS3600 as there have been several iterations of the standard since I last used it about 10 years ago, but I can provide a NZ context which presumably there will be similar provisions in the Australian standard.

In NZ the subject of the stability of the compression end of the wall is treated in a number of way depending on the reinforcement configuration, its usually by meeting both minimum detailing requirements and some specific design checks:-

For singularly reinforced walls (i.e. single layer centrally located in the wall thickness) - There is a check on the buckling of the wall under in plane moment and axial load, this check includes the addition of P-Delta effects from the axial loads. This check is primarily an in-plane check. In real structures subject to earthquake forces there is likely to be some component of face loading as well. For singularly reinforced walls, they must be designed so there is no hinging under face loading at the center of the wall, as this would impact on the stability for in plane actions (like having a column with pins at each end and the middle it forms a mechanism). Our code recently was amended to include many of the lessons learnt from the 2010/2011 Christchurch earthquakes, and more recent 2014 Kaikoura/Wellington earthquakes. These change heavily penalised the use of singularly reinforced walls, essentially now they are to be designed to remain elastic, with limited ductility up to the MCE (maximum credible Earthquake) level. Under face loading if a wall is part of the primary lateral load resisting system in the orthogonal direction, then it is required to be designed for a 'parts' load (like your AS1170.4 chapter 8 loading). This higher parts load ensures under the maximum face loading from higher mode effects, that the wall will remain stable with no face loading mechanism forming.

For doubly reinforced walls (layer of reinforcement located near each face of the wall) - There is a check on the slenderness of the boundary elements (the end of the wall being checked to make sure its thickness is greater than a minimum required thickness), and a check on the stability for buckling if you are over a certain slenderness limit. For most practical wall thicknesses these limits are relatively easy to satisfy, and this by all accounts ensures that any buckling is suppressed. As per above, other recent changes have included limiting the gravity component of the axial load to be less than 0.3 x f'c x Ag, this addresses the effect of high gravity axial loads can have a significant effect on the stability of the wall under lateral loading. Other changes also included increasing the minimum reinforcement levels (both stirrups/confinement and longitudinal reinforcement) in the end regions of walls. While not done specifically for stability, there were a number of walls in the Christchurch earthquake which the wall buckled/sheared off (look up the Grand Chancellor failures for more info on this) and where reinforcement buckled in compression after yielding in tension on the reverse cycle, or where reinforcement fractured completely due to low cycle fatigue. The increase in reinforcement is intended to address these failures, as it was found in the end regions of walls that when the reinforcement ratio was too low, the actual real concrete strength being sometimes 2 x the specified design strength, that this caused a single flexural crack to occur because the concrete section was stronger than the reinforcement provided, and as such the assumed distributed flexural cracking required to dissipate energy did not occur. Bars had large concentrated strains over these short crack lengths rather than the same strain being distributed over a much longer height of wall.

Hopefully someone more familiar with AS3600 can point you in a similar direction to specific AS provisions relating to stability/buckling, but I wouldn't be surprised if there were many provisions with similar intent within AS3600.

 

[rscassar]

There is a risk of the compression end of a wall buckling out of plane. The way I approach shear wall design is to break the wall into wall segments of max 1.5m length, and then design the wall segment based on these forces.