Concrete wall buckling?

[n3jc]

Hi!

We are dealing with the existing RC walls (200 mm thick). Above the walls is the RC slab (200 mm thick)and existing timber roof (will be removed).
The plan is to add a new light storey (timber CLT walls and slabs)on top of existing one (instead of existing timber roof).

This means that the edges of existing RC walls (200 mm thick and 3 m high) will be loaded with large
compression forces from new columns (we are talking about 50 - 100 kN per column).

I'm wondering how do I check existing walls for buckling under new high forces.
I havent found anything usefull in Eurocodes (I found it for columns but not walls).
I'm also interested in ACI standard if someone has anything usefull.

If buckling is gonna be an issue I think the best way to solve this is to add new RC columns at the existing wall edeges.

Im aware we have to make sure that existing structure with new storey has sufficient foundations and seismic resisteance
after adding the new storey, but that's not what this post is about.

Thanks for help.

regards

 

[jayrod12]

A wall is really just a narrow column. At least that's how I've always seen it from a high level.

I'm not familiar with Eurocodes but I have to assume they have a section regarding point loads on walls.

 

[KootK]

In the US and Canadian codes, this is commonly handled by way of determining an effective wall width that resist the loads. It takes the form of some multiple of the bearing area plus an assumed load distribution (2V:1H to mid-height etc). I'd expect you to have gobs of a capacity available for this is a low-rise building without needing to column-ize your walls.

 

[centondollar]

This is a straightforward problem. You may apply Kirchoff-love plate theory and the Rayleigh-Ritz method to estimate the buckling capacity (eigenvalue problem, i.e. upper bound for the buckling force) of the wall. If the load is actually concentrated on a very narrow strip of the wall (as-built in reality, not as envisioned in a sketch), you just need to apply the potential of the load as a triangular distribution and integrate it over the plate edge (=top of the wall, into which the column is connected).

Estimate boundary conditions (pin-pin at top/bottom, free at free edges and pin or stiff at wall-wall connections is most conservative); find suitable displacement trial functions (must be kinematically admissible), such as polynomials or trigonometric functions; evaluate the total potential energy "Pot.total = Potential.StrainEnergy.Plate + Potential.External.Loading; set variation (derivative) of total potential energy w.r.t. unknown parameter in displacement trial function, and solve the critical load (defined previously in the "Potential.External.Loading" step) from the resulting homogeneous equation system by calculating the eigenvalues.

Very straightforward if you´ve done it before for a beam, bar or plate. There are lots of textbooks on the subject, too. Alternatively, do as KootK says and oversimplify the problem to arrive at a solution (sufficiently stiff wall) that is guaranteed to work.

Alternatively, use finite element software to calculate the buckling load.

 

[centondollar]

jayrod12,

to my knowledge, Eurocode requires walls to be treated as plates if their width-to-thickness ratio exceeds some value - it might be 4, I don´t remember exactly. No further guidance is given as far as I know, and the reason seems obvious: this is a problem of structural analysis, which cannot really be completely standardized.

 

[HTURKAK]

..

I am not sure if you are at Eurozone or not.. EC 2-1 provides the requirements for the walls and a figure for walls having different edge conditions.. Pls look EC-2-1 Clause 12.6 and 12.6.5.2 Simplified design method for walls and columns (1) In absence of a more rigorous approach..

I have attached the relevant page of the EC-2

 

[n3jc]

@centondollar
Thank you for reply but I think I won't go there - I think I should take more simple way.

@KootK
I have been thinking the same thing but couldn+t find it in EC2.

@HTURKAK
Thank you,I don't know why but somehow I missed it.