Seismic upgrading of a highly irregular, partially buried, existing reinforced concrete building

[brTCP]

I am trying to retrofit an existing 4 story reinforced concrete building with a typical plan as shown below. The thing is, it has a very long wall on one side, retaining the soil behind, and a very flexible structure over the rest of the floor plan. When I tried to balance the stiffness of the existing wall by adding another wall on the other side, I found out that even with a very long wall, which is impossible due to architectural constraints, I end up with a structure that responds predominantly in torsional modes. The walls perpendicular to the long wall are also causing some headache and uncertainty. What are your thoughts on this? How would you go about retrofitting this building? Should I just accept that this will always respond in predominantly torsional vibration modes and design accordingly? I am neglecting any positive influence of the soil in resisting the vibrations. Thanks!

 

[driftLimiter]

Some ideas are, using thicker walls, using core walls, or a combination of walls and steel frames to get where you need.

Doesn't seem right that if you add an equally stiff wall on the open side you still have torsional modes dominating. It would be a shame to retrotfit the building and it still have a torsional response, but if thats all you can achieve then I guess just make sure you do it right.

 

[brTCP]

I am sorry, I was not clear. If I add an equally stiff wall on the open side, of course I can avoid the torsional modes, but I just can´t due to architectural constraints. I tried to add quite large, but separate, walls on the open side but they are no match for the enormous length wall on the other side. I was wondering if I am missing something, especially since this is a partially buried structure, and I am not sure if my approach of considering the perimeter wall as a shear wall is correct.

 

[HTURKAK]

Yes but you may still need to add some walls or increase the sizes of columns at far row to reduce eccentricity. Apparently your internet country code is (PT) . If so, EN 1998 should be the applicable code .
My approach would be,

- Model the total str. together with retaining wall. FEM the wall and assign lateral soil springs ( half of the horizontal spr. value ) to nodes,
- Check for upper limit ecc. ( eox ≤ 0,30 *rx expression (4.1a) )
- Add shear walls or increase the size of the columns if necessary ,
- Check the retaining wall for static + dynamic earth pressure ( EN 1998-5 )

....


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[brTCP]

Could you explain a bit more what you mean by

assign lateral soil springs ( half of the horizontal spr. value ) to nodes

? I think the soil is perhaps beneficial only when compressed and an nonlinear analysis full of uncertainties is required to model this correctly. Am I right?

You are right, I am using Eurocode 8. My building is irregular in plan, not only due to expression 4.1a, as illustrated below:

And here is how it looks when I add a large wall on the open side:

In fact, the building will be irregular according to Eurocode 8 even if the wall is completely symetrical, because one of the torsional radii is smaller than the radius of gyration of the floor mass:

 

[HTURKAK]

The code allows you to model the fnd. with vertical elastic WINKLER springs. The basement wall ( in this case retaining wall also ) is also supported by horizontal springs. ( You may use the horizontal springs calculated for piles but i proposed half of the value to account the effect of tension springs due to torsional effect . You may model compression only springs ( and elastic-perfectly plastic compression only springs ). if your software is suitable . The realistic soil- structure interaction modelling is a another big story.


Not really . The soil is not modelled for tension springs . I will suggest you to look (DESIGNERS’ GUIDE TO EUROCODE 8


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48