I dont know what to do here...

[greznik91]

I got a new project - reconstruction of an existing building (RC FRAME structure). There is a new extension of this building as shown in picture below:

I have several problems here:
Architect insists to keep 200 mm thick RC walls and a concrete slab. Span between RC walls is approx. 9 m. so the slab is approx 9 m x 2 m (with cantilever of 1 m). At the end of cantilever there is a RC flowerpot (additional load i hate). I was thinking about using this concrete flowerpot as a deep beam (i would extend RC walls below it so it would be supported). Another problem is that there is a new steel staircase that is fixed/supported on a new RC slab. Forces/reactions are not that small and Im already having a problem because of a span itself (9 m). Another issue is a fact that this new extension is not horizontally stable since there is almost no stiffnes perpendicular to the RC walls. So the only option I see is to completely connect new extension to an existing building - anchor new slab to an existing slab (RC beam of existing frame building)and reinforce an existing beam. That way there will be horizontal stability since anchoring of new RC slab to an existing will connect both structures so in case of earthquake/wind forces will be transfered to an existing structure (new and existing structure acting as one). But I still suspect i will have issues above RC slab since RC walls are 11 m high (RC slab is at 4 m) and I ll have to provide some horizontal stiffness but i dont know how...

I was hoping to make this new extension as a separate structure but i just dont see it possible. I was thinking about making a RC moment frame but it would require larger cross section (columns + beam) and i still wouldnt know what to do about a RC slab. I was also thinking about steel structure but I dont know...

i hate it.... What do you guys think?

 

[KootK]

I think that it's a mistake to try and create an independent structure of the extension. With the slabs and vertical elements tied together, I suspect that most of your difficulties will evaporate. The planter beam idea is clever. Go for it. With the structures tied together, your staircase will be supported on a pretty short slab span so that should be okay.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[racookpe1978]

USe the stairway (both steps and "handrails" and guardrail) as a bracing structural assembly.

That would involve ripping out the current stairway and looking at the replacement as a "wide column-to-column triangular member with steps in part of it" of some kind.

 

[greznik91]

tnx for suggestions, but how can I provide horizontal stability above RC slab?

 

[KootK]

Is there an RC roof that you can connect to? Obviously, the system will have to cantilever above the existing roof some degree.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

There is a RC roof but it doesnt make me any good since new structure (extension) doest have a RC slab in that level...

 

[KootK]

There is a RC roof but it doesnt make me any good since new structure (extension) doest have a RC slab in that level...

It'll do you plenty of good if you can attach your walls to that existing roof slab. It'll be your walls that need to cantilever up and support your new roof structure.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

KootK tnx. I think I came up with solution - I ll add new RC slab at both ends and anchor an edge in existing slab and the other edge is supported on new RC wall. That way I think its ok?

One more question about roof beam (beam that supports roof wood structure)...
Since the beam is quite long (9 m) and it is supported on RC walls that are only 200 mm thick, I think I have to consider it as pinned on both ends (design it as simply supported beam). What do you think? I dont know what reinforcement detail to use at connection between a wall and a beam end? Any suggestion? There is also a glass facade under a beam so I deflection might be an issue...

 

[KootK]

I ll add new RC slab at both ends and anchor an edge in existing slab and the other edge is supported on new RC wall. That way I think its ok?

Sounds good. Could probably also just dowel the edge of the walls into the slab generously. Your way seems more robust though.

Since the beam is quite long (9 m) and it is supported on RC walls that are only 200 mm thick, I think I have to consider it as pinned on both ends (design it as simply supported beam). What do you think?

I agree. I dont know what reinforcement detail to use at connection between a wall and a beam end? Any suggestion?

I'd run the wall verticals up to the top of the beam and terminate the beam top steel with standard hooks. I don't think there's a need for anything fancier here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

I was thinking about reinforcing it the way you suggested.

KootK thanks for help. I appreciate it.

regards.

 

[KootK]

You're most welcome gresnik.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.