Retrofitting a 5-storey 1920's non-ductile concrete moment frame structure
Retrofitting a 5-storey 1920's non-ductile concrete moment frame structure
(OP)
Greetings, Eng-tips.
I have a client who wants to add a two storeys on top of an existing structure. I am in a VERY preliminary stage at the moment, i'm just getting my head around what we have to do in order to do this. I have a general plan in my head and I am bouncing this off my colleagues here in my engineering community but i would like to know what the rest of the engineering world has to say about my strategy.
Existing structure:
-5 storeys of non-ductile concrete moment frames, very regular plan and there is floor-to-floor symmetry. rectangular in plan. all existing concrete floors (rigid diaphragm)
-Located in a moderate seismic region
-founded on bell foundations (basically spread footings set several meters below grade)
-infill brick right down the middle and on the west side of the structure, so there are torsional issues
-has a stairwell concrete core
New structure:
-light framed (going to be metal stud or timber framed)
My general strategy:
-Since the existing concrete frame is non-ductile and of questionable concrete strength in some areas, i am going to insert full height steel braced frames. these will be strategically placed to resolve torsion issues as well.
-I am thinking of using buckling restrained braces because of capacity design requirements, so i need to "tune" these braces to buckle when i need them to.
-These braced frames will be founded on piles so that the system is as stiff as possible such that they will attract "enough" loads and so the existing moment frame system should be relatively less stiff.
My question:
1. The reality is that there will be some load sharing between the existing concrete frames and my new braced frames. In order to capture this behavior AFTER the first yield, i would need to setup an ETABS model and then perform a pushover analysis, right? This would inform me of what ductility capacity i have after the retrofit, and inform me of an appropriate ductility value to use when running my static seismic analysis, right?
2. What are some pitfalls you see when setting up a pushover analysis for my situation? I understand EVERY situation is unique just want to know what you all think.
3. Is there any benefit to performing a dynamic analysis in this problem? I mean, the structure in plan will be fairly balanced in terms of rigidity after I strategically place my new braced frames, and the floor masses are pretty much the same except at the top two new stories where they will be lighter. So i think running a static analysis is appropriate, right?
4. What other dynamic "checks" would you do to make sure that a steel bracing retrofit will be compatible? For example, to me, a pushover would confirm compatibility as I need to ensure I don't "blow-up" the existing concrete frames before my new steel bracing gets to help out (i.e. stiffness compatibility).
Don't worry, peeps, I am working with experienced engineers as well, and I have a healthy relationship with my peers here. I just want to know what you have to say.
Cheersbrah
I have a client who wants to add a two storeys on top of an existing structure. I am in a VERY preliminary stage at the moment, i'm just getting my head around what we have to do in order to do this. I have a general plan in my head and I am bouncing this off my colleagues here in my engineering community but i would like to know what the rest of the engineering world has to say about my strategy.
Existing structure:
-5 storeys of non-ductile concrete moment frames, very regular plan and there is floor-to-floor symmetry. rectangular in plan. all existing concrete floors (rigid diaphragm)
-Located in a moderate seismic region
-founded on bell foundations (basically spread footings set several meters below grade)
-infill brick right down the middle and on the west side of the structure, so there are torsional issues
-has a stairwell concrete core
New structure:
-light framed (going to be metal stud or timber framed)
My general strategy:
-Since the existing concrete frame is non-ductile and of questionable concrete strength in some areas, i am going to insert full height steel braced frames. these will be strategically placed to resolve torsion issues as well.
-I am thinking of using buckling restrained braces because of capacity design requirements, so i need to "tune" these braces to buckle when i need them to.
-These braced frames will be founded on piles so that the system is as stiff as possible such that they will attract "enough" loads and so the existing moment frame system should be relatively less stiff.
My question:
1. The reality is that there will be some load sharing between the existing concrete frames and my new braced frames. In order to capture this behavior AFTER the first yield, i would need to setup an ETABS model and then perform a pushover analysis, right? This would inform me of what ductility capacity i have after the retrofit, and inform me of an appropriate ductility value to use when running my static seismic analysis, right?
2. What are some pitfalls you see when setting up a pushover analysis for my situation? I understand EVERY situation is unique just want to know what you all think.
3. Is there any benefit to performing a dynamic analysis in this problem? I mean, the structure in plan will be fairly balanced in terms of rigidity after I strategically place my new braced frames, and the floor masses are pretty much the same except at the top two new stories where they will be lighter. So i think running a static analysis is appropriate, right?
4. What other dynamic "checks" would you do to make sure that a steel bracing retrofit will be compatible? For example, to me, a pushover would confirm compatibility as I need to ensure I don't "blow-up" the existing concrete frames before my new steel bracing gets to help out (i.e. stiffness compatibility).
Don't worry, peeps, I am working with experienced engineers as well, and I have a healthy relationship with my peers here. I just want to know what you have to say.
Cheersbrah





