new structure connected to an existing building - no foundation or column jacketing

[structural87]

hello,

i would like to have your thoughts regarding the below.

i have a new small structure which will serve as extension for an existing old building.
The contractor is very clear regarding the fact of avoiding all types of column / foundation jacketing.
Therefore, i am adding new columns to the new area and will design my structure relying only on the new columns. The columns are being placed 2 meters away from the existing building edge in order not to interfer with the building foundation.
The problem i am facing is that the new structure in subject has a very low stiffness and i would like to brace it against something stiffer. Note that the structure is located in a very low seismic zone and the wind loads arent that much pronounced. Therefore, no need to worry about the lateral loads magnitude.
My question is the following:
1- can i design my beams and slabs as an independant structure and then place some dowel bars in order to connect with the existing structure? For example, can i design the drop beams as cantilever ones and at the end connect them with post anchored dowels ?
2- Can we guarantee that following the above procedure no additional load will be carried by the existing columns?
I believe that the answer is no for questions 1 and 2 but would like to hear your thoughts.
Thank you

 

[jayrod12]

Agreed that the answer is no.

However, if you were so inclined, there should be a way to devise a connection that the vertical load does not get transferred into the existing column but the lateral load can be dumped into the existing structure. In order to review that option we would need way more detail. Framing plans, member sizing, load magnitude etc.

 

[structural87]

Hey jayrod thanks for the input. I draw a typical bay of the structure under discussion. It is a small structure (dimensions in plan 6x10 meters) with 3 stories height.

 

[structural87]

Any comments to the above ? Thanks

 

[jayrod12]

Have you analyzed the existing structure for the additional load it may see if it were connected? You aren't even adding a full metre of the beams load to the column. You may find that you can do that without any modification required. I would likely only look at connecting at the column location personally to avoid overloading the existing slab edge.

But on the topic of slab edge, Have you thought about what you're going to do to avoid a step once things start deflecting?

 

[structural87]

i have analysed the existing structure for the additional load. the code recommends that for elements where additional load is less than 5% of the carried loads, there is no need to strengthen the existing structure.
I am in the range of the 15% if i rely on the existing column (extension is heavily loaded and in some places the span length in the Y direction is higher than 4 meters).
as for the slab edge, i will make sure that deflections are minimal there.
However, my main concern is that i dont have stiff elements in the extension structure and would like to stabilize it somehow.
Thats why i was thinking to brace it against the existing but i cant brace it without loading the existing column.

 

[jayrod12]

You could, but likely not with concrete. You could come up with some sort of steel slip joint detail that would transfer lateral forces but let vertical forces stay in the new structure only.

You say you analyzed the existing structure, but you're saying the total change is 15%. What if those columns had an additional 15% capacity? I would be analyzing the existing column not just for load change, but for overall capacity.

What about some beams between your new columns in the north-south direction to create moment frames?If you account for the continuity of your new beams, you still won't be applying more than 1m of additional beam load to the existing columns. And that load would help balance the moment seem be the column from the existing framing. It may still be good. Do you have reasonably accurate information of the existing column reinforcing and the foundation sizing?