Design of a uncommon shaped RC beam for a building

[D.Jaya]

Dear Structural Engineers

A small commercial development at our area will be such that the owner wants to initially build Ground Floor and a 1st floor and later he want to add 2 more floors (Total of 4 floors)This proposed RC building to be used for commercial purposes. The soil condition is ok with sandy gravel soil profile.

The building having only about 50 square meter footprint area. The owner wants to initially build the ground floor columns along the line marked "X" (in plan),and if any future road widening then that to be shifted to lines "Y" & "Y" as per the authorities instructions.

I analyzed the structure and will design according to the requirements. The critical (the maximum length) of column shifting is from “A” to “B”(1.42 m)Pl. refer to the attached sketch.

I need to clarify
1.Is there any special considerations which to be adopt when designing the uncommon shaped beam marked as “ Beam 1” at gridline 1. Other beams (B2) shall be design in the same manner

2.What are the important facts to consider when joining(if required) the column “B “at a future stage & demolish the Column “A” to take the moment due to the future cantilevered load-(about 600 KNm) (The beam will be casted at the initial construction with the required reinforcement to withstand this moment at this proposed cantilevered edge).

 

[HTURKAK]

My points are ;

- 600 kN-M moment for the future cantilever seems exaggerated.. A moment around 150 kN-m would be more reasonable..

- You may shift the column axis X to Y and column A to position B and design the X-Y portion as cantilever..

- You may delete the frames 2 and 4 .. Frames 1,3,5 should be satisfactory for 9.76 m ..







If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )

 

[Lomarandil]

HTURKAK -- I think your point about the 600kNm may not be correct, particularly if only the ground floor column is shifted. 600kNm does not sound unreasonable to me.

D.Jaya -- the most challenging portion of designing these beams will be that I expect it is unlikely that hydraulics and shoring towers are used to precisely control the transfer of load from column A to B. As such, you'll need to account for how that load transfer will occur according to the actual construction practice -- including the effects of foundation settlement, creep and shrinkage in the structure (will be different if this retrofit occurs in 2 years or 20 years), and how they will cast column B to tie into B1 and B2 while having the floor above.

At the time of the retrofit, beams B1 and B2 will have some deflection at column axis Y. This will be locked into the structure when column B is built and column A demolished, which will cause some redistribution of forces throughout your structure. Then the new foundation at axis Y will settle elastically under the new load, creating additional locked-in deflection.

 

[D.Jaya]

Dear HTURKAK (Structural)and Lomarandil (Structural)

Thanks for your valuable comments.
Also I will check again the value of the Bending moment for the future cantilever.

 

[HTURKAK]

I considered the column axis X will be shifted to Y and Y' for all the floors. In my zone, the planting of column on cantilever beam is not allowed so probably i misunderstood the post.








If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )

 

[jayrod12]

Wouldn't road widening also require, at least in part, the superstructure to be narrowed as well? I don't believe less than 10 feet of overhead clearance on a roadway is acceptable.

I'd also be looking at carrying the thicker beam/slab back further into the building past the proposed new column line. Your beam will still be working hard in negative bending beyond point B, but you've lost the bulk of your effective depth.

 

[D.Jaya]

jayrod12 (Structural)
Thanks for your comments