Column supported by a cantilever beam.

[IngDod]

Greetings,
I was shown architectural drawings for a three story building in which the second and third floor exterior columns rest on the end of a cantilevered beam from the first floor, The cantilever beam is 6.5ft long. I did some preliminary calculations and it seems doable (Section size and rebar)for gravity loads, for lateral loads such a discontinuity would be quite problematic; but I believe this problem could be solved by using shear walls in the interior of the building to take all the lateral loads from the columns. I would like any input you could give me on a case like this, I'm not entirely convinced this is such a good idea (well, I think its a terrible idea..)

Thanks.

 

[KootK]

It sounds as though you've got a pretty good handle on it. Some additional ideas.

1) Keep an eye on long term deflections at the cantilever tip and the impact that may have on cladding joints. A large portion of your load will be permanent.

2) Be sure that the the beam design accounts for vertical seismic accelerations.

3) Use top bars in your beam that can be sufficiently developed past the face of the column.

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.

 

[njkeng]

I would also recommend being aware of the construction phase of the floors above when designing the transfer cantilever and would also allow for pattern live loading where there may be live loading on the back spans on the floors above but none on the backspan of the transfer floor, other combinations should be reviewed, but this one appears to be the one that will most likely lead to the largest tip deflection. As KootK suggested check the deflections at the tip which includes allowances for creep and shrinkage. Also check to see if you can get moment continuity and stiffnes through the beam column joint, this could be occuring on all floors and would add stiffness to the system but also induce bending moments into the columns.

 

[XR250]

@Kootk - Are you suggesting designing for the vert. seismic loads due to the fact that the majority of the load is dead?

 

[KootK]

Yes +/-. I think that designing for the vertical seismic load is mandatory. I'm suggesting that it may be extra important here because a significant portion of the design load will be of the "seismic weight" variety.

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.

 

[IngDod]

Thanks, many important points. I believe the vertical component of the seismic load will be more important than usual in this case due to the cantilevers. One point in favor is that the internal bay of the frame (its a two bay frame with a central column that is continuous unlike the perimeter columns)is relatively large at 33ft, so the dead load on it helps with the cantilever. I will definitely have to take creep and shrinkage into account or the cantilever deflection could be rather large in the long term and the exterior of the building is mostly glass. And the construction load (wood supports most likely) on the cantilever could be rather crucial as well.

Honestly I thought I was going to get a big no-no for this type of building since it places a rather large concentrated load at the tip of the cantilever, but I couldn't find a good reason no to pursue it other than my reservations about it.

 

[BAretired]

Is the cantilever beam concrete, steel or timber? Are the second and third floor columns concrete, structural steel or timber?

BA

 

[IngDod]

@BAretired: Greetings, everything is concrete.

 

[BAretired]

If the column connects to the beam in a moment connection, deflection of the cantilever will tend to cause the top of column (at second floor) to move down and outward so your shear walls will be engaged with or without lateral load. On the other hand, rotational restraint at the tip of the cantilever should reduce vertical deflection. It has to be carefully analyzed.

BA