supporting cantilever ends

[lejam]

Would it hurt to support the open end corner of a 2-meter RC cantilever? Has someone tried it with an i-beam? How would you attach it to the rc cantilever open ends at the corner or sides (you can't put the i-beam in front because it will block the view of the store)?

 

[BAretired]

I guess I am just getting too damn old to understand you young people. I haven't a clue what you are asking.

BA

 

[lejam]

Please check attached picture. The cantilever is about 1.7 meters. How do you put i-beam in front so the weight of the cantilever can bear on the i-beam? The original plan was to just put hollow block at the side with tiny stiffener column, would this be load bearing enough to carry whatever is above the cantilever? The i-beam plan was just to lessen the load on the column holding the cantilever.

 

[MikeHalloran]

Well, the photo didn't help _me_ understand what you are asking.

Perhaps it would help for us to know how this came to be your problem.
... and what problem you are trying to solve.




Mike Halloran
Pembroke Pines, FL, USA

 

[JAE]

If the cantilever only has top reinforcement and you add a support at its end you could possibly introduce positive bending where there might not be bottom reinforcement in the cantilever - or at least enough reinforcement.

 

[lejam]

Mike, it's to be additional support to take some load off the main column because during seismic activity, cantilever are not good.

JAE, the top bars are composed of 4 pieces of 0.78 inch bars and bottom bars are 2 pieces of 0.78 inch bars so you right that a lone support could introduce positive bending.. that is why hollow blocks would be put in between. But hollow blocks are not load bearing.

If I add RC Wall in the section instead of stiffer column and hollow block. I wonder if anyone has tried this? My worry RC wall may have Torsions during seismic movement de-aligning it from the cantilever.

Can anyone think of other solutions?

 

[hokie66]

Who is responsible for the design? That is the engineer who should be making this decision, not a panel on the internet.

 

[NIO85]

lejam, if in the analysis and design it was assumed to be a cantilever, why would you want to alter the system at this stage?

 

[lejam]

NIO

Do not forget that our buildings are designed for certain intensity of earthquakes, let's say magnitude 7.9 so if you can spend some more to make it survive magnitude 8.5. Why not. No buildings are designed for magnitude 11 because of costs.

Cantilever is one of the weakest links in an earthquake because it can act as pendulum.. so since the sides would be originally filled with hollow blocks and there is not much study about the load bearing and seismic performance of hollow blocks. I wonder if using RC walls would give the section more strength.

 

[rb1957]

i'd suggest that the earthquake resistance should be a fundamental design design, taken early on. trying to "improve" the earthquake resistance late in the build is probably futile ('cause the design options are limited) and you can only "add on" pieces (as opposed to a coherent design).

it looks to me as though you've got a concrete pad (upper story floor ?) supported by concrete beams (along the perimeter); one of these edges (the street side ?) has what looks to be a small I beam "supporting" it. It looks to me as though theres a very convenient vertical wall that could support one side ?

but what do i know 'bout this stuff ??

Quando Omni Flunkus Moritati

 

[JAE]

hokie66 is correct here - this is not a good time to be altering a frame in a high seismic area. By adding walls, additional beams, etc. you would be changing the relative stiffnesses of the parts and possibly significantly altering the load path. kejam - are you the engineer of record for this building?

 

[lejam]

My friend is the designer. He agrees cantilever are not good but we can't avoid it if clients want it. He said there is not much research about interactions of hollow blocks and frames so RC walls carrying the cantilever at the sides may not be bad if costs not issue.

Haven't you guys designed cantilever too? The load path of cantilever is not always good because it puts more stresses on the column.

rb, the piece is not an i-beam but a 4 piece 12mm bar where stiffer column will rise as well as hollow block. So I wonder what is the effect of replacing this with RC wall, the designer said no problem and no comment.

For those of you who have designed cantilevers before. Please just comment on your experience or research you have heard about supporting the beam with stiff walls such that no positive moment can develope in the midspan.

 

[kipfoot]

lejam,
It seems that you're asking whether this concrete column could be replaced by a steel column or concrete wall to support 'whatever is above the cantilever'. (When you say 'beam', engineers will generally think you're talking about a horizontal member in bending.)

The answer is, of course, "maybe". If you are relying on a column or wall, introduced under this cantilever beam, to carry gravity load, then it must have an adequate foundation and be considered in relation to entire structural system.

A column or beam under the 'cantilever' may work with the system that is in place, but like others have suggested, it could have ramifications that aren't apparent without considering the system as a whole. The stiffness of a new wall will affect the distribution of lateral load. A lone column may resist an uplift force when the backspan of the cantilever beam is loaded, changing the moment envelope of the beam.

A cantilever isn't necessarily a bad thing, but if you have discovered an overstress in the design, this might be a spot to add capacity to the structure.

 

[FeliceSelene]

Based on the photo provided, it seems that the plan is to provide a column (whether RC or Steel) that will hold the cantilever beam. To answer your question I would like to ask if there is a need to place a column on that cantilever beam? Does this cantilever beam has a concern with regards to deflection? If there is none then there is no use to provide an additional column. This will give only additional cost to the client and will affect the duration of construction even a little.

However, if this support is necessary, you could provide either RC as seen in the photo or Structural Steel. If you use structural steel or I-beam, erect this steel supported on a concrete pedestal at the base and provide steel plates bolted on the bottom of RC cantilever beam. There is a possibility that this I-beam shall be separated into two pieces and be welded at the middle height of column.

 

[dhengr]

We seem to have a number of OP’ers. here who are always involved in projects where it appears there was no or at least inadequate engineering experience and judgement applied in the first place, in a serious structural design effort, before the building ever started; and then it became hurry-up and get er built, we’ll fix that later, maybe it’ll go away if we ignore it. Finally, some non-engineer, without much construction experience either, is responsible for gettin it fixed. He can’t seem to explain the real problem, with enough of the important design information included to allow for a meaningful discussion, or to inspire any confidence that there is any conceptual understanding on the OP’ers. end of the conversation. All-n-all, a really scary situation.

I would fill the volume under that cantilevered slab with compacted granular fill, and call it a slab on grade, and be done with it.