15 meters span cantilever reinforced concrete beam

[istruct1980]

does anyone of you designed or knew someone made a 15meters meters cantilever beam which supports only roof? is it even possible?.
its not post tension, just conventional beam

 

[Agent666]

Anythings possible provided you can fit in enough reinforcement and make it deep enough I guess.

ACI318 for example has the following dimensional limitations for cantilevers (l/8), so for a 15m cantilever the section would at a minimum need to be at least 15/8m = 1.875m deep. Some codes also have a width requirement related to span. I would imagine careful consideration of deflections might actually govern though.

 

[JAE]

Agree with Agent666 - also check your long term deflections - maybe some initial form camber is a good idea.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[EngEstrutural]

a 15m cantilever should be at least 2 meters tall, assuming it's braced, we could have a width of 0.15m
let's say the load is around 0.3 tons/meter, and the beam weighs 0.75 tons/meter.
1.05 x 15^2 / 2 = 118 ton x meter

deflection will be around 30mm instantly, it could reach 100mm on the long term.
To resist the moment, using 60ksi rebar, you'll need over 30cm^2 on the top.
You will also need a place to support that huge cantilever

So, yes, it's possible, expensive, but possible.

 

[jayrod12]

I can't imagine anyone ever doing a 150x2000 beam. The proportions just do not make sense to me. So EngE's prelim calculations are correct, but could likely be worse in terms of instantaneous deflection. A wider beam would also allow for better reinforcing layouts.

 

[EngEstrutural]

I can't imagine anyone doing a 15m cantilever, and yet here we are.

The biggest span I ever achived was 35m simply supported, and the beam was I section, 1920mm tall.

In bridge construction, we can see girders of many times that size.

 

[jayrod12]

The depth doesn't bother me. I've done 2m deep beams. But only 150 wide? yikes. I would be hesitant to even put two horizontals in a 150 beam, once you subtract off clear covers and stirrups, you've got 70mm left to fit bars and spacing between bars.

 

[EngEstrutural]

Depends, there are codes with only 20mm cover for beams, and stirrups small as 4.2mm
That leaves up to 100mm for main reinforcement.
Assuming 25mm rebars(#8), we are left with 50mm for the concrete in 3 or 4 layers.

Many water tanks have similar cross sections for their walls.

 

[hoshang]

What about seismic induced forces on the cantilever?

 

[KootK]

I'd say that much depends on:

a) The particular application. I've encountered things of this sort with, say, an outdoor amphitheater with the top of the beam pitched and the bottom tapered.

b) The nature of the cantilever support. Any rotation there will amplify massively out to the cantilever tip so one should give due consideration to foundation flexibility etc. A healthy back-span situation would be nice.

 

[Agent666]

150mm, this is why ACI needs a minimum width. Standards have minimum widths of members to prevent instability. Similar concept is required in boundary regions of shear walls, a minimum thickness of the wall is required to prevent buckling.

The concrete standard round these parts requires for 15m span a width of at least a meter.

 

[gargarot89]

Maybe you can look into a folded plate/shell structure (and not a beam).

With a quick google search: Groenendael Hippodrome or madrid grandstand from Andre Paduart. "a 13.5m cantilevering folded plate with a thickness ranging from 7 to 12cm only".

Quite impressive.

 

[EngEstrutural]

Pier Luigi Nervi cantilever is not that thick, and of a similar span.

 

[hoshang]

A healthy back-span situation would be nice.

How deep will be that back-span? Is it the same depth of the cantilever? How long will be that back-span? Is it enough 7m or it should be 20m?

 

[MIStructE_IRE]

Ideally with a 15m cantilever, you would have a 30m backspan. Not absolutely essential but i always like the one third two thirds thing with cantilevers.

 

[istruct1980]

hello, the 15meters cantilever beam is supporting a roof of sports complex arena, i would say there is no back span

 

[MIStructE_IRE]

If there’s no backspan then you need to be very careful about rotations/settlements etc since these will multiply massively at the tip.

I think you just need to start designing now to get a feel for the numbers.

 

[hoshang]

i would say there is no back span

If there's no back-span, then I would suggest a cable connected to the top of a column higher than the cantilever elevation.

 

[jayrod12]

As others have said, a large portion of your deflection will come from column rotation. What does the beam frame into?

 

[istruct1980]

The frame will look like and inverted L shape, the 15m cantilever beam has no backspan. If it would be possible and it will be calculated/designed properly/safely, is it possible to have splice all along the bars?

If this is the case i believe that as much as possible the top bars of the cantilever beam would extend all the way down to the column, and m ly question is, if i have 12m max length of revar is it possible to splice the bars? But at the jount, the continous bars should be placed