Cantilever Beam is So Thick - Looking For Counter Proposal

Must be one helluva cantilever, assuming it's an "H" beam. What is the application in more detail.. A balcony of what dimensions with what load... for example.

that's about 18 inches or so, right? By thickness you mean from the outside plates?

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If I'm "getting your drift" corectly, I am of the opinion that anyhthing else you put in there will be more bulky than the steel.

The beam is of reinforced concrete and for a balcony of a villa. The span/ depth is to the limit of 7. Prestressed beams could be the answer .. for which I am looking for the detail design and method.

Regards

Have you looked at steel and stucco?

I assume this is a concrete beam and not a slab that is 450 mm thick. Prestressing the beam is a process of tensioning cables that reinforce the beam inside, placing a compressive force across the entire concrete beam length (since concrete is much stronger under compression, this allows the cables to offset some of the tension on the concrete and increases the compression). This could improve the strength of the structural support from the beams, but 18" is not a particularly thick concrete beam, and you'll need a certain cross-sectional thickness of the beam to add support against the applied bending forces. so, you may not save much thickness by prestressing (maybe a few inches). A good alternative to reduce the beam thickness is to use a Steel beam instead of concrete. You can then cover the beam in stucco or grout just for the aesthetic appearance. Use of a Steel I Beam will require changes to the connections to the supporting structural bearing wall. You can reduce the thickness of a beam a lot by using steel instead of concrete, but you'd still probably need a beam at least 6" thick, and then add any aesthetic covering.

I don't think that a prestressed beam is the most economical, at least not in my neck of the woods. But if you do want a prestressed concrete beam, the methodology is well known.

What is the length of cantilever? What is the tieback span? How much roof, floor or combination of roof and floors does the beam carry, that is, what is the tributary width?

A timber beam or steel beam may be a more economical alternative. Hard to say without seeing your drawings.

Hi everyone ,

Thanks for all the feedbacks. I am yet to receive my consultant's alternative design. Will keep you posted on the final decision.

Thanks again

Regards

Dave

Can anyone help me:

Lot many can:--if only you send drawing of the design you referred to:

Cross section of slab through this 450 thick cantilever beam>the both end supported beam/s,columns, other end cantilever beams.All dimensions,rebars,bend reversals.

If you begin with solid thick heavy concrete slab/s, and long free-hanging ends you may indeed need heavy/deep highly reinforced beams.

If you had opted for hollow precast light-concrete slabs and short freehangs--your home building could have cost 1/4>1/6th.And a lot safer on earthquakes. Lot better insulated too. Like mine-which I designed/built in 1985.AND you would have not bothered to eventhink of prestressing--because NET saving would have been small.

I think he's asking for someone to describe the process of pre-stressing. Too busy right now so I'll leave it to someone else. It's surprising that a consultant who would recommend so involved and capital intensive a process would not be able to describe the process for the client. Maybe a second-consultant opinion is advisable. Better Malay house description would be helpful too. Including whether the beam is new construction or retrofit for some purpose.

It is hard to know just what kind of answer you are seeking. You mention a consultant but also speak of a friend. Are these one and the same person? If not, is your friend's advise based on some notion that (provided the required tools can be gotten) you, yourself, or some small house construction company--or buddies/relatives--will attempt the pre-stress prep and pour? If so, I see that as strongly ill advised, especially where you are located--where, in some provinces, shoddy construction techniques are as much the rule as the exception. Your quest for answers, including answers about preliminary engineering analysis, should--no, must--begin with a respected, commercial construction firm with well-established engineering reputation and previous experience with this construction method.

As I understand, pre stressing of concrete is based on the principles, that concrete reaches greatest strength with increasing cure time and increasing compression loading. The idea behind compressing (pre stressing) during initial cure is that less concrete, and less weight of concrete, can do the same job as a greater amount without pre stressing. For one, a pre-stressed concrete beam might serve where space does not otherwise permit for a non-pre-stressed beam. The trade, as I see it, would also involve the cost of one versus the other method of pour; but pre stressing is normally done, even though more costly, because the extra cost is justified by technical/design/architectural factors. For all but very few residences, the additional cost (engineering studies and implementation) is probably not justified. Again, the best way to find out is to go ahead and solicit a proposal and bid...for both methods.

Cal

The principal role of prestressing ( as i recall..) is to assure that the entirity of the width of the beam remains in compression when bending loads are placed on it.