How comfortable are you with long RC cantilevers? 3

[IJR]

According to most codes, any cantilever is fine w.r.t deflection if its depth is about greater than one fifth of the span. And most codes go on specifying beam reinforcement to ensure no-brittle failure of the beam

Now I have a 10ft cantilever 2.5ft deep supporting landing for a heavily loaded RC staircase(10ft wide public access stair).

Numbers show that both code requirements above can be met. And I am willing to go on.

But peers tend to inject this fear in us all times when cantilevers get a bit longer.

Is there any reason to doubt a "designed" cantilever? Is there anything not established in cantilever designs todate?

regs
IJR

 

[JAE]

A couple of thoughts:

1. A cantilever is by nature a non-redundant piece of structure so you should keep that in mind when designing it. A higher level of safety is appropriate here.

2. Detailing your longitudinal reinforcing is therefore critical (since it is non-redundant). Be sure to develop your top bars well into the supporting concrete beam. We usually hook the top bar down over the end of the cantilever.

3. Do careful calcs on the deflection of the cantilever. Remember to add to the initial dead load deflection an amount that will cover the long term creep and shrinkage deflections. ACI has an empirical formula for this. Adding bottom bars to the cantilever will reduce these long term deflections somewhat.

4. Consider upward cambering of the cantilever to overcome initial dead load deflections as well as a portion of the long term deflections. Be conservative here. I once designed a concrete sskywalk between two buildings. The initial and long term calcs showed a deflection of approximatetely 1 1/2" at midspan. The builder at the site doubled the camber to 3" as he was a little nervous. If you go there today, there is a slight sag downward in the walkway.

 

[IJR]

Thanx JAE for the info and respects for the way you outlined it.

regs
IJR

 

[ishvaaag]

Really there's no special problem in making cantilevers as lond strength and serviceability are met. It seems you can meet them along the guidelines above, and you could even hyperestatically link the landing to the cantilever for some trust effect; that is RC about, great inertia and binding or continuity; but don't do if that leads you to uncozy design unknowns.

Really the deflection issue is something about we mostly can only provide estimates, and it is clear that for long life buildings of RC creep has much to do respect ruin (flat cupolas, for example). The cumulative effect of creep and shrinkage can mean deformations 5 times those evaluated as if elastic with Ec of 28 days age...yet since the average is lower, it is far common settle at about half this. I have seen old continuous foundation beams apparent over the (too soft it seems) ground follow undulating deformations of the same; have seen also a simple span RC beam of early years of the XX century sagging maybe over 12 cm in 4 m or so. Concrete creeps, yes.

 

[breaks]

This topic immediately reminded me of Frank Lloyd Wright's Falling Water in PA. He designed the entire structure, which included some huge cantilevered RC beams. He was insulted when the client went behind his back to have these beams checked by a structural engineer (maybe that's why it should be renamed "Falling House&quot. Apparently, the contractor couldn't believe the small amount of steel in these beams and threw in 2 or 3 times more (for good measure). For whatever reason, these beams have deflected greatly over the lifespan of the structure and are now being repaired.

There's a great article on it in the July/August issue of Structure, put out by the Structural Engineering Institute. I urge everyone to check it out.

 

[ishvaaag]

Yet I have recently read in an architecturally oriented publication (Phaidon, Fallingwater) the big amount of placed steel was so great as to more damage than benefit the integrity of the outfit, in more than producing non-accounted excessive weight. It also makes a positive point on other technical displays of Wright there.

Without the details, I won't argue nor for nor against.

 

[JAE]

The amount of tension rebar in a concrete beam has very little effect on the weight and loading of a concrete beam. I was quite astounded to read that the great one (Wright) actually used the argument of too much rebar to defend himself when his cantilever designs started sagging.

Reinforcing on the compression side of a beam actually reduces deflections and subsequent engineering reviews have all agreed that there wasn't enough reinforcing in the cantilevers.