How comfortable are you with long RC cantilevers?
How comfortable are you with long RC cantilevers?
(OP)
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
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





RE: How comfortable are you with long RC cantilevers?
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.
RE: How comfortable are you with long RC cantilevers?
regs
IJR
RE: How comfortable are you with long RC cantilevers?
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.
RE: How comfortable are you with long RC cantilevers?
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.
RE: How comfortable are you with long RC cantilevers?
Without the details, I won't argue nor for nor against.
RE: How comfortable are you with long RC cantilevers?
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.