×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Effect of Cantilever slab!!!

Effect of Cantilever slab!!!

Effect of Cantilever slab!!!

(OP)
I am performing an analysis of a building which has cantilever slabs.. when i remove the cantilever slab I get quite reasonable reinforcement area. but with the canti slab the required reinforcement area increases. Is it normal to increase the reinforcement or are there any techniques regarding loading to cantilever slabs

canti of almost 3 ft ( 1m)

RE: Effect of Cantilever slab!!!

Cantilever slabs are particular to alternate loading conditions and make sure you have enough reinforcing in the back span.

Dik

RE: Effect of Cantilever slab!!!

The bottom bars in the backspan should not increase due to a cantilever. If they do, there is something wrong with your program, or you don't know how to use it. The top bar requirement will of course increase.

RE: Effect of Cantilever slab!!!

Hokie... might depending on alternate loading conditions... with max load on backspan and min on cantilever... bot rfg can increase. Also, length of top bars can increase into the backspan with alt. loading with max on cant and min on backspan.

Dik

RE: Effect of Cantilever slab!!!

dik,
That logic escapes me. With the same span, adding a cantilever will always relieve load in the span, so the bottom reinforcement would not increase. I didn't take the OP's question to be about patterned loading, but rather about the effect of a cantilever.

RE: Effect of Cantilever slab!!!

Where is the the reinforcing increasing? Comparing a simple span to one with a cantilever, the top steel should increase. Not sure of your loading, and thickness, but bottom reinforcing should not be any more than a simple span, unless you have a heck of a uplift on the cantilever.

RE: Effect of Cantilever slab!!!

(OP)
sorry I din post which part is facing the problem.. I'm talking about the reinforcement area of column. when cantilever is added reinforcement is increased noticeably which is not practically suitable.

RE: Effect of Cantilever slab!!!

Did the reaction greatly change? or eccentricity of load (ie moment transfer?)

RE: Effect of Cantilever slab!!!

strsaurav90, I think it would be good if you provided more information. If you describe your structural system, the material properties, the configuration of the cantilever with respect to columns, walls, structural floors, etc., we might be able to better offer "tips".

A sketch attached to your post would also be of help.

RE: Effect of Cantilever slab!!!

As JAE suggested, a sketch would be the order of the day.

I'm also surprised that the column is being so adversely affected by a 3' cantilever. Sometimes, I intentionally introduce a cantilever to help out with punching shear and/or the design of the adjacent slab and column.

Could it be that, in switching your case from no cantilever to cantilever that you're also changing the boundary condition of your columns from pinned to fixed?

Assuming that your column design is blowing up because of excess moment, one technique that many designers use is to reduce the effective moment of inertia of the perimeter columns such that they attract less moment. I don't remember the magnitude of the modifier but the justification is that, since columns have a relatively low P/M ratio, it makes sense to assume that their moment of inertia is a good deal less than Igross.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Effect of Cantilever slab!!!

Sorry Hokie... a misunderstanding... I was thinking we already had a slab with a cantilever... by alternate loading you can increase the top backspan reinforcing length and by alternate loading you can increase the backspan +ve moment as well as the bar lengths.

Dik

RE: Effect of Cantilever slab!!!

No problem, dik. At least we were both talking about the question asked, and now we find that was the wrong question. Go figure.

RE: Effect of Cantilever slab!!!

(OP)
HAHA.. DIK AND HOKIE66 sorry for the misunderstanding.
JAE, I will post the sketch!!

RE: Effect of Cantilever slab!!!

(OP)
and the sap model is below..
that canti part.. is there any idea about loading
I assigned area load to canti slab as
Uniform to frame(shell) . 2 kN/m(^2) two way
finish load : 0.75 kN/m^2 two way
and i did not mesh the cantilever slabs

RE: Effect of Cantilever slab!!!

Can you run a hand calc and figure it out that way to compare to the computer model?

RE: Effect of Cantilever slab!!!

(OP)
ztengguy ... which method would be appropriate? moment distribution or portal?

RE: Effect of Cantilever slab!!!

Moment distribution if you want to be that fancy. Portal's better suited to lateral loads. You could probably ballpark this pretty well just using the direct design method in ACI.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Effect of Cantilever slab!!!

(OP)
haha.. moment distribution will make me sweat my pants... Portal method would come handy ..
What if I just ignore the cantilever part? :P

RE: Effect of Cantilever slab!!!

Sure. Include the weight of the cantilever slab but not the counterbalancing moment that it provides. That ought to be fine for column and punching shear design. Of course, the moment contributed by the cantilever is statically determinate and thus nice and easy to calculate.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Effect of Cantilever slab!!!

(OP)
thanks kootk. ..
I am a BE graduate .. and has lot more to study and find out the solutions.. Thanks for your help

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources