Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Suspended slab supported on three sides

Status
Not open for further replies.
ataman

ataman

Structural
Dec 7, 2006
53
Hi,

Can anyone give me some assistance in designing a slab on three sides?

I have a 7'-0"x15'-0" balcony and the architect does not want to have a front edge beam. I can either design as a cantilevered slab however I would like to keep the slab thickness to 6". I was considering cantilevering two beams but I'm not sure this will make much of a difference because of the aspect ratio. Can anyone give some advice on designing slabs supported on three sides and if this would make any difference.

Suggestions are also welcome.

Regards,
Adrian
 
Replies continue below

Recommended for you

Sort by date Sort by votes
The architect also doesn't want any columns at the front of the balcony.
 
Upvote 0 Downvote
I am assuming the 7'0" is cantilevering out the 15'0" is the width of the balcony.
I would just cantilever the slab for the 7'. That is not that long of a cantilever and you shouldn't have any trouble keeping the slab at 6" thick.
Doing a very rough check of 100 psf LL, I am getting somewhere in the neighborhood of a #5 @ 12".
 
Upvote 0 Downvote
Thanks MYerges ,

Your assumption is right however, what about deflection? Isn't that an issue, esp. since aci recommends an minimum of L/10 for slab thickness in lieu of more detailed assessment.

Regards,
Adrian
 
Upvote 0 Downvote
ataman-
Yes, you will have to do a quick deflection check (probably by hand). I just did it with #6 @ 12" and the tip is under L/360 for total load.
 
Upvote 0 Downvote
The tip deflection will depend on the backspan, eg. you could get upward deflection with live load inside only.
You could look at a tapered slab thickness; 7" at the building tapering to 5" at the tip results in better performance for the same concrete volume.
 
Upvote 0 Downvote
What about the influence of the boundary conditions along the sides that are 7 feet long?
 
Upvote 0 Downvote
Clefcon

one way spanning slab is actually worse.

Henri2

I'm not sure what you mean exactly

 
Upvote 0 Downvote
MYerges

How do you do a quick calculation for deflection of concrete? Is there some sort of rule of thumb. The procedure I know is pretty invloved and includes calculating effective moments of inertia which is based on the dead and live load stresses. How do you account for long term deflection.

Regards,

Adrian

 
Upvote 0 Downvote
ataman-
You are correct. The procedure is in section 9.5.2.3 of ACI 318-05. The long term deflection multiplier in is 9.5.2.5. Calculating Ie is not overly time consuming.

apsix-
You can get upward deflection with live load on backspan only, depending on magnitude of LL and length of backspan; however, the greater concern (I believe) is downward deflection. In addition, a tapered slab will perform better for deflection, but 7" to 5" is still not complying with the L/10 rule and deflection will still have to be checked. With the tapered slab, it is a much more time consuming process.
 
Upvote 0 Downvote
Rourke's Formulas for Stress and Strain solves a plate with one free edge. You only need to covert stresses to moments.
 
Upvote 0 Downvote
jmiec beat me to it,

Roarks is a good option, otherwise estimate the proportions by deflection compatability and then check deflection based on the proportional loads.

Make sure you include bars for the one way spanning slab as well since the slab will crack otherwise.
 
Upvote 0 Downvote
'one way spanning slab is actually worse.'

In what sense? 15 foot span for a 6" deep slab is not all that unusual.
 
Upvote 0 Downvote
ataman,

I posed the question regarding the influence of the support provided along sides which are 7 feet long because I was concerned that the slab would not behave as a true 7 ft span cantilever under loading...and some bending would occur in 15ft span direction.

IMO, analyzing the slab as a plate supported on three sides with one edge free as proposed by jmiec should be given some serious consideration.
 
Upvote 0 Downvote
I believe the question is if there is any advantage in adding the 2 cantilevered beams at the ends of the slab.

I understand that beyond a slab aspect ratio of about 2 it wouldn't be worth it, so I would design it as a cantilevered slab without end beams, and not rely on an empirical L/10 thickness limit. It will look better without those beams anyway.

 
Upvote 0 Downvote
use hillerborg's method.. provide a concentration of reinforcement along the free edge of your slab spanning in the 15 foot direction. This stiffer "element" will support load from the 7 ft span. Basically providing a beam the same thickness as your slab.
 
Upvote 0 Downvote
In line with calculor's thinking, a 15' span beam at the outer edge of the balcony which could be a sunken beam within the thickness of the slab as a minimum, or you increase the depth of the beam by going below the slab, or above the slab, or a combination of these like a spandrel beam. So with a beam at the outer end of the balcony now you can have a one-way slab spanning only 7 ft.


Yogi Anand, D.Eng, P.E.
Energy Efficient Building Network LLC
ANAND Enterprises LLC
 
Upvote 0 Downvote
Supports on the outside edges will reduce the cantilever moments by around 25-50% and will reduce deflections by about 50%.

I'm sure deflection is all you are worried about...
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

E
  • Question
Suspended Deck Slab Rectifications
Replies
2
Views
650
engoaus
E
psychedomination
  • Locked
  • Question
Suspended Slab Infill
Replies
1
Views
253
JAE
JAE
Enhineyero
  • Question
T-shaped suspended slab movement joint
Replies
1
Views
147
cliff234
cliff234
ITsSB
  • Question
Simply supported two way slab definition
Replies
2
Views
260
Tomfh
Tomfh
Krishna7568
  • Question
C section beam vs Slab attachment for Suspended ceiling
Replies
0
Views
135
Krishna7568
Krishna7568

Part and Inventory Search

Sponsor

Back
Top Bottom