waffle slab analysis

[engoff]

hi friends
i had designed simply supported waffle slab using ACI-318-2008
its dimensions 11.8 x 12.5 M , i chose 450mm depth and 150mm rib width, with 500mm spacing,DL=3KN/m2,LL=2KN/m2.
its depth is more than required by Code, but when i checked deflection ,it was unsafe ,it gives 100mm (short-term & long-term)
i used alpha & beta of grashoff
what is the problem ????

 

[cap4000]

Try converting your waffle into an equivalent solid slab using moments of inertia and then run the calcs. Good Luck.

 

[asixth]

What is the slab "simply-supported" on cmu walls at four sides and do you have 1 or 2 bottom bars in the ribs?

 

[paddingtongreen]

That depth sounds a little skimpy to me. I had to go to 24" (610mm) for similar spans, admittedly mine were under a road but it was a stretch even then, I had to increase the RC wall thickness to provide a large fixing moment at the supports.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[engoff]

friends, thank u 4 ur replies
this waffle is a panel n R.C. foor ,simply supported by Beams around its 4 sides
just today i found out that i used wrong method (grashoff)
which gives unreasonable deflection.
it is mentioned in ACI 435R-95 to use Timoshinko Method
for two-way Slabs.

paddingtongreen , in my case the loads is normal and not that much as urs
if u checked ACI-318, u can notice that the depth which let u avoid deflection calc. is L/33 =12500/33=378Mmm
so 400 is safe depth, but im more conservative to use 450mm

bottom bars 4 diam. 16mm