design of two-way floor

[bento]

Suppose I have a square slab with overall dimension 18*18 m and supported on columns spaced at 6 m(thus, totally 9 panels). There are beams on all column lines. For the center panel, there are also 2 additional orthogonal secondary beams which subdivide the center panel into 4 sub-panels. I understand that two-way actions are needed. I have a few questions;
(1) Is the ACI equivalent frame applicable for this case in which there are beams not on column center lines;
(2) In ACI design of 2-way, the moment in the column strip is shared between slab and longitudinal beams. Is this the only way to obtain the moments for beam design? I mean is there other method to design the beam separately from slab like in one way slab system.
I would appreciate any response.

 

[IJR]

Bento

It is late hour now for me to give you the best response, however no harm trying:

You seem to be having slabs surrounded by beams, and my friend you seem to be having a good arrangement, almost square.

When slabs are designed and as long as there are beams along edges, then why do you worry about presence of columns? Load path is as follows for this case: slabs to joists to beams to columns and that simple.

Unless I got you wrong, you seem to be having all your slabs as 2-way slabs. Design the slabs as 2 way and compute loads transmitted to joists and/or beams and use the reactions from joists and beams to frame axes. And design your frames. Or carry out finite element analysis of the whole thing if you like

As you can see I babble a lot close to sleep

Good luck
IJR

Equivalent frame method is about slabs that act as beams in the load path. In your case your slabs do not transmit load directly to columns.

 

[phuduhudu]

Bento,

I agree with IJR that you just need to design two way slabs with supports on all four edges, allowing for continuity over the beams. Then you calculate the loads to the beams and design them separately (as T beams since you can mobilise some of the slab to help with the beam). Again you would allow for continuity over supports.

You could do it with an FE model but in my view this would be very cumbersome and also no better than a simple hand analysis unless you allow for varying stiffnesses in slabs and beams due to varying amounts of reinforcement and varying states of stress. The nice thing about concrete is that you can allow a bit of redistribution if you need it too.

There is an interesting case where your beams at mid slab in the central panel end. You might find that there is some cracking in the slab at the end of the beam to allow the release of the end moment as the section capacity suddenly changes. Carl Bauer

 

[IJR]

Now that I am sober I can warn you a bit on load transfer from joist to beam.

If your "joist" rests on the span of a beam then you have a shear transfer problem usually handled with what is known in my area as hanger reinforcement. Any good text book in RC design has got that well explained. You simply have to make sure shear in the "joist" will be safely transferred to the beam

Good luck
IJR

 

[austim]

Hi, IJR,

If your first post was written under the influence of some foreign substance, could you please let the rest of us in on your secret. It sounds as if you have got hold of a supply of vintage common sense; I didn't know that it was readily available nowadays, let alone in bottled form.

 

[IJR]

YeaH austim

thanks for the observation. ý wasnt really on the bottle but I was too tired to see the beautiful babe next to me that hour. Funny I could see Bentos post.

When you mention common sense, I really got very little of that stuff. But I have been pushed around so much and I have developed this chronic reaction against design of most structural component.

Guess what? in my area two way slabs usually tend to vibrate. But this has nothing to do with bad design. Has a lot to do with bad politics. Cause someone tried to make everyone own an apartment by playing with some banking stuff. and then everyone had to design slab on the back on the envelope. Man, the slabs vibrate like the envelopes.

Keep in touch pals (Dave, dont censor this one huh)
IJR

 

[dik]

The old National Building Code (of Canada) had coefficients for the thickness and moment values for the design of slabs cast integral with beams. Will see if I can dig up my old copy of the design manual, it will have a reference for the coef. source. The coefficients varied with the aspect ratio of width/length and for dead and live loading.

The method also required that the supporting beams had a minimum stiffness value based on the spans and slab thickness.

The beams in the centre panel can increase the torsional loads where the beams frame into the columns.