Expert: Babak Esmailzadeh Hakimi - 1/26/2012

Question
QUESTION: Dear Sir

Is it possible to design 50'-00" x 50'-00" (15.24m x 15.24)  RC SLAB without intermedite column. waffle slab or any sort of solution pl

ANSWER: Dear   Priyantha

Hi, nothing is impossible. Sure you can design a 15x15 slab and waffle has been used before ( it is for the sake of weight reduction not increase of strength. I can suggest you a X type slab . The X will help you to directly  transfer the loads from middle of slab to columns. But it depends on your architecture too.

Please tell me about your resolution.

Regards
Babak E Hakimi

---------- FOLLOW-UP ----------

QUESTION: Dear sir

There is no any restriction from architectural view.  can you explain something more about X type slab. Is there any  limit  of span (beam or slab)  that we  can safely work with in RC construction

ANSWER: Dear   priyantha

No restriction to have any span in RC slabs. That was an idea, you will have 2 main beams spanning along 2 diagonals. You can even have them prestressed to have reduced sizes. No pressing is allowed under seismic action. Waffle slabs may be applied too, it is depending on what you want.

Please tell me your idea.

Best Regards
Babak Esmailzadeh Hakimi

---------- FOLLOW-UP ----------

QUESTION: Dear Sir

In my case I thought of having main beams in one way (beams at 3.2m interval) and secondary beams at other way( secondary beams also at 3.2m intervals, load from secondary beam act as point load to main beam)

slab thick 150mm
main beam 350x850
secondary  350x350

other than normal calculation I thought of following
*torsion r/f due to deep beam
*In case of deflection basic span/depth ratio multiplied by 10/span

is there any special considerations that I have to attend

In case of long span beams what are the critical points that  structural engineer should concern
Can you guide me to any books or research papers in this regard

I like to know some details with regard to failure of long span RC beams and the reasons

Thank you Sir

Answer
Dear Priyantha

The main feature one has to consider in design of long span slabs and girders is the limitation of deflection + vibration control of roof if it is subjected to human live load. Then you must provide your system with enough stiffness to reduce the frequency of vibration of your system to a degree insuring it being far from human sensation. You can find this topic in Gaylord & Gaylord.

A very important problem to consider in such a big spans is to control the resonace effect of the roof under harmonic loading. In dance halls this may be source of catastrophe if neglected.

I would suggest you to apply prestressing to control the sizes.

Best Regards
Babak E Hakimi