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[AbhiCrux]

Hi Fellow Engineers, I have a query on the above mentioned topic.

ACI 318-19, Fig. 8.7.4.1.3 shows the minimum extension lengths for two way (non-PT) flat slab system. (i.e., 0.33ln & 0.20 ln). Similarly IS code requires 0.33ln & 0.25ln)

Would the same extension lengths apply for two-way PT Flat slab system? If no, can anyone help with the ACI clause suggesting about it. Design is being carried out in ADAPT Builder 2019!

Thanks in advance!!

[vikram.jeet]

ANALYSIS IS UNIVERSAL
DESIGN  IS NATIONAL
DETAILING IS PERSONAL

MODELLING NEEDS TO BE IDEAL

Since design has to be carried as per The specific Country Code where such structure is to be constructed,  for Indian sites, you need to follow IS Code recommendations.

Earlier IS 456 version for general slabs recommended 0.3L ,0.20L but 1978 code revised to 0.3 and 0.25.

[vikram.jeet]

For PT slabs, please also see guidelines in 19.6 of IS 1343 , regarding transmission length.

[pvgraju]

Dear Sir,

Post tensioning can be done for bottom bars since bottom bending will be dominant in slabs. PT will make slab econimical. Generally top bending will be less ,it can be designed like a RC element with 0.3L extension.

These extension lengths are minimum lengths specified in the code. At 0.3l top bending will be zero. So top reinforcement is not needed after that. In case of PT also bending moment values will be same. So extension will be same.

Regards,

P.V.Gavarraju.

[vikram.jeet]

For Cantilever Slabs , the hogging moment at top may require higher top bars reinforcement embedment length at back slab .  Generally it needs to be around 2*L , where L is cantilever span of slab projection. This length takes care of situation ( Full LL on cantilever & No LL on back slab)