Expert: Arshad Khan - 3/26/2010

Question
Dr Khan,

I am told that PT slab is the most cost effective structural system, but I am a little skeptical about this.  I believe that PT slab is only cost effective upto a certain point.  But i do not know where this point is in oder to determine whether to use beam/slab or PT slab.

In my case, we have spans of 10m with live loads of 5kPa and SDL of 6kPa.  Is PT slab still considered the most cost effective since published literature tells me I will require L/D of approx 30-35 hence a slab thickness of approx 300mm.  I have carried out an assessment weight per floor based on the above and find that beam/slab gives me more savings in terms of concrete required (one way slab 160mm thick supported on beam approx 400x600 @ 3.33m c/c), which translate to further saving in foundation.

The only difference therefore is the saving in the steel reinforcement.  Is this saving significant without going further into the more detailed design?

Please can you give me your opinion.

Kind Regards,

Marcus

Answer
Hi Marcus,

You are quite correct. In purely terms of materials (concrete/rebar), RC slabs are more economical than PT. In my experience, for spans of between 6-10m, a one-way ribbed slab (with down-stand beams)is about the most economical.

However, if you take other considerations into account, such as speed of construction, cost/time for shuttering and floor-to-floor heights (where a flat slab is beneficial to keep heights to a minimum, with least restrictions to MEP ducts, etc), then flat slabs (PT or RC) may be desireable.

The decision can vary due to local market conditions- if labour is relatively cheap, then RC may be cheaper- in Europe and USA, flat slabs (RC or PT) may be cheaper all round. Remember also that PT cannot be used in all circumstances (where there are voids or restraint to comopression of slab).

These days building green is in fashion, which means minimising materials, so perhaps speed will be given less importance. Also, in high seismic zones, the lighter the floors, the better!

This is a very good publication, but for the Europe market:

http://www.scribd.com/doc/19184820/Economic-Long-Span-Concrete-Floor-Slabs