Expert: Bruce E. Johnson - 10/24/2008

Question
I Have to design a building but i am not sure from where i start i have to design the roof of parking which is of concrete slab the building dimensions are 50' x 100 ' not providing any column so how can i do it this is a start of my carrer and i am very upset?  please help

Answer
Hi Tarun Garg,
A 50 foot clear span is a long distance for a concrete slab unless you either incorporate intermediate support beams or use a steel bar joist system with corrugated metal decking topped with concrete.

A 50 foot concrete beam is going to have to be substantial, probably at leat 5 feet deep by twelve inches thick with some major reinforcing steel. You would probably need a  beam every 20 feet with a 6" slab integrated into the tops of the beams (the depths of the beam would include the thickness of the slab).
This is not a cost effective system.

Prestressed concrete can be used also.  Prestressed concrete comes in 36" wide slabs and are 6-8" thick.  They are hollow and extruded and have to be filled with concrete to tie them together.  This is a more cost effective system.

Twin tees are also used for longer spans.  The concrete beams are prestressed and incorporated into the slab sections.

Bar joists spaced at 5-0  with corrugated metal decking topped with either lightweight concrete or regular concrete 3" thick is another competitively priced system.

So is your role going to be designing a cost effective roof system or does it have to be poured concrete?

Feel free to write again regarding this or other matters, sincerely bruce e johnson..And for budding engineers and architects please visit my website...www.bejohnsonconsulting.com