Building concrete slab

[kngpenn]

I have a 4 story building that is 119' long by 25' wide. The lower floor has an elevated slab. Which is more economical?

1. Design the slab as a 1 way slab in the 25' direction?
2. Add beams down the 119' direction and design as 1 way slab.
3. Do number 2 and design as 2 way slab.

 

[Ingenuity]

All depends. Does the 'lower floor elevated slab' carry the 3 levels above and act as a podium-type slab?

25' single one-way span slabs will results in pretty thick slabs, that may not be economic, depending on your location and its construction practices.

Continuity via incorporating 'supports' (columns, beams etc) will assist in reducing slab thickness, at the expense of increasing forming costs. If this is a prelim design stage, I have often checked out/presented two options and do prelim costing (or give it to the contractor for pricing info, if a GC is on-board) then base the final design on the selected system.

All adding beams, columns often has impacts to M/E/P and the architect usually has some significant input too.

 

[kngpenn]

The upper floors will have floor trusses spanning the entire 25' width to bear on wood exterior walls. The elevated slab will only carry residential loading and have concrete walls on all four sides as support. Im thinking adding beams and design as one way slab is the way to go.

Project in DC

 

[KootK]

For such a regular building, these one way spanning options might be attractive:

1) Pan joists.

2) Topped precast hollow core planks.

If you have concrete perimeter walls that would be located beneath your beams, I'd definitely be looking to eliminate the beams.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JedClampett]

Double posting is discouraged. I gave some admittedly brilliant advice to this question in the Concrete Engineering Forum. By coincidence, it matches most of the answers above.

 

[DETstru]

If you can post tension the slab you could make it pretty darn thin in the 25' direction.

If you can't do that, I would put 25' long beams every 10' to 15' or so and span a one-way continuous slab in the 119' direction.

 

[Ingenuity]

If you can post tension the slab you could make it pretty darn thin in the 25' direction.

But PT with tendons of 25' long are very UNeconomic. You want to optimize your anchorage hardware/fabrication costs over the longest possible tendon length, and also keep installation and stressing (labor) costs to a minimum by have longer tendon lengths.

 

[DETstru]

Ingenuity, very true. Just throwing it out there in case a thin slab is desirable.

 

[rapt]

Depends on what you call "pretty darn thin". If properly designed, a PT slab spanning 25" simply supported should be at least 9" thick and possibly more!

 

[hokie66]

I would tend to use band beams across and a one way slab in the long direction. As to number of slab spans, do the optimization.