Exist. Concrete Floor Slab

[shuff213]

We're replacing a mechanical unit in a second story mechanical room at a school built in the late 1950s.

According to the existing drawings, the floor construction is "2.5 inch concrete slab on Steeltex Floor Lath". The floor framing below this slab is SJ146 joists spaced not to exceed 2.5 feet.

Although my first concern was the allowable capacity of the existing floor joists, I am not wondering about the span of the floor slab between the joists. Is there any way to determine the capacity of this unreinforced floor?

 

[GoBizon]

"Although my first concern was the allowable capacity of the existing floor joists, I am not wondering about the span of the floor slab between the joists." Is there a typo here???

"Is there any way to determine the capacity of this unreinforced floor?" Try Chapter 22: Structural Plain Concrete of the ACI or a comparable present day form deck capacity (assuming the depth and spacing of flutes are known).

 

[shuff213]

Sorry, that is supposed to read "now wondering...".

 

[GoBizon]

Does the unit sit on a continuous curb? If so, the load to the slab (bearing vs bending/shear) may be marginal with respect to the loading on the joists. If the slab were removed, would the unit and curb still rest on the joists? The opposite would occur if there was not a curb and the unit imposed point loads on the slab. Just my thoughts.

 

[structuresguy]

There may not be a curb for an interior unit. My experience usually has them sitting directly on the slab. Well, actually on an equipment pad raised up 6" above the slab.

SO if there is no existing equipment pad, you could add a 6" pad on to the existing slab, and reinforce your pad to span from joist to joist. Then the strength of the slab is not relevant for the wieght of the unit.

 

[BAretired]

I don't think you can rely on the existing slab to carry the new unit. Either add a concrete curb as suggested by structuresguy or use steel framing to span between joists.

BA

 

[csd72]

BAretired has summed up the two usually options correctly but I would like to highlight a third option that may be more suitable for your situation.

If it helps avoid the need for joist strenghtening then you may want to look at providing a steel spreader beam above the joists that transfers the load to as many joists as necessary. It is often possible to do this without taking up too much space or creating a great deal of trip hazards.

 

[hokie66]

shuff,

Are you certain the floor is unreinforced? There are a lot of 2 1/2" floor slabs around, placed on non-composite corrugated deck and reinforced with light wire fabric. It alway was difficult to justify their behaviour based on normal reinforced concrete design practice, but most have performed satisfactorily.

 

[paddingtongreen]

"Steeltex Floor Lath (Style 48 – 12) has a sturdy cord reinforced waterproof backing with 3” x 4” – 12 ga. galvanized electrically welded mesh. It is supplied in rolls 48” wide x 125’ long for a total area of 500 square feet."

http://www.ivysteel.com/lathing_products.htm

It looks as though it is normally used in swimming pools. Legally, I would think the mesh is worthless because of lack of code bond. I could understand if there was a structural element underneath, and the concrete for finishing.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

The cost of adding a few steel members to span 30" between joists is trivial. Why take a chance with a floor slab you are not certain of?

BA

 

[shuff213]

The unit does sit on a steel curb. Thanks for all the feedback.