Post load on SOG

[RMPE]

Need design procedure for a post load on an existing SOG with WWF. Load is in 20k range and slab thickness is in 9"-10" range.

 

[ishvaaag]

I check punching by a procedure from the ACI Structural Journal in a mathcad sheet rendition and at 8 in thick, reinforced with a square mesh 6 mm diameter every 15 cm amidst and a 8 x 8 in foot print and the punching load is for a less than 3 ksi concrete 294 kip, whilst the yield lines strength of the reinforcement allows for a limit strength value of the load of about 53 kips. It looks then safe enough even with such modest reinforcement, but it is reinforcement what controls.

I can send you the sheet or a rendering of the calculation if you so want.

 

[RMPE]

Thanks for reply. What issue of ACI Journal? Do you use ACI or PCA charts for axle loads, post loads?? Are they applicable to a single post load? What is the zone of influence of a single post load in the middle of a slab, or near an edge!? Very interested in your reply. A new member as of today and fascinated of the breadth of these forums. Hope to add and learn much. Any pointers to use of resources would be appreciated aswell.

 

[Ron]

RMPE...as matter of practicality, zone of influence about equal (laterally) to twice the slab thickness (from center of post in all directions). Shear area for this would be a bit less than 400 square inches (assuming a 4-inch square post). This gives a shear of around 50 psi (similar to ishvaaag's calcs), which is tolerable for concrete of 3000 psi or greater.

 

[RMPE]

This situation occurs priodically and I've never really seen or got a clear solution in my mind. I'm building a steel platform frame within an existing building and have installed 4 posts (W8x18 with 8x8 base plates) and excavated for footings. Turns out the slab is 10" (footing design is 12&quot, so we're trying to justify not using footings on the other four posts. Will core a hole to verify thickness though.

So if I have a 8x8 base plate and 10" slab (reinforcing at 7&quot. Is it simply 4*(8"+2*(7"=d))*(v~70psi wsd)*10"= 61k?

Then I have 61k/(22"*22&quot=18ksf on the soil! If the soil yields, I engage more slab area and then do I need to consider flexural cracking of the slab?? If the platform is used for temporary storage and fully loaded (post load may be 40k if we can accommodate) and unloaded, does this
affect the design?? I suppose it becomes a fatigue problem.

 

[ishvaaag]

I think the reference I used is

ACI SJ Vol. 93 Nº 5 September-October 1996
p. 503
Analytical Computation of the Punching Strength of Reinforced Concrete
Philippe Menétrey

For the yield lines capacity I used a look-up table of formulas in a Jiménez Montoya's "bible" RC text.

Of course after verification of the punching and bending capacity soil comes... punching however extends -you see in practice twice the depth form the plate (if rigid) and then a 20 kip in a 40x40" inches gives a plastified solicitation more than acceptable by almost any competent soils. Not that I was thinking on this when I answered but should neither be a problem. Of course if you can't find a suitably reinforced area around the point the soil will control and you would need additional work.

 

[DMWWEngr]

RMPE,

According to "Concrete Floors on Ground" - PCA:

Assuming: Load=20000lbs
fc=3000psi
Slab Thickness = 10in
CENTER OF SLAB LOADING
MOR=7.5(fc)^1/2=410psi (ACI-318)
8x8 Base plate (64sqin)

Allowable Bearing Stress:
4.2(MOR)=1725psi

Computed Bearing Stress:
20000/(8*8)=312.5psi

Bearing Stress is OK



Allowable Shear Stress:
0.27(MOR)=111psi

Load Periphery(LP)=4(64)^1/2=32in
Computed Shear Stress:

Post Load/(Slab Depth*(LP+(4*slab thickness))

20000/(10*(32+(4*10))=28psi

Shear stress is OK


The above values neglet the structural effects from any reinforcing in the slab. Not knowing the spacings of your frame...
Assuming: 8ft spacing in each direction
k=100pci
FS=2
Joint Factor=1.6

The PCA suggested slab thickness is nearly 10" exactly. Looks like a 20k load will work according to PCA standards.


While a 40k load is acceptable concrete "stress-wise" you could have problems with the soil. PCA suggests using experimental data and deflections to determine if the soil is acceptable (although they don't list a reference to do this?).

P.S. You used the wrong load (61k) in calculating your soil stress above. Plus a larger soil area would resist the load than a 22"x22" area (as ishvaaag mentioned).

HTH!! ---
Andrew