existing elevated concrete slab 2

[deereman]

I've been asked to analyze an existing elevated slab to be used as storage. The slab itself is 6" thick (d=5.75) 3000 psi with #4 @ 6" o.c.spanning 12'-10" from concrete beam to concrete beam. Temp steel in other direction is #3 at 10" o.c. The original design drawings from 1968 say the floor is designed to a live load of 140 psf. Based on moment i get an allowable live load of approx. 120 psf and based on shear it can't withstand its own dead load. Can someone check behind me. I can't believe this slab was undersized by this much. Was shear strength calculated different in 1968? Also, the majority of the beams on one side have exposed reinforcing steel. It looks like some chemical previously used caused spalling of the concrete(these will be replaced.) The beams everywhere else look in good shape and can withstand 140psf. Is a visual inspection of these 'good beams' sufficient or should I require some testing to be done?

 

[rapt]

I think you should redo your calculations. According to me shear works very easily and strength just works for 140psf with 40ksi rebar.

Deflection, on the other hand is definitely a problem.

This all assumes it is a 1 span slab. If it is continuous it is better off.

 

[henri2]

Three questions:

1. Are the size and spacing of top and bottom flexyral steel the same...i.e No.4@6 in o.c ?

2. If the slab is 6 inches thick, size of tension rebar 1/2 in, when cover is taken into account, how do you come up with effective depth d of 5.75 in?

3. Have you considered taking some cores? Perhaps this may allow you to increase the value of f'c used in your analysis.

 

[henri2]

One thing I forgot to mention, if your analysis (core data included...ACI 318 Sec 5.6.5) indicate there could be problems say with serviceability, you could always recommend as a last resort, load testing...ACI 318 Sec 20.3

 

[deereman]

Response to rapt
with 140 psf I get Mu=6.99k-ft, phi Mn=6.59 k-ft, Vu=2.02K, phi Vn=.28k. What numbers are you getting?

answers to henri2
1.) there is only reinforcing in the bottom of the slab.
2.) good question, i looked more closely at the drawings and it shows d=5.75 and 3/4" from the centerline of the reinforcing to the bottom of the slab and calls out the slab thickness as 6". If you add these numbers up though you get a 6 1/2" thick slab.
3.)I may take some cores or do testing.

Thanks for your help.

 

[JAE]

Here's what I get:

DL of slab - 12.5 x 6 = 75 psf
LL on slab - 140 psf

Factored load on 1' strip of slab
75(1.4) + 140(1.7) = 343 plf = .343 kips/ft

Shear at end
Vu = .343 x 12.83/2 = 2.20 kips
Shear capacity
[φ]Vn = 0.85 x 2 x sqrt(3000) x 12 x 5.75 = 6424 kips = 6.42 kips > 2.2 kips OK

Moment at midspan
Mu = .343 x 12.83^2 / 8 = 7.06 ft-kips
Required As = 0.43 sq. in.
Min As = 0.35 sq. in.

#4 @ 6" o.c. = 0.40 sq. in. < 0.43 sq. in. This may be due to the original engineer using ASD instead of ultimate design and the high 140 psf live load penalizes you in ultimate design with the higher LL factor.

 

[henri2]

A few more questions if you don't mind...I find this educative.

1. Is this single span or multi-span?

2. Assuming you are using ACI 318...what edition are you using...is this a ASD or SD approach?

3. In computing wu what lad combination did you use 1.2D + 1.6L...1.4D+1.7L..etc where D = 75 psf and L = 140 psf..if not what did you use?

4. For Mu did you use one of the approximate ACI 318 equations written in the form Mu = wu*ln(squared)/denominator..if so, what was the denominator..did you use clear span ln=12.83'?

5. Do you intend to visit the site and verify the actual thickness of the slab and make an estimation of the cover for the bottom flexural rebar?

 

[henri2]

JAE, seems as if you are a mind reader because while I was typing my response, you were providing answers to some of the questions I had.

BTW, what of the discontinuous ends of the slab...won't there be a bit of fixity...thereby creating some tension at top of slab...which would require some top flexural steel?

 

[JAE]

henri2 - now that you mention it - the condition described does state that it spans from beam to beam. If it is a slab cast monolithically with concrete beams, then ya...there'd be some level of negative moment. If there are other slabs on either side, then definitely I'd design it as a continuous slab.

My calcs above are just for a simple span as that's all the time I had at that moment to post. But a continuous span would posses more capacity so the simple span is conservative.

 

[deereman]

I had the flu last week when I was trying to work on this. things seem much clearer when you get off the drugs and you feel better. I agree with everyones calcs now. I forgot to tell you about a concrete topping above the 6" slab. It slopes to provide drainage. it varies up to 2" thick. They will be driving forklifts around on this slab. Each pallet weighs about 1600 lbs. applying a point load and adding 2" of additional concrete causes the slab to be overstressed (still assuming a 6" slab, because the topping slab wasn't poured integrally with the original slab.)

Do you think these slabs and beams could have been affected any by the chemicals that destroyed the other beams?

 

[jike]

Concentrated loads from the fork lift wheels often create much greater stresses than the uniform load. I would recommend checking that.

 

[cvg]

check your pallet loading also, typical forklifts can pick up more than 1,600 lbs loaded on a pallet.

 

[henri2]

So your DL has increased, the d is lower than originally assumed and moving point load(s) to contend with, which I presume will increase shear when close to a support and maximimize BM when at midspan.

Let us know what the results of your new computations are.

 

[AggieYank]

I copied JAE's post and ran it using my numbers (1.2 and 1.6 load factors, with 0.75 for shear and 0.9 for bending):
Here's what I get:

DL of slab - 12.5 x 6 = 75 psf
LL on slab - 140 psf

Factored load on 1' strip of slab
75(1.2) + 140(1.6) = 314 plf = .314 kips/ft

Shear at end
Vu = .314 x 12.83/2 = 2.01 kips
Shear capacity
?Vn = 0.75 x 2 x sqrt(3000) x 12 x 5.75 = 5.98 kips > 2.01 kips OK

Moment at midspan
Mu = .314 x 12.83^2 / 8 = 6.46 ft-kips
Required As = 0.43 sq. in. (based on fy = 40 ksi)
Required As = 0.29 sq. in. (based on fy = 60 ksi)
Min As = 0.144 sq. in. (slab min. is less than beam min.)

With a pointload:
DL = 75 psf
LL = 20 psf (probably unconservative)
Maximum allowable pointload (with #4 (40 ksi) @ 6" o.c. @ 0.75" from bottom of slab) = 550 lb

Max Moment: 6.04 k-ft (based on wL^2/8 + PL/2)
Max Shear: 1.31 k (based on 0.95*P + wL/2)

Max Allowable Moment: 6.06 k-ft
Max Allowable Shear: 5.98 k
Max Allowable Punching Shear: 24.9 k (assuming a 1" x 1" area, conservative)

NOTES: Slab wasn't checked for deflection. Assuming a simple span with freely rotating ends is unconservative (as JAE mentioned). If the beams are analyzed for torsion, a lower value than wL^2/8 could be used, based on judgement. If analyzed properly for a forklift, a wider width of slab could be used, but that would have to be calculated. I would definitely not allow a forklift on this slab. deereman, you mentioned the slab didn't work in shear. Were you using the 1/2 the allowable load JAE and myself used (which is how it is for unreinforced beams?) This doesn't apply for slabs.

 

[UcfSE]

If you look at ACI table 9.5(a), teh minimum recommended thickness for a slab 154" simply-supported is L/20, or 7.7". At 6" your slab may be ok but you definitiely have to check especially with such a high live load and forklift traffic.

If you look at the phi*Vn capacity provided in your 9 Dec post you'll see that you are off by an order of magnitude from the other posters. That suggests your calc or a math error.

One thing about forklift wheels are they are sometimes the non-pneumatic hard tires. These can exert very high bearing stresses on the concrete and cause wearing problems. If you're expecting a lot of forklift traffic, they may wear down your topping relatively quickly if it isn't a pretty tough mix. I agree with Aggie, I wouldn't allow forklifts on this slab without some new strength.

What is the span in the other direction? Are you actually saving a lot of material by keeping the existing slab instead of replacing it?

 

[deereman]

I used ksi in my concrete strength instead of psi. that is why the shear strength was so low. unfortunately enercalc seems to have an error that does this same thing.