existing elevated concrete slab
existing elevated concrete slab
(OP)
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?





RE: existing elevated concrete slab
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.
RE: existing elevated concrete slab
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.
RE: existing elevated concrete slab
RE: existing elevated concrete slab
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.
RE: existing elevated concrete slab
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
φVn = 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.
RE: existing elevated concrete slab
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?
RE: existing elevated concrete slab
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?
RE: existing elevated concrete 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.
RE: existing elevated concrete slab
Do you think these slabs and beams could have been affected any by the chemicals that destroyed the other beams?
RE: existing elevated concrete slab
RE: existing elevated concrete slab
RE: existing elevated concrete slab
Let us know what the results of your new computations are.
RE: existing elevated concrete slab
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.
RE: existing elevated concrete slab
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?
RE: existing elevated concrete slab