Civil Engineering Reference
In-Depth Information
3. Check limitations for slab analysis by coefficients (ACI 13.6.1)
3 continuous spans in one direction, 5 in the other
rectangular panels with long-to-short span ratio = 24/20 = 1.2 < 2
successive span lengths in each direction are equal
LL/DL = 50/142 = 0.35 < 2
slab system is without beams
Since all requirements are satisfied, the Direct Design Method can be used to determine the moments.
4. Factored moments in slab (N-S direction)
(a) Total panel moment M
o
:
M
o
= q
u
˜
2
˜
n
2
/8
= 0.245 x 24 x 18.83
2
/8 = 260.6 ft-kips
where q
u
= 1.2(142) + 1.6(46.5*) = 245 psf
˜
2
= 24 ft
˜
n
(interior span) = 20 - 1.33 = 18.67 ft
˜
n
(end span) = 20 - 0.67 - 0.50 = 18.83 ft
Use larger value of ˜
n
for both spans.
(b) Negative and positive factored moments:
Division of the total panel moment M
o
into negative and positive moments, and then, column and
middle strip moments, involves direct application of the moment coefficients in Table 4-2.
End Spans
Interior Span
Slab Moments
(ft-kips)
Exterior
Negative
Positive
Interior
Negative
Positive
Total Moment
67.8
135.5
182.4
91.2
Column Strip
67.8
80.8
138.1
54.7
Middle Strip
0
54.7
44.3
36.5
Note: All negative moments are at face of columns.
* Live load reduction: A
I
(1 panel) = 24
x
20 = 480 sq ft
480
L = 50(0.25 + 15/
) = 46.5 psf
Search WWH ::
Custom Search