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Structural Concrete - Hassoun

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17.8 Analysis of Two-Way Slabs by the Direct Design Method 635<br />

where x and y are the shorter and longer dimension of each rectangular component of the section.<br />

The percentages of each design moment to be distributed to column and middle strips for interior<br />

and exterior panels are given in Tables 17.3 through Table 17.6. In a typical interior panel, the<br />

portion of the design moment that is not assigned to the column strip (Table 17.3) must be resisted<br />

by the corresponding half-middle strips. Linear interpolation of values of l 2 /l 1 between 0.5 and 2.0<br />

and of α f1<br />

l 2 ∕l 1 between 0 and 1 is permitted by the ACI Code. From Table 17.3 it can be seen that<br />

when no beams are used, as in the case of flat plates or flat slabs, α f1<br />

= 0. The final percentage of<br />

moments in the column and middle strips as a function of M 0 are given in Table 17.4.<br />

For exterior panels, the portion of the design moment that is not assigned to the column strip<br />

(Table 17.5) must be resisted by the corresponding half-middle strips. Again, linear interpolation<br />

Table 17.3<br />

Percentage of Longitudinal Moment in Column Strips, Interior<br />

Panels (ACI Code, Section 8.10.5)<br />

Aspect Ratio, l 2<br />

/l 1<br />

α f1<br />

l 2<br />

∕l 1<br />

0.5 1.0 2.0<br />

Negative moment at interior support 0 75 75 75<br />

≥1.0 90 75 45<br />

Positive moment near midspan 0 60 60 60<br />

≥1.0 90 75 45<br />

Table 17.4 Percentage of Moments in Two-Way Interior Slabs without Beams (α 1<br />

= 0)<br />

( ) l 2<br />

n1<br />

Total Design Moment = M 0<br />

=(w u<br />

l 2<br />

)<br />

8<br />

n!<br />

r!(n−r)!<br />

Negative Moment<br />

Positive Moment<br />

Longitudinal moments in one panel −0.65M 0 ±0.35M 0<br />

Column strip 0.75(−0.65M 0 ) =−0.49M 0 0.60(0.35M 0 ) = 0.21M 0<br />

Middle strip 0.25(−0.65M 0 ) = 0.16M 0 0.40(0.35M 0 ) = 0.14M 0<br />

Table 17.5<br />

Percentage of Longitudinal Moment in Column Strips, Exterior Panels<br />

(ACI Code, Section 8.10.5)<br />

Aspect Ratio l 2<br />

/l 1<br />

α f1<br />

l 2<br />

∕l 1<br />

β t<br />

0.5 1.0 2.0<br />

Negative moment at exterior support 0 0 100 100 100<br />

≥2.5 75 75 75<br />

≥1.0 0 100 100 100<br />

≥2.5 90 75 45<br />

Positive moment near midspan 0 60 60 60<br />

≥1.0 90 75 45<br />

Negative moment at interior support 0 75 75 75<br />

≥1.0 90 75 45

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