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Structural Concrete - Hassoun

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17.8 Analysis of Two-Way Slabs by the Direct Design Method 637<br />

To determine the design factored moments in continuous structures, the ACI Code,<br />

Section 6.4.3, specifies the following:<br />

1. When the loading pattern is known, the equivalent frame shall be analyzed for that load.<br />

2. When the unfactored live load is variable but does not exceed 3 of the unfactored dead load,<br />

4<br />

q L ≤ 0.75 q D , or when all the panels is almost loaded simultaneously with the live load, it is<br />

permitted to analyze the frame with full factored live load on the entire slab system.<br />

3. For other loading conditions, it is permitted to assume that the maximum positive factored<br />

moment near a midspan occurs with 0.75 of the full factored live load on the panel and alternate<br />

panels. For the maximum negative factored moment in the slab at a support, it is permitted<br />

to assume that 0.75 of the full factored live load is applied on adjacent panels only.<br />

4. Factored moments shall not be taken less than the moments occurring with full factored live<br />

load on all continuous panels.<br />

17.8.6 Reinforcement Details<br />

After all the percentages of the static moments in the column and middle strips are determined, the<br />

steel reinforcement can be calculated for the negative and positive moments in each strip, as was<br />

done for beam sections in Chapter 4:<br />

(<br />

M u = φA s f y d − a )<br />

= R<br />

2 u bd 2 (17.15)<br />

Calculate R u and determine the steel ratio ρ using the tables in Appendix A or use the following<br />

equation:<br />

(<br />

R u = φρf y 1 − ρf )<br />

y<br />

1.7f c<br />

′ (17.16)<br />

where φ equals 0.9. The steel area is A s = ρbd. When the slab thickness limitations, as discussed<br />

in Section 17.4, are met, no compression reinforcement will be required. Figure 13.3.8 of the<br />

ACI Code indicates the minimum length of reinforcing bars and reinforcement details for slabs<br />

without beams; it is reproduced here as Fig. 17.16. The spacing of bars in the slabs must not<br />

exceed the ACI limits of maximum spacing: 18 in. (450 mm) or twice the slab thickness, whichever<br />

is smaller.<br />

17.8.7 Modified Stiffness Method for End Spans<br />

In this method, the stiffnesses of the slab end beam and of the exterior column are replaced by the<br />

stiffness of an equivalent column, K ec . The flexural stiffness of the equivalent column, K ec , can be<br />

calculated from the following expression:<br />

where<br />

1<br />

= ∑ 1 + 1 or K<br />

K ec Kc K ec =<br />

t<br />

K ec = flexural stiffness of equivalent column<br />

K c = flexural stiffness of actual column<br />

K t = torsional stiffness of edge beam<br />

∑<br />

Kc<br />

1 + ∑ K c ∕K t<br />

(17.17)

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