5.18 Direct design method

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The direct design method is a simplified method for the design of concrete

slabs that is applicable if the slab consists of (not too overlong) rectangular

panels and meets other nominal requirements; see for example ACI 318R-95.

Although the method is restricted to evenly distributed loads, it allows an

easy check of FE calculations; see Fig. 5.57.

Other techniques are grid-framework methods and the elastic frame method.

The simplest technique is the slab strip method, in which the slabs are assumed

to carry the load in only one direction (one-way slabs).

The slab in Fig. 5.58 was analyzed a) with finite elements, b) the direct

design method, and c) as a system of one-way slabs. The original design called

for a slab with a thickness of 14 cm. In the “manual method” (one-way slabs),

the T beams had to be analyzed separately. To meet the limitations of the direct

design method the slab thickness was changed to 18 cm. In the following

table the total weight of the reinforcement resulting from the different design

procedures are listed.

Upper Lower Total Difference

Area = 171.53 m2 kg kg kg kg/m2 kg

FE h = 14 cm 378.1 678.0 1056.1 6.16 +20.9

FE h = 18 cm 350.1 664.7 1035.2 6.03 0

Direct design method 557.5 688.1 1245.6 7.26 +210.4

Uniaxial 599.9 662.2 1262.1 7.36 +226.9

5.19 Point supports 477

boundary elements

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