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Design of one-way RC slab

A reinforced concrete slab is a large flat plate, the slab will transfer the load to beam and beam to column then to building a foundation, if the slab is supported by beams in two opposite sides only then the slab is one-way, one-way slab because the bending is in one direction only, if the slab is supported by beam in four directions and the ratio of long side length to short side length is more than 2, it is considered as one-way slab also, because the bending primarily occur in short direction,

Figure 1

 one-way slab assumed as a beam with high width-to-depth ratio, one-way slab design by separating (12 in) or 1 m strip in the long direction as shown in figure 1,  then this portion is designed as a beam, the main reason for using 12 in (1m) width strip is quite convenient and helpful because the load usually specified as pound per square foot(kn/m2). if assumed the slab will support 150 lb/ft2, then the strip will support a linearly distributed load of 150*1=150/ft.

the one-way slab will transfer the loads including self-weight to support beam, therefore the flexural reinforcing steel should be placed perpendicular to supporting beam as shown in figure 2. According to ACI 7.6.5 spacing between flexural reinforcement shall not be more than 3 times of slab depth or 18 in (450 mm). steel will be used in other direction to resist shrinkage and temperature stresses.
Figure 2

ACI provides us with a limitation for span/depth limitations for a concrete flexural member where the deflection not calculated. Table one providing the minimum thickness h for flexural concrete members where deflection not calculated. 
Table 1

when the concrete hardens it shrinks and shorten, the opposite will happen when temperature increase, the length of slab will increase, to resist this stress, shrinkage and temperature steel is added perpendicular to the main reinforcement. ACI code 7.12 specifying the amount of reinforcement as:

  1. steel grade 40 to 50 (280 Mpa-350 Mpa) deformed bar, As equal to 0.002 of the gross cross-section area of the slab=0.002*b*h
  2. Grade 60(420) deformed bars or welded wire fabric is used, As equal to 0.0018 of the gross cross-section area of the slab=0.0018*b*h
  3. when Grade > 60(larger than 420) deformed bars or welded wire fabric is used, As equal to 0.0018*60,000/fy>0.0014(0.0018*420/fy>0.0014) of the gross cross-section area of the slab
shrinkage and temperature steel should not be spaced more than 5 times of slab thickness or 450 mm.

Example 1



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