360R-06 Design of Slabs-on-Ground
CHAPTER 7—DESIGN OF SLABS REINFORCED
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- Điều hướng trang này:
- 7.2—Thickness design methods
- 7.3—Reinforcement for crack-width control only
- 7.4—Reinforcement for moment capacity
| CHAPTER 7—DESIGN OF SLABS REINFORCED
FOR CRACK-WIDTH CONTROL 7.1—Introduction Slabs-on-ground are designed and their thickness selected to prevent cracking due to external loading, as discussed in Chapter 6 . Slab thickness calculations are based on the assumption of an uncracked and unreinforced slab. Steel reinforcement may be used in slabs-on-ground to improve performance of the slab under certain conditions. These include: • Limiting width of shrinkage cracks; • Use of longer joint spacings than unreinforced slabs; and • Providing moment capacity and stability at cracked sections. The use of reinforcement will not prevent cracking, but will actually increase crack frequency while reducing crack widths. Properly proportioned and positioned, reinforcement will limit crack widths such that the cracks will not affect slab serviceability. 7.2—Thickness design methods The inclusion of reinforcement (even in large quantities) has very little effect on the uncracked strength of the slab. The PCA, WRI, and COE thickness design methods described in Chapter 6 may all be applied identically to the design of reinforced slabs-on-ground by simply ignoring the presence of the reinforcement. 7.3—Reinforcement for crack-width control only Reinforcement required for crack-width control is a function of joint spacing and slab thickness. To eliminate sawcut contraction joints, a minimum steel ratio of 0.5% (PCA 2001) of the slab cross-sectional area is recommended. The reinforcement should be located as close to the slab top surface as possible while maintaining minimum concrete coverage over the reinforcement. 7.4—Reinforcement for moment capacity Reinforcement for moment capacity (WRI 2001) provides a cracked, reinforced section equivalent to the uncracked, plain concrete section. This design requires the joint spacing to be as shown in Fig. 5.6 , and the reinforcement is to be discontinuous at the joints. For steel located at mid-depth (in.-lb units) (7-1) (SI units) where A s = cross-sectional area of steel, in. 2 /ft (mm 2 /m) of slab; h = slab thickness, in. (mm); f c ′ = compression strength of concrete, psi (MPa); f y = yield strength of reinforcement, psi (MPa); f s = 75% of f y maximum. (Note: using high steel reinforcement stresses may lead to unacceptable wide crack widths. The designer may want to consider using less than 75% of f y to limit the width of the cracks.), psi (MPa); and MOR = modulus of rupture for the concrete, as used for unreinforced design, generally taken as 9 , psi (0.75 , MPa); may range from 7 to 11 , psi (0.58 to 0.91 , MPa). tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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