Precast Inc. - July/August 2008 - (Page 16) preferable to one large crack. While the current ACI code does not utilize the Z formulas in the text, the Z factor is a convenient method for providing the conditions to limit cracking to an acceptable range. The Z factor applies only when: • Designing one-way slabs • Designing beams • Reinforcing steel has yield strength greater than 40,000 psi After a slab or beam is designed, determine a value Z using the following formula: Z = fs x (dc x A)1/3 (kips/inch) A= 2 x (t – d) x (bw) Number of rebar for slab: (See Figure 3) A = 2 x (t – d) x spacing A = average effective tension area of concrete surrounding one rebar (in.²) bw = width of section considered (in.) t = thickness of slab (in.) d = distance from extreme compression fiber to centroid of longitudinal tension reinforcement (in.) A common limit for Z, where the concrete is exposed to ground or liquids is 115 kips/inch. fs = stress in reinforcing steel at service load (ksi) Conclusion OR fs = 0.6 fy (60% times yield strength of steel) dc = thickness of concrete cover measured from extreme tension fiber to the centerline of the nearest bar or wire (in.) A = average effective tension area of concrete surrounding one rebar (in.²) for beam: (See Figure 2) Knowing when and how to use the correct standards for reinforcing steel will help precasters save money, whether it is by reducing the amount of unnecessary rebar, reducing maintenance costs or just by improving the overall quality of the product. Remember, in this tough economy, knowing your code requirements for placing steel reinforcement will keep you from making costly mistakes. Gary Munkelt is a consulting engineer with Gary K. Munkelt & Associates in North Wales, Pa. Contact him at gkm2001@verizon.net. “PermEntry® Opened My Business to Bigger Markets” — Monocrete Steps used just “Wepre-casttofront sell steps and it really limited our business.” says Chip McLoughlin, co-owner of Monocrete Steps of Connecticut in Bethel, CT. Chip McLoughlin “That’s why we started selling PermEntry about 14 years ago. Since we added PermEntry we’ve been able to tap into new markets and grow our bottom-line. Demand for the product is high great selling feature for homeowners. PermEntry is a product that basically sells itself. If you have it, you can sell it.” To learn more about becoming a PermEntry® dealer, contact The Bilco Company (800) 854-9724 www.bilco.com BOOTH 1239 THE PRECAST SHOW 16 JULY/AUGUST 2008 | PRECAST INC. http://www.bilco.com http://www.bilco.com
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