Precast Inc. - July/August 2008 - (Page 12) TECHNICAL Steel Reinforcement Following code requirements and specifications could save you money. By Gary Munkelt, P.E. T he cost of steel is rising, and many precasters are considering alternative materials to replace costly steel reinforcement in precast products. While steel may be a more expensive option, it may be the only option for a precaster depending on the situation and the scope of the project. If a precaster is to use steel reinforcing rebar in walls or slabs, a refresher on the code requirements and specifications could help reduce the amount of unnecessary rebar and maximize efficiency. This in turn would help save money in this pressing economy. Some industry standards are guidelines to be followed when manufacturing steel reinforced precast products. ACI 318, ASTM C858, ASTM C913, ASTM C1227 and ASTM C1613 are all specifications that cover the topic of selecting and placing the reinforcing steel in underground rectangular structures. ASTM C478 covers placement of reinforcing steel in round manholes used in sewer and water systems. Listed below are some of the key topics that need to be addressed when manufacturing a steel reinforced precast structural member: • Minimum steel requirements • Temperature steel requirements • Maximum reinforcing steel • Concrete protection of reinforcement • Minimum spacing of reinforcing steel • Maximum spacing of reinforcing steel • Crack control (Z factor) But not less than: 200 x bw x d fy As, min = minimum amount of steel reinforcement (in.²) fy = specified yield strength of reinforcement (psi) f’c = compressive strength of concrete (psi) ρ= As (bw x d) (for non-prestressed concrete) As = area of steel reinforcement (in.²) bw = width of section considered (in.) d = distance from extreme compression fiber to centroid of longitudinal tension reinforcement (in.) The purpose of this requirement is to avoid a catastrophic collapse that could occur if the cross section of the beam is large and the amount of steel is small. The problem does not occur in slabs and walls. ACI 318-08 suggests that the minimum amount of steel reinforcement (As, min) for slabs be that required for shrinkage and temperature. Temperature steel requirements The purpose of temperature reinforcement requirements is to provide enough steel to restrain tensile forces in the concrete caused by expansion and contraction of a slab when exposed to extreme hot and cold temperatures of the weather. Temperature reinforcement is required at right angles to the principal reinforcement to minimize cracking and to tie the structure together to ensure it is acting as assumed in the design, according to ACI 318-08 Section 7.12. In a one-way slab, the steel carrying the loads is oriented in one direction and the temperature steel is placed perpendicular to it. When vaults are installed underground, the temperatures are fairly constant and the argument can be made that this steel is not necessary. However, precast concrete units could be exposed to temperature extremes while being stored in the yard waiting to be installed. Also, most engineers will use temperature steel minimum requirements to overcome normal handling stress. For structural slabs and footings of uniform thickness, ACI Minimum steel requirements When designing a precast structural member with steel reinforcement, provisions for a minimum amount of tensile reinforcement are in place for both positive and negative moment regions in the structure. ACI 318-08 Section 10.5.1 defines the minimum amount of steel reinforcement for flexural members, but this specification applies only to beams. Below is an example of how to determine the minimum amount of steel reinforcement in a flexural member: As, min = (3 x √f’c) x (bw x d) fy 12 JULY/AUGUST 2008 | PRECAST INC.
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