Designing the Product When designing cast- ings for a new post tensioning system, Tu- mason says SDI adheres fi rst to a certain number of restrictions that the design requires. He then passes his drawings and CAD fi les off
to the metalcaster, who has a limited amount of leeway to optimize the de- sign for castability.
The variables among the base plate and anchor head castings in each sys- tem are not great. The anchor heads can be a variety of sizes, depending on the project, but the design alterations from system to system are limited primarily to the number of strands the system is required to hold. More strands equal more holes in the fi nished anchor head. “We do between four or fi ve different size anchor heads and bearing plates [for SDI],” Farrar said. “It’s all project driven, from a few strands to multiple strands with 15-16 holes.”
Both types of casting must serve one
primary purpose—hold lots of torque for indefi nite periods of time. This means that not only must the metal- caster deliver the correct specifi cation of iron to SDI, but it must also ensure an adequate amount of material is placed around the holes in the anchor heads to deliver the material properties 80-55-06 ductile iron is rated to deliver. “You have to make sure you have
Schwager Davis makes post tensioning systems, which are composed primarily of a cast anchor head, a cast bearing plate, and a plastic or steel structure that feeds steel wiring through to the base plate.
ductile iron with one shell core and had fi nished casting weights of 69 and 78 lbs.
enough wall thickness to maintain the tension while the bridges are up,” Far- rar said. The holes through which the strands run on the anchor heads also are required to be cast with a slight taper in order to grip the rebar as it is pulled through. Beach said the base plates N.E.W.
produces are standard products for his metalcasting facility’s automated matchplate lines. The two different plates the company produced for the Skyway project were cast in 80-55-06
As long as a few requirements are maintained on both the designer’s and the metalcaster’s side, the process of obtaining quality metal castings for a post tensioning system has been any- thing but tense, according to Farrar. “There’s not very much back and
forth between us anymore,” he said “With the frequency with which they’ve designed castings for us over the last 10 years, a lot of people at SDI are more up to speed on the casting process and what it requires.”
MC
Fig. 1. The design print for one of Schwager Davis’s post tensioning systems shows two metal castings, a bearing plate and an anchor head. MODERN CASTING / February 2010 35
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