Alternate vertical steel reinforcement in prestressed concrete beams

The Texas Department of Transportation (TxDOT) widely uses prestressed concrete I-beams for constructing bridges. Currently, TxDOT prestressed concrete I-beam standard permits the substitution of an equal area of Welded Wire Fabric (WWF) for the traditional standard steel bars used as the vertical steel reinforcement. However, no details are provided to insure the standardization of this allowed substitution. The common practice is to simply make a one-for-one substitution using deformed wire for each conventional deformed bar.

A research project was conducted at Texas Tech University (TTU) to study the behavior of the WWF as a vertical steel reinforcement, specifically the anchorage capacity of the WWF. In addition, alternate vertical steel reinforcement details were proposed using a simplified steel area and an equivalent strength steel area of WWF. The results of this study are reported in this work.

WWF consists in deformed wire bars in the transverse direction and plain wires in the longitudinal direction welded at each intersection using an electrical resistance welding procedure to form flat sheets of WWF. These flat sheets of WWF are bent into the desired shapes and placed into position as units.

The vertical shear reinforcement must be properly anchored at its ends in order to be capable of developing its fiiU shear design strength. Two longitudinal plain wires welded to each vertical leg on the WWF detail and spaced vertically two inches apart are provided to develop this anchorage behavior. This detail requires that the two longitudinal wires and their welds be capable of properly anchoring the vertical wires. This two cross-wire anchoring detail in the WWF has been used in several other similar applications. However, there are some differences between these similar applications and Texas prestressed I-beam details. One of the main objectives of this research work has been to study the effectiveness of this anchorage detail.

The current TxDOT vertical reinforcement detail for I-beams at the ends consists of bars with diameter 1/2 inches (No. 4) spaced at 4 inches, called "R" bars, and bars with diameter 5/8 inches (No. 5) spaced at 4 inches, named "S" bars. Both of these sets of bars must be Grade 60. This project also studied a simplified WWF alternative of reinforcement consisting of using an equivalent wire diameter providing the same steel area. This simplified approach has the advantage of reducing the production cost of the WWF cages, due to the fact that only one wire diameter is needed to be fed during the production process.

In addition to this alternative vertical steel reinforcement, another reinforcement proposal was studied. An equal strength substitution was proposed using Grade 80 wires with a smaller area in such a way that the total strength developed by the vertical WWF reinforcement will be the same of the traditional reinforcement using No. 5 and No. 4 bars of Grade 60. This change in policy would allow the use of the higher yield strength common in WWF material, leading to reduced areas of steel and an associated reduction in cost.

Finally, with the onset of High Performance Concrete, the strength of concrete possibly used in the TxDOT I-beams has increased from 5,000 psi to 10,000 psL Because of this shift, concrete strength was also another study parameter consider in this research.

In order to study the performance of the WWF steel substitution in the particular use of vertical steel reinforcement in the TxDOT I-beams, 14 full-scale tests were conducted at the Texas Tech University (TTU) structural testing laboratory. The concrete strength ranges used were 5,000-7,000 psi for normal concrete strength and 10,000-12,000 psi for high strength concrete.

The beams were tested to observe their performance failure load. The results of this study were used to state recommendations about the current TxDOT policies of using WWF as vertical shear reinforcement.