Continuity of extended strands is essential for all prestressed girders bridges with fixed diaphragms at intermediate piers. Strand continuity may be achieved by directly overlapping extended strands as shown in Figure 1, by use of strand ties as shown in Figure 2, by the use of the crossbeam ties as shown in Figure 3 along with strand ties, or by a combination of all three methods. The following methods in order of hierarchy shall be used for all precast girders for creating continuity of extended strands:
The area of transverse ties considered effective for strand ties development in lower crossbeam shall not exceed:
Aps Area of strand ties, in2
ns Number of extended strands that are spliced with strand and crossbeam ties
fpy Yield strength of extended strands, ksi
fye Expected yield strength of reinforcement, ksi
Two-thirds of As shall be placed directly below the girder and the remaining of As shall be placed outside the bottom flange width as shown in Figure 3.
The size of strand ties shall be the same as the extended strands, and shall be placed at the same level and proximity of the extended strands.
Extended strands at intermediate crossbeams are used to connect the ends of girders with diaphragms and resist loads from creep effects, shrinkage effects, and seismic positive moments. These strands are to be developed within the diaphragm between the two girder ends at intermediate piers. The objective of extending the strands is to ensure that the tensile force carried by the strand can be transferred to the opposite side of the diaphragm, thus ensuring an adequate tensile load path for joint forces. Extended strand that overlaps with strand from the opposite girder or use of strand ties facilitate development of strands within the diaphragm between two girder ends. It is preferable that extended strands, as shown in Figure 1, be used to the maximum extent possible. This arrangement provides the stiffest and most direct tensile load path. However for constructability, strand ties may be utilized to provide extension of the strands into the diaphragm, as shown in Figure 2. The extension is provided by a lap splice between the strand ties and the extended tensile strands. Additionally, the top of the ties in the lower crossbeam may be considered to contribute to the extension of the strand tensile force into the diaphragm, as shown in Figure 3. This force transfer takes place through non-contact lap-splice (strut and tie) action between the extended strands and the lower crossbeam ties.
Because the strand strength is much higher than that of deformed bars, strand ties are recommended. The number and the size of the strand ties shall be the same as that of the extended strands that are being developed with strand ties. Circular strand anchors of 2¾ø x 1should be installed at 4 from the opposite girder end. Circular strand anchors are required for ease of construction. The strand extension beyond the face of strand anchors shall be limited to 1.
The top of lower crossbeam is affected when extended strands are developed by a compressive strut forming between the strand anchors and crossbeam ties. The designer shall calculate the number of crossbeam ties as shown in Equation 1, which assumes that one half of the non-overlapping strand force is transferred from the strand anchors to the lower crossbeam ties through strut action. It is also assumed that only two-thirds of the lower crossbeam ties need to be placed directly below the girders within the width of bottom flange since the affected zone on crossbeam ties is larger than the width of bottom flange. The tie bar size may be increased to meet the design requirement but the spacing should preferably kept the same as the crossbeam stirrup spacing for ease of construction. Ties may be bundled, if necessary. The tie bar size shall not be less than the crossbeam stirrups bar size.
Figure 1: Overlapping Extended Strand
Figure 2: Strand Ties
Figure 3: Lower Crossbeam Ties