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Flexural Behavior of Prestressed
J. O. Bryson and E. F. Carpenter
Prestressed Tee-beams constructed by the split-beam method were tested to failure in flexure to study the behavior and ultimate strength of these beams and to compare their flexural characteristics with those of prestressed beams of conventional construction. The compressive portion of the cross section of the split-beam is cast after the web of the beam has been formed and prestressed. The variables in the study included the percentage of prestressing steel, strength of concrete in the compressive element of the composite split-beams, manner of prestressing and web reinforcement.
Results showed that the composite split-beams behaved similarly to the monolithically constructed beams on the basis of flexural response and ultimate load. The strength of the concrete for the compressive element can be reduced within limits from that required for the prestressed element without sacrificing ultimate load capacity. The required percentage of reinforcing steel is less for the split-beam compared with conventional beams.
Key words: Composite concrete construction; prestressed concrete
A notable departure from the usual concept of composite prestressed concrete design was developed several years ago by A. Amirikian  of the U.S. Department of the Navy. The principal objective of the Amirikian concept is to optimize the application of prestressing for flexural concrete members by prestressing only the area of the cross section normally subjected to tension under bending. This requires that the tensile and compressive areas of the beam's cross section be constructed separately in order to restrict the precompression of the concrete to the tensile section. Therefore, this procedure can be considered as a special case of composite construction in which the interface of the two elements is set at the neutral axis of the composite section instead of at the junction of the flange and web as in normal composite construction. Beams constructed by this procedure are called "split-beams” and feature reduced prestressing forces for the same working load capacity compared with similar beams of conventional design.
A series of prestressed Tee-beams constructed by the split-beam technique were tested to failure to study the behavior and ultimate strength of these beams and
to compare their performances with those of conventional monolithic prestressed beams. The variables in the study included the percentage of prestressing steel, strength of concrete in the compressive element of the split-beam, manner of prestressing, and web reinforcement.
The work reported here is an extension of an earlier study  with split-beams of rectangular cross section. With the rectangular beams the principal difference between the split-beam and conventional beam was in the required prestressing force and location of the prestressing tendon in the cross section. The crosssectional properties of Tee-beams lend themselves to an additional advantageous feature in the split-beam technique. With Tee-beams under fexural loading the strain on the compressive surface (top of the flange) is usually considerably less than that on the tensile surface (bottom of the stem) due to the position of the elastic neutral axis. This means that the strength of the concrete in the flange section, provided to resist compressive stresses, need not be as high as that required in the tensile section which is initially cast and prestressed. This allows for savings in materials.
procedure is to determine the overall cross section for the beam as would be done for a conventional monolithic prestressed beam. From the properties of the full cross section, the area that will experience tension under loading is defined by the location of the elastic neutral axis. This area will be cast separately in the split-beam construction and is termed the tensile element. After prestressing the tensile element, the zone of the split-beam that will resist compression is cast. on and is practically stress free prior to the application of live load.
The specimens in this investigation included beams of conventional monolithic construction as well as the split-beams of composite construction. They were all Tee shaped in cross section with a 3-in by 15-in flange, an overall depth of 18 in, and were 19 ft long. The two monolithic beams were post-tensioned while the split-beams included both post-tensioned and pretensioned specimens.
Figure 1 shows the nominal dimensions of the beams with the location of the prestressing tendon in the cross section given for both the monolithic beam and the split-beam. Also, the positions of support and points of loading for tests are indicated.
fixed in position at the ends of the form and at the third-point and midspan locations.
A single steel bar, threaded on both ends, was used as the prestressing tendon in the post-tensioned beams. In each case, the tendon was straight and located at a constant depth in the cross section throughout the length of the beam.
Figure 2 shows a tensile element setup for posttensioning. The tensioning force in the tendon was measured with a steel dynamometer attached to the tendon at the end of the beam opposite to the jacking end. This force was distributed over the ends of the prestressed element with l-in thick bearing plates. Heavy duty steel nuts bore against the dynamometer on one end and the bearing plate on the other end to maintain the prestressing force in the element.
The pretensioned beams were each prestressed with two strands of steel cable. In profile, the strands were straight and parallel and spaced approximately l.in center to center in a vertical plane throughout the central 12-ft section of the span of the beam (6 ft to each side of the midspan section). From these points they spread apart at equal angles to about 3-in