Shawn P. Gross, Villanova University
Camber is an important serviceability consideration for the design of precast, prestressed concrete bridge girders. In simple terms, net camber is the difference between an upward component due to prestress and several downward components caused by loads. Both material properties and structural parameters influence girder camber. Consequently, camber behavior of high strength concrete (HSC) girders can be significantly different than for conventional strength concrete girders.
The higher modulus of elasticity and lower specific creep of HSC tend to reduce the elastic and time-dependent components of upward camber or downward deflection. Prestress losses are also affected by differences in these material properties. In general, losses are lower per unit prestress, which results in slightly greater camber for HSC girders.
The use of HSC in bridge girders is usually associated with very efficient structural designs. Longer span lengths lead to a significant increase in the magnitude of individual components of camber. The use of higher prestressing forces, which are required to accommodate these efficient designs, usually leads to a significant increase in the upward component of camber associated with prestressing. Larger girder spacing can significantly increase the superimposed loads, thereby increasing the downward components of deflection.
An analytical comparison of the predicted camber at the time of girder erection for HSC and conventional concrete designs is shown in Fig. 1. The HSC girders in this example are actual designs selected from the Louetta* and San Angelo Texas HPC Showcase Projects. The analyses for these girders utilized a computerized time-step procedure and material properties measured as part of the research projects for these bridges. The HSC designs exhibit large camber values at erection, with predicted values of 5.0 to 8.3 in. (130 to 210 mm). This can be attributed directly to the use of higher prestressing forces in these girders and longer span lengths.
Although camber can be difficult to predict accurately even for conventional concrete girders, the variation in key material properties and structural parameters will generally have a more substantial impact for HSC. In Fig. 1, the range of potential camber values at erection were calculated using an upper- and lower-bound approach that considered variations in creep, modulus of elasticity, prestressing force, and dead load. The range of values can be seen to be as large as 6.0 in. (150 mm) for the HSC designs. The sensitivity of these HSC designs is more than twice that of the conventional strength concrete design.
As part of the Texas HPC Showcase projects, camber measurements were recorded from release through service on 22 HSC girders. Typical results from this research program are shown in Fig. 2. In general, measured values were somewhat lower than predicted values (based on measured properties), but showed an acceptable level of accuracy. Probable reasons for this lower-than-predicted camber include variations in material properties between test specimens and the actual girders, and the effect of thermal gradients and differential shrinkage.
When designing with HSC, the increased camber sensitivity of HSC designs must be considered. Simplified methods of camber prediction are not applicable for HSC because of the many possible combinations of material properties and structural configurations. Even with a detailed procedure such as a time-step or computer analysis, accuracy can only be achieved when important material properties and structural parameters are known.
Material properties may be determined by trial batching or by past experience with a given mix design in similar conditions. An upper- and lower-bound approach is suggested so that a reasonable range of expected values may be defined.
Further Information
For more detailed information on camber in HSC girders, see:
Gross, S. P. and Burns, N. H., “Field Performance of Prestressed High Performance Concrete Highway Bridges in Texas,” Center for Transportation Research, The University of Texas at Austin, Research Report 580/589-2, to be published.
*The Louetta Road project was described in HPC Bridge Views No. 1.