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May 2, 2003
Photos courtesy of Cary Kopczynski & Co.
Partial post-tensioning of this slab reduced the post-tensioning quantities and correspondingly increased the rebar quantities. This benefited the building’s performance and reduced its cost.
To post-tension or not post-tension? That is the question. At least that’s the question confronted by structural engineers in the design of most cast-in-place concrete structures.
How should one go about answering the question? What issues should be evaluated? What are the long-term serviceability aspects? What about future building modifications? What is the impact of recent advancements in materials? What about cost?
These and other questions need answers before an intelligent decision can be made.
For most cast-in-place concrete structures, post-tensioning provides several distinct advantages. First, the uplift provided by the tendons allows a reduction in slab and beam depth. For example, a flat slab spanning 25 feet without post-tensioning would typically have a thickness of 8 to 9 inches. That same slab with post-tensioning would be 6 to 7 inches thick.
Post-tensioning also reduces deflection. Profiling the tendons to high points over supports and low points at mid-span has the effect of carrying the beam or slab dead load directly to the supports. Thus, the net dead load carried by the member can be significantly reduced or even eliminated. Beams and slabs that would otherwise show significant deflection can be post-tensioned to result in little or no deflection. This creates major serviceability advantages in all sorts of structures, from office buildings to hotels to condominiums to parking structures.
Another major advantage to post-tensioning is that it generally reduces the structural frame cost because it significantly reduces dead load for most structures. Assuming an average reduction in slab thickness of 2 inches, a building’s dead load is reduced by 25 pounds per square foot. The corresponding reduction in column loads, foundation loads and seismic loads translates into cost savings throughout the structure.
Despite this, there are situations where post-tensioning is inadvisable. Buildings whose occupants make major structural modifications may be better off with conventionally reinforced slabs and beams. Remodeling, renovating, core drilling or penetrating post-tensioned buildings is more difficult than for conventionally reinforced structures.
The use of post-tensioning in The Olympus in Seattle resulted in longer spans, shallower floors and reduced slab deflection.
The use of post-tensioning in remote, rural areas should also be carefully evaluated. Since post-tensioning requires a higher level of construction sophistication, it is sometimes inappropriate in these out-of-the-way structures. Proper installation of post-tensioning requires a labor force highly skilled in its use and execution. This skilled labor may not be available in remote areas.
Another factor in the evaluation of post-tensioning versus conventional reinforcing is shrinkage. Post-tensioned slabs and beams shrink more than conventionally reinforced slabs and beams due to the axial anchor forces. With modern design using partial post-tensioning, however, this shrinkage can be reduced to manageable levels. Essentially, partial post-tensioning reduces tendon quantities and increases rebar quantities.
It should also be kept in mind that the majority of slab shrinkage is caused by drying of the concrete, not by post-tensioning. Thus, the dominant cause of slab shrinkage remains, whether or not it is post-tensioned.
By carefully optimizing the balance of post-tensioning versus mild reinforcing in a slab or beam, the appropriate balance can be achieved. For some underground structures, it may be appropriate to reduce post-tensioning quantities and increase mild reinforcing quantities to achieve this optimization.
Post-tensioning is not without its problems. If improperly used, long-term durability can be jeopardized. Recent improvements in post-tensioning materials and procedures, however, have combined to enhance its long-term durability. These improvements include thicker sheathing, better anchors and full encapsulation of cable. The proper use of these new materials and components improves the overall quality of the installed system.
The question remains. To post-tension or not post-tension?
Cary Kopczynski is president of Cary Kopczynski & Co., a structural engineering firm based in Bellevue.