SIX PROBLEMS WITH POST-TENSIONING FORCE SPECIFICATIONS
Point 1: Tendon elongations are calculated based on the assumed value of tendon stress of 189 ksi. This is an industrywide non-secret because it’s always on the first sheet of shop drawings.
Point 2: Shop drawing tendon quantities are based on the magic number of 26.8 kips per tendon of post-tensioning stress. The PT supplier’s friction calculations are rarely used to calculate tendon quantities. This is another industrywide non-secret.
Point 3: Force specifications introduce ambiguities. Where along the tendon is this magic force supposed to be taken? Or is it the average force, the value that’s historically been used?
If the specification requires force to be 26.8 at any point along the tendon, additional tendons must be added if just one point falls below that. This may well overstress/overbalance spans closer to the stressing end. ACI’s commentary section states “overestimation of prestress losses can be almost as detrimental as underestimation, since the former can result in excessive camber and horizontal movement.” In one case I know of, the tendon was 2% low in only one span; adding tendons to meet the required force increased the effective force in the span adjacent to the stressing end to 18% above the specified force.
This is why friction losses should be considered during design.
Today’s design software is so sophisticated there’s no excuse for structural engineers to still be using force specifications instead of considering seating and friction losses when determining required number of tendons.
Point 4: Another problem with force specifications is the actual number of tendons is not explicit. What is the magic number for force? We used 26.8 kips during my years as head of a detailing department, but the problem is the round-off. Slight changes in the force requirement can increase the number of tendons. In this fiercely competitive industry, that can mean the difference between winning or losing a bid. When you have multiple banded tendons on multiple floors or distributed tendons where the force is highly variable, round-off can result in a significant increase in the quantity of tendons.
In this fiercely competitive industry, that can mean the difference between winning or losing a bid. When you have multiple banded tendons on multiple floors or distributed tendons where the force is highly variable, round-off can significantly increase the number of tendons. Expecting the PT estimator to figure all of this out during take-off is unreasonable.
Point 5: These calculations should be done by the structural engineer and the required number of tendons specified on structural plans. The actual effective force has a direct effect on the nominal flexural capacity and tensile stresses in the concrete. Due to friction losses, this force varies along the tendon. It also affects balance loading, which in turn reflects in the deflections of the member.
In my opinion, the engineer hasn’t completed the design unless these calculations are done. Shop drawing detailers are excellent interpreters of structural drawings, but they’re not engineers; they don’t know anything about loads, member deflections, or member stresses. They only know the codes that directly affect their drawings.
Why are they being asked to do part of the structural engineer’s job?
Point 6: The other issue with shop drawings is the requirement for a professional engineer (P.E.) stamp. This is also directly related to force specifications.
The definition of engineering is met when the forces are used to calculate the number of tendons required. Design software lays out the tendons during modeling. If these quantities are transferred to the design drawings, engineering isn’t required during shop drawing creation and therefore the stamp shouldn’t be required. However, I believe all jurisdictions require shop drawings to be stamped even when tendon quantities are specified.
In a perfect world, engineers would specify tendon quantities. Designs would be easier to take off for bidding, easier for the supplier to detail, easier for the engineer to check the shop drawings, and this would eliminate the need for suppliers to have their shop drawings stamped.