Case Studies

Post-tensioned Walls

The purpose of the proposed research is to obtain test data to confirm and codify a design protocol for a new type of concrete shear-wall system that incorporates vertical post-tensioned tendons. The defining feature of the hybrid wall system is the rocking/flexural response and recentering capability provided by unbonded, vertical post-tensioned tendons, coupled with the energy dissipation provided by the reinforcing bars. In concrete construction, a building’s lateral bracing system, particularly for mid- to high-rise buildings in seismically active areas, accounts for a substantial portion of total building costs and can impose significant architectural constraints on building design.

Currently, buildings using conventional concrete wall systems to resist earthquake forces are at a disadvantage owing to the assignment of lower R values. Moreover, building height limitations also require the use of moment-resisting frame systems in tall structures, further limiting design alternatives and increasing construction costs.

One of the goals of the proposed research is to confirm that the proposed shear-wall system has sufficient ductility to be assigned an R value of 8 rather than the current R value of 6, which is assigned to a well-detailed conventional wall. The assignment of a higher R value for the shear wall system will result in much more economical designs that can be constructed more quickly.

Post-tensioned walls have unique characteristics which provide the ability to re-center after a major earthquake. This gives them an advantage over all conventional seismic systems, which rely only on yielding and can sustain permanent deformations after large earthquake excitations. The recentering response of post-tensioned walls greatly adds value to buildings when measured on a life-cycle basis. This feature is an essential part of comprehensive, sustainable design strategies to reduce earthquake losses and material waste in seismically active areas.

Prestressing can reduce the quantity of vertical mild steel reinforcement in the walls by fifty percent or more compared to conventional construction. This results in more efficient material use and reduced congestion.

By demonstrating through experimental research that increased ductility and enhanced performance can be specifically and directly achieved through a more deliberate, careful approach to detailing and proportioning, we believe an appropriate adjustment can be made in the code to differentiate between the special capacity-designed, post-tensioned walls proposed herein and traditional reinforced-concrete shear walls designed according to current code provisions. This recognition would allow for greater flexibility and more consistency in the design and construction of seismic-resisting concrete structures, broadening the range of possibilities in the construction market, while reducing construction costs.

Tipping Structural Engineers has used the post-tensioned wall system to provide lateral bracing for several new construction and seismic retrofit projects.