Which End to Stress First?
In post-tensioned floors, tendons are stressed after cast concrete gains adequate strength. The stressing operation is monitored to ensure that tendons receive their design-intended force.
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Author Archives : Bijan O. Aalami, S.E., Ph.D., C.Eng.
Bijan Aalami is a Life Member of ASCE, a Principal of PT-Structures Inc., an Emeritus Professor at San Francisco State University, and a former Professor at Arya-Mehr University (now the Sharif University of Technology). (bijan@pt-structures.com)
A Simple, Serviceable, and Safe Option
There are three fundamentally different methods to design a post-tensioned concrete member: load balancing, rigorous, and straight. All three methods, when followed correctly, result in serviceable and safe members. They differ substantially, however, both in the computational effort and potentially in the economy of the final design. …
Part 2
Part 1 of this article (STRUCTURE, October 2017) showed that if column and wall supports restrain the shortening of a post-tensioned member at stressing, part or all of the precompression intended for the post-tensioned member will be diverted to the supports. …
Part 1
When an unrestrained post-tensioned member is stressed, the member will shorten because of the pre-compression imparted by the stressing. If column and wall supports restrain this shortening, part or all of the pre-compression intended for the post-tensioned member is diverted to the supports. The loss of pre-compression to the supports leads, in turn, to a reduction of the member’s moment capacity. …
When major building codes are not in agreement on specific structural members, confusion and uncertainty are propagated among design engineers. In this case, the industry may need to start a conversation with code authorities for clarification.
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Three issues specific to the design of post-tensioned members are currently treated differently by the American Concrete Institute’s Building Code Requirements for Structural Concrete and Commentary, ACI-318 (2014), and Europe’s Design of Concrete Structures – Part 1-1 General rules and rules for buildings, European Code EC2 (2004). For any designer working globally, these two code documents define the design of post-tensioned members. …