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Post-tensioning Software (was Connections)

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Over the last two days there have been a few postings that have discouraged
the use of finite element programs for the design of post-tensioned
structures. As a developer of finite element post-tensioning software I feel
I need to respond.

Having working in the post-tensioning industry for many years (designing,
detailing, inspecting, stressing and detensioning tendons...), I have seen a
large number of good and bad designs. The quality of the designer (not the
software) has the largest impact on the quality of the design.

Novice designers tend to favor frame-based programs because the programs
work with a simplified sub-structure (avoiding the complexities of the real
structure) and the programs can even select the number of tendons and choose
the profiles. These novice designers don't realize that the "automated"
designs sometimes are ludicrous designs (simply complying with 6 root f'c
tension limits  - the cornerstone of a number of frame programs - does not
necessarily deliver a economic or constructible design).

Experienced designers tend to favor finite element programs, because they
give a more accurate assessment of what is actually happening in the
structure (and the finite element programs now allow the engineers to design
the structure more quickly). Experienced designers want to see the effects
of openings, irregular column layouts, point loads, etc.. While all
engineers are reluctant to switch from an old familiar program to new
software, many experienced designers have switched from their frame programs
to finite element programs because of these advantages.

One of the previous posts suggested that finite element based design moved
the designer further away from the realities of construction - I believe the
opposite is true. In a finite element program the engineer has to confront
constructability issues that are totally ignored in frame-based methods
("how do I get those 15 tendons around that opening?").

A post also suggested that finite element design somehow led to engineers
ignoring loads that need to be transferred to the foundation - again, I
believe the opposite is true. Finite element programs give more accurate
reactions and present the reactions in a easier to read format (what could
be better than a plan with each column reaction presented at the column,
with a table of reactions as an alternate presentation). With frame-based
programs, the engineers need to hunt through dozens of strips for the
reactions and then need to decide how to consider the mismatch between
reactions of the perpendicular strips - this process does not lead to a
simple means of ensuring a complete and consistent load path.

Allan Bommer
Structural Concrete Software, Inc.


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