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Re: Post-tensioning Software (was Connections)

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Allan,

I don't think that Gail for one ever really said anything "bad" about FE
programs...but rather tried to make the same point that you made.  FE
programs (for PT or otherwise) are a tool.  And any tool in the hand of a
fool is dangerous.  I think she was trying to point out that there are
many people who use PT FE programs that don't really understand PT so they
then lack the ability to understand what a PT FE program shows them.  In
other words, the old addage..."garbage in = garbage out" applies but the
fool using the tool may not realize that their wonderful FE model is crap.

Plus, I will second Gail's most recent post.  To a large degree, using FE
for concrete design (PT or otherwise) is to some degree like using a
scapel where a sledge hammer is more appropriate.  As she pointed out,
concrete is _NOT_ anywhere near as precise as what is typically "modeled"
in analysis/FE programs.  Too many engineers get nice warm fuzzies when
their analysis model spit out numbers with 5 decimal places, when the
"fuzziness" of concrete means that in reality the theoretical design could
be off by 5%, 10%, or even more.

Regards,

Scott
Ypsilanti, MI


On Wed, 3 Dec 2003, Allan Bommer wrote:

> 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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