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RE: Alternatives to Post-Tensioning Construction

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nicholas,

we use RAPT (www.raptsoftware.com) for 2D PT and RC design. it is the most
user friendly software i have ever used for RC and PT design and analysis.
it is able to model the most complicated of models, with the simplist of
ease. additionally, it accepts partial prestress, voided sections, steps,
tapers, etc, etc. it is way better than ADAPT PT in my experiences. i am a
structural engineer who undertakes specialized structural construction,
mainly PT, concrete repair, CFRP applications, load testing etc. the
software assists us daily in these construction applications.


for residential construction....with PT (or RC) flat plates, it is very
common not to use drop panels or capitala, and with reduced sized columns
for maximum floor area punching shear becomes an issue and is often best
solved with the the use of large-headed studs (for example DECON Studrails).


also, ....

as an alternative to unbonded PT also consider BONDED/GROUTED PT. we use
galv or HDPE flat ducts (3" x 1") accomodating upto 5x0.5" uncoated strands,
stressed with a monostrand jack, then grouted. with a good "system" approach
to PT (ie inclusive of shop drawings, supply, install AND supervise) grouted
PT is structurally superior to unbonded, most especially for ultimate moment
capacity, future openings etc.

we have cut large openings (>20' x 20') into bonded PT buildings with a
minimum of effort wrt the existing PT tendons.

the Post-Tensioning Institute (www.pti.org) has just released the guide spec
for grouting of PT tendons and is a good reference for those interested.

sure there has been some problems with bonded PT around the world (UK, US
etc), but this is somewhat isolated/infrequent compared to the problems with
unbonded PT in building in the US when you compare PT work of similar
ages/construction period for both systems. for those with bonded PT tendons
with problems, in most of those instances it was due to a GC doing the PT
(and grouting). grouting of PT is not difficult with a little experience,
but in the hands of a GC (or sub)without any experience it is potentially
disasterous. the PTI spec is an attempt to compile all the issues etc and
resolve them.

in my experiences, the grouting is NOT an expensive exercise as has been the
point of arguement for not using bonded PT in the US, and the grouting is
near always off the critical path for construction.


mark geoghegan
STRUCTURAL TECH
mgeoghegan(--nospam--at)structural-tech.com






Post tensioning systems have come a long way since the 70's.  A typical
modern system includes complete factory-encasement of the strands in a
waterproof grease and then an extruded plastic sheath and either of the two
main types of anchorage available:

1) standard anchors which are bare castings and provide minimum protection
to the gripper zone and the transition between the anchor and the strand.

2) fully encapsulated, electrically isolated anchors that are completely
covered in a thick plastic coat, have a grease cap over the gripper pocket
and a plastic trumpet that fully engages and seals the strand sheathing to
the anchor sheathing.

A properly detailed PT system will provide excellent performance over the
life span of the structure.  In areas where salt exposure is a problem the
typical system recommended is the fully encapsulated one.  Further
protection can be provided with proper specification of low-shrinkage,
high-density concrete containing fly ash and/or silica fume.

The durability of any concrete structure, PT reinforced or otherwise, is
predicated on proper and timely maintenance coupled with good detailing and
construction practices.  The reasons previously stated by others on the list
note the benefits of PT.  I would however try to locate penetrations early
on in the design process and only locate conduit parallel with and in the
slab. Plumbing should be kept out of the decks for maintenance reasons if
nothing else.


Nicholas Blackburn, PE

ps: On a related note what software does the List use for PT design.  We use
PT Data typically but I am familiar with Floor and ADAPT PT.


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