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RE: Broken Tendons

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Hi Mark, Hi Syed,

	I just got notification that this reply didn't go through the first time,
so, I'm going to try again.

Original message:

	I'm going to reply to both of your responses together, so that anybody
reading this won't have to go through the same stuff twice.  I'd like to
start by thanking both of you for your responses.  You both covered a great
deal of material, and clearly spent quite a bit of time on it.  I really
appreciate your time and effort.

	The slab is a solid slab for a parking lot that is currently under
construction.  I assumed that the tendons were prestressed, since that was
the only situation that made sense to me: if it's post-tensioned on site,
then all they had to do is replace the tendon and start over.  Other than
that, I don't have much information.  Part of this is my fault: having
several jobs going on, I didn't spend the time that I should have getting
answers.  I did send a fax to the contractor for more info, but have yet to
have any of my questions answered; I don't even know which tendon broke or
under what circumstances.  All that I've been told is that the tendon
supplier did some calcs, and that that was the procedure.

	After reading your letters, I will send a request for further info, and
look further into ACI 318.  I apologize for having not thought of that
before; I haven't done anything with prestressing since I was an undergrad,
so I hope that you'll bear with my ignorance on the subject.

	I was given a copy of the calcs, and when I called the engineer, I had a
difficult time making myself understood.  She just went over the logic
behind the procedure (just the stress-strain relationship).  I feel a
necessity to have this procedure confirmed by an unbiased engineer.  After
all, the supplier has a reason to say that everything is OK.  Nobody that I
know has come accross this problem before, so I decided to try the SEA.

	Again, I greatly appreciate your responses.

John

-----Original Message-----
From: Mark Geoghegan [mailto:mgeoghegan(--nospam--at)structural-tech.com]
Sent: Sunday, June 10, 2001 9:49 PM
To: Seaint
Subject: Re: Broken Tendons


john,

re your broken tendons - i presume that when you say "prestressed slab" you
mean unbonded post-tensioned, probably flat plat/slab.

the info was a little vague, so i present two cases.

did the strand break during:

1.	 the on-site PT stressing operation?

 OR

2.	after several years in service? if the slab has been in service for
several years and it is in an aggressive environment, then the strand break
may have been due to corrosion whereby you will need to investigate a lot
further for other potential areas with similar distress etc etc.

i think you are talking CASE 1, probably about a single strand failure on a
monostrand unbonded PT tendon during the stressing operation. this can occur
due to misalignment of the wedges, overstressing, or internal damage of the
tendon. since you have the tendon elongations you can check the actual vs.
theoretical and compare to ACI 318 clause 18.20.1 (+/- 7%). this may/may not
eliminate overstressing as the cause. for wedges misalignment you can
sometimes check if you have the broken strand tail and visually observe the
wedge marks, but often the marks "let go" with the breakage.  but seems that
you do not need to explain WHY it failed, only what is its effect! ACI
clause 18.20.4 permits total loss of prestress due to unreplaced broken
tendons not to exceed 2% of the total prestress.

you will have to then define the "total prestress" for that element - maybe
effective width, middle strip, or other - ACI probably will allow you to use
the full panel width, although technically this is not correct.

the tendon was probably 0.5", low-relaxation, stressed to 33 kips (0.80
Fpu). this is the jacking force BUT you need to calculate the effective
prestress force, considering stressing losses (short and long-term) for you
check analysis.

if then tendon breakage account for less than 2%, then the ACI is a
deemed-to-comply clause that says effectively you do not have to do any
more. if >2%, or you wish to do it any way, use the revised EFFECTIVE
prestress force(accounting for the breakage) in your check analysis and see
the effect. i would not be really concerned about flexural or compressive
stresses, and would typically only check the ultimate moment capacity. if
the % breakage is high, then you do need consider all effects - including
punching shear depending on the circumstances of the breakage and columns
etc. for a single strand isolated tendon you should have no real problems
for proving the slab has sufficient capacity.

i would follow up with the contractor/PT subcontractor to explain the
nature/cause of the break. if this is an ongoing project, it is always good
to correct some site practices that may cause these breakages.



regarding the reply/follow-up with hollow-core. hollow core is a precast
product and hence pre-tensioned and hence bonded by definition, and for
tendons (strands) to break this usually occurs (although very seldom as they
are stressed < 75% fpu, no curvature etc) at the precast site during
stressing, so has little to do with in-service breakages, so i do not see
the relevance for your case - if indeed i interpret your enquiry correctly.

HTH

Regards,

Mark Geoghegan BE (Hons.-Structural)

S T R U C T U R A L   T E C H
AUSTRALIA  -  GUAM  -  HAWAII


>From: "syed faiz ahmad" <syedfaiz23(--nospam--at)hotmail.com>
>To: seaint(--nospam--at)seaint.org
>Subject: Re: Broken Tendons
>
>John
>Well, first of all what kind of slab is this?
>If its hollow core slab its no problem. In holllow core slab, we often
>>need
>to provision for openings ( to cater for mechanical equipments etc ) and
>the
>section is analysed & designed with extra tendons, keeping in mind that one
>or more tendons will be cut at site to allow provisioned openings to be
used
>for the purpose it was created for. So, in case of your problem, you can
>analyze the cross section to examine the stress history, that is to know
>if
>the stresses are alright ( within allowable limits )at service & ultimate
>loads. Another important aspect is the deflection, especially due to
>sustained loads. Try it, you will find it  a simple problem.
>SYED FAIZ AHMAD
>SENIOR STRUCTURAL ENGINEER
>SAUDI OGER LTD
>RIYADH, SAUDI ARABIA.


>From: "John Dreger" <jdreger(--nospam--at)collinsengr.com>
>Reply-To: seaint(--nospam--at)seaint.org
>To: <seaint(--nospam--at)seaint.org>
>Subject: Broken Tendons
>Date: Fri, 8 Jun 2001 13:16:12 -0500
>
>Hi Everybody,
>
>I've been asked to analyze a prestressed slab in which one of several
>tendons has broken.  We have the actual (initial) elongations of the
>tendons, and, assuming that everything is still in the elastic range, can
>calculate the initial forces in each tendon.  Eliminating the broken
>tendon,
>and comparing the required total force with the actual (initial) forces
>indicate that the slab should be OK.  However, as I've never come across
>this problem before, I'd like to find out if here is a common procedure to
>check the adequacy of slabs (or beams, for that matter) in which a tendon
>has broken.  Unfortunately, I don't have any more specific information
>right
>now.
>
>Thanks for any help.
>
>John Dreger


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