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Re: Balanced Moment vs PT Moment

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"as long as you include all of the loads induced"

Right,

Balanced load plus all other loads gives the balanced (total PT) moment.

 What brought this up is that I was looking at the output in ADAPT and it was giving the "balanced moment" right next to the "balanced load" and it confused me (or I confused myself).  I wasn't sure what this "balanced moment" it was giving me represented, apparently what it always represents (total PT moment).  I just think it would be less confusing to someone who is new at this to just call it the damn "PT moment" instead.

 




>From: Gil Brock <gil(--nospam--at)raptsoftware.com>
>Reply-To: <seaint(--nospam--at)seaint.org>
>To: <seaint(--nospam--at)seaint.org>
>Subject: Re: Balanced Moment vs PT Moment
>Date: Thu, 02 Jun 2005 08:32:06 +1000
>
>Will,
>
>The Balanced Moment at any point can be calculated directly from the
>  balanced loads as long as you include all of the loads induced.
>These include effects from anchorage eccentricity and changes in
>concrete section (and change in force if you want to get accurate).
>
>In the case of a straight tendon at constant eccentricity, there is
>a moment equal to P*e at each end due to the eccentricity of the
>strands where they anchor giving a continuous moment diagram over
>the length of the strands of Pe. This is assuming no loss in force
>over the length of the strand which is not really correct as there
>is a transmission length for pretensioned strands.
>
>In the case of a parabolic tendon profile with end eccentricity
>there is a UDL from the parabolic profile plus end moments due to
>the anchorage eccentricity.
>
>It all adds up to P*e at every cross-section along the member if the
>member is determinant.
>
>At 09:35 PM 1/06/2005, you wrote:
>
>
>
>>Yes,
>>
>>I was confusing the "balanced load" (from the load balancing
>>method) with generating a "balanced moment". When apparently the
>>"balanced moment" is just the resulting PT moment and cannot always
>>be derived directly from the "balanced load".
>>
>>  It would seem more logical just to call it the "PT moment" but it
>>appears as the standard for some reason to refer to it as the
>>"balanced moment".
>>
>>
>>
>>
>> >From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
>> >Reply-To: <seaint(--nospam--at)seaint.org>
>> >To: seaint(--nospam--at)seaint.org
>> >Subject: Re: Balanced Moment vs PT Moment
>> >Date: Tue, 31 May 2005 20:26:07 -0400 (EDT)
>> >
>> >The short answer is "yes".
>> >
>> >The longer answer...first, I believe you are thinking of
>>"balancing loads"
>> >not "balancing moments" (but then I may be misunderstanding).  If
>>you are
>> >using the methodology of "balancing load" that I have seen, then
>>the end
>> >goal is to produce uniform compressive stresses over the entire
>>cross-section
>> >of the member over the entire length of the member.  This means
>>that you
>> >need to find a loading that completely replicates the bending
>>stresses in
>> >reverse that the prestressing/tendon profile creates.  An example
>>would be
>> >you have a parabolic tendon profile with the eccentricity of 0 at
>>the
>> >ends.  A balancing load would be a uniform load (as the moment
>>diagram is
>> >a parabola).
>> >
>> >Now, in practice, this rather difficult to completely do.  Take a
>>simple
>> >drapped tendon profile (eccentricity 0 at ends, max at midspan
>>and a
>> >straight line connecting the two points in between).  The
>>balancing load
>> >would be a point load (creates nice "triangular" moment diagram
>>like the
>> >tendon profile).  Yet, you would still in reality have a uniform
>>load also
>> >(the dead weight of the beam itself), so it would not be truly
>> >"balanced".
>> >
>> >Thus, I am used to the concept of "balanced loads" primary use to
>> >illustrate basics of prestressing and how tendon profiles can be
>>thought
>> >of in terms of applied loading in "conventional" terms (usually
>>balanced
>> >loading leads to discussion of "equivalent loads").  This can
>>help people
>> >new to prestressing get a grasp of how prestressing can induce
>>stresses in
>> >the members.  I will note that the basic concept of "balanced
>>loads" can
>> >be useful, even if you completely balance forces created by the
>>tendon
>> >profile...the closer your are to balanced, the less deflection or
>>camber
>> >issues to deal worry about and the member should be crack free as
>>it it
>> >should be largely under uniform or darn close to uniform
>>compression.
>> >
>> >HTH,
>> >
>> >Scott
>> >Adrian, MI
>> >
>> >
>> >On Tue, 31 May 2005, Will Haynes wrote:
>> >
>> > > Is the "balanced moment" always equal to the moment from the
>>post
>> > > tensioning?
>> > >
>> > > In other words, if I have beam with a straight tendon at a
>>constant
>> > > eccentricity of 6" and a force of 150 kips, the moment from
>>the post
>> > > tensioning is a constant 75 ft*kips. Would it be correct to
>>also call
>> > > this the "balanced moment"?  Even though this is not a
>>parabolic profile?
>> > >
>> > >
>> > > Or, if the same beam has a parabolic tendon profile, but the
>>tendon is
>> > > eccentrically placed at each end. Would the balanced moment at
>>the beam
>> > > midspan be the sum of the (eccentric end moment + the moment
>>from the
>> > > drape), or would it be just the moment from the drape?
>> > >
>> > > I am not sure what the definition of "balanced moment" is
>>supposed to be
>> > > for beams with tendons that are eccentrically placed at the
>>ends.
>> > >
>> > >
>> > >
>> > >
>> > >
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>Regards  Gil Brock
>Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163
>586)
>5 Cameron Street Beenleigh Qld 4207 Australia
>Ph +61 7 3807 8022              Fax +61 7 3807 8422
>email:          gil(--nospam--at)raptsoftware.com
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