Interview help - post tension design using hand calculations 5

[sparta7963]

Hi, I am a attending an interview with a company which does Post tension work. The job position is 'Structural Engineer - Post Tension Design'. I was told by the recruitment agency that the company will do a technical round interview where i have to use hand calculation in post tension design. I have about 5 years experience as a structural engineer but never done post tension design. Can you please tell me the some question that I might be asked in 'Post tension design' and what sort of hand calculations i have to do. I am in UK, so please provide me some study materials based of BS/EURO code. Thank you.

 

[Trenno]

Been a while since I've seen that classic ^^

 

[TehMightyEngineer]

That image would actually make a great interview question; "what's wrong with this picture?".

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[slickdeals]

At least the stressing end is shown on the right side

 

[dik]

almost always terminated the strand at the kern point to account for some -ve moment and to increase the sag a tad.

 

[rapt]

Dik

OK for an increase the negative moment capacity, but you also increase your negative moment due to secondary prestress effects, and you are also applying an end moment which completely negates the extra uplift and does nothing to help with deflections or service stresses at mid-span.

Further on the negative side, if the member is not deep enough you are creating congestion problems at the top with anchorages needing to fit in with transverse tendons and reinforcement in both directions.

So unless you really need the extra negative moment capacity (which you normally do not, as I would always add some passive top reinforcement at end columns anyway), don't waste your time.

 

[dik]

agreed to some extent... and add regular reinforcing as noted... running the sums it does improve things a bit... just take advantage of the added sag in the end span.

thanks, Dik

 

[rapt]

What is the added sag doing for you, after you take into account the added end moment which completely negates the extra uplift from the added sag?

 

[cal91]

I don't have any experience in PT, could someone clarify what is wrong with the picture? Thanks!

 

[KootK]

Prestressing tendons experience tension losses due to anchor seating, relaxation, elastic shortening, creep etc. With very short tendons, those losses decrease the prestress to the point that the prestressing ceases to add meaningful benefit to the system. It can also be a bit tricky to jack short tendons accurately as small increments of end stretch produce relatively large increments of tendon force (d = PL/AE).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

almost always terminated the strand at the kern point to account for some -ve moment and to increase the sag a tad.

What is the added sag doing for you, after you take into account the added end moment which completely negates the extra uplift from the added sag?

dik,

What rapt refers can be easily understood for a simplistic case of a simply supported beam with end eccentricities ABOVE the CGS. Such end eccentricities makes the midspan deflection worse (less uplift), as per the attached two cases, where I just rearranged 5wL⁴/384EI and P(e₁+e₂)L²/8EI for midspan displacements, under the actions due to prestress due to w_bal and end-eccentricity of prestress, respectively.

The PL²e₂/48EI term is making the midspan uplift LESS compared to CASE II with the same prestress, less "sag", yet with no end eccentricity.

 

[dik]

Thanks... fully understood...

Dik

 

[rapt]

Ingenuity,

Nice pictures. Wish I could do that!

In Case 1, the drape is e1 + e2, not e1, so the balanced load deflection is 5PL^2(e1 + e2)/48EI.

After subtracting off the end moment effect, Case 1 result is 5PL^2(e1)/48EI, exactly the same as Casse 11.

 

[KootK]

I was curious too so I did a little fact checking myself last night ('tis the season). Turns out Inginuity is fit to lead the free world after all. Or at lest design a little PT.

Excuse the quality. I wan't originally intended for public consumption.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

rapt - just some mathematical 'gymnastics' to get the reduced form. See below.

Inginuity

KootK - your 'thumb-nail-dipped-in-tar' presentation is as good as your spelling, but that's ok, injinrs are not here on this earth to look good or be great spellers - well at least I'm not!

 

[Ingenuity]

Nice pictures. Wish I could do that!

I did a FEM analysis too - want to see that? Just kidding ;-)

 

[KootK]

I'm actually a first rate speller. I'm just usually doing this on my phone and correcting the autocorrects isn't worth the effort. Even so, I've gotta be in the upper 50% round here with respect to readability.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I did a FEM analysis too - want to see that? Just kidding winky smile

If you've already got it prepped then... yeah. I think that midspan moments do come out a tie.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

Sorry Ingenuity, I had always assumed it exactly cancelled the increased uplift effect and commented before looking into the derivation. But looking into the math you are correct, it actually makes your deflection slightly worse, contrary to some designers expectations.

Which would explain why lowering the tendon at the anchorage below the centroid in a simply supported span (and the reverse in a cantilever), reducing the "drape uplift" but introducing a beneficial set of end moments, actually helps reduce deflections.

This is one of the reasons I have always tried to talk designers out of thinking in terms of load balancing. They tend to think of the drape effect only and not the "secondary profiling effects" (as distinct from Secondary Prestress (parasitic moment)) like moments from end eccentricities. "Moment Balancing" is a much safer concept!

Some early PT programs used to actually make those incorrect assumptions regarding raising end eccentricities, even one developed by a PT company you used to work for in Sydney which was taken to America by an employee and has developed and been used extensively in the USA.

 

[rapt]

KootK,

Yes, with a determinate member, the mid span moment is P * e1 no matter what!

 

[Ingenuity]

Sorry Ingenuity

No worries.

Which would explain why lowering the tendon at the anchorage below the centroid in a simply supported span (and the reverse in a cantilever), reducing the "drape uplift" but introducing a beneficial set of end moments, actually helps reduce deflections.

Yep, even if you consider a prestressed plank (and ignore strand end-slip etc) with straight strand of constant eccentricity, e₁, [CASE III below] there is 20% more midspan uplift than a parabolic drape of e₁ 'sag' with the same prestress [CASE II]:

If you've already got it prepped then... yeah.

No, I didn't do a FE analysis. But I concur with rapt with a midspan moment of P*e₁.