Interview help - post tension design using hand calculations
Interview help - post tension design using hand calculations
(OP)
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.






RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
I'd learn about jacking procedures, transfer design stages, and prestressing losses; to name a few problems they may ask. Some other aspects might be various steps in checking capacity or whether the member remains uncracked. I'm familiar with the US codes so I can't help with any specifics beyond just generalizations.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Interview help - post tension design using hand calculations
I got the job, but I will say that it was "balls to the wall" upon starting on the first day. Thrown some interesting, multiple, fast-paced projects in the first week!
Not wanting to turn you off - but if you do go past the interview phase and land the job, I am going to assume it will be fast-paced from the get-go. If you don't have the fundamental basic understanding of PT, they may not be willing to slow down and train/mentor you.
Design software has (unfortunately?) changed a few things over the years, on what you can 'get away with' on assumed-knowledge.
Did you take a class at uni on prestressed concrete?
Are you in AU or the UK?
RE: Interview help - post tension design using hand calculations
I understand the risks of a new job, but i can either sit at home dreaming about an ideal job Or work hard and make use of a given opportunity. Hopefully i should be able to manage will try to study what you have mentioned..
any particular study material u propose other than TR43 (Concrete Society Technical Report No. 43: Post-tensioned concrete floors)?
RE: Interview help - post tension design using hand calculations
Since you are currently in AU I would grab a copy of one or both of the following AU underdrad text books on prestressed concrete:
2. Design of Prestressed Concrete to AS3600-2009, Second Edition, Raymond Ian Gilbert, Neil Colin Mickleborough, Gianluca Ranzi
Available from most uni bookshops.
Even if you get an older 2nd-hand copy of Warner/Faulkes book (late 80's or early 90's edition I think), that has some good practical design info that will be an advantage to you at minimal cost.
The second text does a better treatment of the theoretical aspects of prestressed concrete (especially creep, shrinkage, deflection etc), but the first text has some good info for practical design of slabs and beams: step-by-step procedures and design examples.
And forget about UNbonded PT - AU is all bonded/grouted PT.
All the very best to you.
Also - if you want a heads-up on PT software in AU - most (all?) consulting companies in AU (and beyond) use RAPT (www.raptsoftware.com) and if you go to their website I think there is a trial download where you can take it for a spin. The author and owner of RAPT visits here at Eng-Tips frequently, so hopefully he will chip in too.
RE: Interview help - post tension design using hand calculations
The RAPT manual is a good resource (all 740 pages of it!). There is good discussion on preliminary sizing, load balancing and level of prestress in the beginning of the "Theory" section.
RE: Interview help - post tension design using hand calculations
If you really understand concrete design, convince them of that. The step up to PT design is not that large if you understand engineering, structures and concrete design (and I mean understand, not just know how to design an RC slab using software). By all means read the books to get started, but I would be honest. They will not expect many people coming from Europe to have much building PT experience.
RE: Interview help - post tension design using hand calculations
From what I have seen in my job hunting, australian companies don't consider or value your experience outside australia (that what the ALL the job recruitment consultants told me and the salary offered for that position was between AUD$60000 - $70000). So i am guessing its more of a junior level position.
RE: Interview help - post tension design using hand calculations
Employer: vet you to ensure that they're getting what they need.
Candidate: chase down your dreams with as much courage and tenacity as you can muster.
If I could somehow convince NASA to hire me to train for the next manned space flight, would I give it a go? Darn right.
Must the next prime minister have previous experience being prime minister? Of course not. Many jobs are learned on the job. Meaningful personal growth usually requires reaching a little.
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.
RE: Interview help - post tension design using hand calculations
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.
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
"I haven't done any PT projects, but I have reviewed some technical references ..."
I'd be interested in when to use PT beams, and why (are they used in those situations).
another day in paradise, or is paradise one day closer ?
RE: Interview help - post tension design using hand calculations
You just want them aware of your experience level......especially if they want you to come in and immediately stamp stuff (you may not be comfortable with that).
One of the best jobs I ever had was when I told the guy in the interview: "Your ad asked for [this or that skill]....here are some of those skills I have: [ ], and here are some I don't have: [ ]."
RE: Interview help - post tension design using hand calculations
That won't be an issue - there is no PE/SE stamping in AU, per se.
Stamping/sealing is a somewhat unique North American "restrictive trade practice".
RE: Interview help - post tension design using hand calculations
Is that you asking, rb?
If so: Typically, PT (and prestressing generally) is used to 1) accommodate larger spans at 2) less structural depth than non-prestressed members, with 3) superior deflection control due to the active 'actions' of eccentricity/curvature of the prestressing tendons, and 4) less cracking due to the effective P/A. Also good for fatigue resistance too - more appropriate for bridges than buildings.
RE: Interview help - post tension design using hand calculations
Coming more from the practical perspective, I highly recommend downloading the material available from the Post Tensioning Institute of Australia.
http://www.ptia.org.au/Downloads.aspx
In particular the 'Practical Prestress Detailing' guide. I've attached it for anyone too lazy to click the link.
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
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.
RE: Interview help - post tension design using hand calculations
thanks, Dik
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
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.
RE: Interview help - post tension design using hand calculations
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 wbal 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.
RE: Interview help - post tension design using hand calculations
Dik
RE: Interview help - post tension design using hand calculations
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.
RE: Interview help - post tension design using hand calculations
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.
RE: Interview help - post tension design using hand calculations
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!
RE: Interview help - post tension design using hand calculations
I did a FEM analysis too - want to see that? Just kidding
RE: Interview help - post tension design using hand calculations
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.
RE: Interview help - post tension design using hand calculations
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.
RE: Interview help - post tension design using hand calculations
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.
RE: Interview help - post tension design using hand calculations
Yes, with a determinate member, the mid span moment is P * e1 no matter what!
RE: Interview help - post tension design using hand calculations
No worries.
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]:
No, I didn't do a FE analysis. But I concur with rapt with a midspan moment of P*e1.
RE: Interview help - post tension design using hand calculations
What is affected by the tendon disposition elsewhere is the total area under the moment diagram and, by extension, the total area under the curvature diagram and, ultimately, deflection. Since a level tendon profile msximizes the area under the moment diagram, it makes sense that arrangement would maximize deflection.
So, if one were to design a cantilever with the singular goal of inducing upwards tip deflection, the best strategy would be to:
1) Raise the end anchorage as much as possible.
2) Use a concave down parabola to get from the end anchorage elevation to the support tendon elevation.
Both of these things because they "fatten" the moment diagram. But, then, we typically do neither of these things in conventional design for a number of practical reasons.
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.
RE: Interview help - post tension design using hand calculations
RE: Interview help - post tension design using hand calculations
Yes, the best for deflection. But when pushed too far you start causing transfer stress problems!
If he was alive today, poor old TY Lin would not like our conclusions regarding Load Balancing vs Moment Balancing, but Leonhardt and Guyon surely would.