Post Tensioning as a Deferred Submittal
Post Tensioning as a Deferred Submittal
(OP)
I have recently seen drawings for a run of the mill condo building where the floors consist of post tensioned slabs. The thing that surprised me is that the engineer for the building specifies the design of the post tensioning to be 'by others' as a deferred submittal.
Has anyone seen this before?
Has anyone seen this before?






RE: Post Tensioning as a Deferred Submittal
Actually, the final PT layout is almost always done by others around here. However, a conceptual layout and stressing requirements should be included on the drawings. Also, the conventional reinforcement needs to be determined for banded construction prior to the bid.
RE: Post Tensioning as a Deferred Submittal
RE: Post Tensioning as a Deferred Submittal
http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992
RE: Post Tensioning as a Deferred Submittal
Mike McCann
MMC Engineering
RE: Post Tensioning as a Deferred Submittal
I don't like this particular way of project delivery either, but even in the conventional way, foundation design often proceeds before the superstructure design is complete. It requires perhaps conservative assumptions on loading, coupled with checks on the footings and columns before things proceed too far. As RE says, the project design engineer should maintain control over the slab and beam sizes.
RE: Post Tensioning as a Deferred Submittal
RE: Post Tensioning as a Deferred Submittal
How would the EOR design the columns, bearing walls, and lateral system, let alone the foundation, without the slabs already being designed?
Do you just assume an 8" PT slab at each floor, and maybe 11" at the podium slab if there is one, and run with the design assuming simple spans? Seems like a serious waste of materials to me to "save" time, and, to me, really opens the door to create problems.
Mike McCann
MMC Engineering
RE: Post Tensioning as a Deferred Submittal
RE: Post Tensioning as a Deferred Submittal
Mike McCann
MMC Engineering
RE: Post Tensioning as a Deferred Submittal
http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992
RE: Post Tensioning as a Deferred Submittal
That is really the "old" way of doing it. Most of us would accept changes which benefit the constructability while maintaining the design integrity.
This "new" way involves the contractor's engineer doing his own design of the "flatwork".
RE: Post Tensioning as a Deferred Submittal
http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992
RE: Post Tensioning as a Deferred Submittal
Put it in a conspicuous space, but don't make it special...
Dik
RE: Post Tensioning as a Deferred Submittal
The bottom line drives this practice of performance design. If you design around a proprietary system, you are socking it to your client with no clear benefit. If you allow room for different systems, competition reigns. Its more work for the EOR but it has become an expected practice.
The work around is to get your client to buy into a system ahead of time and bring the PT designer on board at the beginning of the project.
RE: Post Tensioning as a Deferred Submittal
Also, it is important that the structural engineer has experience in PT slabs so he can interrogate the design from the sub-contractor. It should not be an avenue for the structural engineer to get away from designing it, because he does not have the expertise to.
We are like RE as well.......we design our slabs to make sure we know what to expect from the sub-contractor. Unless you design a few typical slabs on the project, you won't get a feel for the results since every project is unique in certain ways.
In the US, mostly the design gets done by the EOR (which is what I was used to). It appears that in India the design is done by the PT sub, with the EOR providing a big box saying "PT SLAB".
RE: Post Tensioning as a Deferred Submittal
RE: Post Tensioning as a Deferred Submittal
But really it doesn't matter where in the world you are, I don't much care for this practice unless of course I get paid for the design and then don't have to do it or take responsibility for it.
In that case I am all in.
RE: Post Tensioning as a Deferred Submittal
And therein lies the reason the practice has taken hold.
RE: Post Tensioning as a Deferred Submittal
RE: Post Tensioning as a Deferred Submittal
Now they did do their original designs of columns, supplemental beams, etc. and spent a lot of time in the design phase consulting with the PT entity to help develop their designs.
I see nothing wrong with this as long as the EOR doesn't just let the PT supplier do the design without input, limiting parameters, on-going oversight, follow up after the PT design to ensure consistency with the rest of the structure, and follow up afterwards to document what was actually built.
This is similar, but quite a bit more complex, to and EOR designing a pre-manufactured metal building and having the PEMB designer do the actual member designs.
RE: Post Tensioning as a Deferred Submittal
"Precast, bar joists, PEMB, etc"...same loss/sharing of control and responsibility, so I fail to see the distinction. In the case of precast and bar joists, they do form the "floor itself".
I don't like sharing the PT design, but I like precast even less.
RE: Post Tensioning as a Deferred Submittal
I mean, how many of the EOR here wants to take the responsibility to design the pre-manufactured wood trusses, economically or the review and seal of the truss manufacturer designs, at the price the truss engineer is being paid?
Basically anything the contractor can tell the owner they can get a lower price for, if they have some other engineer do, it is going to become a deferred submittal item.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Post Tensioning as a Deferred Submittal
RE: Post Tensioning as a Deferred Submittal
If they grouted the cables in the US, there wouldn't be so many problems of that type. Unbonded PT should be outlawed, as it is in Australia.
RE: Post Tensioning as a Deferred Submittal
Watch out, the PTI will put you on the watch list and will not let you back in if you want to go back for a holiday again! Thinking like this, you must just about have an Aussie accent by now!
In too mny of these cases, the design is done by the PT contractor without any checking /co-ordination with the consultant. It leads to lots of problems because a lot of consultants who take this route know nothing about PT and are not interested so just accept what they get. I think as a minimum the EOR should be required to to a full design check and take responsibility for the design (if it goes to court he will cop the blame anyway!!). That way he might show some interest.
Rememebr that the PT companies designer might really know no more about PT design than you (or even less). Who knows if he put the correct numbers into the black box he uses and then interpreted (yes he still has to do that) the ones that it produced correctly! Who knows if he can design/detail a slab correctly.
Slickdeals is working in India where I know for certain that a lot of PT company designers are taking some wonderous short cuts in their designs (copying certain other "experts" who I have criticised previously) which are producing under strength slabs. This practice is rampant throughout SE Asia, India and Middle east at least, and they justify it all by manipulating the software to defy statics and equilibrium.
RE different systems, in any particular market, the systems are basically all the same (they have been copying each other for decades). If the designer nominates his design parameters properly, then any PT contractor quoting on it can put in a non-conforming bid to allow for his system and pay the designer to fit his system into the project if he wins!
RE: Post Tensioning as a Deferred Submittal
It has always seemed odd to me that an architect/owner hires a consultant based on qualifications then allows an engineer that they have never met design their floor system.
Also the company designing the PT has a financial interest in using less material.
RE: Post Tensioning as a Deferred Submittal
Thanks for the differing viewpoints however. I found this such an interesting topic.
RE: Post Tensioning as a Deferred Submittal
- whether he uses 5/8" diameter strand or 1/2"
- Stress the strands and Strip the forms at 1 day or 3 days (different concrete mixes)
- Stressing sequence
- In some cases, allow the contractor to use bonded or unbonded systems
- Banded or equally distributed tendons
- etc...
Unlike a strict PT design by the EOR, the EOR for a performance design PT system must be even more educated on the current available PT systems and how the different variables may work together. More engineering time is required during the submittal review process and more thought must be provided in the specifications in order to limit the PT contractors to an intended result.
RE: Post Tensioning as a Deferred Submittal
We review the layout in the shop drawing process.
RE: Post Tensioning as a Deferred Submittal
I always do a full review of the PT when designed by others and give them grief when they do something dodgy (like setting the Ieff/Ig ratio to 1.0, using 100% column stiffness and deflection design, use two-way slabs which use torsional stiffness and neglect Mxy for design and even laying out the shear stud reinforcement in the wrong direction).
Some guys are just dodgy and do not have any more expertise than yourself, but overall they seem to be better than truss engineers.
RE: Post Tensioning as a Deferred Submittal
Can you elaborate a little on the column stiffness criteria?
RE: Post Tensioning as a Deferred Submittal
RE: Post Tensioning as a Deferred Submittal
Is there any literature out there that says that you should be using something other than Ig for slabs?
RE: Post Tensioning as a Deferred Submittal
For some reason, you're a lot more likely to be able to find the original design than any deferred submittal. If you want to change a structure (cut an opening, add a concentrated load, etc.) you're screwed without the submittal drawings. They're submitted to the engineer, they might or might not review them, and they go in some kind of cold storage (or are just promptly lost). It's like the last scene from "Raiders of the Lost Ark" You may be able to contact the actual design firm, if you can figure out who they are. And a lot of times their record keeping is poor.
How many posts on this thread concern someone trying to figure out OWSJ capacities? It's the same thing, where the design drawings defer the design of a critical element and the final materials are not documented.
This is a comment without solution. I don't want to design prestressing. But I've had to try to find actual submittals for it, and it was impossible.
RE: Post Tensioning as a Deferred Submittal
lets start a new thread about Ief=Ig for Pt design.
http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992
RE: Post Tensioning as a Deferred Submittal
Just because ACI allows something does not mean it is correct. ACI and Bransons formula both grossly underestimate deflections in lightly reinforced RC members. This has been shown extensively in tests. But ACI still allows it.
Part of the reason is Bransons formula for tension stiffening which is completely incorrect at the point of cracking, saying that there is no increase in deflection when the first crack occurs (Ms = Mcr). The other problem is that a lot of stresses that are created in a memeber are ignored by most designers eg shrinkage restraint by internal reinforcement (in a normal member this will be between .5 and 2MPa, depending on how heavily reinforced it is). So some sensible codes have allowed for this in an attempt to get results from simplified code deflection calculation methods to match real world deflections. There was a paper several years ago in an ACI journal by Gilbert that gave an example of this. Actual deflection of the slab after 9 months was 29.5mm, while ACI predicted about 9.3mm (interestingly RAPT got 30.1!).
In PT design, shrinkage restraint by the reinforcement is much less as the percentage of steel is reduced. But it still occurs.
But the other thing that is ignored in most deflection calculations is tension stress induced by external restraints to shrinkage and temperature change shortening. This just about always exists and in many structures is very significant. It needs to be allowed for. One way, without calculating it, is to make a blanket assumption that Ieff is limited to a percentage of Igross. Another way is to add a tension stress into all sections in the member, or alternatively, reduce the tensile strength of the concrete by an equivalent amount.
The most stupid thing to do is to say ACI does not mention it so I do not have to worry about it. ACI does mention it where it says shringage and temperature effects must be considered in calculating deflections. Most designers simply ignore this as either "too hard to do" or as "I dont know how so lets forget it" and put their heads into the sand.
RE: Post Tensioning as a Deferred Submittal
so much for my new thread, I'm going to copy your response across and reply there.
http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992
RE: Post Tensioning as a Deferred Submittal
RE: Post Tensioning as a Deferred Submittal