Post Tensioning as a Deferred Submittal 4

[dcarr82775]

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?

 

[JAE]

We had an entire retail/commercial/residential complex designed by the EOR who outsourced the PT design to another entity.

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.

 

[hokie66]

dcarr,
"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.

 

[woodman88]

This is the same for any deferred submittal. The EOR farms out the design (or first sometimes does a basic design of the item) to another engineer. The other engineer has (hopefully) greater expertise about the item and the owner gets the most economical design.
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.

 

[a2mfk]

Like Ron said, very common in Florida. I think its a combination of this being a bit of a specialty (I have never had an opportunity to do it in 13 years), the specialty contractors having their "own guy" who does it their "own way", and liability. A previous firm I worked for, very respected, would not do them because of the liability. Most of the buildings they worked on in that category they talked the owner/arch into flat plate conventional reinforcing. There was at least one high-profile failure in the Miami area that forced residents to move out I think due to some sudden cable failures. Then of course there are other lesser known problems with PT projects near the coast because of the salt and corrosion...

 

[hokie66]

a2mfk,
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.

 

[rapt]

Hokie66,

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!

 

[ron9876]

rapt I am with you all the way. How can someone be the EOR on a project that has a floor system that they don't understand. Makes no sense to me at all.

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.

 

[dcarr82775]

I do not but the argument a PT floor is the same as any other deferred submittal. A bar joist is not even remotely the same animal as a PT floor slab. A PEMB is entirely separate from the foundation it rests on. A PT floor is designed integrally with the columns and walls so therefore IMO can't simply be farmed out.

Thanks for the differing viewpoints however. I found this such an interesting topic.

 

[Teguci]

The PT issue as a defferred submittal should not be due to the inexperience of the EOR with PT but as an opportunity for different PT systems to compete. The EOR should (read-must) be well versed in the problems and design of PT. The purpose of performance designing of the PT is to allow the contractor to choose:
- 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.

 

[Lion06]

We do the actual PT design showing PT forces and mild steel on the drawings. We leave it up to the PT supplier to detail it and provide the friction calculations.

We review the layout in the shop drawing process.

 

[asixth]

PT as a deferred item is common practice in Australia because it is cheaper (with possibly less integrity). I also offer to do it for cheaper and give the PT designer hell when they are designing my slabs which aren't in accordance with my performance specification. With PT as a design and construct item, I will issue my concrete profiles and without doubt the PT designer will come back with concrete profiles with 5-10mm shaved off.

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.

 

[slickdeals]

@asixth,
Can you elaborate a little on the column stiffness criteria?

 

[asixth]

I reckon for designing for serviceability and ultimate design criteria a cracked column stiffness 20-50% should be used. It may only mean 1-2mm in long-term deflection. Except punching shear where 100% column stiffness should be used.

 

[Lion06]

I don't think there's anything dodgy about using Ieff/Ig = 1.0 for slabs. ACI explicitly allows you to do this. I do this for PT design and every engineer I know that does PT (and precast, prestressed) does this as well.

Is there any literature out there that says that you should be using something other than Ig for slabs?

 

[JedClampett]

Another underlooked aspect in deferring this kind of design concerns record drawings (or any design data).
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.

 

[rowingengineer]

Lion06,
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

 

[rapt]

Lion06,

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.

 

[rowingengineer]

Rapt,
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

 

[rrmiv]

Does anyone know of a standard or guideline regarding the relationship of the EOR to the P/T designer? Does PTI or ACI address this anywhere such as a "Code of Standard Practice", describing the responsibilities of all parties?

 

[ron9876]

Check your local/state requirements.