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?

 

[Teguci]

On my projects I usually draw a box and add a note "See architectural drawings for additional information."

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.

 

[hokie66]

It is increasingly common practice where I am to outsource the design of the "horizontal components" when post-tensioning is used. I don't like it, and know of lots of problems which have reared their heads where this split of responsibility has been employed. Just another version of design-build.

 

[rowingengineer]

We do it quite often; however we do a complete design, but give the PT contractor the opportunity to submit changes to our design. The changes are normally the number or type. This way they don’t get to stuff our thickness up, but dose give the client the best solution.

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

 

[msquared48]

How can you defer the design of the PT when the vertical reactions determine the foundation design? Who is kidding whom here?

Mike McCann
MMC Engineering

 

[hokie66]

Mike,

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.

 

[Ron]

It is common practice in my area to delegate the PT design to a specialty engineer...usually employed by the PT contractor. The EOR still has the obligation to review the design for compliance with the overall design intent, including checking loadings and reactions.

 

[msquared48]

I still don't get it I guess.

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

 

[hokie66]

You base the assumptions of the concrete profiles on experience with similar structures. The preliminary concrete profile plan is then usually given to the PT contractor's consultant. Both design offices work at the same time, and communicate with each other...at least in theory.

 

[msquared48]

I guess I'm just too used to doing things all in the same office. Oh well. Times change I guess.

Mike McCann
MMC Engineering

 

[rowingengineer]

We do a full design, with profile runs, reo ect, we just allow the PT contractors engineers to make changes to the layouts to suit construction.

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

 

[hokie66]

RE,
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".

 

[rowingengineer]

I do let him do his own design; I even supply the rapt runs I am using. He just can't go outside the parameters I set. I am also so kind as to let him back certify the 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

 

[dik]

I was always taught that you don't put 'boxes' around items on a drawing or bold, italise, or use a larger font for specs... with the reasoning that, "What is the reason for making that 'more important'?"

Put it in a conspicuous space, but don't make it special...

Dik

 

[Teguci]

When I hear PT performance design, I am thinking about the size, spacing and stressing of the strands. The dimensions of the slabs, beams, girders and columns can be predetermined by the EOR. It is also important for the EOR to determine the pour sequence and allow for the volume change that occurs when the slab is stressed.

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.

 

[slickdeals]

I think the key to having a successful design is to have very defined set of design parameters that the PT contractor has to abide by. Otherwise, it will end up being a numbers game......

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".

 

[dcarr82775]

Wow, I figured this was a rare thing, but I guess it is more common. I don't like the idea personally. The next logical thing is for your standard steel/wood floor beam and column system, or the lateral system to also be designed by a specialty 3rd party.

 

[dcarr82775]

I always forget this is a world wide board. I was more interested in what occurs in the US.

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.

 

[hokie66]

darr,
And therein lies the reason the practice has taken hold.

 

[dcarr82775]

My comment was tongue and cheek, I still don't care for the idea. Precast, bar joists, PEMB, etc I get, but not the design of the floor itself.