PT design by others 1

[Enhineyero]

There seems to be a variety of opinions on whether structural engineering firms should include PT design on their documentation or leave it to specialist. Most structural engineers have a good understanding of PT (noting that some heavily rely on software such as RAPT) but not to the same extent as the guys working in PT. Obviously, guys working in a PT company design PT on a day to day basis, while a structural engineer in a consulting firm might do this once in every 6 months (depending on the profile of the firm).

The main point of engineers who include PT in their documentation is that the documentation gets a bit grey in the area where PT elements and non-PT elements intersect, and if anything happens in the project it would be a bit challenging in a liability point of view.

The arguement of engineers who nominates PT design by specialist is that specialist know more about this trade. PT firms would likely avoid mistakes that most consulting engineer would make and have the tools towards a more efficient process (i.e. their PT design can be directly exported to shop drawings). Engineering firms would put in a lot of effort (and probably lose money) on documenting PT, which might end-up being changed by the PT specialist for various reasons (i.e. design detailing issues, constructability).

I've always leaned towards the latter and would like to know what others think.

 

[r13]

Most structural engineers have a good understanding of PT (noting that some heavily rely on software such as RAPT)

If one can't do the design by hand, then it shall be left to the specialist with a well draft action plan. A computer program can help to achieve efficiency, only if one has solid understanding on the subject in concern.

 

[Retrograde]

I prefer to leave it to the PT guys if possible. Some clients don't like that though and want everything done by the one engineer.

If one can't do the design by hand, then it shall be left to the specialist with a well draft action plan.

Not sure what to make of this comment. I can assure you the specialists are not doing the design by hand.

 

[r13]

Simple, many people think computer can do any design just plug in the numerical values of prescribed parameters without even know the logic behind. Read the whole sentence.

 

[Tomfh]

I prefer to leave it to them. They understand how their operation works.

 

[STrctPono]

Apparently I'm unaware how Structural Engineers do PT design in buildings as I wasn't aware that passing the PT design off to the Contractor's Engineer was even a thing. Does this mean that you size the slab thickness, provide the design loads, and then have the entire design provided by them for you to review and approve? In my industry, we do 95% of the PT design and detailing so I'm just curious.

 

[Retrograde]

Does this mean that you size the slab thickness, provide the design loads, and then have the entire design provided by them for you to review and approve?

Generally yes but we do not "approve" their design as such, just review and comment.

 

[KootK]

In my PT practice, limited to North American buildings, the EOR (me) specified all aspects of the 3slab material and geometry, the amount of effective slab prestress, the typical details, the strand profiles, the mild reinforcing, integrity reinforcing, and the shear reinforcing. The PT supplier really only detailed out the individual strands and achorages, shop drawing style, and sorted out the jacking required to hit the specified effective prestress after losses.

So much of the PT design work has serious implications for the overall building planning that I struggle to imagine handing that off to a downstream party except for the simplest and least aggressive of slabs. But, then I guess anything is possible with enough experience and communication. High end timber design has gone a similar route in my area with specialty firms handling the fire and connections which is, of course, most of what matters

 

[rapt]

I spent 10 years in a PT company trying to encourage and help consultants to do their own PT designs while some opposition PT companies tried to convince everyone it was a dark art that only they could perform, using exactly the arguments above. Then have spent the last 30+ years providing consultants with a design tool to be able to do it easily.

And they still keep coming up with the same excuses.

Come on. Those consultants that let the PT company do it do so because it saves them money or allows them to quote lower fees! And by doing so will always be in a position where they will never learn to do it themselves.

 

[STrctPono]

Well it appears that the consensus from Engineers in Australia is to have the PT design kicked off to the Contractor. Other than KootK, I'm curious what other Engineers do in North America.

 

[hokie66]

STrctPono,
I don't think you have a broad enough sample to declare a consensus. I am retired now, but I worked for a large consultant, and we always did our own PT design as part of the package. For PT work, we had 4 plans for each level: profile, bottom steel, top steel, and post-tensioning.

My only exposure to the split responsibility scheme was in investigating jobs by others where things went wrong.

 

[Enhineyero]

Does this mean that you size the slab thickness, provide the design loads, and then have the entire design provided by them for you to review and approve?

I usually use a span table to determine the preliminary size of the slab/beam and estimate the volume of reinforcement. The detailed design is by the PT consultant. The result of prelim and detailed in terms of cost is usually not that far off.

Those consultants that let the PT company do it do so because it saves them money or allows them to quote lower fees! And by doing so will always be in a position where they will never learn to do it themselves.

I've seen some engineer charge the same fee, PT or no PT. I always make it a point to charge extra if using a certain technology provides value for the client (esp. it requires more work for the consultant), otherwise why work hard for nothing. If PT becomes the norm in the building industry then that's a whole new game, until then PT remains an exception to the rule.

Well it appears that the consensus from Engineers in Australia is to have the PT design kicked off to the Contractor. Other than KootK, I'm curious what other Engineers do in North America.

Its a hit and miss in Australia, some firms do some firms dont. What I can say is that they don't get hired because they know PT (its more of a bonus).

 

[WARose]

The arguement of engineers who nominates PT design by specialist is that specialist know more about this trade. PT firms would likely avoid mistakes that most consulting engineer would make and have the tools towards a more efficient process (i.e. their PT design can be directly exported to shop drawings). Engineering firms would put in a lot of effort (and probably lose money) on documenting PT, which might end-up being changed by the PT specialist for various reasons (i.e. design detailing issues, constructability).

I think this is the main reason. When I've reviewed calculations from engineers who don't do it on a daily basis.....I see all sorts of missed items. (Probably most common: failure to account for bursting forces from jacking at the ends.)

It's better to leave it to people who have it streamlined.

 

[r13]

A side question, how popular is PT in the US, or North America?

 

[KootK]

Pretty popular in some markets. I've seen tons of it in the midwest, Florida, and California. My understanding is that NY and Chicago have issues with labor unions and very fast floor cycling that mostly tilt things in favor of mildly reinforced slabs.

Use in Canada seems sporadic in part, I think, because we were early adopters and got burned by the corrosion issues associated with earlier versions of the technology. For the most part, I only see PT in Canada on the occasional Vancouver or Toronto job. We do a lot of PT repairing in Alberta.

 

[slickdeals]

My understanding is that NY and Chicago have issues with labor unions and very fast floor cycling that mostly tilt things in favor of mildly reinforced slabs

Wouldn't this tilt it more towards PT? The good concrete subs are doing 3 day cycles here in Chicago with PT slabs and a jumped core.

 

[KootK]

You're in Chicago, you tell me. PT requires some curing time prior to prestress so I guess it comes down to how long you're willing to leave your formwork/shoring in. In NY, a three day cycle times is 50% too long.

 

[KootK]

In NY, my understanding is that the labor issues make it such that the guys doing rebar are different from the guys doing PT. Unless you're floor plates are big enough to be split into two, offset pours (rare in Manhattan), having to schedule one team after another pretty much kills the two day turnaround. In that sense, the labor issues and the scheduling issues are intertwined. The US is a big country and I'm sure you'll find examples of all kinds of things being done or not done in various places at various times. This is what I've experienced.

 

[rapt]

Enhineyero

I would have said the percentage of PT used in Australia is relatively high compared to other countries.

The biggest problem is getting it taught in universities and practiced in consultancies. When I started, PT companies offered to at least help with the design as there were no design tools available and the design work required was new and required more time than RC. So the PT industry did the prelims, shop drawings etc and assisted with design theory, basically teaching consultants who were new to PT and covering the extra costs involved. Keeping the design inhouse in PT companies also meant that PT startups were difficult to get going as they needed an experienced design group. What we are tending to get now is that a lot of the smaller PT companies use a selected group of PT consultancies who do not do a lot more than PT company design work.

As Hokie said and I have heard from many others, a lot of the problem designs actually come out of PT company design. The logic that they know their system is rubbish. The building PT systems are all basically the same now, all copied from the original VSL and Freyssinet systems from the 1960-70's.

Some builders/developers have been convinced by PT companies that they should be given the design as part of the contract because they would get a more economical building out of it. "Economical" is not necessarily "good".

 

[r13]

In the US, the prestressed concrete course was either a common core, or recommended selective in the junior/senior year of college, but I am not aware which colleges offer PT as mandate though.