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.

 

[Tomfh]

We had a recent battle with a PT company over some minor detail. We wanted some extra bars put in to satisfy a code requirement. It was black and white as far as code compliance went. The PT designer said to me I was being ridiculous to demand compliance with every last little thing. It was an interesting insight into their mindset, and into the general disconnect between increasingly economical industry, and increasingly detailed code requirements.

 

[rapt]

Tomfh,

I had one after I moved into consulting where I had to do a design check on a very large building designed by a PT company and they eventually had to add 200t of extra reinforcement, which then made them more expensive than the competing bids that they had beaten to get the job!

On another one they had designed the typical bays perfectly well, about 8.4m spans, but had made three non-typical 9.5m end bays that differed from the typical exactly the same as the typical bays. They then tried to bluff through a fudge to make them work and it was even worse than the original, taking it from under strength to the same amount under strength and non-ductile as well. I had to give them a lesson in PT design to get that one fixed, and it was one of the biggest PT companies in the world at the time.

 

[dflva]

I designed a number of post-tensioned buildings in the early 2000s. Design process for all of them was exactly as KootK described above. Post-tensioning is alive and well in the Mid-Atlantic US.

 

[Agent666]

"Economical" is not necessarily "good"

This has been my one experience with specialist PT designers coming out of Australia. They are so focussed on getting everything down to the bare bones, absolute minimum reinforcement imaginable, that they ignore any code requirements that they simply don't agree with or which would otherwise arbitrarily increase reinforcement. Unfortunately, it seemed like if they didn't do this then the competitor would be and a few kg/m^3 of reinforcement is the difference between getting a job and having no job.

Being in NZ we had a whole lot of other requirements that related to seismic which means more reinforcement typically. Most of the time we got, is this necessary, I think it's being heavy handed.... we've never done this before.... you know the story.

Had one situation where I looked at the crack widths in a partially prestressed design, I thought at ~1.0mm the crack widths were too high. I was told Australian codes never had crack width calculations, so it wasn't something they typically checked, so didn't see an issue with what was being proposed.

Had another situation where there was a significant amount of compression side reinforcement which was a continuation of the spans positive moment reinforcement, this was working really hard and was actually yielding (500MPa). But they refused to apply the appropriate confinement and anti-buckling requirements. Because they were not specifically 'using it' for the design in the negative moment region. If it's there, and it's carrying significant loads I believe putting your blinkers on and ignoring the obvious effects certain clauses are trying to protect against is flawed.

 

[blihpandgeorge]

another aussie here... there a few reasons the many consultancies prefer to not do PT design, and in my experience it is basically down to fee's.

it is not just the technical design aspect - but also the coordination with services penetrations, cast in services, cladding fixings and the final architect setout that comes with doing the FINAL documentation. There is alot of time, and associated risk in this, and if someone wants to do this for a lower fee most larger consultancies are happy to let them do it. Having someone else do it also muddies the water if there is a problem.

Delivery PT is also complicated (in the eyes of non-PT designers) by the additional construction supervision of PT (shop drawings, grouting operations, reviewing stressing timing, tendon extensions, etc). This history of how this developed has already been discussed, but it now appears to suit many structural design firms who feel their 'value' doesn't extend to doing all the details.

Saying it is too specialist may be valid to some, but if people really think this then get the PT specialist in early and not as a downstream aspect where their ability to inform the design is limited. There are lots of other examples of this, ie steel connection design in the UK is not done by the main designer etc.

i personally dont like it, i think one person should be responsible for an entire design. The Aus practice of design and construct of PT (and other aspects) I feel is a significant problem in our industry in my opinion.

 

[rscassar]

Yes, also from an Australia, I'm not a fan of Design & Construct deferral for floor plates. A horizontal slab rarely ever acts to support 'vertical' floor loading alone. Issues that I have seen with D&C slabs are:

- Co-ordination of concrete profiles provided by architect.
- Co-ordination of floor occupancy types and loading allowances for partitions and facades.
- Complete avoidance of column transition design.
- Complete avoidance of in-plane diaphragm forces.
- Lack of responsibility taken at horizontal to vertical interface. Shear connectors to shoring walls and core walls.

 

[Trenno]

Having worked both in Australia and the UK, I too agree that sub-contracted PT design definitely has flaws.

I think it's also similar to the UK practice of not designing the steel connections - there are always too many grey areas that just aren't as coordinated, in terms of design philosophy and practicality, as they should be.

 

[Enhineyero]

Thanks for the response, didn't know that there are a lot of folks here from Australia, I thought most users in this forum are from North America.

I get that there are flaws in the PT designed by the contractors. However, there is a valid point that some engineering consultancies are yet to reach a point where doing PT design makes financial sense. I don't see a lot of seminars or training towards PT design (compared to pre-cast), this to me signals that the PT industry is not really looking to gain more ground in the market.

Also, observed a more than usual amount of PT questions in the forum, hopefully this is not attributed to this post

 

[rapt]

Enhineyero

I think you will find a lot of the training is done in house, especially in larger consultancies. The PT industry approach is still to want D&C projects so they can compete on the scheme as well as PT price.

RE the economics, that would be because consulting engineers grossly undervalue their contribution to the design (in many countries, not just Australia) to win work. The only one who benefits is the developer as long as the problems do not show up before his warranty period ends.

 

[Trenno]

Check out the PTIA, I've found some of their resources helpful when I was starting out, especially the Practical Prestress Detailing guide.

Looks like their website is being renovated, many of the links to their documents haven't been updated.

[URL unfurl="true"]http://www.ptia.org.au/[/url]

 

[MOJOJOHN]

This is the rule we are required to follow in Florida.

61G15‐31.004 Design of Cast‐in‐Place Post‐Tensioned Concrete Structural Systems.
(1) Structural engineering documents shall show the complete structural configuration and loading requirements of the post‐tensioned system including: member sizes, type of post‐tensioning system, location of all prestressing tendons (in plans and elevation), magnitude of all prestressing forces, and all design assumptions. Structural engineering documents shall also show all required non post‐tensioned reinforcing steel including size, spacing, and lengths required for the post‐tensioned system.
(2) If the engineer of record (EOR) elects to delegate the responsibility for preparation of calculations and installation drawings to a delegated engineer for the post‐tensioning system, the EOR shall require the submission of installation drawings for review. Calculations shall also be submitted by the delegated engineer which show sufficient information to document that the number and size of tendons provided are adequate to carry all loads shown on the structural engineering documents. The member dimensions and tendon directions shall match those on the structural engineering documents, unless otherwise agreed to with the EOR, via modified structural engineering documents. Installation drawings shall include the following as a minimum: identification of all the structural elements designed by the delegated engineer, all details of post‐tensioned and non post‐tensioned materials to be used including necessary accessories, and instructions for construction. If the delegated engineer utilizes or requires any additional reinforcing to maintain the member sizes shown on the structural engineering documents, the delegated engineer shall inform the EOR. If any moments, shears or axial loads are required for the lateral force resisting system the EOR shall provide them to the delegated engineer for inclusion in the preparation of the delegated engineering documents. All forces imposed on the load supporting members from the post‐tensioned system shall be reported to the EOR. The installation drawings and calculations shall bear the seal, date, and signature of the delegated engineer who prepared them and shall be reviewed by the EOR for the structure. (3) It is the responsibility of the EOR for the structure to review the post‐tensioning system installation drawings together with the shop drawings of all required reinforcing steel needed for a complete structural design.
(4) The effect of post‐tensioning on other parts of the structure is the responsibility of the EOR.
Rulemaking Authority 471.033(2), 471.008 FS. Law Implemented 471.033(1)(g), (j) FS. History–New 1‐26‐93, Formerly 21H‐31.004, Amended 9‐28‐ 10, 2‐28‐16.