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Re: Post-tensioned or reinforced ?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Post-tensioned or reinforced ?
- From: Daniel Popp <drp181(--nospam--at)yahoo.com>
- Date: Fri, 16 Nov 2007 21:28:56 -0800 (PST)
Others in my office have far more experience than I do, but I have picked up a few things. My thoughts concerning your questions:
Post-tensioned slabs are very common throughout the US -- I would venture that PT slabs are built far more than RC slabs. In most areas you can find contractors that are very familiar with PT, which does require more expertise than RC, both in design and construction.
- Longer spans or less thickness - PT makes more efficient use of both the concrete and the reinforcement, and as a result typically uses less of both. Less material usage is generally seen as "green".
- Less deflection - the post-tensioning force can counteract the self-weight of the slab, resulting in much less dead load deflection. The designer can control the deflections to a much greater extent than with RC. However, the longer spans possible with PT can lead to live load deflection issues.
- Durability - the post-tensioning force places the concrete permanently in compression, leading to fewer small cracks and less opportunity for water and chlorides to infiltrate the slab over time. Many engineers will specify higher PT forces at the roof levels of parking garages and other structures for this reason.
- More control over the design - the structural engineer has more tools to influence the slab behavior.
- More complexity - PT construction requires much more skill on the contractor's part than does RC. Small errors can have large impacts. Tendons are stressed to a high force level when the concrete is only several days old.
- Shrinkage and movement - The early stressing of the green concrete leads to more shrinkage than would be seen in an RC slab. If restraint is present that resists the shrinkage (e.g. a shear wall), cracks can develop in unexpected places.
- Durability - PT slabs rely on the anchorages at the slab edges in order to maintain the force. In the past, there have been corrosion issues with anchorages which have led to costly repairs. The current state of the art promises much better long-term durability.
- More control over the design - the structural engineer is responsible for more aspects of the slab behavior.
As a structural engineer and not a contractor, I cannot fully address the issue of cost. However, here in Hawaii, nearly every high-rise project utilizes PT to minimize the slab thickness. Since the building height is limited, a reduction in the slab height will allow an additional story or stories to be added to the project, which increases the value of the building. Many projects here have an 8'-6" floor-to-floor height: 8'-0" clear with a 6" slab. This is typically only possible with PT, unless the spans are short. Spans of 25 to 30 feet are routinely done with 6" slabs.
Many engineers are very familiar with PT, and many excellent resources are available online. It sounds as though you have an opportunity to become a leader and innovator in your region, with lots of experience to draw upon.
From: Kevin Below <kbofoz(--nospam--at)gmail.com>
Sent: Friday, November 16, 2007 4:52:37 PM
Subject: Post-tensioned or reinforced ?
Daniel, since you have a lot of experience with PT, I would like to ask you something I have been wondering about for a long time : What are the bottom-line advantages and disadvantages of PT vs RC slabs ?
In Quebec, I have never seen PT slabs being used, so there is little current field experience, although that could be brought in if it was worthwhile.
Is there an appreciable cost difference ? Advantage PT or RC ?
Deflection control ?
Slab thickness ?
What are the green aspects ?
Our office has done a great deal of high-rise PT design recently. We use both ADAPT and RAM Concept, which was known as Floor before being bought by RAM International (which is now owned by Bentley). Both programs have their strengths and weaknesses.
ADAPT is considered by many to be the industry standard, and is very good for two-dimensional design (design strips). I would recommend using the program to become more familiar with the design process, as it gives the user a simple two-dimensional design interface which makes adjusting the design easy. ADAPT has more recently introduced a three-dimensional finite element analysis option, which we have not used much yet. ADAPT Support is also beyond compare -- they even offer to model your slabs for you for a fee.
For complete design of two-way PT slab systems, RAM Concept is hard to beat. It performs a full three-dimensional analysis of the slab, including beams, drops, thickened areas, and support. The meshing of the slab is the best I've seen in any structural program. All design checks can be performed by the program, as well as punching shear checks and stud rail design. Concept is now integrated with RAM Structural System, meaning that geometry and loads can be sent both directions. This has not benefited our office, as we do not use RAM Concrete; I have been told by RAM that their Structural System cannot handle two-way slabs, meaning that extraneous beams must be added to the RSS model. Another downside: preliminary design is not as simple as in ADAPT, especially if the engineer does not have a good feel for the design.
Feel free to email me directly for more specific observations on the programs and PT design in general.
----- Original Message ----
From: Stan Caldwell <stancaldwell(--nospam--at)gmail.com >
To: SEAINT Listserv <seaint(--nospam--at)seaint.org>
Sent: Friday, November 16, 2007 1:20:13 PM
Subject: ADAPT or POSTEN ??
For the last few years, my practice has mostly been bridge design. However, every now and then, a significant building project comes along. This is one of those times. By the end of the year, my staff will start designing two building projects with a total of more than 40 floors. Most of the floors will likely be post-tensioned concrete construction. Both projects are located in Texas, well away from the Gulf Coast, so seismic and wind are not anticipated to be major issues.The last time we maintained licenses on P/T software was back in the days of DOS. I assume that the two leading options today are the same that they were back then, ADAPT and POSTEN.From recent hands-on experience, do any of you have recommendations on either of these programs?Are there any other reasonable options that I am not yet aware of?Thank you in advance for sharing your wisdom on this subject.Best regards,Stan R. Caldwell, P.E.Richardson, Texas
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