Effects of fire on PT slab and PT beams 2

[AlbertStructural]

Dear All,
I would like to get your expert opinion regarding effects of fire on post-tensioned (PT) slab system supported by PT beams. Recently one of the newly malls in middle east region got fire (During finishing phase of project) and the fire badly affected the PT slab as well as the adjoining hollow-core slab and also the steel structure frame as an architectural covering for a large opening of mall.
How does fire exactly reduce the strength and integrity of PT slab as well as PT beams and what are the safe or reasonable assumptions while designing a repair system for such a fire-affected slab system?
Much appreciated.

Regards.

 

[Ingenuity]

Depending on the duration and the sustained temperature of the fire it can have a severely detrimental effect on the prestressing strand properties.

At about 600°F the strands are going to undergo increased relaxation. At 1200°F any mild-steel rebar will yield.

Concrete will typically spall at fire temps of 300°F.

Was the system bonded or unbonded?

The article from Structure Magazine entitled Fire Damaged Post-Tensioned Slabs has a bit of info. Link

There are several research-based papers by John Gales and Kevin McNeal that present some lab-based testing results. I think I have copies of them if you need.

Also, I think the Institution of Structural Engineers (UK-based) has a document on PT and fire from a design perspective.

 

[hokie66]

Fire under a slab is much more damaging than a fire over a slab. The case presented in the article which Ingenuity linked was a fire over the slab, and the slab was salvageable.

 

[AlbertStructural]

Dear Ingenuity and hokie66,
In our case the fire was caused due to some short-circuit and quickly spread throughout the open area of Mall and then flames were escaping from some dome areas (covered with steel structure). We did lot of core testing as well as carbonation testing. Carbonation depth was max. 7mm which is satisfactory. Most slab areas had soot on them and some had spalling. But my concern is how the rise in temperature affects the bonding as well as stresses in PT system. Yes our system is BONDED. Please do send me some articles or relevant information regarding effects of fire on PT structure. Attached link was good but I need more technical insight.
Regards.

 

[KootK]

Link

Thesis, but very relevant. Might get some mileage out of the references.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

Albert,

I agree with hokie66. A fire to a structure soffit is worse than to the top surface.

Similar to the link that KootK provided (author MacLean), Dr John Gales did a related Masters thesis in 2009 at Queen's University too, entitled: Transient High-Temperature Prestress Relaxation of Unbonded Prestressing Tendons for use in Concrete Slabs Gales Thesis

Gales is now at Carleton University, but he does have a website that has further info, especially if you go to "Publications" tab, it has links to PT fire related papers. Some are free, some cost $.Gales Web Link

Another researcher is Luke Bisby, from University of Edinburgh - but he too is a Canuck, orignally from Queen's University. His recent joint paper is entitled: "Structural fire performance of contemporary post-tensioned concrete construction". Bisby Paper

The above are good references/information from a theoretical/research perspective. In the practical world, where we have real clients (and their lawyers!), you will need to get a qualified testing lab and/or specialized fire consultant on board. If this was in North America, a lab like Construction Technology Laboratories (CTLGroup.com) would be able to assist in lab testing of some samples of strand that are removed from the structure. Not a easy task to remove 4+ feet long samples from a BONDED PT structure, but if deemed necessary, it can be done. You will be mainly be interested in testing data for strength (UTS) (quick, and easy to do, and costs less than US$1,000 per test). But, if you need strand relaxation data, that can be costly and time consuming. There is an accelerated test procedure that some labs undertake.

Anyway, the above is a starting point.

Do you have any "hot" photos of the fire damage you can share?

 

[Ingenuity]

I found an old list of references that I put together some time ago that may also be of assistance:

1.

Structural performance of a post-tensioned concrete floor during horizontally travelling fires
E. Ellobody, C. G. Bailey
This paper highlights the structural performance of a bonded post-tensioned concrete floor subject to fires that travel horizontally between zones within the floorplate. The floorplate was previously analysed by the authors based on experimental and numerical investigations on one-way spanning bonded post-tensioned concrete slab strips. In the previous studies, a nonlinear finite element model was developed for ...
Journal: Engineering Structures - ENG STRUCT , vol. 33, no. 6, pp. 1908-1917, 2011​

2.

Modelling of unbonded post-tensioned concrete slabs under fire conditions
Ehab Ellobody, Colin G. Bailey
This paper investigates the structural behaviour of unbonded post-tensioned one-way spanning concrete slabs in fire conditions. The slabs were simply supported and reinforced with 15.7mm nominal diameter seven-wire mono-strand tendons. A nonlinear finite element model for the analysis of post-tensioned unbonded concrete slabs at elevated temperatures was developed. The mechanical and thermal material nonlinearities ...
Journal: Fire Safety Journal - FIRE SAFETY J , vol. 44, no. 2, pp. 159-167, 2009​

3.

Fire tests on bonded post-tensioned concrete slabs
Colin G. Bailey, Ehab Ellobody
The results from eight fire tests conducted on bonded post-tensioned one-way spanning concrete slabs are presented in this paper. The fire tests were augmented with two additional tests at ambient temperature, carried out to failure on slabs with identical geometry and prestressing tendons. The different structural response between using plastic and metallic ducts, Limestone and Thames Gravel aggregates, ...
Journal: Engineering Structures - ENG STRUCT , vol. 31, no. 3, pp. 686-696, 2009​

4.

Whole-building behaviour of bonded post-tensioned concrete floor plates exposed to fire
Colin G. Bailey, Ehab Ellobody
This paper discusses the whole-building behaviour of post-tensioned concrete floor plates under fire conditions. Based on the results of eight fire tests on one-way spanning bonded post-tensioned concrete slab strips, recently conducted by the authors, a nonlinear finite element model was developed to model an entire typical concrete floor plate. The considered floor plate was post-...
Journal: Engineering Structures - ENG STRUCT , vol. 31, no. 8, pp. 1800-1810, 2009​