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  • 1.  Retrofit of Precast Double Tee Floor System

    Posted 10-10-2018 08:55 PM
    Edited by Tirza Austin 10-10-2018 08:55 PM
    An owner would like to convert the ground floor of an empty two story retail building to a grocery store and food court.  The ground floor consists of 10'-0" wide x 24" deep precast double tees spanning between precast inverted t-beams, with a 2'-6" crawl space underneath.  The existing floor has few openings/penetrations and those that are there were accommodated in the original design and construction in the mid '80s.  

    The grocery layout requires a significant amount of drains, plumbing, trench drains, sloped slabs, etc.  I need some technical assistance regarding retrofit of existing precast double tees.  Are there any options other than removing a large portion of the existing floor and replacing with a cast-in-place solution?  Original manufacturer is not in business and shop drawings do not exist.  I have warned Architect and owner that cutting of precast members is not advisable.

    Any precedents or advice?

    Thank you in advance.

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    Kirk Ellison P.E., M.ASCE
    Principal
    Ellison Gage & Associates, PLLC
    Plano TX
    9723548855
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  • 2.  RE: Retrofit of Precast Double Tee Floor System

    Posted 10-11-2018 10:19 AM
    Edited by Tirza Austin 10-11-2018 10:18 AM
    I do a lot of work with repairing parking structures.  Some of these structures consist of precast double tee planks. Obviously, parking structures need drains.  Most often you can insert a drain along a joint between two panels.  Basically each end is a cantilever.  We have had some success with attaching a thickened portion of concrete to the stems of the precast to support what is above.  I would assume that the load for your grocery store is much higher than the parking structure so it may not work.  The problem with the double tees is that the thickness of concrete makes it difficult to attach anything to it.  However, in my experience the concrete used in double tees is very good quality.

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    Rich Thomas A.M.ASCE
    Project Engineer
    Duluth MN
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    Original Message


  • 3.  RE: Retrofit of Precast Double Tee Floor System

    Posted 10-11-2018 10:20 AM
    Edited by Tirza Austin 10-11-2018 10:20 AM
    Dear Kirk Ellison,

    I have good information about retrofitting old buildings. I'm glad to be able to help you. 
    Can you provide information details on why retrofitting is not enough?
    Please provide a few photos of the project and architectural plans.
    Depending on the project, a suitable method can be provided.

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    Reza] [Mokarramaydenlou Ph.D., P.E., C.Eng, S.E., M.ASCE
    Structural Engineering and Seismic rehabilitation Consultant
    Mokarramandpartners LLC

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    Original Message


  • 4.  RE: Retrofit of Precast Double Tee Floor System

    Posted 10-14-2018 09:09 AM
    ​Dear Kirk,

    I believe the main issue is the absence of shop drawings. It's a liability issue: Once we touch an existing structure, we become liable to its performance, including issues that might be exisisting in its current condition before we even touched it. When it comes to rehabilitation/ alteration work, design notes should start with full information about the existing structure. Design notes should, as always, be able to stand in a court of justice, if need be for whatever reason (can be due to some other party's fault) .

    If I was to do alterations to DTs, I would need shop drawings showing:

    - Strands profile; number, configuration (straight/ one point depression/ two point depression), de-bonding
    - Ultimate strength (270 ksi?)
    - Low relaxation or stress relieved?
    - Jacking force
    - Concrete strength at release (f'ci).
    - Concrete strength at 28 days (f'c) + site inspection/ testing to confirm that f'c is met or exceeded.

    Drains can be added at DT ends, centered at the joint between two DTs to minimise the reduction to cross section.

    Run design using a trusted prestressed / precast design program: contact PCI or CPCI

    In the case you have: There is problem with the loading too: Parking garages LL= 50 psf. Retail LL, if I remember correctly, is double that. Besides, there is added thickness to topping to make up for the difference between parking and retail finished surface. Most likely, the original capacity will be exceeded, unless it was designed with ample extra capacity, which is very unlikely.

    If was given a project like this one, I would play it safe as noted above.



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    Neil Kazen, M.Eng., M.Sc., P.Eng.
    FASCE, FCPCI, FEC,
    Retired Structural Engineering Manager, Transportation Division, SNC-Lavalin
    Toronto, Ontario, Canada
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    Original Message


  • 5.  RE: Retrofit of Precast Double Tee Floor System

    Posted 10-12-2018 10:03 AM
    Edited by Tirza Austin 10-12-2018 10:03 AM
    Maybe you could build up a floor with cellular concrete or some other lightweight material (assuming you have the capacity to work with) the the double Ts. 

    I was recently introduced to cellular concrete (our brand was CellCrete) which has apparently been in use extensive for decades. It's a proprietary foam/air entrained concrete that can range from very lightweight to relatively strong (for a fill) based on the design mixture. 

    Let us know if you can work out a solution - that's an interesting situation. 



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    Kellen O'Connor A.M.ASCE
    Cushing Associates Inc
    Pinole CA
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    Original Message


  • 6.  RE: Retrofit of Precast Double Tee Floor System

    Posted 10-16-2018 11:26 PM
    Edited by Tirza Austin 10-16-2018 11:25 PM
    One solution we have implemented for strengthening concrete bridge decks is to apply a bonded layer of fiber-reinforced ultra-high performance concrete (UHPC) ranging from 1-3" thick to the top of the existing deck. UHPC has a compressive strength exceeding 17,000 PSI and an elastic modulus ~2x that of conventional concrete. It can also sustain tensile strength (due to the fibers) and can be supplemented with additional reinforcing steel if needed. This could be a way to add additional capacity to the existing double tee floor system.

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    Gregory Nault P.E., S.E., M.ASCE
    Project Manager, Ductal UHPC
    LafargeHolcim, Chicago IL
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    Original Message


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