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Underpinning yes or no ?

frubens | Posted in Construction Techniques on November 16, 2014 04:26am

 Hi,

 

I have a question regarding the need for underpinning. 

I am planning to add a 3rd story + mezzanine to an existing 2 story semi-deteached townhouse with 8″ brick masonry walls.

The additional floor and mezaninne will be built up in wooden frame with brick veneer or hardy panel.

 

The allowable soil bearing capacity (with safety coefficient 2.5) was calculated at 3573 psf and the footings are 20″ wide brick.

DCP values were recorded from 16 to 20 + in the 2 testpits by the footings.

According to my strucutral engineer the building needs to be underpinned to allow for a 3rd story, but

every single contractor that has visited the site is surprized with these findings.

Of course I would love to avoid having to underpin the building if not absolutly necessary.

Could anyone advise ?

 

kind regards,

 

frederik

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Replies

  1. mark122 | Nov 16, 2014 06:58pm | #1

    You went through the process to have a SE look at your situation and give you an assesment. You dont underpin and build as is and something happens your a fool. You dont underpin and nothing happens you feel like a fool...

    Underpin and build as recommended would be my advice, or get a second opinion from a different SE without letting him know what the first one suggested.

    To build going against the SE recommendations would be a mistake in my opinion. SE over kill sometimes? absolutely, but do you really want to invest all that money to find out he was right and your investment us in jeopardy?

    1. sapwood | Nov 17, 2014 11:39am | #2

      I will second this opinion. Get another professional to give you a foundation design. If he agrees with the first's anaylysis, then talk to different contractors. The one's you've already spoken with are going to be obstinate from the get-go. 

      Or, if the cost of underpinning is lower than another engineering study, just do it. 

  2. User avater
    deadnuts | Nov 18, 2014 09:59pm | #3

    I've seen contractors surprised by less.

    Bottom line is that structural engineer trumps contractor in this case. If you don't think your structural engineer is correct, then get a second opinion--from another structural engineer.

    You can look at it another way: If your doctor diagnoses you with diabetes, it doesn't matter how many non-doctor family members or friends you ask whether they think you have diabetes--you still have diabetes. Until maybe you find another doctor that says you don't. Chances are that won't happen, but at that point you have the choice seek mulitple qualified (meaning doctor) opinons..and feel reasonably accurate going with the average.

  3. User avater
    deadnuts | Nov 18, 2014 10:05pm | #4

    finally getting with the program

    Replies 1 & 2 are priceless. Keep these in mind next time these two want to rant and rave about the fallacy of referring engineering issues to professional engineers.

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