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RE: Foundation Repair - Large Reciprocating Compressor

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The grout will crack and reflect existing cracks.  It is just a matter of degree.  I would urge your client to add some temperature steel at least.  I presume that the skid would still be set on shim stacks prior to grouting. 
I would consider providing new anchor rods.  I would consider the DUC anchor which are custom built.  You can get any length that you want.  I would use a hydraulic tensioner and tension the anchor rods beyond the calculated service load.  This precludes fatigue in the threaded areas and engages the foundation more definitively. 

Regards, Harold Sprague


From: h.d.richardson(--nospam--at)
To: seaint(--nospam--at)
Subject: Foundation Repair - Large Reciprocating Compressor
Date: Wed, 20 May 2009 11:54:01 -0600

Fellow engineers,
        This posting is going to get rather lengthy; so if you're not interested in compressor foundation dynamics and repairs I suggest you skip this one.
        I have a client who wants to repair a large compressor foundation.  Some of the particulars are as follows:
1.)  The equipment is in the single digit thousands of horsepower.  The skid is about seven by fifteen feet plan dimension and held down by about twelve 1.5" diameter anchor bolts.  It has been in operation for something like 30 to 40 years.  The intent is for the owner to increase the throughput by means that would probably include increasing the operating speed of the compressor.
2.)  One corner (including one of the anchor bolts has broken off and been repaired at some time in the past.  I estimate the broken out piece would have been about 5 to 6 cubic feet of concrete.  This repair seems to be holding out fairly well; but other areas of the grouting are quite a lot the worse for wear.
3.)  The foundation has been painted white, which shows up cracks very well.  There are four or five visible hairline cracks ( a couple are quite prominent) but nothing you could stick a knife blade into.
4.)  Vibration is quite noticeable (I would even venture to say very undesirable; but no one is complaining.) although the equipment has only recently been rebalanced.  The operating speed is several hundred r.p.m.  Touching the equipment and the foundation I sense that the equipment vibration amplitude is significantly more than that of the foundation.  This could most likely (in my estimation) mean: there is significant elastic deformation within the steel skid and, therefore, the amplitude of the equipment IS significantly more than the foundation; or the vibrations are mainly in the lateral rocking mode, therefore, the equipment, being significantly further away from the centre of rotation than the foundation, would have a significantly higher amplitude of vibration ( I suspect the latter to be the case as metal fatigue would likely be a problem if the former).
        My experience with foundation design from this time period is that no dynamic analysis would have been performed; and the weight of concrete in the foundation would have been about five times the weight of the equipment.  There are no drawings available showing the installation.
5.)  My client, the contractor, wants to lift the compressor, remove all old grout plus several inches of concrete, all of the old repair, (leaving a very rough concrete surface for grouting) and regrout the equipment using epoxy grout.  This could require 8 to 12 inches of grout plus the corner repair; this seems a lot to me.  Furthermore, he is adamant that there should be no steel reinforcing in the grout itself while I would like to see the top reinforcing in the block remain intact.  Also the work would involve extending the anchor bolts using threaded couplings.
6.) My responsibility would be to retain a suitable concrete testing company to take and test cores and to confirm that the foundation after preparation, is sound enough for the reinstallation.  My concerns: it's a lot of grout; and how do I assess the extent and seriousness of the cracks.  My plan is to take at least four cores, two through cracks in an attempt to assess the depth of the cracks, and two through apparently sound concrete, plus another two depending upon the other four cores.
        Any comments or suggestions would be more than welcome and I would be grateful to receive them.
H. Daryl Richardson

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