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Annoying Floor Vibrations

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Gentlemen,
 

    First let me thank you for your responses.  They haven't solved the problem yet but they have inspired some additional thoughts on my part.

    Some of you asked for more information so here is a more detailed description of the situation as it is known.  The following sketch (N.T.S.) illustrates the floor plan.

                          South
                            O
                            |
                            |Beam A
                            |9'
        Beam C 18'          |                (area 1)
   O------------------------|
        (area 4)            |5'    (area 3)
                            |-----------------------------O
                            |3'  Beam B   23' xxxxxx
<<<<==East                  |                West ==>>>>
                            X
                                    (area 2)
                          North

     O = 3x3x0.25 hollow steel column;
     X = adjustable column (28 kip capacity)
xxxxxx = fireplace location
Beam A = W12x26, 17' total span
Beam B = Two C10x25 (with three 2x10 in between)
Beam c = Two C10x15 (with three 2x10 in between)

    2x10 joists @ 16" o/c span north/south as simple spans lapping over the top Beams B and C.  The original 2x10 beams and the new C10 and W12x26 beams are dropped below the floor.  There are double floor joists above beam A supporting a full length and full height partition separating area 1 (living room) from area 4 (bedroom.  The steel beams were installed as a means of removing undesirable columns and partitions from the basement area.  All floors are covered with thick, high quality carpet; all ceilings are finished with 1/2" drywall and painted.  All of this part of the house from the main floor up is original, having been built by one of Calgary's better builders for his own use about 30 years ago.  The new steel beams were set on the steel posts, O, then jacked into position using a jack near the adjustable post, X, so that the steel beams are in full contact with the 2x10 joists.  Through bolts were placed at about 3' spacing as high as possible to prevent twisting of the channels with no transfer of vertical load through the bolts.

    The owner initially noticed, and complained of, excessive undesirable vibrations when he was seated in area 1 and clearly able to detect people walking in area 2.  Note that area 1 and area 2 are not on the same line of floor joists, therefore, any vibrations must be transferred laterally through Beam B.

    My initial investigation indicated that a "heal drop test" would produce very noticeable vibrations for the person doing the test in any of the four areas identified on the sketch.  An observer on area 4 could not detect vibrations from another person doing a "heel drop test" in and of the other areas; and an observer on any of the other areas could not detect vibrations from a person doing a "heel drop test" on area 4 suggesting that Beam A plus the partition bearing directly on it completely absorb all vibrations.

    To answer Dennis's question, the vibrations would seem to be maximum for the tester on area 3, and larger on area 1 than on area 2.  Vibrations on area 4 are the smallest of the lot and had not yet been noticed by the owner.

    To answer Thor's questions, all structural components are bearing one upon the other; there are no joist hangers.  Each foot fall would appear to produce vibrations which die out before the next foot fall (say less than 1/2 second).

    While I was on site I made an attempt to guess the natural frequencies of the beams: my guesses were about 10 Hz for Beam B; and 6+ Hz for Beam C.  I sensed that Beam B definitely had a higher natural frequency than Beam C.  My calculation of 5 Hz for Beam B was surprisingly low; but I accepted the calculation.  I just now checked the natural frequency calculation for Beam C and got a value of about 8 Hz.  This suggests that I may have greatly over estimated the mass of the fireplace for dynamic calculations; but there is about 60 cubic feet of masonry material and I wasn't about to under estimate the weight for the design of a beam.

    I think I will reassess the natural frequency of the beam considering lighter masonry.

    Possible remedies include: do nothing and tolerate the vibrations; stiffen Beam B; Install a new beam parallel to Beam B with 23' span about 7 feet south of Beam B, supported on Beam A c/w new column and footing 23' west of Beam A; and install a new beam parallel to Beam B with 23' span 3' or 4' south of Beam B, supported on Beam A and an existing column which has sufficient reserve to accommodate the new loading.  This last solution is looking more and more appealing as I think about it.

    Any further comments you might have would be more than welcome.

Thanks, and best regards,

H. Daryl Richardson