Drilling through four 2x8s? Can it be done to run a drain line

Discussion in 'Remodel Forum & Blog' started by hd99flhr, Nov 1, 2010.

  1. hd99flhr

    hd99flhr New Member

    Messages:
    2
    Location:
    Virginia
    I am remodeling a bathroom on the top story of my house, where the bathroom overhangs the first level by 18". The floor joists in house run east and west, but they installed 4 2x8s (east/west) to support the joists (running north and south) for a portion of the bathroom. I am wondering if I can drill a hole through the 4 2x8 for an 1.5" drain line? The hole would be within the first 1/3 of the overall span of the 2x8s.

    Thanks in advance.
  2. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    Holes in the centerline, the "neutral axis", shouldn't affect the joist deflection.
    For other locations you need to find a reference that shows how much E, the modulus of elasticity, is affected by the hole and then do the tedious but not difficult calculations for deflection.
    Normal values of E for this wood range from 0.8 million PSI to 2.2 million.

    I'll see if I can scrounge up my Strength of Materials text.

    There are also patents on a metal sleeve that bolts to joists and allows you to remove a large chunk of the joist cross-section. It works on the principle that steel is about 18x stronger than fir.
    Last edited: Nov 1, 2010
  3. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA
  4. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    Here's another way.

    Assuming a 10% reduction in beam strength is "down in the noise" have a bunch of people stand over this composite 8x8 beam so that you can measure deflection due to this concentrated force.

    Drill a hole that is 1/3 the area of a 1.5" hole, which is sq rt of [(1.5^2)/3] = 0.87" in diameter. Recheck your deflection with the same concentrated load and estimate what lost area would give you a 10% loss in strength.

    Based on your estimate, enlarge the hole to 2/3 the area of a 1.5" hole, which is sq rt of [(1.5^2)/(3/2)] = 1.22" in diameter. If this larger hole still results in significantly less than 110% of the original deflection drill your 1.5" hole.
    Last edited: Nov 1, 2010
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