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My home was built in the late 50's with 2x8 floor joists. When my 130 lbs dog runs across the room, the floor bounces slightly. We are wanting to reinforce that area before adding a heavier dining table. My plan is to add a double or triple 2x8 beam at two points 1/3 and 2/3 across the room below. Running perpendicular to the existing joists. The span is just under 15'. Is this feasible without having to add a post in the middle of each beam?

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    What is the span of the 2x8 floor joists now? 15 feet is kind of pushing 2x8s free span limits, so might not help with the bouncing. Will there be a problem adding post/s?
    – crip659
    Jan 10 at 22:00
  • Also about 15'. The posts would be right in the middle of our exercise room on top of a rowing machine and treadmill. Jan 10 at 22:45
  • 15 foot span should only require one beam. You do have plus or minus a few(2 or 3) feet either way from exactly centre. Rule of thumb for joists is one inch per foot, span tables might give 2x8s maybe about 12 feet span. 15 feet does give bouncy floors.
    – crip659
    Jan 10 at 23:02
  • You should sister all the joists. Use a hydraulic jack and some lumber in the middle of the span to get them to 'crown' a little bit, then attach a sister that's as long as possible. Running perpendicular to the existing joists you lose 8" of headroom and likely start to violate code.
    – Mazura
    Jan 11 at 1:35
  • How widely spaced are the 15 ft spanning joists?
    – popham
    Jan 11 at 2:13

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The beams aren't holding the floor up, they are just to make it stiffer, so no additional posts are needed.

Even just adding cross-blocking between the joists can work to stiffen a floor.

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