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I've seen similar questions but I'm still stuck for an answer, so I hope someone can help...

I've just built what is essentially a two-storey cube, 6m x 6m x 6m. It's a steel construction, a little over-specified (very solid foundations, H-beam for the posts and beams, and box beams for the floor supports). The second floor is cement-wood board, 20mm thick.

When the frame was completed, it all seemed to be a bit too bendy, so I added additional 6" box posts inside at 2m and 4m, as well as triangular steel supports at the corners. That seemed to make the whole structure more stable.

However, now that I've laid down the tiles and have a good solid construction, the floor still wobbles whenever someone walks on it. It's very slight, but if I walk anywhere in the room, each table and everything on it shakes slightly for a good 10 seconds or so. You can feel the wobbling noticeably.

Is there anything that can be (easily) done to make the floor wobble-free?

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    needs more crossbracing
    – ratchet freak
    Commented Apr 26, 2016 at 10:45
  • I only see crossbracing going in one direction... is there any perpendicular to the ones we can see?
    – Aloysius Defenestrate
    Commented Apr 26, 2016 at 13:25
  • I am quite curious what your intended use for the structure is?
    – bigbull15
    Commented Apr 26, 2016 at 14:11
  • thanks - more crossbracing it is then... (they're in two directions, but too short perhaps). We're using the structure as a kind of home/office - upstairs is the office, downstairs is the living room.
    – Gary
    Commented Apr 26, 2016 at 16:13

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You are probably quite far from over-specified, if you have significant deflection from walking around. Such are the breaks of self-taught structural engineering, particularly if you don't use the resources of formal structural engineering in your self-teaching.

That which will stand up, and that which will stand up without unacceptable deflection from loading are different things. How this structure behaves in the face of various external loads should also be a concern; you refer to it as a house, and when houses collapse (say, in hurricane force winds) people get hurt or die.

Steel-framed floors are almost always built with a truss structure which has considerable depth, to provide stiffness to the structure. You appear to have a single layer of square tubes.

I would strongly suggest a consultation with a (licensed, professional) structural engineer.

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So your issue is one of "deflection", which is resulting from the h-beam joists spanning too far for your liking.

The fix would be to determine the deflection you want and then to make sure there are enough cross beams supported by posts that will effectively reduce the span of the original h-beam joists.

By placing a single cross beam across the middle (supported by posts), you will cut the span of those joists in half and substantially reduce deflection. Whether that beam alone is enough to make your floor comfortable is a matter of looking up h-beam deflection/span tables or using a deflection calculator.

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  • thanks - I did add a cross beam in the middle (and 3 more supporting posts) to reduce the span down to 4m and 2m (so now there are four horizontal rectangles: two lots of 3mx4m and two lots of 3mx2m) - but I'm still getting the wibbly wobblies! :o
    – Gary
    Commented Apr 26, 2016 at 16:18
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    Then I would definitely post your issue over here: engineering.stackexchange.com and tag it as "structural engineering". The more details you can give, the more ideas they will give you to check for. If you do, that update your question and add a comment that you have decided to go that route so that a moderator can close this question so it's not cross-posted.
    – alfreema
    Commented Apr 26, 2016 at 16:36
  • If you don't post it over on engineering.stackexchange.com, then you might update the above question with more details about the wobble. Is the entire structure/floor wobbling on a horizontal plan, or would you describe the wobbling only as "bouncy" (up and down)?
    – alfreema
    Commented Apr 26, 2016 at 16:40
  • @Gary - if you have changed the structure, an updated picture might be helpful. Generically, the concept of "X-Bracing" will probably help if you are wiggling horizontally rather than bouncing vertically. Triangles are stable, rectangles are not.
    – Ecnerwal
    Commented Apr 28, 2016 at 0:30
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I get the impression that the "wobbling" you describe is mostly horizontal. The explanation is fairly simple. Steel is elastic, and therefore requires diagonal bracing to attain rigidity. If you look at the height of your small diagonal braces relative to the height of your corner posts, you can see that they're undersized. I'd shoot for diagonal bracing that is at least 1/3 the height of the corner posts, individually or in combination.

An actual engineer can give you more specific advice. My suggestion comes from personal experience, which I admit is mostly based in wood framing.

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  • thanks, that makes a lot of sense (trying to find a local engineer where I live hasn't been possible; the engineers I engaged online all said no diagonal bracing required, oh well...)
    – Gary
    Commented Apr 26, 2016 at 16:08
  • They were right--it's not required. Unless you don't want to be seasick. :)
    – isherwood
    Commented Apr 26, 2016 at 16:12
  • @Gary I'm curious now. Where do you live?
    – kinar
    Commented Apr 27, 2016 at 13:57

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