I am building with framework-type trusses (see image. Ignore the blue arrows). The trusses are located at 1000-1200 c/c and have a span of 4120 between the top plates. Between each truss, I have placed a 45x170 beam. Below this, a 28x70 spaced board and ceiling are nailed.
The idea was to place 45x95 c/c 600 on the roof beams and then chipboard flooring. But now I feel there might be a bit limited headroom up there, so I probably need to come up with something else. Suggestions?
The idea was to place 45x95 c/c 600 on the roof beams and then chipboard flooring. But now I feel there might be a bit limited headroom up there, so I probably need to come up with something else. Suggestions?
If it gets crowded, you can place the 45x95 beams at 600cc as noggins, i.e., cut quite short and at the same level as the rafters.
Consider using joist hangers for those in the middle. You can advantageously use well-fitted 170x45 beams in the middle. Here it should be a press fit, so you have to hammer the piece in. But it ends up being the same as cross-bracing.
But start easily and measure how much it bends down when you stand on the beam in the middle of the attic room. If it flexes a few mm, you should be good to go. Then you can lay the floorboards!
Consider using joist hangers for those in the middle. You can advantageously use well-fitted 170x45 beams in the middle. Here it should be a press fit, so you have to hammer the piece in. But it ends up being the same as cross-bracing.
But start easily and measure how much it bends down when you stand on the beam in the middle of the attic room. If it flexes a few mm, you should be good to go. Then you can lay the floorboards!
Shortlings I see as a purely theoretical construction that cannot possibly work with normal wood.
But now I have at least cross-braced all the sections. When I maneuver my 92 kilograms onto the middle of one of the extra beams, it flexes 2 mm. With screwed and glued flooring chipboard on top and screwed 28x70 on the underside, it should be fully sufficient for a sleeping loft slash storage space?
I'm wondering if one should move the diffusion plastic under the sparse panel, then I could screw and glue the sparse panel as well...
But now I have at least cross-braced all the sections. When I maneuver my 92 kilograms onto the middle of one of the extra beams, it flexes 2 mm. With screwed and glued flooring chipboard on top and screwed 28x70 on the underside, it should be fully sufficient for a sleeping loft slash storage space?
I'm wondering if one should move the diffusion plastic under the sparse panel, then I could screw and glue the sparse panel as well...
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· Etelä Pohjanmaa
· 2 467 posts
What will you use the space for, storage? Go out to the countryside, find the old man who owns the local sawmill, the planing mill, and have him plane tongue and groove onto 45mm planks to get a proper floor.
Slightly more than half the length will be a sleeping loft (guests), the rest will be storage space for lighter items (summer/winter clothes, Christmas/Easter things, etc.). The former part will have electric underfloor heating in the form of foil, so I'm thinking that chipboard and a 10 mm veneer floor will be good there.
If the 45x170 joists are to function as floor beams across the entire width (4120 mm) without underlying support in the middle, they are far too weak for residential purposes. A 100 kg person standing in the middle of such a beam will instantly bend it down 7 mm, which is unpleasantly much. Cross bracing won't help here. For perfectly optimal flex conditions, glulam is required, e.g., 78x225 mm.
The best solution to keep the floor height down is to do as @Finndjävel suggests in post #4.
The best solution to keep the floor height down is to do as @Finndjävel suggests in post #4.
I have 170x60 in the countryside with a span of 3.9 m and a center-to-center distance of 70 in the floor joist which are cross-braced with wood ceiling panels on the underside and 22 mm chipboard as well as 23 mm thick solid wood flooring screwed down into the beams on the top side. There is no hint of deflection for my part. However, it only contains closets, which are part of the construction, and only beds. If one wants heavy furniture, I would like to increase the size.
The c/c distance has less significance for deflection. 60 mm wide lumber immediately provides better conditions than 45 mm of the same. Additionally, if the chipboard and floorboards interact with the beams, the deflection is further reduced. Cross bracing has a marginal effect as it distributes the impact of point loads to adjacent beams. If the deflection from a 100 kg point load on a 45x170 C 24 beam with a 3.9 m span is 6 mm, it decreases to 4.5 mm with 60x170. Theoretical values without considering floor material and cross bracing, etc., which still provides an indication.
I would argue that the cross-bracing in this case made quite a difference to the free beams. From ~5 mm deflection @ 92 kg to <2 mm at the same weight, before the spaced panel and floor chipboard. But the conditions are somewhat complex, as only every other beam spans 4120 mm freely - the trusses are indeed on the same purlin, but the support legs mean the free span is only ~2500 mm.
The interior ceiling height on the second floor is 760-1614 mm before floor and ceiling coverings, so there won't be any running or heavy furniture
The interior ceiling height on the second floor is 760-1614 mm before floor and ceiling coverings, so there won't be any running or heavy furniture
It is possible to understand the effect of cross bracing quite simply by studying the formula for calculating a centered point load, PL^3/48EI. Cross bracing means that the load P might decrease by perhaps 25%. However, a shortening of the span has a much greater effect because it is the cube of the change that matters.
I might have been in a bit of a rush when I wrote something that requires a good explanation.
The formula can be read: P (point load) times L (span) raised to the power of 3 divided by 48 times E (modulus of elasticity) times I (moment of inertia) gives the deflection. The modulus of elasticity has different values for different materials and strength classes. For example, construction timber C14 7000 MPa, likewise C24 11000 MPa, common glulam 13000 MPa, normal steel 210000 MPa. The moment of inertia depends on the geometric shape of the beam's cross-section. For common square cross-sections, you calculate the section according to the formula B*H^3/12. That is, the width of the beam times its height raised to the power of 3 divided by 12.
Switching from C14 timber to glulam will therefore almost halve the deflection if other conditions remain unchanged. If the value of the point load P can be reduced by cross bracing to 50%, it has the same effect as doubling the width of the beam. The two most effective ways to reduce deflection are either to reduce the span or to increase the height of the beam. Small changes there have large effects. The ideal value for deflection evaluation (which is the same as the instantaneous deflection) is that a 100 kg person should cause a maximum deflection of 1.5 mm regardless of the span.
The formula can be read: P (point load) times L (span) raised to the power of 3 divided by 48 times E (modulus of elasticity) times I (moment of inertia) gives the deflection. The modulus of elasticity has different values for different materials and strength classes. For example, construction timber C14 7000 MPa, likewise C24 11000 MPa, common glulam 13000 MPa, normal steel 210000 MPa. The moment of inertia depends on the geometric shape of the beam's cross-section. For common square cross-sections, you calculate the section according to the formula B*H^3/12. That is, the width of the beam times its height raised to the power of 3 divided by 12.
Switching from C14 timber to glulam will therefore almost halve the deflection if other conditions remain unchanged. If the value of the point load P can be reduced by cross bracing to 50%, it has the same effect as doubling the width of the beam. The two most effective ways to reduce deflection are either to reduce the span or to increase the height of the beam. Small changes there have large effects. The ideal value for deflection evaluation (which is the same as the instantaneous deflection) is that a 100 kg person should cause a maximum deflection of 1.5 mm regardless of the span.