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I'm going to reinforce a 2.2m span of shelf made out of 18mm OSB bord with two 60*4 mm steel strips running underneath the board. The shelf will be 1m deep front-to-back and carry a 100-kg load (consider it uniformly distributed over the shelf).

What's the best method of connecting the strip to the board?

Screws are obvious answer, what size should I get and how many? Or would glue or rivets suit me better?

My understanding is that connection points would mostly have sliding forces when the shelf is bent under load, but I'm not sure.

  • Are you adding the steel strips to stiffen the shelf or is there another purpose? Is it supporting machinery? What is the load it will carry? – Ashlar Mar 18 '16 at 14:22
  • @Ashlar Yes, to stiffen the shelf. Max load is 100kg more or less evenly distributed. Shelf's depth is 1m. More details. – Gleb Mar 18 '16 at 14:44
  • Which is it? In this post you say that you want to attach both strips of steel under the OSB, and in the similar post an DIY you say that you want the steel strips to sandwich the OSB. Further, is the shelf supported only on the ends, or is it also supported on the back edge against a wall? This all affects which answer is going to be good, correct, or usable by others who may have a similar question. – Ast Pace Mar 23 '16 at 17:09
  • @AstPace I've considered two designs -- sandwich, and single strip.Calculations proved me that single strip would be enough. Although sandwich would have created stronger beam with same amount of materials -- I've chose single strip, because of labor of drilling and painting halved comparing to sandwich. (Strips thinner than 4 mm are hard to find here) – Gleb Jun 5 '16 at 20:02
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I'm going to reinforce a 2.2m span of shelf made out of 18mm OSB bord with two 60*4 mm steel strips running underneath the board.

My understanding is that connection points would mostly have sliding forces when the shelf is bent under load, but I'm not sure.

I'll answer this from a structural engineer's standpoint (being that I am one).

What you're doing here is making a composite shape out of your wood shelf with steel reinforcement. As this shape bends, in order to behave compositely, you need to restrain the relative slipping of the OSB layer over the steel layer and develop what's called shear flow between the two layers.

The math behind the answer is beyond the scope of this SE, but I'll try to give you a belt-and-suspenders answer.

Screws are obvious answer, what size should I get and how many? Or would glue or rivets suit me better?

You have 18mm (~ 11/16") thick OSB and 4mm (~ 3/16") thick steel straps for a total depth of 22mm (~ 7/8"). I would go with 19mm (3/4") long round head or pan head screws, say a #8 or #10 screw size, spaced at 100-150mm (4"-6") on center. You'll have to drill through the steel strap, obviously.

Personally, I'm a fan of SPAX screws for OSB since they have an aggressive, self-drilling thread pattern. They make a pan-head screw that would be just about perfect.

screw

Most glues will not form a long-term or reliable bond between steel and wood, so I would not recommend that. Rivets might work, but I'm not too optimistic about their long-term holding power in wood compared to screws (and they're harder to install).

@Ashlar Yes, to stiffen the shelf. Max load is 100kg more or less evenly distributed. Shelf's depth is 1m.

I would space the straps at the approximate third points of the depth of the shelf (i.e., 33cm from either the front or back).


Edit: I just did a quick back of the envelope calculation on your shelf. The shear flow between the two layers will be on the order of 200 lb/in (or 100 lb/in per strap). So, the #10 by 3/4" screws at 4" centers should work. See this PDF for screw shear capacities in plywood.

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  • About composite materials and how they'll respond to bending, surely glue could be enough given the expected bend would be very modest? (Precisely because the OSB is being reinforced.) Re. this: "Glue will not form a long-term bond with the steel" is that in relation to the amount of movement you're visualising or just a general glue & steel comment? – Graphus Mar 18 '16 at 15:47
  • @Graphus, it's more of a bond issue that a strength issue. If the glue could bond well to both the steel and the wood, that would probably work ok. However, given the dissimilar materials, I feel that screws are the better choice. surely glue could be enough given the expected bend would be very modest? It's not really an issue of the bend, more of the expected forces at the interface between the two materials. – grfrazee Mar 18 '16 at 15:50
  • @grfrazee Great answer! Further away from woodworking -- can you explain why would pan-head serve my purpose. My query comes from the estimation that within centimeters of sag -- elongation of the strip will be in the magnitude of decimal fraction of millimeter. The hole in the strip has to be slightly bigger than the shaft of the screw to let the threads through. So I'm perceiving that relative slipping would be restrained only by friction. Am I nitpicking and it would all be covered by deformation of the hole and screw head, or would countersunk head be better in this regard? – Gleb Mar 19 '16 at 0:54
  • @Gleb, counter sink heads would work fine too, but require more machining. I'd recommend making the hole in the steel strap as small as possible while the screw still fits through. – grfrazee Mar 19 '16 at 2:40
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    @Graphus, as you say, the format here isn't particularly suited for extended discussion. In order not to clutter up the comments here, I'll just post this link for further information on shear flow. – grfrazee Mar 19 '16 at 16:16
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I would actually tend toward bolts. Bolts with rounded heads or slightly recessed into the wood will provide holding strength with the nut at the bottom while producing the least amount if bother on the top of the shelf. This spreads the holding power over a larger area than the threads of a screw.

enter image description here

Screws will have to go through the steel up into the wood and they will need to be short enough not to poke through. and OSB will slowly 'release' the screws over time as pressure is applied.

Glue might work, for a while anyway, but you'd have to get a glue to hold metal and wood, and I'm not sure how good of a surface OSB is for gluing to, someone else would have to answer that one.

Rivets could work, but personally I'd go with bolts instead since they are easier to remove should you need to.

Add for reinforcement, I'd go with angle iron, as the 'triangle' is very strong and you can get away with a much smaller piece for the same strength.

enter image description here

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  • I would only entertain glue if it was going to be bolted but it seems redundant in that case. Between moisture and the individual chips I would expect a glued iron to fail long before a bolted one. – Matt Mar 18 '16 at 15:11
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    @Matt That is my thoughts too. – bowlturner Mar 18 '16 at 15:12
  • Carriage bolts are designed to be used with steel or wrought iron frames with a square hole punched in them. They are not properly used in wood. – Treow Wyrhta Mar 18 '16 at 15:41
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    @TreowWyrhta I frequently use Carriage bolts with wood, and they work great. – bowlturner Mar 18 '16 at 15:45
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I know I am not really answering the question, but putting a steel strip as you describe under a shelf will do nothing to support it. For a 6 foot shelf you would need a 1.5" high steel bar (thickness does not matter too much). So, unless your "strip" is a 60mm x 4mm x 40mm bar then it will not do anything.

I would not recommend a pan head screw (as in a different answer) for anything on a bookshelf, because it sticks out. All the fittings on a bookshelf must be flush, otherwise the books gets damaged. You may think that because the pan head is on the bottom of the shelf it will not damage any book, but that is not the case, it will. I know this because I have a very large library.

With a 6-foot shelf you will need tall cross member (such as I describe above). For steel the cross member needs to be about 1.5" tall and if it is wood about 2.5" tall. Alternatively you can use a supporting posts in the middle of the shelf, effectively making them into 2 x 3' shelf instead of one 6' shelf.

UPDATE

Maybe I am misunderstanding your question. Are you saying the dimension of the strip is 2.2m x 60mm x 4mm ? In that case, it will work as long as the bar is oriented so that that 60mm dimension is vertically under the shelf.

To fix such a bar, what I would do is not use screws at all. Make a 4mm x 2mm groove in the shelf then just fit the bar into the slot. On the side supports I would cut a hole 60mm x 4mm all the way through the side walls because you will need a lot of support there. You may even want to add small angle braces at the side walls.

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  • OP never said it was a bookshelf...and 1m depth would be pretty deep for a bookshelf anyway. – grfrazee Mar 18 '16 at 15:59
  • Yes, it's not a bookshelf. The whole point of this design is to make OSB board itself make it's contribution to the load bearing. Something like composite wood - steel L-beam, the board being the web, and the steel being the flange of the beam. – Gleb Mar 18 '16 at 16:36
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I'm in a totally different place. I'd make a shelf sandwich. Cut the OSB into strips, say 40mm in width. Obtain thin sheet metal, 2.2m x 17mm x 0.85mm. Drill a dowel hole every 100mm thru the strip steel. Use locators to drill matching holes in the OSB strips.

dowel locator pins

Insert the wooden dowels in each hole.

wooden dowels

Use epoxy to glue the whole thing up like a triple stack sandwich (bread / meat/ bread / more meat and lettuce / bread). In this case it will be OSB / strip of steel on edge/ OSB / another strip of steel / OSB (etc...) Way strong, no screws. Cheaper, too. Metal held on edge in this way is quite strong. You are burying the strength inside the OSB board. Epoxy will bond the steel to the OSB quite well.

Maximize the strengths of materials in the best way possible.

Edit.. You can also purchase pre-drilled steel strapping. This would save a lot of time. Its also pretty cheap... I'm in the USA, where HomeDepot sells a ten foot long roll of this stuff (3/4" wide) for $3 USD.

Steel strapping.

And here's a sketch of the sandwich, less holes and dowels... Can you envision the design from this?

shelf sandwich

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  • I'm not clear about what you're suggesting. Is it to make my 1 meter depth of shelf out of 25 OSB 40mm strips+steel strip? Can you make a drawing of the sandwich. – Gleb Mar 31 '16 at 14:40
  • Oops. Yes exactly that. But truthfully I was thinking a bookcase shelf width. What's you load numbers? What's your deflection criteria (generally the weak link in bookcase shelf / furniture design. )? 25 strips glued may be overkill for your loads. Maybe you settle for ten x 100mm sections with nine strips of steel buried in the board. – zipzit Mar 31 '16 at 16:12
  • An interesting and creative idea, but using 28ga strapping, one would need about twice as many straps as you mention in order to come anywhere close to the same stiffness as the composite structure described @grfrazee. Plus, there's all that drilling, gluing, clamping that would be even more challenging than all the screwing advocated by grfrazee. – Ast Pace Mar 31 '16 at 20:38

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