Cantilevered Glue Seam Strength

[Pwk5017]

In the process of working out the details for a walnut island top. I always knew they wanted a 12" overhang, but I assumed I would tackle the issue by running my 8/4 boards perpendicular to the overhang. Unfortunately, the client wants the boards running the full 63" length instead. This now has me questioning how to support the cantilever, so I dont have a 2' wide walnut top headed back to my place 8 months from now. I would prefer not to route recesses and install metal hardware, but will if thats my only choice. For a 12" cantilever, I can assume I will have 1-2 unsupported glue seams over that span. My friend owns a domino 500, which has me thinking about using it for joint reinforcement in this case. Would a handful of 40-50mm long dominoes provide adequate strength for this scenario? I ultimately think 1.75" thick hardwood with a perfectly jointed gluebond would be able to span that distance without sagging, but I dont want to chance it. 

[Brendon_t]

Hmm, I would proabably try to convince the client that 3 corbels should be best to strengthen and support the top.

[bob493]

any reason you cant run a support beam or bracket underneath? 8/4 stock does give you plenty of surface area for joinery work tho.  if you want more surface area, you can cut your joints at an angle if youre comfortable with that.

[wdwerker]

Use 10 mm dominos every 8 or 10" on the overhung and next seam inwards. But add a 1 3/4" square crosspiece in 3 places using elongated holes to allow the top to move. You can bevel the end of the crosspiece and stop it 1/4 of the width of the last board from the edge. You could also round over the two bottom corners to protect knees.

[Ronn W]

Assuming that the joints are stronger than the wood.  The most stress will be at the edge of the base cabinet.  When load is applied to a cantilever the cantilever bends downward (no surprise there). The wood on the top surface experiences tension and the lower surface experiences compression.  Compression is not a problem but the tension can be.  Wood is very weak in tension perpendicular to the grain (what you have).  The worst case is if someone sits on the corner of the cantilever.   I ran some numbers for you and barring any imperfection in the wood or the joints (weakness) and assuming that the allowable cross grain bending stress in the walnut is 570 psi (the only number I could find) and assuming a 200# load. You will be stressing the wood to about 70% of its allowable stress.  So if everything is perfect you are probably OK.

To be honest with you, though, I strongly recommend a 2 or three corbels as Brendon suggests.

[gee-dub]

Or a mechanical solution with 2 or 3 pieces of angle iron installed in mortises from below with slotted holes for expansion. The Mortise would be simple enough with a router nd the iron would not be visible unless you were laying on the floor.  No corbels to bump into.

Bottom view shown.

 

[Pwk5017]

Wow, thanks for running the numbers, Ronn. Too bad it confirmed what I already feared. Yeah, if dominoes werent enough, angled iron mortised into hidden channels was my solve. Should be easy peasy with 1400 and a guide rail. I will spend more time cutting the iron with a saws all or grinder. As always, thanks for the gut check.

[Mark J]

On 5/2/2016 at 7:36 AM, gee-dub said:

2 or 3 pieces of angle iron installed in mortises

You know, maybe consider aluminum instead of iron or steel.  I'm thinking of corrosion over the next 10 to 20 years. Not that the iron would fail, but rust would rub off on clothing or hands.  Is there such a thing as stainless steel angle?

[Tpt life]

Aluminum sheds and turns stuff gray. I think the point is to not leave it where it can wear. Underneath like that, paint or epoxy will last indefinitely. 

[wtnhighlander]

Mark J, there certainly is stainless angle, but it costs about 10x the price of normal carbon steel.