How Do You Reinforce a Slab? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 18 Old 08-09-2012, 01:32 PM Thread Starter
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How Do You Reinforce a Slab?

I am working on a blanket chest (will post photos later). It is designed to double as a sitting bench. The lid is made from 4/4 walnut milled to about 3/4". I am thinking about ways to reinforce the lid but also allow for wood movement. If I simply glue support ribs on the underside I am certain this will cause problems later on. The lid is glued up from 4 pieces to create the panel.

Any suggestions would be greatly appreciated.

Thanks
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post #2 of 18 Old 08-09-2012, 02:00 PM
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Wouldn't the four sides be support enough? Depending on the size of the box that is. The vertical sides would, to my thinking, hold the weight of someone sitting on the lid. If the span isn't too great, it should not bow under their weight. Course, that's just my opinion.
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post #3 of 18 Old 08-09-2012, 02:07 PM
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How about just screwing the support ribs to the top.

You could elongate the hole in the direction of the width of the lid if you were concerned about allowing for movement of the lid.
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post #4 of 18 Old 08-09-2012, 02:46 PM Thread Starter
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Quote:
Originally Posted by rayking49 View Post
Wouldn't the four sides be support enough? Depending on the size of the box that is. The vertical sides would, to my thinking, hold the weight of someone sitting on the lid. If the span isn't too great, it should not bow under their weight. Course, that's just my opinion.
Yes, the lid is supported on four sides and I don't think there is risk of it braking. But it will flex and I think it will be noticeable. I would like the lid to be stiffer to avoid the flexing when you sit on it.
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post #5 of 18 Old 08-09-2012, 02:49 PM Thread Starter
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Quote:
Originally Posted by Dave Paine View Post
How about just screwing the support ribs to the top.

You could elongate the hole in the direction of the width of the lid if you were concerned about allowing for movement of the lid.

up to this point (aside from the the hinges) I have been able to avoid any kind of hardware relying purely on the joint design for strength ( I don't even need glue,). I was trying to stay on this path. So, if I went with screws, I would probably counter-bore the holes and plug them. I am just trying to see if anyone has thought of anything else before going this route

Thanks
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post #6 of 18 Old 08-09-2012, 03:29 PM
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Originally Posted by SLAC_Engineer View Post
up to this point (aside from the the hinges) I have been able to avoid any kind of hardware relying purely on the joint design for strength ( I don't even need glue,). I was trying to stay on this path. So, if I went with screws, I would probably counter-bore the holes and plug them. I am just trying to see if anyone has thought of anything else before going this route

Thanks
Well if you are trying to avoid fasteners, then perhaps you need to cut a big dovetail in the top and make the ribs beveled to fit the dovetail.

Would have strength in the joint and not need to be glued.

Easier way would be to glue two beveled sections of the rib and have the middle section be the dovetail, but this would mean gluing.

You do not have much in the way of options if you want to avoid glue and fasteners.
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post #7 of 18 Old 08-09-2012, 04:38 PM Thread Starter
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Originally Posted by Dave Paine View Post
Well if you are trying to avoid fasteners, then perhaps you need to cut a big dovetail in the top and make the ribs beveled to fit the dovetail.

Would have strength in the joint and not need to be glued.

Easier way would be to glue two beveled sections of the rib and have the middle section be the dovetail, but this would mean gluing.

You do not have much in the way of options if you want to avoid glue and fasteners.
I don't mind glue and plan on gluing the assembly when done. I have already had to glue the top and bottom panels as well as the raised panels on the front and rear. I'm just trying to avoid fasteners that may come loose over time.

I also thought about a sliding dovetail but then you lose longitudinal stiffness in the top when you cut the groove in the lid.

Your idea with the two piece rib sounds interesting. But I am having a hard time envisioning it.

Some other ideas I had include

-a rib that is made from endgrain so that it would move with the lid. Though, the lid is walnut and the rib will probably be maple to match the raised panels. I don't know if it is ok to use dissimilar woods. I assume different species grow and contract at different rates.

-I also have thought about a floating rib the is held in place at the ends with u-shaped blocks glued to the boards. The rib would be able to float transversely and offer strength to the lid.

-last idea is to orient the rib longitudinal so the wood movement is not an issue. It would primarily improve longitudinal stiffness and indirectly stiffen the slab in the transverse. I don't know if that makes sense.

Last edited by SLAC_Engineer; 08-09-2012 at 04:41 PM.
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post #8 of 18 Old 08-09-2012, 04:51 PM
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Typically a cross brace is the first idea

But that introduces the "wood movement" factor. What about a ridge back down the center, that tapers as it reaches the sides or ends since less strength is needed there. If 2 people sit on it, the weight will be somewhat on the ends anyway, but certainly the center will need support. What about a vertical post or a center brace that goes down to the floor of the chest? That way the top can stay as is.... no braces or ribs. bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 08-09-2012 at 06:15 PM.
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post #9 of 18 Old 08-09-2012, 05:10 PM
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I would just make a couple of 1x4's out of walnut like the top and attach it to the underneath side with a few screws. More than likely if you glue them on it will cause the top to split when the top shrinks. If you use too many screws it would probably have the same effect as using glue causint the top to split.
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post #10 of 18 Old 08-09-2012, 06:00 PM
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If you have four boards that make up the panel, glue down the center of the underside of each board one edge mounted rib, e.g., ¾"x 1½". Glued to the center of the board (with the grain), it will give support to each board, and you need no fasteners, and it won't impede cross grain movement





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post #11 of 18 Old 08-11-2012, 03:56 AM Thread Starter
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Thanks again for the ideas. Here is a photo of a dry assembly. All the joinery is finished I need to do a lot of shaping now and implement a stiffening device to the lid.



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post #12 of 18 Old 08-11-2012, 07:41 AM
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I don't see a problem

The lid isn't that long, so there is less tendency for it to bend in it's length. The depth appears reasonable also. If the ends of the lid are supported by the side pieces, in a rabbet or with a bottom cleat, that will add additional resistance to bending.

You might post the dimensions of the lid to make sure it isn't a photographic illusion. I'm guessing the lid is about 40" long and about 19" deep.... bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #13 of 18 Old 08-11-2012, 08:02 AM
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Quote:
Originally Posted by woodnthings View Post
I'm guessing the lid is about 40" long and about 19" deep
My guess is 42" x 20". Being supported on all 4 edges if there was any deflection it would be minimal, and likely not noticeable.





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post #14 of 18 Old 08-11-2012, 08:20 AM
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Quote:
Originally Posted by SLAC_Engineer
Thanks again for the ideas. Here is a photo of a dry assembly. All the joinery is finished I need to do a lot of shaping now and implement a stiffening device to the lid.
You have engineer in your name: this a sophomore level mechanics of material question. Even if your an electrical or one of the others that don't have to take the class you should have enough math skills to understand the equations with a little reading.

Go to the USDA Forest Products Lab's web site and download the "Wood As An Engineered Material" book. Everything you need to know to answer your question is in it. All the equations, and all the strength data on just about every wood that grows in the US or Canada, and most commercially available species from somewhere else.

I think your design is ok. The doubt I have is in the ability of the frame and panel to support a load at mid span without sagging. There you have a pair of 1X 2s carrying the load. Adding a pair of 1X 4s full length on the inside will give you enough strength to carry the load if they are properly secured. I'd glue and screw, but if you don't want fasteners try miller dowels. Do not glue to the panels just the frame.

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post #15 of 18 Old 08-11-2012, 12:46 PM Thread Starter
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Originally Posted by jigs-n-fixtures

You have engineer in your name: this a sophomore level mechanics of material question. Even if your an electrical or one of the others that don't have to take the class you should have enough math skills to understand the equations with a little reading.

Go to the USDA Forest Products Lab's web site and download the "Wood As An Engineered Material" book. Everything you need to know to answer your question is in it. All the equations, and all the strength data on just about every wood that grows in the US or Canada, and most commercially available species from somewhere else.

I think your design is ok. The doubt I have is in the ability of the frame and panel to support a load at mid span without sagging. There you have a pair of 1X 2s carrying the load. Adding a pair of 1X 4s full length on the inside will give you enough strength to carry the load if they are properly secured. I'd glue and screw, but if you don't want fasteners try miller dowels. Do not glue to the panels just the frame.

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You make a lot of assumptions in your statement some of which are insulting. I'm an ME and a good one.

If you read through the post you will see that I intend to use glue. I also intend to draw bore the frame. the only 1x2's are on the side.

I am concerned with flex in the lid which is a 3 dimensional deflection and by no means a sophomoric level problem. I could solve it with complex equations from roarks formulas or I could do an FEA analysis on the lid or I could just shoot from the hip and glue a ridge back/rib.

And last time I checked, sophomore engineering classes don't discuss wood movement as a function of ambient humidity levels. Which is what I am con concerned with so as to not over-constrain the lid.
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post #16 of 18 Old 08-11-2012, 01:52 PM
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Quote:
Originally Posted by SLAC_Engineer

You make a lot of assumptions in your statement some of which are insulting. I'm an ME and a good one.

If you read through the post you will see that I intend to use glue. I also intend to draw bore the frame. the only 1x2's are on the side.

I am concerned with flex in the lid which is a 3 dimensional deflection and by no means a sophomoric level problem. I could solve it with complex equations from roarks formulas or I could do an FEA analysis on the lid or I could just shoot from the hip and glue a ridge back/rib.

And last time I checked, sophomore engineering classes don't discuss wood movement as a function of ambient humidity levels. Which is what I am con concerned with so as to not over-constrain the lid.
Not intended to insult. If you were a Database Engineer you probably wouldn't understand the equations. My Intent is to get you thinking through the problem to a real solution. using your full skill set.

I was quite serious in my recommendation that you get and read the USDA wood manual. A lot of folks don't have the background knowledge to fully understand the contents. You apparently do, and it will change the way you design. The base knowledge is taught in Mechanics of Materials, a sophomore level class.

Wood is a pretty straight forward material. The USDA manual has all the design values for it, including the shrink/swell rates for varying humidity.

The problem is complicated only by the frame and panel front, and back if you used it there. The panels are free floating, and don't contribute until there is already deflection enough to bind them up. So you have in essence two stringers held apart by the verticals. Assume a point load at mid span and see how much it deflects. If that is more than you are comfortable with add more wood at the top, the bottom or both.

Now analyze the top, it is long and narrow, and supported continuously on the sides by the front and back, which since you now know will carry your design load without excess deflection, you can concern yourself only with the deflection mid span front to back. The glue joint, if done right, is at least half again as strong as the wood. So you don't need to consider it.

My guess based on training and experience is: the front and back need to be beefed up a bit, and the top is fine as is.

But take the twenty minutes or less to download the book. Then look up the engineering properties of your wood and do the calcs, so you know the answer, and aren't just guessing.

This is obviously a piece you care about. Use your full set of skills to make it the best you can produce.

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post #17 of 18 Old 08-11-2012, 02:28 PM
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Quote:
Originally Posted by jigs-n-fixtures View Post

I was quite serious in my recommendation that you get and read the USDA wood manual. A lot of folks don't have the background knowledge to fully understand the contents. You apparently do, and it will change the way you design. The base knowledge is taught in Mechanics of Materials, a sophomore level class.

Wood is a pretty straight forward material. The USDA manual has all the design values for it, including the shrink/swell rates for varying humidity.
I'm all for technical data, and would suggest that much of the data can be helpful. When this type of work is done on a daily basis, it becomes fairly predictable how a design will fare, with what is used. If this type of work or level of experience is minimal, a question can be posted on forums like this and addressed by those that do have that type of experience. So, in essence I'm not answering his question by telling him to read a book (which may have too many variables for a short answer).

Quote:
Originally Posted by jigs-n-fixtures View Post
The problem is complicated only by the frame and panel front, and back if you used it there. The panels are free floating, and don't contribute until there is already deflection enough to bind them up. So you have in essence two stringers held apart by the verticals. Assume a point load at mid span and see how much it deflects. If that is more than you are comfortable with add more wood at the top, the bottom or both.
With the R&S front design, it appears more than substantial for its configuration to support the lid. If the back is done similarly, or a plywood back, the same holds true IMO.






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post #18 of 18 Old 08-11-2012, 04:47 PM
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My architectural experience suggests that woods have a design factor and it is usually 20%+ stronger than the calcs call for in all scenarios in deflection mode and generally 50%+ stronger than breaking strengths call for. Don't get me wrong, but when calculating wood, I always go by the design tables + a safety factor, so anything I designed is severely overkill.
I know this isn't addressing the issues directly but the point Cabinetman makes about having a sense of what works without the books is something learned by time/experience, and the books are generally a last resort to double check yourself.

Gut feeling on this is the top is fine unless Big Bertha sits on it on a regular occasion (and if big enough, just once will do it in). Also if someone is to be standing on it to reach something, then point loads come into play (Big Bertha is a Uniform Load...Spike heels are a point load).
You see, the usage is a factor that books don't play out. If it was me, even knowing the top is fine, I would overkill and add a few 1" x 2" laid flat cross braces, against the grain on the underside.

On the floating panels, I'll refer you back to Cabinetman. Thats his expertise.... I don't do fine joinery.

I will say. I know few furniture/cabinet designers that waste time and effort doing calcs before assembling or building furniture. The book(s) might be in their shop, but likely it has a inch of dust on it. My architectural design/engineering books do, and they're inside the house.
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