Maximum allowed weight on a kiln shelf

[Cristobal]

I'm trying to find cordierite shelf specification data to find out the maximum allowed weight that the shelf can withstand at a given temperature and 3 supporting columns.

My half shelves are 26x13" and 5/8' thick, Firing up to cone 05 and planning to put 45 lbs on each shelf.

[Hulk]

That's a good question.
Foundry Service & Supplies has some spec on cordierite shelving*, but not the weight limit(s).
You might call and ask them. If you get some info, please do post an update.

*https://www.foundryservice.com/

[Cristobal]

I called them - they did not know, the guess was 15 lbs.

I called my local ceramic store where I purchased the shelves and they said no more than 50 lbs

I called another store in Bay Area and they said 25 lbs.

[Rae Reich]

I think it might depend on load distribution. 

[Mark C.]

1 hour ago, Rae Reich said:

I think it might depend on load distribution. 

yes for sure

[neilestrick]

What shape are the forms you're using? Do they have a small base that will put all the weight in a small area, or a wide base that covers a lot of the shelf. With a small base I would move the posts in closer to the base rather than out at the edges of the shelf like they normally all. All posts must align vertically to distribute the weight to the floor. Personally, I would not put 45 pounds on a 5/8" shelf, and definitely wouldn't do it 3 rows high. I would consider firing each separately.

[Cristobal]

I will be glazing bricks. One weighs around 7 lbs.
The half shelves are a little bit smaller now - I needed to trim the ends to provide better exhaust flow for better temperature uniformity in the kiln. Now they are 25x12", so it should help a little. I decided to add a fourth post - in the center of the shelf's semicirlce. This way I can fit 4 full bricks (2 on each side of the center) and one cut brick (between middle posts) on a shelf.

In theory I could buy thicker shelves or even buy some more expensive ones, but I would need to do more test firings to get results I got after 7 tests and since my workshop is completely outside I can not waste another season, before the weather becomes wet.

[Rae Reich]

At low temp that might work but why not load only one shelf with the full 45 lbs and put less weight on the others for the first firing?

[Cristobal]

Rae,

I have 310 bricks to glaze. I can put 5 per shelf, it's 10 per bottom level, 10 per medium and I can only fit 8 on top level, due to flue shape (4 posts and 8x8" shelf on top). This way I will be able to fire 28 at once, so I will need at least 11 firings. Each firing takes me at least 3 days. I also work, take care of the homestead and help to finish the house. If I fit less then it will take proportionally longer.

I will give it a try and if it fails or not, I will report here for reference. The middle shelf hardly gets cone 06 and the top one even less.

[neilestrick]

Be careful about adding a 4th support. Make sure the shelf does not rock at all, otherwise the high point becomes the breaking point. I would use clay wads on the posts to make sure it sits evenly.

[Cristobal]

Neil,

I was actually worried about rocking possibility. What are "clay wads"?

[neilestrick]

Just wads of your clay. Set them on the posts and let the shelf squish them down. Cover them in alumina or kiln wash so they don't stick.

[Cristobal]

Should I wait for them to dry? Could I use ceramic fiber instead?

[Bill Kielb]

2 hours ago, Cristobal said:

I will give it a try and if it fails or not, I will report here for reference. The middle shelf hardly gets cone 06 and the top one even less.

You could check the allowable stress and apply a factor of safety. None of it would apply to the shelves heated however as manufactures likely won’t publish the tested heated data. For the most part it’s fairly straight forward so the bending moment divided by the allowable stress provides a minimum section modulus. 

These things can be calculated online these days. The ending result would only serve to provide reasonable confidence or if this loading is even close to the capacity of the shelf. To this I will try and get to this later today (no promises - it’s been 50 years so need to go slow) just to see if a shelf appears 1x or 50x the anticipated load. Since you or I do not do this daily we are not good at it and will need to include everything we can think of. The final result will be just a best attempt or way to develop confidence. 
 

The devil is always in the details so I like like the idea of doing it (as apprentices) as it likely will uncover some other things to address. I think of the bottom set of columns failing or punching through the bottom shelf or worse the bottom brick. Unforeseen stuff like that that could be a problem so best to try and look at it as close as practical.

Mullite properties seem very available

 

 

[Cristobal]

Bill,

For posts I'm actually using pieces of firebrick that I cut to size.

Piercing the bottom. Hmm.. I put entire kiln on 0.25" plate and the plate rests on the kiln legs.

Cold crushing strength of IFB-2600 brick is on average 2.0 MPa, which is roughly 20 kg/cm2. My posts are 6.5x6.5 cm so the area affected is about 42 cm2 and for 4 posts it's 168 cm2. The weight of 14 bricks + 3 half shelves would be transferred to this area. The weight would be around 45 kg + 9 kg shelves = 54 kg.

54 kg distributed over 168 cm2 gives 0.32 kg/cm2, so 62 times less than the cold crushing strength.

 

[neilestrick]

1 hour ago, Cristobal said:

Should I wait for them to dry? Could I use ceramic fiber instead?

Fiber would just compress. I would run a preheat so they dry out before steam explosions.

[neilestrick]

17 minutes ago, Cristobal said:

Bill,

For posts I'm actually using pieces of firebrick that I cut to size.

Piercing the bottom. Hmm.. I put entire kiln on 0.25" plate and the plate rests on the kiln legs.

Cold crushing strength of IFB-2600 brick is on average 2.0 MPa, which is roughly 20 kg/cm2. My posts are 6.5x6.5 cm so the area affected is about 42 cm2 and for 4 posts it's 168 cm2. The weight of 14 bricks + 3 half shelves would be transferred to this area. The weight would be around 45 kg + 9 kg shelves = 54 kg.

54 kg distributed over 168 cm2 gives 0.32 kg/cm2, so 62 times less than the cold crushing strength.

 

What type of kiln are you using?

[Bill Kielb]

57 minutes ago, Cristobal said:

For posts I'm actually using pieces of firebrick that I cut to size.

Piercing the bottom. Hmm.. I put entire kiln on 0.25" plate and the plate rests on the kiln legs.

Nice! I like the thinking. Glad to hear the posts are brick, much better footprint. The kiln probably is made of 2300 f firebrick so maybe a bit less 20kg/cm sq. Just thinking about this, you may place the posts so a bit of the shelf cantilevers each side reducing the moment considerably unloading the center of the shelf. Since your loads are all uniform, this should be easy to offset to a large degree. With large footprint supports, I think your worries decrease a bunch. Will run the section tonight if I get a chance, but double cantilever loading might be an easy solution without much thought.

Reasonable alignment of all columns including those under the starter shelf if present. You are right though my worry was not taking the column loads all the way down to the bottom brick. Double cantilever does not affect the load on the kiln bottom for sure it must support the full stack.

Edited August 27 by Bill Kielb

[Cristobal]

5 hours ago, neilestrick said:

What type of kiln are you using?

I think it is Skutt 1227, 3 phase. I removed all electrical parts and converted to gas.

[neilestrick]

So as Bill said, those are 23K bricks. Sounds like with the steel plate underneath the floor should be fine..

[Mark C.]

clay wads rolled  in alumina and applied to top of stilt -then squish the shleve on them to even out any rocking-load shelve and fire slowly as clay wads dry out.Since its a gas kiln go very slow at 1st.Since its just a pile of bricks why use selves at all? You could pile the bricks up leaving some air space to dry them and stack them up to top of kiln lid. Since its a conversion kiln (now a gas kiln) the trick will be going slow at 1st to dry them out without exploding any. 

Are you making soft bricks or hard bricks?

[Cristobal]

Mark,

I will be glazing heavy firebricks that I purchased, not making them from scratch. If I did, I would use no shelves.

Bill,

I have found hot load strength for BNZ bricks (byr PTI Thermal). It says that at 2000 F at 10 psi load for 1.5 h it's 0.1%. 10 psi would be equivalent to 0.7 kg/cm2 so still two times more that my calculated load of 0.32 kg/cm2.

I would like to do the firing as soon as possible, but other tasks are drawing me away from it.

[Bill Kielb]

3 hours ago, Cristobal said:

10 psi would be equivalent to 0.7 kg/cm2 so still two times more that my calculated load of 0.32 kg/cm2.

Good find - would use that number as it’s at some elevated temperature. I don’t know if the double cantilever idea makes sense, but is a way to reduce the forces in the shelf with the placement of the columns in from the edge so an opposing moment is created. It means if you bring the support posts in from the edge, even let’s say 1 inch but load the shelf 100% out to the end, it will reduce or offset the force  in the center of the span. So any small loaded overhang reduces the necessary strength in the center of the span. Setting the shelf posts some minor amount, say  1-2” from the edge but loading it fully with bricks out to the edge should add to your shelf safety margin. I think your published number is likely better than using the mullite cold temp data and computing assuming it is the shelf material. Even a small cantilever improves your center span safety margin. A factor of 2 is 200% - often used for possible impulse loads (dropping the bricks on the shelf), so quite high. The entire stack has to sit on the columns so stick with you cut brick columns with there larger footprint under the starter shelf as well. Align ALL the columns vertically. I think you have done all reasonable research to gain confidence in the concept.

Firing a single shelf of bricks probably reasonably confirms the shelf will be fine.

See below 

Edited August 28 by Bill Kielb

[Mark C.]

Since your mullite shelves are only 5/8 but solid ones not the hollowcore I would stack the bricks as you do pots. Just make sure to pay close attention to all the stilts lining up . If you warp a shelve  or some its no big deal as 5/8 codalite are the cheapest shelves out there..I assume the bricks are not glazed on the thin side( 2.5 x 9 inch side). The stilts on the floor need to be on hard brick as that stack of bricks will be really heavy on the floor.

Let us know how this turns out in your conversion kiln

[Cristobal]

Thank you Bill.

I just checked and I can move the end posts 1' away from the edge and still fit 4 full and one short brick on the shelf.

Mark,

Bricks will be glazed on the stretcher side (the 2.5x9" side) and some both on stretcher and header side.