Hijack!

[Alan G]

7 minutes ago, wdwerker said:

It might look better with a 1 1/4" lip all the way around for appearances. 

+1

I usually make benches/countertops 1-1/4" thick. It is rarely for strength. It is just looks better with that thickness of top. If you are going to use ply you can either laminate up some to get to 1-1/4" or add edging to give that look.

[legenddc]

10 minutes ago, Isaac said:

That is pretty thin. I would try to stiffen it, regardless if you use solid wood or plywood. You could make the entire thing much thicker, like how woodworking work benches are made. One drawback is you use a lot of material and get a massively heavy piece of furniture. The typical alternative is to stiffen the top with a perimeter skirt. It also gives you a place to both hide and attach your legs. 

In general, you are unlikely to actually fracture or snap a desk top, but a flimsy one will not give a good feel. 

I was thinking I would be better off starting with at least 5/4, maybe 6/4 wood.  Is 8/4 overkill?

A perimeter skirt would add some stiffness but then I would have to run braces underneath so it doesn't feel flimsy. 

Right now there's a temporary 3/4" sheet of plywood there which seems okay but it's only been a day.

[Isaac]

16 minutes ago, wdwerker said:

A 33" span approx 24" deep would handle 500lbs using either 3/4 ply or walnut. That should allow for that occasional couple using the desk while being spontaneous!

It might look better with a 1 1/4" lip all the way around for appearances.

How did you arrive at the 500lbs?

[legenddc]

15 minutes ago, Alan G said:

+1

I usually make benches/countertops 1-1/4" thick. It is rarely for strength. It is just looks better with that thickness of top. If you are going to use ply you can either laminate up some to get to 1-1/4" or add edging to give that look.

I think thicker would look better as well. I could just laminate 2 3/4" sheets together and add solid wood around it so it would be 1.5" thick.

[Chestnut]

1 hour ago, Isaac said:

How did you arrive at the 500lbs?

Dynamic loading? ....

1 hour ago, legenddc said:

I think thicker would look better as well. I could just laminate 2 3/4" sheets together and add solid wood around it so it would be 1.5" thick.

You could just save the weight and ply and put a 1.5" thick edge on 3/4" ply. Steve is right that's a lot more strength than you'd never need. 500 lb center load is .07" of sag with a 1.5" x .75" edge. Other than steve's example i don't know how many times you'd get half that weight on a desk. Computers don't weight what they once did. OSHA frowns upon using using a desk as a ladder.

[Isaac]

12 minutes ago, Chestnut said:

Dynamic loading? ....

You could just save the weight and ply and put a 1.5" thick edge on 3/4" ply. Steve is right that's a lot more strength than you'd never need. 500 lb center load is .07" of sag with a 1.5" x .75" edge. Other than steve's example i don't know how many times you'd get half that weight on a desk. Computers don't weight what they once did. OSHA frowns upon using using a desk as a ladder.

I think whatever sag calculator you guys are utilizing is using the wrong assumptions about the boundary conditions and treating this like a fixed condition. A table top will virtually always be a simply supported condition, which will mean greater defection than you are computing. 

[Chestnut]

42 minutes ago, Isaac said:

I think whatever sag calculator you guys are utilizing is using the wrong assumptions about the boundary conditions and treating this like a fixed condition. A table top will virtually always be a simply supported condition, which will mean greater defection than you are computing. 

Not if the top is attached to two cabinets on either side so i amused fixed. If it was simple legs i'd agree. Even still 500lbs .25" deflection doesn't meet criteria but for a short duration load you're not going to break the shelf it's just going to sag a bit. The point still stands someone resting their arms on the desk is not going to cause problems at .75". It would even support a person standing on the desk but maybe not 2 people doing activities steve infered.

https://www.woodbin.com/calcs/sagulator/

[Isaac]

10 minutes ago, Chestnut said:

Not if the top is attached to two cabinets on either side so i amused fixed. If it was simple legs i'd agree. Even still 500lbs .25" deflection doesn't meet criteria but for a short duration load you're not going to break the shelf it's just going to sag a bit. The point still stands someone resting their arms on the desk is not going to cause problems at .75". It would even support a person standing on the desk but maybe not 2 people doing activities steve infered.

https://www.woodbin.com/calcs/sagulator/

Ahh you are right, I misunderstood the installation condition here. I agree in either case the shelf wouldn't break, just wanted to make sure if we are throwing around specific deflection estimates we are using the right assumptions. 

[Chestnut]

Just now, Isaac said:

Ahh you are right, I misunderstood the installation condition here. I agree in either case the shelf wouldn't break, just wanted to make sure if we are throwing around specific deflection estimates we are using the right assumptions. 

The reality is probably somewhere in between. The cabinet is goign to have enough weight to act like a fixed span untill you get heavy loads then it'll left up the cabinet or at least shift weight. 

Awe man i just nerd snipped myself. Now I'm trying to figure out what beam moment diagram would look like. Also the 3d diagram would be interesting as well.

[legenddc]

It withstands a 40 lb kid standing in the middle without being attached to the cabinets okay. I'll have to pull out a straightedge to see if it moves at all when the kid is on it.

Maybe the ply with a 1.5" edge will work. If it does eventually sag I can build a stronger top.

 

3 minutes ago, Chestnut said:

The reality is probably somewhere in between. The cabinet is goign to have enough weight to act like a fixed span untill you get heavy loads then it'll left up the cabinet or at least shift weight. 

Awe man i just nerd snipped myself. Now I'm trying to figure out what beam moment diagram would look like. Also the 3d diagram would be interesting as well.

I know what you mean. I started trying to google it and ended up on physics websites before I decided I didn't get enough sleep for this and it'd be easier to ask here.

[Isaac]

haha yeah its ok.

I think at a minimum you are going to put some kind of hardwood edge on that thing, for aesthetics, and it will look better with a beefier edge. 

[Chestnut]

I'd attach it to the cabinets just to stop it from sliding around. I'd get annoyed with a desk that when i leaned against it the top would move. your mileage may vary.

[legenddc]

I will attach the top. Need to look at it when I get home to figure out I add the edge and not interfere with the cabinets.

[wdwerker]

I've been using the Sagulator for years and testing the results against reality.  32-33" spans are rarely a problem for 3/4" stock. Adding a wider/thicker lip greatly increases the capacity without noticeable sag. 3/4" plywood top with a 500lb total load, center load, yields a .09 total sag~ .034 per foot. A 600 lb uniform load yields a .06 total sag ~.020 per ft. and both of those are without the benefit of a lip. 

A solid walnut 7/8" thick top , 600lb centered load yeilds .05 total sag ~ .018 per ft. If you add a 7/8 x 1 1/4" lip a 750lb centered load yeilds .06 total sag ~ .020 per ft.      Same top w 1 1/4" lip would handle a 400 lb. uniform load with a .08 total sag~ .027 per ft. If treated as an adjustable shelf i.e. tossed on the side cabinets unattached.

If I started with 5/4 walnut a finished top 7/8" thick seems realistic after planing, glue up and sanding.

Its when the shelf is adjustable, 36" or wider & made out of thin or man made materials that load limits get lower. Textbooks and hardback novels usually average around 25 lbs per ft. The calculators default load is 30 lbs per ft uniform load.  

[wdwerker]

3 hours ago, legenddc said:

I will attach the top. Need to look at it when I get home to figure out I add the edge and not interfere with the cabinets.

If they are metal or wood 2 drawer filing cabinets a 1/2" lip hanging down from the top should clear with no problem.  My dad had an office supply business and I have made plenty of plywood & laminate desktops supported by 2 drawer metal filing cabinets. One was 28 ft. wide with 30" spaces for office chairs between roughly 18" wide cabinets. 

[Just Bob]

9 hours ago, wdwerker said:

It might look better with a 1 1/4" lip all the way around for appearances.

Might reduce spontaneous splinters too.

[Coop]

Can “ nerd snipped” be found in the urban dictionary? 

[Chestnut]

@K Cooper

[Coop]

There’s help out there somewhere for ya nut 

[treeslayer]

Just now, K Cooper said:

There’s help out there somewhere for ya nut 

if you find someone check on a group rate Coop 

[Tom King]

All my Physicist friends are also Mathematicians, and one does not take precedence over the other.   To be a Mathematician is sort of a prerequisite in order to be a Physicist.  I don't think there are two separate categories, but the rest of the cartoon is absolutely correct.  My best friend is one of the World's top Physicists, and if he wasn't also one of the World's top Mathematicians, he wouldn't be.

[drzaius]

Oh crap! I can't get that resistor grid problem outta my head.

[Chestnut]

1 hour ago, drzaius said:

Oh crap! I can't get that resistor grid problem outta my head.

I tried to just ignore the infinite grid and assumed the resistors in the close proximity would govern but i got half way through even that and realized my mistake.

So you like circuits

 

[drzaius]

HAHA! Maybe not that much.

I quit the resistor thing when I felt my kidneys start to leak outta my ears.

[wtnhighlander]

You have to realize that ANY problem that involves the term "infinite" is designed specifically to push our nerd buttons. Put down the pencil and walk away.